MECHANICAL PROPERTIES OF
SKELETAL MUSCLE
• Define a motor unit and describe the order of recruitment of motor units
during skeletal muscle contraction of varying strengths.
• Describe the relationship between the skeletal muscle action potential
and a muscle twitch.
• Distinguish between an isometric and isotonic contraction.
• Explain the relationship of preload, afterload and total load in the time
course of an isotonic contraction.
• Draw the length versus force diagram for muscle and label the three lines
that represent passive (resting), active, and total force. Describe the
molecular origin of theseforces.
• Explain the interaction of the length:force and the force:velocity
relationships.
• Distinguish between a twitch and a tetanus in skeletal muscle and explain
why a twitch is smaller in amplitude than a tetanus.
• List the energy sources of muscle contraction and rank the sources with
respect to their relative speed and capacity to supply ATP for contraction
Motor Unit
• A motor unit consists of a somatic motor
neuron plus all the skeletal muscle fibers it
stimulates
• A single somatic motor neuron makes contact
with an average of 150 skeletal muscle fibers,
and all of the muscle fibers in one motor unit
contract in unison
Motor Unit
A twitch contraction is the brief
contraction of all the muscle fibers in a
motor unit in response to a single action
potential in its motor neuron
Action pot
Q. In muscle physiology, the latent period refers to
a. the period of lost excitability that occurs when
two stimuli are applied immediately one after the
other.
b. the brief contraction of a motor unit.
c. the period of elevated oxygen use after exercise.
d. an inability of a muscle to contract forcefully
after prolonged activity.
e. a brief delay that occurs between application of a
stimulus and the beginning of contraction
• Note that a brief delay occurs between application of the
stimulus (time zero on the graph) and the beginning of
contraction. The delay, which lasts about two milliseconds,
is termed the latent period.
• During the latent period, the muscle action potential
sweeps over the sarcolemma and calcium ions are released
from the sarcoplasmic reticulum.
• The second phase, the contraction period, lasts 10–100
msec. During this time, Ca2 binds to troponin, myosinbinding sites on actin are exposed, and crossbridges form.
Peak tension develops in the muscle fiber.
• During the third phase, the relaxation period, also lasting
10–100 msec, Ca2 is no more released ;and as it is
continuously transported back into the sarcoplasmic
reticulum, Ca level fall, myosin-binding sites are covered by
tropomyosin, myosin heads detach from actin, and tension
in the muscle fiber decreases
PROPERTIES OF SKELETAL MUSCLE
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Excitability
Conductivity
Contractility
Tonicity
Extensibility & elasticity
Refractory period( brief)
Fatigue
• Excitability: Ability of the muscle to
respond to a stimulus.(electrical ,thermal
,chemical,mechanical
• Conductivity: Ability of the muscle to
transmit an impulse (AP) from one part
of the fibre to another part.
• Contractility: Ability of the muscle to
shorten or contract in response to a
stimulus.
• Tone:The state of partial sustained contraction
seen in all muscles.
• Extensibility: Ability of the muscle to elongate
when stretched.
• Elasticity: Ability of the muscle to return to its
original length when stretch is removed.
REFRACTORY PERIOD: . It is a period of
action potential in which another stimulus
applied will not produce a response in a muscle.
• The refractory period is short in skeletal muscle, but very long in cardiac
muscle – 250 msec
• This means that skeletal muscle can undergo summation and tetanus,
via repeated stimulation
• Cardiac muscle CAN NOT sum action potentials or contractions and
can’t be tetanized
How do we control the strength of contraction?
1.
2.
3.
4.
Large Motor unit involved
More motor units recruited
More fast type II b types of fiber
Increasing the rate of stimulation
More motor units recruited
Twitch, Summation, and Tetanus
• Twitch:
– Muscle is stimulated with a single electrical
shock (above threshold).
• Quickly contracts and then relaxes.
• Summation:
– If second electrical shock is administered
before complete relaxation of muscle.
Increasing the rate of stimulation
Twitch, Summation, and Tetanus
(continued)
• Incomplete tetanus:
– Stimulator delivers an increasing frequency of
electrical shocks.
• Relaxation period shortens between twitches.
• Strength of contraction increases.
• Complete tetanus:
– Fusion frequency of stimulation.
– No visible relaxation between twitches.
– Smooth sustained contraction.
Simple Twitch, Summation, and
Tetanus
Q. In a certain muscle, it takes 25 msec for a
single twitch to develop its peak force in
response to a single stimulus. If this muscle
were stimulated with two stimuli spaced 15
msec apart, the result would be
(A) A single twitch identical to the one-stimulus
twitch
(B) A contraction similar to a single twitch, but
of higher amplitude
(C) Two distinct contractions of very short
duration
(D) A failure of the muscle to contract at all
• CONTRACTILITY: Ability to contract in
response to a stimulus.
