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.
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…..
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
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.
Importance:
It allows gradation of force to occur for weak & strong
contractions.
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.
• If repeated APs are separated by long intervals of time, muscle
fibers have time to relax completely between stimuli.
• If interval of time between APs is 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. What happens when a second action potential
occurs immediately again in the same 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 one 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. (see diagram on the next slide)
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…
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.
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.
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
ISOTONIC VS. ISOMETRIC
CONTRACTION
ISOTONIC CONTRACTION
Not all muscle contractions shorten muscles. They can generate
force or tension and at the same time stay the same length.
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.
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: Muscle that reacts rapidly is composed of
Fast fibers & muscles that react slowly with long
contractions are composed of Slow fibers
Color is determined by the protein myoglobin
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









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
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.
Cause:
An increase in the number of actin and myosin, causing an
increase in thickness of individual muscle fibers.
Example:
Body building
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. When 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