MECHANICS OF SKELETAL
MUSCLE
Dr. Ayisha Qureshi
Assistant Professor
MBBS, MPhil
A MUSCLE TWITCH
The Muscle Twitch
A single action potential causes a brief contraction
followed by relaxation in the muscle. This is called a
single Muscle twitch.
• Electrical and mechanical events in a muscle
always occur in relation to one another: The
electrical event (Action potential) is followed by
the mechanical events (contraction). The whole
process is called Excitation-contraction coupling.
• Twitch starts 2 ms after depolarization of the
membrane, before repolarization is complete----Why the delay?
Contractile activity and
electrical activity in
skeletal muscle:
A single action potential in a
skeletal muscle fiber lasts only 1 to
2 msec, while a skeletal muscle
contraction and relaxation lasts for
about 100 msec.
The onset of the resulting
contractile response lags behind
the action potential because the
entire excitation–contraction
coupling must occur before crossbridge activity begins. In fact, the
action potential is completed before
the contraction even begins.
Time is take for the following
processes:
• AP to spread down the t-tubule.
• Release of Ca2+
• Ca2+ to attach to Troponin C
• Power stroke
• Ca2+ uptake by the ATPase
pump in the SR.
LENGTH & TENSION RELATIONSHIP:
Length & Tension Relationship
• A relationship exists between the length of the muscle
before the onset of contraction and the tension (force
developed in the muscle) that each contracting fiber can
develop at that length.
• For every muscle there is an optimal length (lo) at which
maximal force can be achieved on a subsequent
contraction.
• More tension can be achieved when beginning at the
optimal muscle length than when the contraction begins
with the muscle less than or greater than its optimal length.
This length–tension relationship can be explained by the
sliding filament mechanism of muscle contraction.
Length & Tension relationship
Length (L) and Force (F) or tension of a muscle are closely related:
1.
Optimal length (lo): (In the previous slide seen as point A) This is the point where thin
filaments optimally overlap the thick filaments. This is also the normal length of the
sarcomere. At this point, maximal no. of cross-bridges & actin filaments are accessible
to each other for binding & bending.
2. At lengths greater than Optimal length (lo): (in the previous slide seen as point C) This
is when the muscle is passively stretched. The thin filaments are pulled out from
between the thick filaments, decreasing the number of actin sites available for crossbridge binding. So some of the cross-bridge and actin sites “do not match up” and go
“unused”. So, NO actin –myosin overlap, tension developed by the muscle is zero.
3. At lengths less than Optimal length (lo): (in the previous slide seen as point D) If a
muscle is shorter less tension is developed for the following reasons:
- The thin filaments from the opposite sides become overlapped.
- The ends of the filament become forced against the z-discs so no further
shortening can take place.
Length-Tension Relationship
Points to Remember:
1. When the muscle is at its Optimal length, it
contracts with the maximum tension.
2. Force of contraction (tension generated) is
maximal at the resting (Optimal) length &
decreases if the muscle is longer or shorter.
ENERGETICS OF MUSCLE
CONTRACTION:
Energy sources
The main source of energy for muscle contraction is ATP. ATP is used
in 3 different steps in contraction-relaxation process. These steps
are:
1. Splitting of ATP by myosin ATPase provides the energy for the
power stroke of the cross bridge.
2. Binding (but not splitting) of a fresh molecule of ATP to myosin lets
the bridge detach from the actin filament at the end of a power stroke
so that the cycle can be repeated. This ATP is later split to provide
energy for the next stroke of the cross bridge.
3. Active transport of Ca2+ back into the sarcoplasmic reticulum
during relaxation depends on energy derived from the breakdown
of ATP and is used by the ATP- dependant Calcium Pump.
The concentration of ATP in a Muscle fiber= 4mmole. It is sufficient to
maintain full contraction for only 1 to 2 seconds at most.
SOURCES OF ATP
There are 3 main sources of ATP:
1. Creatine Phosphate/ Phosphagen Energy system:
- takes place within the muscle
-uses the Phosphate bond from Creatine phosphate
- First source of ATP when exercise begins; instantaneous energy available.