• Types :-
• Isotonic contraction
• Isometric contraction
Isotonic and Isometric Contractions
• Isotonic contraction:The contraction that
occurs when the muscle is allowed to freely
shorten,so that tension in the muscle is kept
constant.
• Isometric contraction: the contraction that
occurs without any shortening of the muscle,
so that the tension increases, but the length
of the muscle remains constant.
Isotonic Contraction
Isometric Contraction
1. Same Tension In The Muscle 1. Increase In Tension
2 .Muscle Length Changes
dec – concentric
incre- ecentric
3. Work Is Done –Weight Lifted
4.Extra Heat produced.
Relaxation Heat produced
After Contraction
5. Greater Energy Is Used
6 Eg-1. Muscles of upper limb
while lifting weight ,lifting
the leg while walking
2. Same Length-muscle Length
Remains Constant
3. Work Performed Is Not Seen
4. Less Heat Produced
5. Less Energy Is Used
6. Eg:- Calf muscles on
standing
Preload
• Preload is the load on a muscle in a relaxed state,
that is, prior to contraction.
• Applying preload to muscle does two things:
1.Causes the muscle to stretch. The greater the
preload added, the greater the stretch of the
muscle. Along with stretching the muscle,
preload stretches the sarcomere. The greater
the preload, the greater the pre- stretch of the
sarcomere.
2.Causes the muscle to develop passive tension. If a
2-g weight is suspended from a muscle, a 2-g
force also develops within that muscle. This force
is the passive tension. The greater the preload,
the greater the passive tension in the muscle.
LENGTH-TENSION CURVES
Preload-length Tension Curve
– resting skeletal muscle acts as a simple spring. As
preload is added, the muscle stretches and
develops a passive tension. The passive tension
can be considered an internal force that opposes
and equals the preload force.
Q. All of the following will occur when an
unstimulated muscle is stretched except:
A. increased preload
B. increased afterload
C. increased muscle length
D. increased passive tension
Afterload
• After load is the load the muscle is working
against or trying to move during stimulation.
• If the muscle is trying to lift 100 lb. during
stimulation, then the afterload is 100 lb.
• During contraction, the muscle does not
necessarily lift or move the afterload.
ISOMETRIC CONTRACTION OF THE ISOLATED
SKELETAL MUSCLE
• During an isometric (same length) contraction,
the overall muscle length will not change.
• the cross-bridge cycling will produce active
tension
Active Tension Development
• The active tension developed during an isometric
contraction is proportional to the number of
these cross-bridges that cycle. The more crossbridges that cycle, the greater the developed
active tension.
• the active tension is maximal when there is
maximal overlap of thick and thin filaments and
maximal possible cross-bridges which is at
resting state.
• When the muscle is stretched to longer lengths,
the number of possible cross-bridges is reduced,
and active tension is reduced.
• When muscle length is decreased, the thin
filaments don’t have enough space to slide on
thick filaments so more active tension can’t be
generated.
Total Tension
• The preload creates a passive tension prior to
contraction, and cross-bridge cycling during
contraction adds an active tension
component.
• The total tension in the active muscle is the
passive or preload tension plus the active
tension.
• Length-tension relationship in skeletal muscle. Maximal active
tension occurs at muscle lengths where there is maximal overlap of thick
and thin filaments.
Q. The figure depicts the isometric lengthtension relationship of skeletal muscle. Identify
the region where actin and myosin overlap is the
least
The velocity of shortening
• reflects the speed of cross-bridge cycling.
• the velocity of shortening will be maximal (Vmax)
when the afterload on the muscle is zero.
• As the afterload on the muscle increases, the
velocity will be decreased because cross-bridges
can cycle less rapidly against the higher
resistance.
• As the afterload increases to even higher levels,
the velocity of shortening is reduced to zero.
When the muscle
is maximally
stimulated,
lighter loads are
lifted quickly and
heavier loads
more slowly. If
the applied load
is greater than
the maximal
force capability
of the muscle,
known as Fmax,
no shortening
will result and
the contraction
will be isometric.
If no
load is applied,
the muscle will
shorten at its
greatest possible
speed, a velocity
known as Vmax
A- max load
D- min load
FORCE-VELOCITY RELATIONSHIP
• describes the velocity of shortening when the
force against which the muscle contracts i.e. the
afterload, is varied
When all the
initial velocity
measurements
are related to
each
corresponding
afterload lifted,
an inverse
relationship
known as the
force-velocity
curve is obtained.
Q.In a series of afterloaded isotonic twitches, as
the load is increased, the
(A) Force developed by the muscle increases and
the shortening velocity decreases
(B) Force developed by the muscle increases,
while the velocity remains the same
(C) Velocity increases to compensate for the
increased afterload
(D) Force developed is determined by the
velocity of shortening