Download Lab VI

LAB VI
Muscle Function I:
Factors Influencing Muscular Force Production &
Introduction To Electromyography
The next several labs will focus on muscle and neuromuscular function as they relate to
human performance. Today’s lab focuses on introducing students to electromyography and
factors that influence muscular force production. In order to better understand muscle
physiology, and thus how muscles perform, one must first have a solid understanding of basic
anatomical, biochemical, and structural properties of skeletal muscle. Understanding these
properties will also help the student to better understand how muscles adapt to repeated use
(exercise training) or disuse. Some of this pre-lab reading is included to remind students of some
of the basic properties of skeletal muscle that influence muscle performance.
Classification of Muscle Fiber Types & Fiber Type Plasticity
Considerable skeletal muscle research has been based on the red and white appearance of
muscle tissue on gross inspection. The color is based on the abundance of myoglobin in the
tissue. Those muscles undergoing continuous, tonic activity (i.e., postural muscles) have greater
myoglobin content and appear more red. On a microscopic inspection it is now known that most
human muscles are heterogeneous with a relative amount of interspersed red fibers and white
fibers, although some muscles may have more red than white fibers, or visa versa. The
distribution of red and white fibers gives the muscle a checkerboard appearance with varying
ratios of red to white tissues depending on the function of the muscle.
A second characteristic on which muscle is classified is that of contractile rate. Twitch
time was first correlated with red vs. white fibers in 1873 by Ranvier. At that time it was
observed that white muscle contracted faster than red muscle. This resulted in a simple
dichotomy: slow-red muscle and fast-white muscle. Much of the current knowledge in muscle
rehabilitation is based on this finding. The tonic, extensor, antigravity functions of the slow-red
muscle and phasic, flexor functions of the fast-white muscle are basic considerations recognized
in therapeutic exercise of neurological patients. Two years after Ranvier’s initial findings, in
1875, it was found that some of the red muscles contracted at the same rate as the white muscle.
By 1904 it was known that all slow muscle is red but not all red muscle is slow.
Since the early 1960’s the differentiation of mammalian muscle fiber type has been based
primarily on their histochemical profiles. When specific stains are applied to the sectioned
muscle, those fibers with the greatest activity of the specific enzyme will stain the darkest. The
resulting stain profile allows identification of specific skeletal muscle fiber types.
Several classification systems have developed based on a variety of histochemical
(appearance, qualitative) and biochemical (assay, quantitative) characteristics. The prevalence of
such systems made the translation of results across studies difficult particularly when human
studies did not coincide precisely with those of other mammals.
The histochemistry and biochemistry of human muscle initially delineated just two fiber
types: Type I (slow, small, high oxidative, low glycolytic) and Type II (fast, large, low oxidative,
high glycolytic). ATPase activity was used to distinguish the two types until it became clear that
Type II fibers included types with both high oxidative capacities (IIA fibers), and low oxidative
capacities (IIX fibers). Please note that human IIX fibers used to be referred to as IIB fibers. The
greater ATPase activity of type II fibers allows for faster breakdown of ATP and therefore faster
cycling of myosin-actin interactions during the contraction.
The different muscle fiber types reflect the fact that these fibers express different types of
myosin heavy chain (from here on simply referred to as myosin). Myosin is a protein, and is thus
encoded by our DNA. In order to produce a protein, DNA is first transcribed into mRNA, which
Lab VI - 1
is then translated into a chain of amino acids that we call a protein. If a particular gene is “turned
on” the gene is transcribed and translated, and the cell will “express” this protein. Thus, a type I
fiber is a type I fiber because the gene that encodes the type I myosin heavy chain is turned on
and the genes that encode type IIA and IIX myosin are turned off. Likewise a IIX fiber is a II X
fiber because it expresses type IIX myosin (gene for myosin IIX turned on and genes for IIA and
I are turned off).
The different types of myosin vary in their ability to break down ATP. Types IIA and IIX
can break down ATP much more quickly than type I myosin. This accounts for the faster rate of
cross bridge cycling in these fibers. Faster cycling of cross bridges between actin and myosin
means that these fibers can produce force more quickly and have a shorter twitch time. Thus, an
individual with a large proportion of fast twitch fibers (IIA and IIX) would be able to move more
quickly and would excel in events such as sprinting events.
Types I, IIA, and IIX fibers, based on myosin ATPase histochemistry, make up the vast
majority of adult human skeletal muscle fibers. These types can be distinguished using
histochemical procedures to determine the stability of the myosin ATPase enzyme under three
different pH conditions (10.6, 4.3 and 4.6). By comparing the staining intensity of serial sections
of a muscle sample the staining characteristics of a single fiber can be compared under all
conditions. This procedure also allows the identification of three additional intermediate fibers,
IC, IIC and IIAX. The IIAX fibers represent an intermediate fiber between IIA and IIX and may
reflect a transitional stage. The IC and IIC fibers are express some amount of type I myosin and
some amount of type IIA myosin. These intermediate fibers illustrate that an individual muscle
fiber may express more than one type of myosin. For example, type IIAX fibers express myosin
types IIA and IIX. Intermediate fibers such as IC and IIC appear to be in transition between
slow and fast fibers; the genes for both type I and IIA myosin heavy chain are both turned on to
some extent.
Exercise training studies have shown the possibility of fiber plasticity or the transition of
one type to another. Although some studies have shown transitions from type I to type II and the
opposite with training, there is relatively little evidence to support this type of transition in
humans under normal conditions. However, there is strong evidence for the transition among the
subtypes of the II fibers i.e. type IIX fibers to IIA fibers, and visa versa. Extreme changes in fiber
types between slow and fast fibers have been observed in some unusual or experimental
circumstances. For example, in humans (and animals), spinal cord injury and deinnervation of a
muscle can result in transitions from type I fibers to type IIX fibers. Under experimental
circumstances in rabbits and rodents, chronic low frequency stimulation (>12 hrs/day for 30
days) can result in IIX  IIA  I transitions. Likewise, experimental deinnervationreinnervation studies in small mammals have shown that if you innervate a muscle that is
composed of principally type I fibers with a nerve that usually innervates a muscle with
principally type IIX fibers, the muscle will become almost all type IIX. Thus, the innervating
nerve, or its stimulation pattern, plays a role in dictating the fiber type.
So, how do these properties of skeletal muscle influence muscle performance attributes
such as strength, speed, power, and endurance? Several factors influence each of these types of
performance so only a few examples will be provided here. The types of muscle fibers found
within a muscle is obviously an important determinant of the speed of muscular contraction. The
more fast twitch fibers there are within a muscle, the faster the speed of muscle contraction.
