Download muscle purpose and function

I. Types of Muscle
1. Cardiac
2. Smooth
3. Skeletal
a. 40-45% of the body weight
b. more than 430 muscles found in pairs (r/l)
c. most movements use less than 80 muscles
II. Purpose
1. To provide strength and protection to the skeleton by distributing loads and absorbing shock
2. To enable bones to move at the joints
III. Properties
1. Extensibility - it can be stretched
2. Elasticity - it will return to its original size and shape after being stretched
3. Contractility
IV. Structure
b.Epimysium
d.Perimysium
e.Endomysium
Sarcomere
Sarcolemma
(Kapit and Elson, 1977)
IV. Structure
2. Striations: Hill ModelA-band, I-Band, H-zone, M-line, Z-line, etc.
Consult your diagram of the components of a sarcomere
IV. Structure
3. Myofibrils - actin and myosin contractile proteins
a. the previous slide presented the longitudinal arrangement
b. the cross-sectional arrangement is less impressive
IV. Structure
4. The role of the contractile proteins and muscle contraction
a. the contractile proteins are:
1. Myosin (thick filaments)
2. Actin (thin filaments)
3. Troponin
4. Tropomyosin
5. Tropomyosin and tropinin are 2 strands that are twisited about each other.
6. The tropomyosin lies along the valleys of the actin, while the troponin look like ovals
on the tropomyosin.
7. When calcium binds to the troponin, the tropomyosin is removed from the active
binding sites
on the actin and the cross-bridge attachment can occur. (sliding of actin over myosin)
5. Fiber types
a. Slow twitch
Oxidative
b. Fast Twitch
Glycolytic
c. mechanical properties
1. Contraction (twitch)
2. Contraction time
3. Time to 1/2 relaxation time
4. Fatigue rate
6. The Muscle Model-The Musculotendinous Unit
a. A mechanical model that includes the components that distribute loads
1. The tendons (SEC) are a spring-like elastic component that is located in series with the
contractile
component (CC - the contractile proteins of the myofibril, actin and myosin), and the
epimysium,
perimysium, endomysium and sarcolemma (PEC) represent a second elastic component
located in
parallel with the contractile component
b. CC=contractile component
PEC = parallel elastic component
SEC=series elastic component
(Nordin & Frankel, 1989)
c. The series elastic fibers seem to be more important for tension development than are the
parallel elastic fibers.
V. Muscle Fiber Architecture
A. Fiber arrangement
1. Longitudinal or Parallel
a. the fibers lie parallel to its long axis
b. e.g sartorious, rectus abdominus
2. Fusiform or Spindle-shaped
a. Rounded muscle which tapers at either end
b. E.g. biceps brachii, brachialis, brachioradialis
3. Fan-shaped or Triangle or Radiate
a. Flat type of muscle whose fibers radiate from a narrow attachment at one end to a broad
attachment at
the other
b. E.g pectoralis major, and minor, gluteus medius and minimus, internal oblique
4. Penniform
a. muscle fibers are arranged in a feather-like pattern
b. there are 2 types
1 - Unipennate
a. the fibers extend diagonally from one side of the long tendon
b. e.g. tibialis posterior, flexor pollicis longus, flexor and extensor digitorum longus,
semimembranosus
and peroneus tertius
2 - Bipennate
a. along central tendon with fibers extending diagnolly in pairs from either side of the
tendon
b. e.g. rectus femoris, soleus, vastus medialis and lateralis, flexor hallicus longus
3 - Multipennate
a. a combination of several bipennate fibers
b. e.g. deltoid, gluteus maximus, infraspinatus
B. Muscle force production is proportional to the product of the size and number of fibers (i.e.,
the
cross-sectional area)
C. The anatomical cross-sectional areas is different from the physiological cross-sectional area
1. For a given anatomical cross-sectional areas, the fiber arrangement will affect the
number of muscle
fibers within the same physiological cross-sectional area.
a - Anatomical Cross Sectional Area (CSA) - is taken perpendicular to the long axis
of the muscle
b - Physiological Cross Sectional Area (PSCA) - is taken perpendicular to the muscle
fibers
2. Penniform fibers, when compared to the longitudinal fibers, with a given anatomical
cross-sectional areas will have:
a. a greater number of fibers
b. a greater force production potential
c. a smaller range of motion
VI. Muscle-tendon length ration
A. Penniform muscles tend to have short tendons when compared to longitudinal, fusiform,
and radiate
muscles. This will result and increased range of motion
B. Greater muscle-to-tendon length ratio of non-pennate muscles will decrease the amount of
work of a
contracting muscle
VII. Muscle Function
A. Roles of Muscles
1. Agonist - Prime Mover
2. Synergist - Stabilizers, Neutralizers
3. Antagonists - Responsible for the opposite motion of the Agonist
4. Co-contractors - Simultaneous contraction of opposing muscles
B. Not all synergists are co-contractors
1. Stabilizers - these muscles
a. maintain a joint position against the pull of gravity, an opposing muscle, or against
the momentum of an
action
b. the goal is NOT joint action
2. Neutralizers are co-contractors
a. Neutralizers - these muscles contract to negate an undesired component motion
brought about by
activation of the Agonist
C. Types of Static Work and Contractions
1. Static Work
a. Isometric
b. Co-contractions
2. Dynamic Work
a. Concentric Contraction - the muscle shortens
b. Eccentric Contraction - the muscle lengthens under tension
c. Isokinetic Contraction- contraction produces constant joint action velocity (max.
