1. A motor unit consists of a motor neuron and the muscle fibers it stimulates. A single motor neuron stimulates around 150 muscle fibers to contract together. 2. The latent period refers to the brief delay between stimulation and the beginning of muscle contraction, which allows the muscle action potential and calcium release to occur. 3. The force-velocity relationship describes how the velocity of muscle shortening decreases as the force or load against which the muscle contracts (the afterload) increases. Greater afterloads require slower cross-bridge cycling.

1. MECHANICAL PROPERTIES OF
SKELETAL MUSCLE

2. 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

4. 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

6. 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

7. • 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, myosin-
binding 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, Ca2is actively transported back into the
sarcoplasmic reticulum, myosin-binding sites are covered
by tropomyosin, myosin heads detach from actin, and
tension in the muscle fiber decreases

8. PROPERTIES OF SKELETAL MUSCLE
• Extensibility & elasticity
• Excitability
• Conductivity
• Contractility
• Tonicity
• Refractory period
• Fatigue

9. • Extensibility:Ability of the muscle to
elongate when stretched.
• Elasticity: Ability of the muscle to return
to its original length when stretch is
removed.
• Excitability: Ability of the muscle to
respond to a stimulus.(electrical ,thermal
,chemical,mechanical).
• Contractility: Ability of the muscle to
shorten or contract in response to a
stimulus.

10. • Conductivity: Ability of the muscle to transmit
an impulse (AP) from one part of the fibre to
another part.
• The condution velocity of an impulse in
skeletal muscle fiber is slow. it is about 5
meters/sec
• Tone:The state of partial sustained contraction
seen in all muscles.

11. REFRACTORY PERIOD: It is about 30-
50millisec . It is a period of action potential in
which another stimulus applied will not produce
a response in a muscle.
Types: Absolute
Relative

12. • 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

13. • CONTRACTILITY: Ability to contract in
response to a stimulus.
• Types :-
•Isotonic contraction
•Isometric contraction

14. 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.

18. Isotonic Contraction
1 Same Tension In The Muscle
2 Muscle Length Decreases
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 upperlimb
while lifting weight
,lifting the leg while
walking
Isometric Contraction
1same Length-muscle Length
Remains Constant
2 Increase In Tension
3 Work Performed Is Not Seen
4 Less Heat Produced
5 Less Energy Is Used
6 Eg:- Calf muscles on
standing

19. How do we control the strength of
contraction?
1. Large Motor unit involved
2. More motor units recruited
3. More fast type II b types of fiber
4. Increasing the rate of stimulation

20. SUMMATION
• When the strength of the stimulus is increased
the contractile response also increases.The
increase in response is due to recruitment of
motor units. This is called quantal summation
or multimotor unit summation

22. 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.

24. 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.

26. Simple Twitch, Summation, and
Tetanus

27. TETANUS
• When the number of stimuli are more than
two but less than tetanizable frequency then
partial tetanus is obtained –clonus 30/sec
• Tetanus- when stimuli are applied at a higher
frequency, tetanus-40/sec (continuous
contraction) is produced.
• Infection by clostridium tetani is due to
synchronous discharge from all the nerve
fibers-leading to tetanic contraction of all
skeletal muscles .

28. Preload
• Preload is the load on a muscle in a relaxed state,
that is, prior to contraction.
• Applying preload to muscle does two things:
• 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.
• 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.

29. Afterload
• Afterload 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.

30. 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.

32. 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 cross-
bridges that cycle, the greater the developed
active tension.

33. • 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.

35. 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.

37. • Length-tension relationship in skeletal muscle. Maximal active
tension occurs at muscle lengths where there is maximal overlap of thick
and thin filaments.

38. 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

39. Q. The figure depicts the isometric length-
tension relationship of skeletal muscle. Identify
the region where actin and myosin overlap is the
least

40. FORCE-VELOCITY RELATIONSHIP
• describes the velocity of shortening when the
force against which the muscle contracts i.e. the
afterload, is varied

41. • Isotonic (same tone) contraction - The force,
rather than the length, is fixed i.e. the muscle
contracts with the same force
• 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.

42. Classification of Fiber Types in Skeletal
Muscles

43. Examples:
• Type I Red fibers: in postural muscles
– Large myoglobin content and many mitochondria
• Type IIa Red fibers: in muscles needed for
activities like middle distance running,
swimming, etc.
– Large myoglobin content and many mitochondria
• Type IIb White fibers: needed for activities like
sprinting
• Low myoglobin content and few mitochondria