Properties of Skeletal Muscle & Muscle Fibers that Influence Performance
Skeletal muscles and the fibers that make them up have evolved produce force, and to do
so under varying conditions. For some of our evolutionary ancestors, the maximal force
produced was less important than being able to produce force quickly (needed for fast
locomotion; perhaps to catch prey or avoid predation). For others, the ability to maintain
Lab VI - 2
repeated, submaximal forces for long periods of time (e.g. for gathering food) were more
important than either maximal force or maximal velocity. Thus, it is no surprise that among the
human population we find individuals whose muscles have conferred on them the ability to
produce amazing forces (strength athletes), achieve incredible speeds (sprinters), produce great
forces rapidly (power athletes like throwers), or travel long distances (endurance athletes). These
different performance attributes (force, speed, power, endurance) are dependent upon specific
properties of our skeletal muscles, and of the fibers that make them up. Because different
properties confer different advantages upon the muscle or muscle fiber, one can understand why
it would be difficult, or impossible, to find someone who is simultaneously the fastest, strongest,
and most durable athlete. That is, there is good reason why a particular type of athlete will excel
in one, or perhaps a few, sports or events, but not in others.
The major reason why fast twitch fibers contract faster than slow twitch fibers is that they
have a greater myosin ATPase activity, and therefore can breakdown ATP faster than slow
twitch fibers. This allows for faster cycling of cross-bridges during the contraction cycle.
Additionally, fast twitch fibers also are well equipped to make ATP quickly. This fast ATP
production is associated with a great ability to perform anaerobic glycolysis, which proceeds
more quickly than aerobic glycolysis. Fast twitch fibers store a great deal of glycogen, which
along with glucose from the blood can serve as the substrate for glycolysis. Additionally,
because fast twitch fibers also have a great deal of glycolytic enzymes, such as lactate
dehydrogenase (LDH), they can produce ATP very rapidly. Unfortunately, anaerobic glycolysis
results in the production of lactic acid. Like other carboxylic acids, it readily dissociates from
hydrogen ions under physiological conditions, and hydrogen ions may, in turn, predispose the
individual to fatigue. The fast rate of ATP degradation may also increase the amount of
inorganic phosphate (Pi), which may predispose the individual to fatigue. Thus, fast twitch fibers
can contract quickly but have poor fatigue resistance.
Muscle fiber length also influences the velocity of muscle contraction. Longer fibers can
contract faster than shorter fibers. For example, we generally find in humans that the flexor
muscles (e.g. hamstrings and biceps brachii) are arranged as fusiform muscles with long fibers
oriented down the long axis of the muscle. Extensors, on the other hand (like quadriceps femoris
and triceps brachii) tend to be arranged as pennate or bipennate muscles with numerous, short
muscle fibers. The long fiber length of the flexor muscles enables them to produce modest
forces, but with great velocity. These long fibers provide another potential advantage; they allow
for greater excursion. That is, they can shorten to a much greater extent than a short fiber, and
thus they may allow for a greater range of motion.
If you consider other species, some of the fastest contracting fibers can be found in
rattlesnake tail muscle and hummingbird flight muscle. These muscle fibers can be stimulated
over 100 times per second (>250x/sec in rattlesnake tail muscles). This means both turning on
and turning off muscle contraction very rapidly. This can only be accomplished by having a
very large volume of sarcoplasmic reticulum that can both release and re-sequester calcium very
rapidly. It is also noteworthy that the properties of the sarcoplasmic reticulum, and of calcium
handling proteins, in general, vary between fast and slow twitch fibers. For example, fast twitch
fibers many have up to seven times more SERCAs (sarcoplasmic reticulum calcium ATPases)
than slow twitch fibers. This would allow a fast twich fiber to relax much faster than slow twitch
fibers, and thus contributes to the shorter duration of the twitch time in fast fibers.
While muscle length does influence the velocity of movement, it does not influence
muscle strength (maximal force production). Muscle fiber size, however does influence muscular
force production, but in a different dimension. Specifically, the cross-sectional area (XSA) of a
fiber is the major property of a fiber that dictates its ability to produce force. A fiber with a
larger cross sectional area would have more contractile proteins in parallel that can contribute to
force production. Pennate, bipennate, and multipennate muscles pack a lot more fibers into the
Lab VI - 3
cross section of the muscle which allows them to produce a great deal of force. As you can see
from the above discussion, the design of the muscle, thee muscle architecture can influence
muscle performance in many ways by varying the length of the fibers, the arrangement of the
fibers, the angle of pennation, etc.
Cross-sectional area is influenced by a number of factors. Gender, age, exercise training,
and disuse are just a few of a long list of factors that influence fiber cross-sectional area. These
influence XSA by influencing the rate of protein synthesis and/or protein degradation within the
fiber. Related to this is regulation of the ratio of myonuclei per amount of fiber (the myonuclear
domain). The DNA that encodes proteins within a cell is found in the nucleus. If a fiber is
growing, it may need to increase the number of nucelei in order to be able to produce the
additional protein needed for the muscle to grow and to become stronger. Satellite cells are cells
just outside the cell membrane that can act as a source of new nucelei during muscle growth,
repair, or regeneration.
Muscle hypertrophy is the term used when muscle fibers are increasing in size, resulting
in growth of the whole muscle. Another potential mechanism of increasing the size of a whole
muscle is increasing the number of fibers present; this is called muscle hyperplasia. There is a
large body of evidence that suggests that muscle growth with training is a result of muscle
hypertrophy, not hyperplasia. However, it does seem that individuals who are predisposed to
having very large muscles (like your instructor) may have the benefit of being born with more
muscle fibers.
The loss of muscle mass is called muscle atrophy, and is likely due to a decrease in
muscle fiber size (as a result of loss of contractile proteins). The loss of muscle mass with aging
is referred to as sarcopenia. When a muscle cell becomes smaller in size, it does not need as
many nuclei to support this now smaller fiber, so some of the nuclei undergo a type of
programmed cell death called apoptosis.
Muscle hypertrophy and atrophy are thus influenced by the proliferation of satellite cells,
the rate of myonucelar apoptosis, and the rates of protein synthesis and degradation. These
processes, in turn, are regulated by multiple mechanisms including a number of hormones and
growth factors. The hormones and growth factors that regulate these processes do so by a
number of intracellular signaling mechanisms.
Muscular strength is influenced by a multitude of factors. As previously stated, the
greater the cross-sectional area of the fiber, the greater the number of contractile filaments there
will be for contractile processes. Thus, a large diameter fiber can produce more force than small
diameter fibers. Students should also be reminded that force (related to strength) and speed of
movement are the major determinants of power. Therefore, muscular power is ultimately
dependent on muscular strength and the speed of muscular contraction. Many events that we
traditionally think of as requiring a great deal of strength really require a great deal of power. For
example, force must be generated very rapidly in order to jump or to throw heavy objects.