- short bursts of high-intensity exercise. E.g. high jump, sprints…
2. Oxidative phosphorylation: aerobic or endurance type exercise.
- takes place in the mitochondria
- requires oxygen & uses fatty acids, glucose in blood and glycogen
stores
- to sustain long duration mild to moderate aerobic exercise. E.g. walks,
jogging, swimming, marathon runners….
3. Glycolysis: anaerobic or high-intensity exercise
- when oxygen demands are not met & oxygen NOT available.
- uses glycogen stores of the muscle
- proceeds very rapidly and leads to formation of lactic acid.
- moderate to severe exercise. E.g. 800 meter run. Cannot be sustained for
long time.
CHARACTERISTICS/ PROPERTIES OF WHOLE
MUSCLE CONTRACTION :
We have been talking about muscle fibers as a
single muscle cell…..
Now we will consider Muscle as a whole
consisting of several to several hundred muscle
fibers….
1. MUSCLE FATIGUE
Definition:
Fatigue occurs when prolonged & strong stimulation of an
exercising muscle reaches a stage when the muscle is no
longer able to respond to the stimulation with the same
degree of contractile activity.
• Is of 2 main types:
1. Muscle fatigue: occurs in the muscle & is a defense
mechanism that protects the muscle by preventing it
from reaching a point where no ATP will be available.
2. Central fatigue: more psychological. Occurs when CNS
no longer activates the motor neurons supplying the
muscles. Person stops exercising even though the
muscles can still perform.
1. MUSCLE FATIGUE
CAUSES:
1. Depletion of Glycogen energy stores.
2. Accumulation of Hydrogen ions from lactic acidinterfere with cross- bridge functions.
3. Intracellular acidosis from lactic acid inhibits
glycolysis enzymes & slows ATP production.
4. NT depletion at the NMJ.
5. Central fatigue- lack of will & sleep.
6. Accumulation of extracellular K+
2. OXYGEN DEBT
• The body normally
contains about 2 liters
of oxygen:
0.5 liters
Air in lungs
0.25 liters
Body Fluids
1 liter
Hb of Blood
0.3 liters
Muscle with
Myoglobin
2. OXYGEN DEBT
•
•
•
•


1)
2)
3)
During muscular exercise, a lot more Oxygen is supplied
to the muscle than is present.
↑ O2 consumption = ↑ energy expended
All stored O2 is used within a minute or so
After exercise is over:
2 liters of normally present blood must be replenished
9 liters extra must be provided for:
Resynthesis of the Creatine Phosphate.
Conversion of lactate into pyruvate.
Form fresh supplies of ATP through oxidative
phosphorylation.
2. OXYGEN DEBT
• All this extra Oxygen that must be “repaid”
(11.5liters) to the body is called the Oxygen
Debt.
SO,
A person must breathe rapidly even after the
exercise is over!
3. MUSCLE TONE
Even when muscles are at rest, a certain amount
of tautness usually remains—This is called
Muscle Tone.
Cause:
Low rate of nerve impulses coming from the
spinal cord which are controlled by the:
1. Signals from the brain to the spinal cordanterior motor neurons
2. Signals that originate in the muscle spindles
located in the muscle itself-Intrafusal fibers
4. MOTOR UNIT
Definition:
All the muscle fibers innervated by a single nerve fiber are called a MOTOR
UNIT.
OR
Each single motor neuron plus all the muscle fibers it innervates is called a
MOTOR UNIT.
• One motor neuron innervates a number of muscle fibers, but each muscle
fiber is supplied by only one motor neuron. When this neuron is
stimulated, all the muscle fibers supplied by it contract together.
• Each muscle consists of a number of mixed motor units.
• For a weak contraction of the whole muscle, only one or a few of
its motor units are activated.
• The number of muscle fibers per motor unit and the number of motor
units per muscle vary widely, depending on the specific function of the
muscle. E.g. the kind of work that the muscle performs…..
4. MOTOR UNIT
• Number of muscle fibers in a motor unit vary in
different muscles from 2 or 3 to more than 1000.
• Average: 80-100 muscle fibers to a motor unit.