moment
development)
d. Isotonic Contraction - external load is constant through-out the motion (sub-max
moment
development)
3. Tension development
a. Isometric contractions produce more tension than does concentric contractions.
b. Eccentric contractions have been suggested to develop more tension than isometric
contractions.
c. The ability of isometric and eccentric contractions to develop more tension that
concentric
contractions are considered to be due to:
1. supplemental tension developed in the series elastic component and
2. the contraction time
4. The increase contraction time allows for:
a. more time to transfer tension to the elastic component as the musculotendinous unit is
stretched
b. more time for the forming of cross-bridges
c. more time, more recruitment of motor units
D. Force-Length Relationship
1. Muscle force/tension is dependent on the length of the muscle.
2. When the muscle fiber is at its “resting” length, maximum tension is developed
Muscles - Relationships
The Tension-Length Curve
(Nordin and
Frankel, 1989)
The tension-length curve from part of an isolated muscle fiber stimulated at different lengths.
The tension is maximal at the slack length, or resting length, of the sarcomere (2.0 m), where
overlap is greatest, and falls to zero at the length where overlap no longer occurs (3.6 m).
The tension also decreases when the sarcomere length is reduced below the resting length, as
extensive overlap interferes with cross-bridge formation.
Length-Tension for a whole muscle. The curves indicate the contributions of the active and passive
components of the muscle.
(Nordin and Frankel, 1989)
FORCE-LENGTH RELATIONSHIPDevelopment of Force/Tension
-At less than 50% of resting length, the muscle cannot develop contractile tension
- At normal resting length, the muscle is already in slight passive elastic tension. At
this length the muscle produces it's greatest tension.
- At greater than resting length, although total tension is greater than tension at
resting length, developed tension is less and decreases with increasing muscle length
- At extreme lengths the muscle tension developed is zero, and the total tension is
equal to the elastic tension
The Load-Velocity Curve
(Nordin and Frankel, 1989)
At negligible external load - concentric contraction at max velocity
 When load = max muscle force, zero velocity (isometric)
When load is greater than max muscle force, the muscle lengthens and the larger the
load, the faster the lengthening
FORCE-VELOCITY RELATIONSHIP
Power = Force x Velocity [P = (F) (v)]
- If v = 0 (i.e., an isometric contraction), then P = 0
- If force is minimal, power will also be minimal
- Maximal power will be generated by finding the combination that would
optimize the highest force that can be developed with the highest velocity
The velocity at which a muscle shortens is affected by the force it must produce to
move the load
- In a concentric contraction, velocity of contraction decreases as the load
increases
- With zero load, velocity contraction is maximal and reflects the shortening
speed of the
contractile component
- In an isometric contraction velocity equals zero, and maximal force can be
produced
- In a maximal eccentric contraction total tension developed increases as velocity
of contraction
increases (This will continue to the point of failure)
Force-Time Curve for a whole muscle
 Slower contractions lead to greater force development
This increased time allows the tension created by the contractile components to be
transferred to the parallel and then the series elastic components
(Nordin and Frankel, 1989)
Force-time curve for a whole muscle contracting isometrically. The force exerted by the muscle
is greater when the contraction time is longer, since time is required for the tension created by the
contractile components to be transferred to the parallel elastic component and then to the series
elastic component as the musculotendinous unit is stretched (Adapted from Kroll, 1987)
DETERMINANTS OF FORCE AND VELOCITY
FORCE IS AFFECTED BY:
-The number of myosin cross-bridges in parallel that can interact with actin. HOWEVER
- not all types of cross-bridges generate the same amount of force and
- only a percentage of cross-bridge may be activated, even during a maximal
contraction
- Muscle fiber CSA
- Muscle CSA
- Muscle fiber arrangement (architecture)
VELOCITY IS AFFECTED BY:
- Muscle length (the number of sarcomere in series)
- Shortening rate per sarcomere per muscle fiber. This is dependent upon
- fiber type
- combinations of protein isoforms
- Fiber arrangement. Multipennate muscles will be relatively shorter and have slower
velocity of shortening
Force-length and force velocity relationships may estimate if sarcomeres behave uniformly.
However
- not all sarcomeres act at exactly equal lengths (sarcomeres at the ends of fibers being
typically shorter that fibers in the
middle)
- not all sarcomeres have identical velocities
OPTIMAL VELOCITY
- Optimal velocity of movement is dependent on the load to be moved
- The greater the load, the lower the optimal velocity BECAUSE
- increased load requires increased number of cross-bridges. Therefore, more muscle fibers
need to be recruited and more time needed to form cross-bridges (Force-Velocity)
- A rapidly contracting muscle generates less force than one contracting more slowly BECAUSE
- the faster the actin and myosin filaments move past each other, the less time and fewer
cross-bridges that can be formed and therefore the lower the tension (Froce-Velocity)
- Velocities above optimum are uneconomical BECAUSE
- greater number of fibers are needed to produce the same force therefore wastes energy
- Velocities below optimum are uneconomical BECAUSE
- force must be maintained over a longer period of time, therefore more energy is expended for
the same amount of shortening
- Most individuals will unconsciously perform at an optimal velocity for a given load
- Optimal velocity for the same load will vary with different activities (e.g.. arm curl vs a bench
press; or a wide grip bench press vs a narrow grip bench press)
- Optimal velocity for the same load can vary with different individuals in the same activity (e.g..,
bench pressing with a 100 lb weight, where person has a 1RM of 150 lbs compared to someone who
has a 1 RM of 200 lbs)
- Velocity of muscle shortening appear to be increased with a warm-up