Muscular endurance, just like speed and strength, is affected by multiple properties of the
muscle fiber. In the literature, much attention has been given to the role of metabolic processes
in determining a fiber's resistance to fatigue (endurance is dependent on resisting fatigue). Slow
twitch fibers have greater fatigue resistance because they are especially well equipped for
aerobic metabolism as a result of several properties. For example, slow twitch fibers have a
greater number of capillaries compared to fast twitch fibers, which would allow for greater blood
flow. Slow twitch fibers also have a greater amount of oxidative enzymes, such as succinate
dehydrogenase (SDH), than fast twitch fibers. These higher levels of SDH can be partially
explained by the greater amount of mitochondria in slow twitch fibers. Additionally, glucose,
glycogen, and fatty acids can be used for aerobic metabolism, and this ability to use fats explains
why more lipids are stored in slow twitch fibers than in fast twitch. All of these properties relate
to the slow twitch fibers' ability to use aerobic metabolism. The human body has a great deal of
Lab VI - 4
substrate (glucose, glycogen, and lipids) available for aerobic metabolism. Furthermore, while
aerobic metabolism proceeds more slowly than anaerobic glycolysis, it does not result in the
production of lactic acid. Thus, it is easy to see how aerobic metabolism can allow for sustained
energy production for long periods of time, and thus avoid fatigue. It is also noteworthy that
slow twitch fibers contract more slowly because of slower ATPase activity and thus consume
less ATP for a given workload, making them much more efficient. Furthermore, they are
stimulated less frequently, which also reduces the energy required to achieve a given workload.
This greater efficiency is also of great benefit in long distance performance as covered in
previous labs.
Some major characteristics of human skeletal muscle fiber types
Fiber Type
Characteristic
I
IIA
IIX
Diameter – men*
small
large
intermediate
Diameter – women*
large
intermediate
small
Z-line diameter
wide
intermediate
narrow
Twitch time
120ms
64ms
64ms
ATPase activity
low
high
high
SDH, oxidative enzymes
high
med-high
low
LDH, glycolytic enzymes
low
med-high
high
Mitochondria
high
intermediate
low
Capillary density
high
intermediate
low
Myoglobin
high
high
low
Lipid content
high
medium
low
Glycogen content
low
intermediate
high
Recruitment order
1
2
3
Resistance to fatigue
high
high
low
* For young men and women. From Staron et al. Journal of Histochemistry and Cytochemistry
48(5): 623-9, 2000. Available at www.jhc.org/
Muscle Actions
The distinguishing characteristic of muscle tissue is its ability to produce active tension.
This tension is the result of the interaction of actin and myosin filaments and the “power stroke”
of the acto-myosin cross-bridges (the sliding filament theory). The tension developed by these
myofilaments can produce three types of “actions” depending on the amount of resistance placed
on the muscle. If the active tension produced by the muscle is equal to the resistance placed on
the muscle the result is an isometric (meaning same length) or static action. By definition an
isometric muscle action occurs when the muscle is activated but its length remains static. This
definition works well when examining isolated muscle, however, we usually examine muscle
function within the confines of a bony lever system. Therefore, an isometric action is frequently
described as a muscle action that results in no joint movement or change in joint angle.
If a muscle produces more active tension than the resistance placed on the muscle, a
shortening or concentric action results. This type of action is the easiest to understand in the
confines of the sliding filament theory; as the cross-bridges rotate, the actin and myosin
filaments slide past one another shortening the length of the sarcomere and the muscle fiber. If
the amount of active tension produced by the muscle is less than the resistance placed on the
muscle, a lengthening or eccentric action occurs. This type of muscle action is not as easily
explained by the Sliding Filament Theory. The acto-myosin cross-bridges are formed but the
sarcomere and fiber are lengthened.
Due to the changes in muscle length, concentric and eccentric actions are also referred to
as dynamic actions. Concentric and eccentric actions used to be referred to as isotonic actions
Lab VI - 5
because the amount of weight being lifted was constant, and thus it was thought that the “tone”
in the muscle remained constant (i.e. isotonic, same tone). It is now clear that during normal
dynamic actions the muscle’s “tone” does not remain constant. Therefore the term isotonic may
be outdated. Nevertheless, concentric and eccentric actions are still frequently referred to as
isotonic actions. Many physiologists prefer the term dynamic, when referring to concentric and
eccentric, to isotonic because it does not imply same tone, but rather implies simply that the
muscle is moving and the joint angle is changing.
Motor Unit Recruitment and Electromyography
When a muscle fiber is stimulated to produce a single twitch it produces force in an all or
nothing fashion. A large fiber (which contains more actin and myosin) will produce more force
than a smaller fiber because it can create more cross-bridges. However, axons of motor neurons
branch to innervate several muscle fibers. Thus, the smallest amount of muscular contraction
possible is when all of the fibers innervated by a motor neuron contract together. A motor
neuron and all of the muscle fibers it innervates is called a motor unit. The number of muscle
fibers per motor unit varies between different muscle groups and within each muscle (i.e. each
muscle has many different sizes of motor units). Motor units that contain more and/or larger
muscle fibers will produce greater amounts of force. Muscles concerned with fine, graded
movements such as those in the fingers or eyes may have as few as three to six muscle fibers per
motor unit. Muscles involved with more gross movements such as those in the legs or back may
have hundreds or thousands of muscle fibers. For example, the gastrocnemius muscle averages
over 1,000 fibers per motor unit.
Activation of motor units can be easily observed by electromyography (EMG). This
process involves measuring the electrical activity of motor units for various muscle groups. The
electrical potentials are picked up using small needle electrodes that are either inserted directly
into the muscle or surface electrodes placed on the skin over the muscle group to be measured.
Small needle electrodes can allow for the assessment of action potentials of individual motor
units. Surface electrodes, on the other hand, pick up the electrical activity from many different
motor units simultaneously. Impulses picked up by the electrodes can be amplified and
recorded. Only surface electrodes will be used in our EMG experiments. Remember, with
surface EMG we will not be able to assess individual action potentials. Instead, what we will be
observing using surface EMG is the electrical activity that results from all action potentials from
all activated muscle fibers at each point in time. For this reason surface electrode recordings
appear rather disordered.
It should also be pointed out that the amount of muscle electrical activity that actually
gets to the surface of the skin, and therefore can be recorded using surface EMG, is influenced
by the amount of resistance (impedance) encountered by the electrical current between the
muscle and the surface. Several factors affect this resistance. For example, the amount of oil or
moisture on the skin, the thickness of the layer of dead skin cells on the surface of the skin, and
the amount of subcutaneous fat all alter the amount of resistance the electrical current will
encounter between the skin’s surface and the muscle. As a result of this resistance, not all
electrical activity actually gets to the surface of the skin. Thus, one must be cautious when
comparing EMG activity between individuals, between different sites on the body, or even
comparing EMG activity at different times (e.g. before and after training). To minimize this
resistance to electrical flow, an alcohol prep pad is usually used to scrub the area where the
electrode will be placed. The alcohol pad will help remove any oil, moisture, and perhaps some
dead skin cells present on the surface of the skin.
In a normal muscle at rest there is very little electrical activity and the muscle is said to
be electrically silent. However, keep in mind that postural muscles are involved simply in
helping our body to resist gravity (and therefore help us to maintain an upright posture). Thus,
Lab VI - 6
these anti-gravity muscles will maintain some amount of “tonic” activity even when the body is
at rest. That is, there will always be some EMG activity in these postural muscles whenever the
person is upright. When any muscle becomes active, for example when lifting a weight, there
will be an increase in electrical activity. Remember, the electrical activity observed, using
surface EMG, at any point in time is a result of all of the electrical activity in all fibers that are
electrically active. It is also worth pointing out that while activation of muscle fibers is the result
of neural stimulation, the EMG tracing represents only the electrical activity in the muscle, not
the motor neurons.