• Muscles which have to perform fine grade, intricate
movements have motor units with as few as 3-5
muscle fibers to a unit .e.g. hand, eye
• Muscles with relatively crude movements, number of
muscle fibers is quite large. E.g. muscles of lower
limbs
• In one whole muscle, different motor units overlap
5. ALL OR NONE LAW
In a single muscle fiber exactly the same as in the single
nerve fiber.
• A sub-threshold stimulus does not produce a response
while a threshold or supra-threshold stimulus produces a
maximal response.
In whole muscle the response is different.
• A gradual ↑ in stimulus strength causes a gradual ↑ in
muscle contraction till a maximum is obtained. This is
because with each ↑ in stimulus strength more & more
motor units are stimulated.
• When all motor units are activated---all muscle fibers are
contracted , then a further ↑ in the strength of the
stimulus is without any additional contractile effect.
6. Force of Contraction Summation:
Summation: is the process of adding together of
individual twitch contractions to increase the
intensity of whole muscle contraction.
There are 2 types of summation:
1. Multiple Fiber Summation (No. of motor
units stimulated)
2. Frequency Summation
6. a: Multiple Fiber Summation
Definition:
It is the summation of individual muscle fiber contractions by
increasing the number of motor units contracting simultaneously.
• Initially, with a weak signal from the CNS-only smaller units are
stimulated.
• Later, when signal from CNS becomes stronger, larger motor units
are excited----This is called SIZE PRINCIPLE.
Importance:
It allows gradation of force to occur for weak & strong contractions.
Cause:
Smaller motor units are driven by smaller motor nerves & are more
excitable than large ones---so are excited first! Then, if greater strength
is required, then larger motor units are recruited.
6. b: FREQUENCY SUMMATION
Definitions:
Force of contraction increases by increasing the frequency of contractions.
Two twitches from 2 action potentials add together to produce greater
tension in the fiber than produced by a single action potential. This is called
twitch summation or frequency summation.
• Force generated by the contraction of a single muscle fiber can be ↑ by
increasing the rate at which the action potentials stimulate the muscle
fiber.
• If repeated APs are separated by long intervals of time, muscle fibers have
time to relax completely between stimuli.
• If interval of time between AP shortened, the Muscle fiber will not have
relaxed completely at time of 2nd stimulus, resulting in a more forceful
contraction.
• A single action potential in a muscle fiber produces
only a twitch. Let us see what happens when a second
action potential occurs in a muscle fiber. If the muscle
fiber has completely relaxed before the next action
potential takes place, a second twitch of the same
magnitude as the first occurs. The same excitationcontraction events take place each time, resulting in
identical twitch responses. If, however, the muscle fiber
is stimulated a second time before it has completely
relaxed from the first twitch, a second action potential
causes a second contractile response, which is added
“piggyback” on top of the first twitch.
FREQUENCY SUMMATION
When APs come one after the
other after the relaxation of the
muscle is complete…….
When APs come one after the
other before relaxation of the
muscle is complete…
6. b: FREQUENCY SUMMATION
If APs continue to stimulate the muscle repeatedly at short
intervals, there is no time for complete relaxation between
contractions
↓
Individual twitches fuse into one continuous contraction
↓
Whole muscle contraction appears to be smooth, sustained & of
maximal strength
↓
This is called TETANIZATION or TETANUS
(A tetanic contraction is usually three to four times stronger than
a single twitch.)
• Physiologic basis of twitch summation & Tetanus:
The main reason is the sustained elevation in cytosolic
Ca2+ permitting greater cross-bridge cycling. As the
frequency of action potentials increases, the duration of
elevated cytosolic Ca2+ concentration increases, and
contractile activity likewise increases until a maximum
tetanic contraction is reached. With tetanus, the
maximum number of cross-bridge binding sites remain
uncovered so that cross-bridge cycling, and consequently
tension development, is at its peak.
6.b: FREQUENCY SUMMATION & TETANUS
Two types of Tetanus:
1. COMPLETE or FUSED TETANUS: If repeated stimuli are
applied at fast rate, then no relaxation occurs between the
stimuli, muscle reaches max. tension and remains there & a
sustained contraction phase is obtained.
2. INCOMPLETE or UNFUSED TETANUS: if repeated stimuli at a
slower rate, then muscle fiber relaxes slightly/incompletely
between summated stimuli but the relaxation remains
incomplete.
