Download Skeletal Muscle Contractions and Exercise

Skeletal Muscle Contractions and
Exercise
Muscle Tension
Force exerted by a contracting
muscle on an object.
Load
Force exerted on a muscle by the
weight of an object to be moved
Motor Unit
• Includes the motor
neuron and all the
muscle fibers it
innervates
• May be as few as four or
as high as several
hundred
• Fine motor control
muscles have smaller
units (fingers) while
gross muscle control
involves larger units.
Force of Muscle Contraction
Depends on four things
a. Number of muscle fibers
stimulated
b. Size of muscle fibers
c. Frequency of stimulation
d. Degree of muscle stretch
Number of Muscle Fibers
• As the number
of motor units
increase, the
greater the
muscle force
Size of Muscle Fibers Stimulated
• The bulkier the
muscle/greater the
cross-section size,
the greater the
muscle tension and
the more weight can
be lifted.
• Uses large fibers of
large motor units
Hypertrophy vs. Atrophy
Hypertrophy is
the increase in
muscle size
Atrophy is the
decrease in
muscle size
Degree of Muscle Stretch
• Ideal length-tension relationship
has a slight overlapping of the thin
and thick filaments
• Permits sliding nearly the entire
length of thin filament
• No overlap, no cross-bridge
connections, no tension, no
contraction
Ideal Relationships and Muscles
• Optimal operational length is 80% to
120% of normal resting length
• Severely stretched or pulled muscle
(180% of optimal length) cannot
develop tension for contraction
• Muscles contracted to 75% cannot
shorten further.
• This is why muscles must be stretched
prior to any form of exercise
Disuse Atrophy
• Use it or loose it!
• If you don’t work your muscles, they
will weaken over time due to
degeneration and loss of muscle mass
• Can decrease at 5% per day until it
stablizes at a certain mass
• Very problematic for bed-bound
patients
Frequency of Stimulation
• How often a muscle is stimulated will
determine the rate of contraction, or
whether a contraction occurs at all.
Muscle Twitch
Detected with a myogram
Involves a single action potential of a motor
neuron
Three phases
1. Latent period
2. Period of Contraction
3. Period of Relaxation
Graded Muscle Responses
Variations in muscle contractions permit
variation in response by muscles to a
task.
Occurs by
1. Frequency of stimulation change
2. Change the strength of a stimulus
Muscle Response to Frequency
Change
• Involves wave summation
• Second and further responses are
stronger than the first until the
responses are summed
• Unfused or incomplete tetanus vs. fuse
or complete tetanus – What and why?
Muscle Response to Stronger
Stimulation
• Controlled contraction force due to multiple motor
unit summation
• Occurs via motor unit recruitment
• First visible contraction referred to as the threshold
stimulus
• Maximal stimulus produces increased force, when
all motor units are recruited – going beyond will not
create a stronger contraction
• What is the difference between a pat and a slap, as
far as muscle are concerned?
Trepping the Stairway to Heaven
• Increased Ca2+ in the sarcoplasm permits greater
availability for troponin attachment at the crossbridge
• Enzymes become more efficient as temperature
increases
• Also temperature aids muscle pliability (this is
why you WARM UP first before exercising)
• Result – stronger contractions that occur in steps
forming a staircase myogram
Muscle Tone
• Muscles are always slightly
contracting.
• Due to spinal reflexes
continually ennervating muscles
at a low level
• Keeps muscles healthy and firm
Isotonic
Contractions
• Iso = same, ton
= tension
• Occurs when
tension in
muscle
develops to
move a load
Isometric Contractions
• Muscle is attempting to move
a load greater than the tension
the muscle can develop
• Muscles that naturally involve
isometric contractions include
back muscles to hold posture
or joints in a stationary
positions, such as salutes,
squatting for a period of time,
standing
Isometric contractions
• Or “Same measure”
contractions
• Occurs when tension
in muscle develops,
but the load doesn’t
move
• Muscle cannot
shorten or lengthen
Concentric vs. Eccentric
•
•
•
•
Concentric – muscle shortens
Ex. Picking up book, flex bicep
Eccentric – muscle lengthens
Ex. Coordination of body,
purposeful movement like walking
Eccentric are 50% more forceful
• Are also more
like to cause
delayed-onset
muscle
soreness (like
after a brisk
walk or run)
When do we use both types of
contractions?
• Jumping
• Squats or deep
knee bends
• Throwing
• Lifting
Effects Of Exercise
• Aerobic or Endurance Exercising
– Includes running, fast walking
swimming, racing, biking
– Increases number of capillaries
surrounding muscle fibers; Increase
number of mitochondria within muscle
fibers
– Most dramatic in slow-twitch muscle
fibers (white or red?)