It is easy to demonstrate, using EMG, that the greater the force produced by the muscle,
the greater the electrical activity in the muscle. This increase in electrical activity suggests that
more motor units are active. This process, known as "recruitment of motor units", permits us to
control, with some precession, the amount of force created by the muscle. Therefore, it also
allows us to make smooth, graded movements. Recruitment can be controlled in two major ways:
by altering the frequency with of firing of the motor units or by altering the number of activated
motor units. Twitch (Wave) Summation relates to the frequency of motor unit stimulation and
Multiple Motor Unit Summation (MMUS) relates to the number of motor units that are being
stimulated at a given time.
The process of recruitment can be seen on an EMG trace by either an increase in
amplitude or frequency of the EMG signal. In general, if a motor unit is stimulated at a high
frequency the twitches will summate (twitch summation) and will increase force production.
Using needle EMG one can observe twitch summation by changes in the frequency of the EMG
signal. Noting changes in frequency of stimulation of individual motor units is not possible
using surface EMG. Muscular force production can also be increased by recruiting more motor
units (multiple motor unit summation). The more motor units there are active at any given time,
the greater the amplitude of the EMG signal. Our central nervous system uses both recruitment
processes, wave summation and MMUS, to control the amount of force generated within a
muscle. Several other considerations related to recruitment (e.g. recruitment order and
synchronization of MU activation) will be discussed in the muscle function III lab.
However, motor unit recruitment is more complex than just altering the number of motor
units being used and the firing rate of those motor units. During sub-maximal contractions the
motor units ”take turns” firing and relaxing in order to avoid fatigue. For example consider an
isometric action that requires the use of only two motor units at a time, and the motor units being
used are named A, B, and C. At first motor units A and B may be used, while motor unit C is
relaxing. Then, motor units B and C are fired, allowing motor unit A to relax. Following this,
motor units A and C are fired together allowing motor unit B to relax. Then back to A and B
firing together and so on. This process of constantly switching the motor units being fired is
called asynchronous motor unit recruitment. Keep in mind that this is a highly simplified version
of events because most movements require many more than just two motor units at a time.
During dynamic muscle actions the CNS controls this asynchronous recruitment of motor units,
while at the same time altering the total number of motor units fired, as well as altering the firing
rate of the motor units. In this way we can produce smooth movements, altering force
production as needed, and at the same time delaying the onset of fatigue. Interestingly,
resistance exercise training appears to increase a person’s ability to fire motor units in synchrony
during forceful contractions. The end result is that during forceful contractions more motor units
can simultaneously be stimulated.
In order to objectively evaluate surface EMG activity, the clinician or researcher must first
process the signal in several ways. Processing of the EMG signal frequently involves filtration,
rectification, integration, and normalization. The EMG activity in a muscle will have just as
much positive as negative electrical activity (just as much electrical activity is moving towards
the electrodes as there is going away from them). Thus, if one were to try and quantify this
Lab VI - 7
electrical activity, one would probably come up with about zero electrical activity (for example
summing positive 5 µV and negative 5 µV = zero). In order to quantify the EMG signal one
must first make all of the negative electrical activity positive, this process is called rectification.
Using integrated EMG (iEMG), the computer will integrate this rectified signal, and allow us to
quantify the electrical activity (the computer will calculate the area under the curve, which will
be recorded on the screen as an integral).
An additional consideration when discussing the complexity of even the simplest
movements is the order in which motor units are recruited. Smaller motor units tend to have
lower thresholds for recruitment. That is, they do not need as great a stimulus to create an action
potential. The smallest motor units tend to be composed of type I muscle fibers. Thus, small
motor units made up of type I muscle fibers are recruited first because they have the lowest
thresholds for recruitment. Type IIX muscle fibers tend to be included in large motor units and
Type IIA muscle fibers tend to included in intermediate sized motor units. Because of these
differences in motor unit size and threshold for recruitment, motor units with type I muscle fibers
tend to be recruited first, no matter how much force is required. With increasing force
requirements, more motor units with type I muscle fibers will be recruited. Eventually, as the
force requirement becomes greater, motor units with type IIA muscle fibers will be recruited
next, and finally during contractions that require maximal or near maximal force production
motor units with type IIX muscle fibers will be recruited. This orderly process of recruiting the
motor units with the smallest motor neurons first followed by those with larger motor neurons is
called the size principle, which was first described by Henneman in 1965.
There is some evidence, albeit mostly from animal studies, that there are exceptions to
the rules of the size principle. For example during extremely quick, powerful movements, such
as when your cat jumps from the floor to the top of your refrigerator, type IIX fibers may be
recruited first. Additionally, it appears possible that during eccentric muscle actions the higher
threshold motor units (type IIA or IIX) may be recruited first.
What is the importance of this size principle? Well, consider some of the movements
from your daily life. When you are picking up a pencil it would not make sense to use the
largest, strongest motor units. On the other hand, when you are lifting a large amount of weight
adding the force of a very small motor unit would not be very helpful. Recall that type I fibers
are especially equipped for aerobic metabolism and type IIX fibers are especially equipped for
anaerobic metabolism. Aerobic metabolism is more efficient than anaerobic metabolism (you
get approximately 18 times more ATP from a single glucose molecule aerobically than
anaerobically) and fats can be used aerobically, but not anaerobically. Therefore, the animal
can produce more energy for a given amount of fuel substrate and at the same time spare
glucose for the brain. So, from an energy standpoint, this orderly recruitment of motor units
makes us energetically much more efficient.
If comparisons of integrals are to be made between iEMG recordings it is important to
make sure that the integral is obtained from a similar time period (similar delta T) from each
recording. If this is not taken into consideration, one might erroneously conclude that one iEMG
recording had more electrical activity than another simply because the integral was obtained over
a longer period of time. For example, if a subject were to perform two isometric actions using
the same weight, and perform these actions in exactly the same manner on both occasions one
would expect the integral to be almost exactly equal. But, if the integral was measured over .2
seconds of the first iEMG recording and over .4 seconds of the second iEMG recording, clearly
there would be more electrical activity during a long period of time than during a short period of
time. The different integrals in this example would not be due to actual differences in the
number of MU recruited, instead they would be due to differences in time period over which the
integral was calculated.
Lab VI - 8
Comparing surface EMG activity between subjects or between two parts of the body is, at
best, difficult. This is partly due to the fact that skin resistance varies greatly between subjects
and between different parts of the body even on the same subject. For this reason, if you want to
compare EMG results between subjects or between different parts of the body, the EMG activity
must first be normalized. The integrals obtained from surface iEMG recordings can be
normalized in several ways. Normalized EMG activity is usually calculated as the recorded
iEMG integral divided by the iEMG integral obtained during a standard task. One of the most
commonly methods of normalizing iEMG activity is by normalizing the electrical activity
relative to the iEMG activity recorded during a maximal voluntary contraction (the most force a
person can voluntarily produce during an isometric action). Thus, the integrals obtained from
various muscle activities can be reported simply as a percent of the electrical activity during
maximal voluntary contraction. Sometimes iEMG activity is normalized to the integral obtained
while producing a standard force (or holding a standard weight).