CAUSE: Enough Ca2+ ions are maintained in the muscle
sarcoplasm so that contractile state is sustained without
allowing relaxation between AP.
7. THE STAIRCASE/ TREPPE EFFECT
• DEFINITION:
When a series of maximal stimuli are delivered to the muscle at
a frequency just below tetanizing frequency
(when muscle twitch due to previous stimulus has just
completed), the tension/amplitude developed during each
twitch increases till a max. height is reached & a plateau is
formed. This is called the Treppe/ staircase effect.
Because the tension rises in stages, like the steps in a staircase,
this phenomenon is called treppe, a German word meaning
"stairs."
• CAUSE: The rise is thought to result from a gradual increase
in the concentration of calcium ions in the sarcoplasm, in
part because the ion pumps in the sarcoplasmic reticulum
are unable to recapture them in the time between
stimulations.
Treppe Effect
8. ISOTONIC VS. ISOMETRIC
CONTRACTION
ISOTONIC CONTRACTION
There are two primary types of contraction, depending on
whether the muscle changes length during contraction.
They are:
• Isotonic contraction: occurs when muscle contracts with
shortening of length but against a constant load, thus,
the tension on the muscle remains constant (iso= same,
tonic= tension)
OR
A contraction that creates force & moves a load.
Isotonic contractions are used for body movements and for
moving external objects. E.g. picking up a book, a box.
ISOMETRIC CONTRACTION
• Isometric contraction: occurs when muscle contracts
without shortening in length.
(iso= same, metric= measure or length)
OR
A contraction that creates force without movement.
Isometric contractions can be seen in 2 cases:
1. If the object you are trying to lift is too heavy.
2. If the tension developed in the muscle is deliberately
less than needed to move the load. E.g. standing for
long time or holding up a glass of water while taking
sips.
Physiologic basis of Isometric & Isotonic contractions:
The same internal events occur in both isotonic and isometric
contractions:
Muscle excitation starts the sliding filament cycling; the cross bridges
start cycling; and filament sliding shortens the sarcomeres, which exert
force on the bone at the site of the muscle’s insertion.
During a given time, a muscle may shift between isotonic & isometric
contractions. E.g. when you lift a book up it is isotonic contraction and
when you keep holding the book up while reading it is isometric
contraction.
NOTE:
Since Work=Distance X Load,
Isotonic contractions do work where as Isometric do not.
9. ELECTROMYOGRAPHY
• Activity of motor units can be studied by
electromyography, the process of recording the
electrical activities of the muscle on a cathode ray
oscilloscope.
• No anesthesia is required. Small metal discs are
placed on the skin overlying the muscle as pick-up
electrodes or hypodermic needle electrodes are
used.
• The record obtained with such electrodes is the
Electromyogram (EMG).
10. RECRUITMENT
• If each motor unit contracts in an all-or-none manner,
how then can muscle create graded contractions of
varying force & duration?
The answer lies in the fact that muscles are composed of
multiple motor units of different types. This allows the
muscle to vary contraction by:
1. Changing the types of motor units that are active OR
2. Changing the number of motor units that are responding
at any one time.
For a weak contraction of the whole muscle, only one or a
few of its motor units are activated. For stronger & stronger
contraction, more & more motor units are recruited. This is
called Motor Unit Recruitment.
10. RECRUITMENT
• At rest EMG shows little or no activity
• With minimum voluntary activity a few motor units discharge, & with
increasing voluntary effort more & more are brought into play----Recruitment of motor units
• Asynchronous Recruitment: One way that CNS avoids fatigue in a
sustained contraction
The CNS alternates between the different motor units supplying the same
muscle so that some of the motor units rest between contractions,
preventing fatigue. e.g. during a sustained contraction, only a portion of
the muscle’s motor units is involved as is necessary in muscles supporting
the weight of the body against the force of gravity. The body alternates
the motor units as shifts at a factory, to give the motor units that have
been active an opportunity to rest while others take over. Changing of the
shifts is carefully co-ordinated so that the sustained contraction is smooth
rather than jerky.
11. FAST vs SLOW FIBERS
•
1.
2.