Aerobic Exercise
• Does not
contribute to
large muscle
fiber gain!
Resistance Exercise
• Includes weight-lifting or isometric
exercises
• Strength, not stamina
• Increases muscle size by 50% if
exercise every other day
• Is an increase in size of muscle
units than the number
• If resistance muscle
training isn’t balanced
between antagonistic
muscles groups/pairs,
person becomes muscle
bound
• Means they lack
flexibility, awkward
stance, can’t make good
use of their muscles
• This is not always true,
but it looks pretty
weird!
Muscle Bound!
Train Smart!
• Get in shape before you start a sport!
• Overload principle - Forcing muscles
to work will promote increased muscle
strength and endurance.
• However - Remember to rest between
exercise days to prevent overuse
injuries!
How muscles grow
• Disruption to muscle cell organelles activates
satellite cells, which are located on the outside of
the muscle fibers between the basal lamina
(basement membrane) and the plasma membrane
(sarcolemma) of muscles fibers to proliferate to
the injury site (Charge and Rudnicki 2004).
• In essence, a biological effort to repair or replace
damaged muscle fibers begins with the satellite
cells fusing together and to the muscles fibers,
often leading to increases in muscle fiber crosssectional area or hypertrophy.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of
muscle regeneration. Physiological Reviews, Volume 84, 209-238.
How muscles grow
• As the satellite cells multiply, some remain as
organelles on the muscle fiber where as the
majority differentiate (the process cells undergo as
they mature into normal cells) and fuse to muscle
fibers to form new muscle protein stands (or
myofibrils) and/or repair damaged fibers.
• Thus, the muscle cells’ myofibrils will increase in
thickness and number.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of
muscle regeneration. Physiological Reviews, Volume 84, 209-238.
Role of Growth Hormone
• Resistance exercise stimulates the release of growth
hormone from the anterior pituitary gland, with released
levels being very dependent on exercise intensity.
• Growth hormone helps to trigger fat metabolism for energy
use in the muscle growth process.
• Growth hormone stimulates the uptake and incorporation
of amino acids into protein in skeletal muscle.
• This is why you take amino acids/protein shakes AFTER
you exercise, within 30 minutes of stopping.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of
muscle regeneration. Physiological Reviews, Volume 84, 209-238.
Testosterone’s Role (works in
women as well!)
• Can stimulate growth hormone responses in the pituitary,
which enhances cellular amino acid uptake and protein
synthesis in skeletal muscle.
• In addition, testosterone can increase the presence of
neurotransmitters at the fiber site, which can help to
activate tissue growth with the satellite cells.
• As a steroid hormone, testosterone can interact with
nuclear receptors on the DNA, resulting in protein
synthesis.
• Testosterone may also have some type of regulatory effect
on satellite cells.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of
muscle regeneration. Physiological Reviews, Volume 84, 209-238.
CAUTION!
• Is injecting growth hormone or
anabolic steroids worth it?
– significant increase concerning
morbidity and mortality among
anabolic steroid users
– increased incidence of premature
mortality among power lifters
– may cause irreversible
cardiovascular toxicity, especially
atherosclerotic effects and
cardiomyopathy
– possibly irreversible
neuropsychiatric toxicity through
apoptosis of neurons
Further risks
• Risks for Females
• Specific risks for females
associated with anabolic
steroids include:
• increased facial hair
growth
• development of masculine
traits, such as deepening
of the voice, and loss of
feminine body
characteristics, such as
shrinking of the breasts
• enlargement of the clitoris
• menstrual cycle changes
or stops
Further risks
• Risks for Males
• Specific risks for males
include:
• Muscle overgrowth
• testicular shrinkage
• pain when urinating
• gynecomastia
• impotence (inability to get
an erection)
• sterility (inability to have
children)
Steroid-induced acne
Response to resistance training
• All studies show that men and women respond to
a resistance training stimulus very similarly.
• Due to gender differences in body size, body
composition and hormone levels, gender will have
a varying effect on the extent of hypertrophy one
may possibly attain.
• Greater changes in muscle mass will occur in
individuals with more muscle mass at the start of a
training program.
Charge, S. B. P., and Rudnicki, M.A. (2004). Cellular and molecular regulation of
muscle regeneration. Physiological Reviews, Volume 84, 209-238.
On Your Own
• Review Smooth and Cardiac Muscle
contractions and compare to Skeletal
• Use Table 9.3 to help you!
• Making Connections! On page 316 –
Homeostatic Interrelationships
• Developmental Aspects of Muscles
• Homeostatic Disorder Imbalances