In addition to all of this processing of the EMG signal, it is also frequently necessary to
filter out other sources of electrical “noise”, such as background electrical current associated
with nearby electrical outlets. Electrical activity of the heart may also be observed if electrodes
are placed near the heart; this electrical activity can also be filtered. One problem with filtering
out electrical activity on an EMG is that you will also lose some of the muscle electrical activity.
Factors affecting muscular force production
In addition to motor unit recruitment there are several other factors that influence
muscular force production (see table below). In this lab we will focus on how motor unit
recruitment, joint angle, type of action, the stretch-shortening cycle, and velocity influence
muscular force production.
The joint angle that a muscle is acting about is an important factor determining the
amount of force that will be produced. There are several potential mechanisms by which joint
angle can influence muscular force production. In human physiology you learned about the
length-tension relationship of skeletal muscle. This relationship explains how the length of the
sarcomere influences the amount of actin myosin overlap, and therefore affects the ability of the
muscle to actively produce force. As joint angle changes through the range of motion, the length
of the muscle, and thus the length of the sarcomeres also changes, thus affecting the amount of
active tension that can be produced. Furthermore, the more a muscle is stretched the greater the
amount of passive tension there will be (tension associated with stretching of the elastic
components of skeletal muscle). In vivo the muscle moves through a relatively small portion of
the length tension relationship that is depicted in textbooks. For example, while early
experiments by Huxley demonstrated that almost no force is produced if a muscle is shortened to
than 70% of its original length or stretched to170% of its original length, these extremes are
never observed during normal human movement. Thus, there must also be other mechanisms by
which joint angle influences force production.
Leverage has a large effect on the amount of force that can be produced. Leverage
changes as joint angle changes, and it is affected by several factors. First, the angle of force
application, which is the angle at which the muscle inserts on the bone at a particular joint angle
will influence leverage. Leverage is better, and we are more mechanically efficient, if the
muscle is pulling on the bone at a 90 degree angle. This maximizes the amount of muscular
force that is transmitted to the bone. The further the angle of force application is from 90
degrees (above or below 90 degrees) will result in poorer leverage. A second consideration is
the force arm: resistance arm ratio. Humans produce meaningful movement by moving bones
about a joint. Each joint is arranged as lever system. The force arm of a lever is the distance
(perpendicular to the pull of gravity) from the fulcrum of the lever to where the force is being
applied. The resistance arm of a lever is the distance (perpendicular to the pull of gravity) from
Lab VI - 9
the fulcrum of a lever to location of the resistance. For example, the distance from the elbow to
the insertion of the biceps brachii would be the force arm during a bicep curl and the resistance
arm would be the distance from the elbow to the weight in the subject’s hand. The greater the
force arm: resistance arm ratio, the greater the leverage.
A firm understanding of leverage has several practical applications. First, the greater the
leverage, the greater the force the subject can be produce at that joint angle. Second, if a subject
were to hold a specific weight at two different joint angles, one with a poor leverage and one
with good leverage, fewer motor units would be needed to hold the weight when there is good
leverage. The point in the range of motion where a subject has the poorest leverage is frequently
referred to as a “sticking point”. Sticking points therefore are the points in the range of motion
where we will require the most motor unit recruitment to lift a given weight. When performing
isokinetic movements, the sticking point or points can be identified as the joint angles where the
subject produced the least force.
Nearly all skeletal muscles result in the movement of a bone around a particular axis of
rotation. Thus, the muscle is creating force in a somewhat linear direction, but the force that is
created by moving the limb is being created in an angular direction. Torque is essentially force
applied in an angular direction. As discussed in previous laboratories, the Newton is the
internationally recognized unit of force. The internationally recognized unit for torque is the
Newton-meter (Nm) and the traditionally used Americanized unit is the foot-pound (ft-Lb). The
torque can be calculated if you know the force being exerted on an object (e.g. the force applied
at the hand if one is performing a dumbbell curl with a 10 pound weight is 10 pounds or 44.5
Newtons) and if you know the distance from the axis of rotation (the elbow in this case) to where
the force is being applied (the hand in this case). Torque can be calculated as the force exerted
on an object times the distance between the object and the axis of rotation. In the previous
example if the distance from the elbow to the hand was .3m and the force was 44.5 Newtons,
then the torque would be 13.35 Nm (= 44.5 x .3)
The effect of joint velocity on force production is best illustrated by the Force - Velocity
Curve. During concentric actions, as velocity increases force production decreases, but during
an eccentric action force production increases with velocity. Thus, the ability of muscle to
produce force is also affected by the type of action being performed, the transition between
actions and the speed at which the action is being performed. Maximal eccentric force
production has been reported to be 30-40% greater than that recorded for maximal concentric
efforts. An explanation for this finding is unclear, partly because the sliding filament theory
does not explain a muscle produces force during eccentric actions. Furthermore, isometric
actions, where there is no movement can be thought of as infinitely slow moving contractions.
Thus isometric force production is greater than concentric force production and would be
expected to be lower than eccentric force production.
The stretch-shortening cycle or pre-loading of the muscle with an eccentric stretch has
been reported to produce greater force production in the following concentric action. This
phenomena may be the result of stored elastic energy during the pre-stretch (stretching the elastic
components of the muscle) or by increasing MU recruitment via a stretch reflex. There are many
possible contributors to the elastic properties of muscle (and thus to passive tension). One of the
most important elastic components of muscle is the protein titin, which extends between the
thick filaments and the z-lines. Additionally, collagen and elastin found in the tendons and
muscle connective tissue (epimysium, perimysium, and endomysium) may also contribute.
There are additional considerations in describing what can influence muscular force
production. For example, it has been demonstrated that strong muscle contractions around 8-12
minutes prior to strength, power, jump, or sprint performance may actually enhance
performance. This phenomenon is referred to as post-activation potentiation.
Lab VI - 10
Table 1. Major Physiological Factors that affect muscular force production:
Size of the muscle
# of motor units
Size of motor units
# of fibers/MU
Size of fibers ( Amount of contractile proteins in each fiber)
Recruitment
# of active motor units
Frequency of activation of motor units
Type of action (eccentric, concentric, or isometric)
Joint angle
Leverage
Length-tension relationship
Stretch-shortening cycle
Stretch reflexes
Use of elastic energy
Velocity of movement
Fatigue
In summary there are several factors that influence muscular force production (see table
above). This list is not all encompassing or comprehensive – this list simply summarizes the
major properties of a muscle (e.g. size) and how a muscle is activated (e.g. slow, fast, eccentric,
concentric, etc.) that influence muscular force production. Other factors such as exercise
training, aging, gender, and the presence of neuromuscular diseases can influence muscular force
production, but each of these will generally influence force production by altering one of the
physiological variables listed above. For example, resistance training results in hypertrophy of
muscle fibers and increases the ability to recruit motor units. Aging, on the other hand, is
associated with atrophy of muscle fibers and a reduced ability to recruit all available motor units.