•
•
•
The skeletal muscle fibers are mainly of 2 types:
SLOW or RED or TYPE I MUSCLE FIBERS
FAST or WHITE or TYPE II MUSCLE FIBERS
Every muscle of the body is composed of a mixture
of both fast & slow fibers.
Simply: Fibers that react rapidly are Fast fibers &
muscles that react slowly with long contractions are
Slow fibers
Color is determined by the protein myoglobin
11. FAST vs SLOW FIBERS
SLOW-TWTCH/ RED/ Type I
 Small diameter
 More myoglobin
 Fatigue resistant
 Mostly Oxidative
 Slow rate of contraction
 Myosin ATPase activity LOW
 ↓ no. of myofilaments
 Red
 Posture maintenance

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






FAST-TWITCH/ WHTE/Type II
Large diameter
Less myoglobin
Easily fatigue
Mostly glycolytic & oxidative
Fast rate of contraction
Myosin ATPase activity HIGH
↑ no. of myofilaments
White
Forceful & rapid movements
12. MUSCLE HYPERTROPHY
Definition:
When the total mass of a muscle increases, this is called Muscle
Hypertrophy. The resulting muscle enlargement comes from
an increase in diameter of the muscle fibers. It is in response
to a regular & intensive use of that particular muscle. e.g.
body building.
Physiologic Basis:
• ↑in the number of actin & myosin filaments causing increase
in thickness of individual muscle fibers---called fiber
hypertrophy
• Rate of synthesis of actin & myosin far greater
• Signaling proteins triggered that turn on genes that direct the
synthesis of more of these contractile proteins.
13. MUSCLE ATROPHY
Definition:
When the total mass of a muscle decreases, it is called Muscle
Atrophy. If a muscle is not used, its actin and myosin content
decreases, its filaments become smaller and the muscle decreases
in mass and becomes weaker.
Physiologic Basis:
1. When the muscle is prevented from doing work even though the
nerve supply is intact. e.g. in bed-ridden patients, in a limb in a
plaster of Paris cast. This type is thus called Disuse Atrophy.
2. Atrophy also seen nerve supply to the muscle is lost. This can be
due to an accident or when motor neurons supplying a muscle are
destroyed .e.g. Poliomyelitis.
• Muscle fiber becomes thin & low in proteins, glycogen and ATP.
• When muscle continuously shortened then sarcomeres at the end
of the muscle fiber actually disappear
14. MUSCLE HYPERPLASIA
• Under rare conditions of extreme muscle force
generation, the actual number of muscle
fibers increase, in addition to the fiber
hypertrophy ----This increase in fiber number
is called Muscle Hyperplasia.
Mechanism: Linear splitting of previously
enlarged fibers
MUSCLE DISEASES
MUSCLE CRAMPS
Definition:
Painful, sustained & involuntary
contractions of the muscle with
motor units contracting
repeatedly.
CAUSE: There can be many
causes the most common of
which are:
• Due to increased excitability
of the peripheral parts of the
nerves
• Electrolyte disturbance
• Nocturnal cramps (night
cramps)
• Cramps due to strenous
exercise
• Dehydration.
DUCHENNE MUSCULAR DYSTROPHY
Duchenne Muscular
Dystrophy
Definition:
It is a fatal musclewasting disease that
primarily strikes boys
and leads to their death
before the age of 20.
There is progressive
degeneration of
contractile proteins of
the muscle and their
replacement with
fibrous tissue.
It is a genetic X-linked
disease.
DUCHENNE MUSCULAR DYSTROPHY
Mutation in the Dystrophin gene located on X-chromosome
↓
Skeletal muscle lacks protein dystrophin (a large protein that provides
structural stability to the muscle cell’s plasma membrane)
↓
Its absence leads to constant leakage of Ca into the muscle cell
↓
Ca activates proteases that start damaging the muscle
↓
Leads to increasing muscle weakness & fibrosis
↓
Symptoms start at 2-3 years, patient wheel-bound at 10-12 years
Usually die at about 25-30 years of age (usually Males)
↓
Death is usually due to respiratory failure or heart failure as the
respiratory or heart muscles become too weak.
↓
Milder disease is Becker’s muscular dystrophy