Muscle Performance
The performance capability of muscle can be expressed in three ways; muscular strength,
muscular endurance and muscular power. Muscular strength is a measure of the maximal
tension or force that a specific muscle or group of muscles can generate. Maximal values of
strength are defined as maximal force production in a single effort. Strength, therefore,
represents the maximal number of acto-myosin cross-bridges that can be formed at one time.
Muscular endurance is the ability of a muscle or group of muscles to sustain a level of tension or
force over an extended period of time. Endurance capacity is not dependent on the maximal
number of cross-bridges that can be formed at any one, but the ability of the energy systems to
continuously recycle ATP to continue cross-bridge activity.
By definition power is the rate at which work is accomplished or work per unit time.
Remembering that
work = force x distance, therefore power = (force x distance) / time.
To better understand muscular power this formula can also be expressed as
power = force x (distance / time) or force x velocity.
Muscular power reflects the explosive nature of strength. Power is optimized when high forces
are being produced at high speeds. However, remember that during a concentric action, as
movement speed increases maximal force production decreases.
Isokinetic Actions
In the next few labs we will also be using a unique, man-made instrument called an
isokinetic dynamometer, to study various properties of muscular force production and muscular
Lab VI - 11
performance. Isokinetic dynamometers have several potential clinical and athletic applications.
Muscle actions performed on isokinetic dynamometers are frequently referred to as isokinetic
actions. Isokinetic devices can be used to perform isometric, concentric, or eccentric actions.
An isokinetic action is one where angular velocity (not the velocity of muscle shortening) is
constant, and is performed against an accommodating resistance, thus maximally loading the
muscle at all joint angles. Because isokinetic dynamometers use an accommodating resistance,
the harder the subject pushes against the lever arm of the dynamometer, the greater the resistance
the dynamometer will provide. When working with an isokinetic device the subject is asked to
contract maximally every contraction. Thus, the accommodating resistance theoretically allows
for maximal motor unit recruitment at all points throughout the range of motion. When we refer
to isokinetic actions, the term “action” refers specifically to how the muscle is performing the
activity (maximal contraction against an accommodating resistance with a constant angular
velocity). But when we use the terms concentric, eccentric, or isometric actions we are referring
to what the muscle is doing as it produces force (shortening, lengthening, or staying the same
length), but does not consider how it is performing the activity.
When isokinetic dynamometers were first developed it was thought that they would be a
great way to train muscles. It was thought that by allowing for maximal recruitment through all
joint angles, that subjects might be able to better strengthen their muscles. However, studies
performed over the last few decades have not demonstrated greater strength or muscle mass
gains when compared to more traditional resistance training regimens. Furthermore, proponents
of isokinetic machines neglected to consider specificity of training. There are very few, if any,
examples of normal human activities that involve isokinetic actions. Thus, an athlete who trains
on an isokinetic dynamometer, performing constant velocity movements against an
accommodating resistance, is unlikely to see great improvements in strength during their normal
athletic events. Whenever possible, in order to achieve optimal results, an athlete should be
tested and trained the same way they will be competing (specificity).
Pros and Cons of using Isokinetic devices:
Pro
Velocity can be controlled, potentially important for research applications.
The use of an accommodating resistance ensures that injured patients or individuals
undergoing rehabilitation will not be required to produce such great forces as to risk
injury.
Computerized system records multiple variables during each contraction (torque, power,
work, etc.).
Con
Lack of specificity (This is a major concern):
It controls velocity unlike most natural movements.
It is performed against an accommodating resistance unlike most natural movements.
Most movements performed on an isokinetic dynamometer isolate a single muscle or
muscle group unlike most natural movements
Devices are very expensive compared to free-weights.
Technicians running the device require considerable training time in order to consistently
get accurate results.
It takes a long time to set up each subject, thus fewer subjects can be trained or tested in a
given period of time.
Devices take up a lot of space.
Lab VI - 12
LABORATORY PROCEDURES
*remember to clean up after each experiment and return all equipment to its original location
EMG set up
Computer and amplifier setup. For this experiment you will be using the Gateway
computers with the Acknowledge software. Data will be collected from electrodes connected to
a differential amplifier (DAM 50). The DAM 50 will be connected to the BIOPAC system
which is connected to the computer. All of the proper connections and adjustments will be made
for you before lab starts. The only adjustment you will need to make will be turning on the
power switch and the input select switch on the DAM 50. Once all electrical connections
(including electrode connections) have been made turn the Power on and turn the Input
select from ground to A-B. Make sure you turn these controls to ground positions when
you remove or adjust your electrodes.
Electrode Placement. To perform this experiment you will place an electrodes over the
biceps brachii. Cleanse the skin on the anterior arm with alcohol. Rub the skin hard to remove
oil and dirt. Apply electrode gel to the electrode chambers. Remove the paper from the back of
the electrodes and adhere two of the electrodes to the anterior arm (over the biceps brachii) the
third (ground) can be placed anywhere. Connect the three electrode wires to the electrode
buttons. You can now turn the DAM 50 Power to on and change the Input select from ground to
A-B. IMPORTANT, after completing the experiment you must change the input select on the
DAM 50 back to ground and turn the DAM 50 off before unhooking the subject. Failure to do
this before unhooking the subject may blow a very, very expensive fuse in the DAM 50.
Recording the iEMG. We will be recording EMGs using the "Acknowledge" software.
In the bottom right hand corner of the screen that serves as both the start and stop button. To
begin recording an EMG simply press start. To stop recording press stop.
Screen display adjustment. If you move the cursor to the voltage bar (vertical) on the
right hand side of the display and click the mouse you will be able to adjust the size of the
voltage display. The same procedure can be used to adjust the size of the time scale (on the
bottom of the display, usually between 0.5sec/div and 2sec/div work well.). To optimally fit
your tracing on the screen go to the :display menu” at the top of the screen near the center.
Choose “autoscale waveforms”, this should optimally fit all information on the screen. You can
change the voltage and time scale of the recording at any time. However, it is recommended that
you not change these scales while you are actually recording the EMG.
Quantification of iEMG activity. After recording the EMG you can quantify the iEMG
activity by clicking and dragging over the desired portion of the screen. Remember, when
comparing different contractions to use the same time period (same “delta T” for reach
measurement), usually 0.1 to 0.3 seconds near the peak activity of the contraction works well).
Click and drag over the appropriate “delta T” and record the “integral”. This “integral” indicates
the amount of electrical activity during that time period. The “integral” and “delta T”
information can be seen on the button bar near the top of the screen. In some cases it is
beneficial to calculate an integral average by dividing the integral by the delta T, which allows
one to compare EMG data over varying time frames (different delta T values).
In some of the experiments you will be asked to "normalize" the EMG activity to the
EMG recorded during a maximal voluntary contraction (MVC). This normalized iEMG can
only be calculated after you have recorded the iEMG integral during an isometric maximal
voluntary contraction. You simply divide the iEMG integral from the experiment and divide it
into the iEMG recorded during the MVC and multiply by 100. In this case the normalized iEMG
is simply expressed as a percent of the maximal value.
Lab VI - 13
I. Isometric Actions
A. Motor Unit Recruitment. This experiment involves the recording of EMG activity of
the biceps brachii muscle. Prepare your subject and place the electrodes as described
above. You will be recording EMG and iEMG activity at rest and during light,
moderate, and heavy isometric contractions. Follow the instructions above for setting
up your subject and recording the EMG. In addition to making a rough drawing of
the EMG and iEMG activity you will also record the iEMG integral for each of these
contractions following the procedures above.
EMG activity
0 mV
Rest
Light
Moderate
Heavy
Integrated EMG (iEMG) activity
0 mV
Rest
Light
Moderate
Heavy
What are some major differences between the EMG and iEMG recordings?
Delta T used: _________
rest
Weight:
__zero___
Integral:
_________
Light
_________
_________
Moderate
_________
_________
Heavy
_________
_________
From this data, and visual inspection of these EMG recordings, is there any
evidence of multiple motor unit summation? Explain your answer.
Is there any evidence of wave summation? Explain your answer.
When one needs to produce more force would you expect only multiple motor
unit summation, only wave summation, or both types of summation to occur?
Lab VI - 14
B. Compare the EMG activity of the biceps brachii with the elbow at a 90° angle
with the forearm 1) in a supinated and 2) in a pronated position.
Delta T: _______ Weight:_____
1. supinated integral _________
2. pronated integral_________
Is there a difference in electrical activity of the biceps brachii between the
supinated and pronated position? If so, why do you think they are different?
If you are assessing a subject’s ability to perform elbow flexion and you are
recording the EMG activity of only the biceps brachii, how might this affect your
ability to interpret results? Why?
C. Joint Angle & normalization of iEMG integrals to maximal iEMG integrals. Use the
same biceps EMG preparation from above (using the same subject is recommended).
Collect EMG data during 1) a maximal voluntary contraction and 2) from series of
sub-maximal isometric actions at several different elbow joint angles. Note: If
possible choose subjects within the class with varying size elbow flexors (arm girth
measurements can help) for comparison of class data. For part 2, where a submaximal
weight is used, a similar weight should be used for all subjects.
Arm girth: ________________
1. Maximal Voluntary Contraction (MVC). Set up the computerized force transducer
(similar to a cable tensiometers) so that the subject will be able to perform an
isometric biceps curl at a joint angle of approximately 90 to 100. Press start on
the computer, which will record both the EMG and force simultaneously. Have the
subject pull up on the cable as hard as possible while trying to minimize recruitment
of other muscles and muscle groups. Determine the force from the maximal
voluntary contraction by clicking and dragging from the baseline to the peak
(highest point) on the force recording (Note: make sure that you are recording data
from the correct channel), the number in the button bar labeled "delta" will give you
the MVC in units of pounds. Then, determine the iEMG integral at the point where
maximum force was developed. It is recommended that you have the subject
perform this MVC two to three times with about a one minute rest in between
attempts.
Delta T: _______
Effort 1: MVC: ___________ pounds
Integral:_____
Effort 2: MVC: ___________ pounds
Integral:_____
Effort 3: MVC: ___________ pounds
Integral:_____
Lab VI - 15
2. Leave your subject hooked up to the EMG device. You will now have your subject
perform isometric biceps curls with a dumbbell at different joint angles while
recording the biceps EMG (you do not need the computerized force transducer
anymore). Use a goniometer to determine joint angles. Use a weight that is moderate
to heavy (but not maximum) for your subject. Use this same weight at each joint angle.
Use the same delta T as above (from the previous MVC experiment, C part 1). After
you have recorded the iEMG integral at each of these joint angles, you can now
normalize these values relative the MVC integral (determine what percent the
electrical activity at each joint angle is relative to the maximum electrical activity). It
is very easy to calculate the normalized iEMG. Simply divide each of the integrals
recorded (below) by the integral from the best MVC effort above and express these
normalized values as a percent of maximum (integral at each joint angle  integral
from MVC x 100 = % of MVC).
Delta T: _______ Weight:_____
Joint Angle
150°
120°
90°
60°
30°
integral
________
________
________
________
________
Normalized iEMG activity
( integral to left by best MVC integral)
________
________
________
________
________
After collecting your data, enter your best MVC data from part 1, your data from
90 degreees in part 2. and arm girth into the class spreadsheet.
Are there differences in electrical activities of the biceps brachii between the
different joint angles?
What are two biomechanical and physiological changes that occur when the joint
angle changes? Why do these changes require alterations in motor unit
recruitment, even if the weight is the same? (What major factors account for the
differences in force production at different joint angles?)
Do you think each of these is of equal importance? If not, which one do you think
is more important in this example?
What joint angle required the greatest amount of biceps motor unit recruitment?
What joint angle required the least amount of biceps motor unit recruitment?
What is the definition of a sticking point?
Lab VI - 16
If the biceps were the only muscle involved in elbow flexion, based on this data,
what joint angle would represent the point where your subject had the greatest
leverage? How about the worst leverage? What angle (or angles) would
represent the sticking point(s)?
Can we accurately define sticking points if we only record EMG activity from one
muscle? Explain your answer.
At what joint angle(s) would you expect the subject to be able to exert the greatest
force when maximally recruiting all of their motor units? Explain your answer.
What are the benefits of using normalized iEMG data versus using the raw (not
normalized) iEMG data?
3. CLASS DATA questions/ Effect of Muscle Size using class data from above
Did there appear to be a relationship between arm girth and MVC? If so, how
might arm girth influence maximal force production?
How were the normalized iEMG numbers different between subjects with
different arm girth? (compare normalized data from 90 degrees)
What do these differences suggest about the effect of muscle size on force
production, neural recruitement of the muscle, and perhaps other areas of muscle
perforamance?
II. Dynamic Actions
A. Motor Unit Recruitment & Type of Action. Have your subject perform dynamic
actions with three different weights. Obtain integral measurements near the peak
EMG activity during the concentric and eccentric phases of their contractions.
Weight:
Concentric Integral:
Eccentric Integral:
Light
_________
_________
_________
Lab VI - 17
Moderate
_________
_________
_________
Heavy
_________
_________
_________
B. What type or types of summation can we observe from these findings? Type of
Action & Joint Angle. Using the same EMG setup as before for the isometric actions,
evaluate the iEMG activity of this muscle with a dumbbell as the subject moves
through the range of motion. Record the iEMG activity at three points during the
concentric phase and eccentric phase of elbow flexion. Delta T: _________
Draw observed iEMG activity
180
|
90
(Concentric)
|
0
|
90
|
180
(Eccentric)
Elbow Joint Angle
Concentric
Joint angle
integral
Near full extension (~180°)______
Mid-flexion (~90°)
______
Near full flexion (~30°) ______
Eccentric
Joint angle
integral
Near full flexion
______
Mid-flexion
______
Near full extension
______
Based on this recording, if the biceps were the only muscle involved in this
activity, what angle(s) do you think would be the “sticking points”?
How does the recruitment pattern for these dynamic actions compare to the
recruitment pattern during isometric actions? If they are different, explain why?
How does the recruitment pattern for these dynamic actions compare to the
recruitment pattern during isokinetic actions? If they are different, explain why?
How does EMG activity compare between concentric and eccentric actions? If
they are different, explain why?
Lab VI - 18
III. Isokinetic Actions
A. Motor Unit Recruitment & Joint Angle. Prep the vastus lateralis for EMG recordings
as described above. Evaluate iEMG activity of the quadriceps and knee extensor
torque during maximal knee extensions at an angular velocity of 30°/second.
Draw observed iEMG activity during knee extension
Knee Extensors
mV
90°
135°
180°
Knee Angle
Draw observed torque curves for the knee extensors and flexors
Knee Extensors
Torque
90°
Knee Flexors
Torque
135°
180°
180°
Knee Angle
135°
90°
Knee Angle
Why does EMG appear this way during isokinetic actions?
What is the definition of Torque?
Why is the torque different at different joint angles?
How do these EMG and torque curve findings compare to your experiments using
dynamic actions?
Did there appear to be any “sticking points” during this action? If so, how can
you tell?
Lab VI - 19
How did the Extensor and Flexor Torque curves compare? Which produced more
torque? Did the curves appear similar or different? Explain your answer.
B. Velocity & Type of Action. A Biodex Isokinetic Dynamometer will be used to
evaluate the effects of movement velocity on muscular force production. Peak torque
will be measured at several concentric and eccentric velocities. Note: if possible also
collect data from two different types of subjects in your class for comparison (e.g.
endurance athletes, strength athletes, sprinters, cyclists, runners, etc)
Concentric
Speed
Extensor Peak Torque
30°/sec
__________
60°/sec
__________
90°/sec
__________
180°/sec
__________
240°/sec
__________
300°/sec
__________
Flexor Peak Torque
__________
__________
__________
__________
__________
__________
Eccentric
Extensor Peak Torque
__________
__________
__________
Flexor Peak Torque
__________
__________
__________
Speed
10°/sec
30°/sec
60°/sec
Draw your Torque velocity curves below for extensors, flexors, and from
other groups in your class
Torque
(ft lbs)
|
60
|
|
|
|
|
|
|
30 10
0
30
60
90
120
(Eccentric)
(Concentric)
Velocity (degrees/sec)
|
180
|
300
Explain in your own words what happens to force production as the velocity
increases for both concentric and eccentric actions.
Lab VI - 20
How does force production compare between concentric, eccentric and isometric
actions? (i.e. which produces the greatest and least amounts of force)
How many degrees per second would you have a subject move if you wanted
them to perform an isometric action?
Based on this data, what do you think the isometric peak torque would be
(approximately) if we had our subject perform an isometric action on the
isokinetic dynamometer?
We have previously discussed the relationship between power, force, and
velocity. Based these relationships and on the data you obtained above, do you
think that subjects would produce more power concentrically during slow,
medium, or fast speeds? Do you think that subjects would produce more power
eccentrically during slow, medium, or fast speeds? Explain your answers.
How did the curves compare between extensors and flexors? What does this tell
us about the properties of the extensor and flexor muscles?
How did the curves compare between subjects? What can account for any
differences observed?
What properties of muscles and muscle fibers influence force production and
speed of movement?
C. Type of Action. Using the data recorded in part B (above) compare the concentric and
eccentric peak torques at 30° per second.
Concentric peak torque:
____________
Eccentric peak torque:
____________
How does force output compare between concentric and eccentric actions?
Explain.
How does the isokinetic data compare to the experiment assessing iEMG activity
during concentric and eccentric actions using a dumbbell? How are they different
and how are they similar?
Lab VI - 21
Study questions
1. Explain how wave summation and multiple motor unit summation relate to force production. How do
each of these influence what we see on an EMG recording.
2. What are advantages and disadvantages of using surface EMG and needle-electrode EMG?
3.
By what mechanisms does joint angle influence force production?
4. How does leverage relate to force production? What factors influence leverage? (be specific)
5. How are sticking points related to a) leverage, b) joint angle, c) EMG activity when performing
concentric dumbbell curls, and d) torque during isokinetic knee extensions?
6. How are the following terms related: leverage, force arm to resistance arm ratio, angle of force
application, sticking point.
7. _________________________ actions are performed with constant (maximal) motor unit recruitment
and against an accommodating resistance. For this reason the EMG activity and MU recruitment
during these activities is usually variable / constant while force/torque/resistance is variable / constant.
On the other hand, when using free weights EMG and MU recruitment are variable / constant and
resistance/force is variable / constant.
8. What are the major properties of isokinetic actions?
9. Would you advise an athlete to use an isokinetic device for strength training or would you
recommend free weights? Explain your answer.
10. List several pros and cons of using isokinetic devices versus using free weights.
Lab VI - 22
11. Define the roles of the following terms as they relate to analyzing surface EMG activity:
a. Filtration
b. Rectification
c. Integration
d. Normalization
12. What considerations must be taken into account if one wants to compare iEMG activity between
subjects? For example, can you think of any potential problems in comparing iEMG activity of
abdominal muscles of well trained and untrained subjects?
13. According to table 1 (near the beginning of this lab) there are a number of physiological factors that
influence force production. Which of these physiological factors do think would best explain
differences in strength with differences in (more than one answer is possible):
a. Gender
b. Age
c. Strength training
d. Limb immobilization
e. Body Size
14. What is the difference between torque and force? If you know the force produced, what other
information do you need to calculate Torque?
15. What is responsible for giving red fibers their red color? What anatomical and physiological
differences would you expect between red and white fibers?
16. How do twitch times compare between red and white fibers? What generalizations can and
can not be made regarding fiber color and twitch time? How does myosin ATPase activity
relate to twitch time?
17. What fiber types can be delineated using myosin ATPase histochemistry?
Lab VI - 23
18. What types of fiber type changes can and can not occur in humans under normal
circumstances (e.g. training, detraining)?
19. Muscular strength, muscular endurance, and the speed of muscle contraction are all partly
dependent on specific attributes of the muscle and muscle fibers performing the activity.
Name several characteristics of muscles, or muscle fibers, which influence muscular speed,
strength, power, and endurance.
Strength
Speed
Power
Endurance
20. What is succinate dehydrogenase? What is lactate dehydrogenase? Which of these would be
more associated with aerobic, endurance activities? Which of these would be more
associated with high intensity, anaerobic activities? Explain your answer.
21. How are the following characteristics of type I fibers inter-related: slow twitch time, low
ATPase activity, high SDH levels, low LDH levels, high mitochondrial content, high
capillary density, high myoglobin content, high lipid content, and high resistance to fatigue?
22.
How are the following characteristics of type IIX fibers inter-related: fast twitch time, high
ATPase activity, high LDH levels, low SDH levels, low mitochondrial content, low capillary
density, low myoglobin content, low lipid content, high glycogen stores, and poor resistance
to fatigue?
23. Review from Human Physiology. On a separate piece of paper list all events that occur during a
stretch reflex, including events involved in action potentials, synaptic and neuromuscular
transmission, and excitation, coupling, contraction, and relaxation (according to the sliding filament
theory) in muscle fibers.
Lab VI - 24