Physiology of Muscle
There are different types of muscle but we are describing specifically skeletal muscle.
Skeletal muscle is well supplied with blood and nerves. Muscle contraction requires a large amount of
energy and thus requires a good supply of nutrients and oxygen, as well as an efficient method to
eliminate waste products.
Sliding filament theory: thin myofilaments slide past thick myofilaments, shortening the muscle length.
This is achieved by cross-bridges moving in one direction, like oars. The stimulus for this to happen
comes from a motor neuron. A motor neuron, together with all the muscle cells it stimulates is called a
motor unit. Muscles controlling precise movements have fewer than 10 muscle fibres for each motor
unit, whereas muscles for gross movements may have up to 500 muscle fibres for each unit.
Total tension can be varied by adjusting the number of motor units activated - called recruitment. The
motor neurons fire asynchronously - this helps prevent fatigue and provides better control.
If exercising aerobically, glucose is used to produce pyruvic acid plus energy and the pyruvic acid then
breaks down into water, carbon dioxide and energy.
If exercising anaerobically, incomplete breakdown of pyruvic acid leads to lactic acid, which diffuses
to the liver and muscles. This has to be ultimately broken down and for this oxygen is needed – known
as oxygen debt. Lactic acid has toxic effects and contributes to muscle fatigue. In Hatha Yoga, asanas
are to be carried out aerobically in order to prevent muscle fatigue and the build up of lactic acid.
Skeletal muscle consists of many muscle fibres (cells) arranged in parallel bundles. Muscles can grow
in diameter by increasing the thickness and number of miofibrils, and in length by forming additional
sarcomeres - the functional units of a muscle cell. They have the ability to contract, and if relaxed, are
very extensible. When a muscle contracts, two kinds of protein (actin and myosin) in the sarcomeres of
its cells slide along one another. In the body, a muscle can be contracted to 70% or stretched to 130%
of its normal resting length. (Normal resting length is the length that the muscle takes up in the body in
a typical resting attitude.) Outside the body, the muscle can be contracted to 50% of its length and
stretched more than 130%. As a muscle is stretched beyond its normal resting length, its force of
contraction gradually drops, reaching zero at 175% of resting length. The diminishing strength of
contraction is caused by a decreasing amount of overlap between actin and myosin.
Amount of overlap of actin and myosin in a sarcomere a) of contracted, b) resting, and c) stretched muscle
Muscle contracts with greatest force at its normal resting length. Whole muscle is encased in a fibrous
connective tissue sheath (epimysium). Bundles and even single cells are also surrounded by the same
tissue (perimysium and endomysium). The tension generated by muscle cells is transferred to the fibres
of connective tissue.
Tendons are cordlike extensions of this tissue. Collagen fibres, a major element of fibrous connective
tissue, have great strength, little extensibility, and no ability to contract. These fibres are arranged in
wavy bundles allowing motion until the slack of these bundles is taken up. Extension of a tendon
beyond four percent of its length causes irreversible deformation. An improper use of Isometric or
eccentric tensions can put too much stress on collagen fibres, damaging them and causing muscle
soreness-a result of the disintegration of collagen and the release of hydroxyproline (one of Its
components) into the muscle. With age, molecules of collagen change by becoming more rigid, which
is reflected in general body stiffness.
Collagen fibres surrounding muscle fibres at their junction with the tendon
You can permanently elongate tendons and connective tissue sheaths, with minimal structural
weakening by low-force long-duration stretching with temperatures of the tendons at more than 1O3
degrees F. To increase the amount of permanent elongation, you maintain the stretch achieved while
tendons and sheaths were warm while they cool down. This fits the description of a relaxed stretch
done after the main part of your workout during the cool-down, with this qualifier: the stretch must be
at the range of motion at which muscle fibres exert less tension than the connective tissue.
The joint capsule is a connective tissue sleeve that completely surrounds each movable joint.
Immobilization for a few weeks causes chemical changes in the collagen fibres of the joint capsule that
will restrict your flexibility.
The ligaments holding your joints together are made primarily of collagen fibres. They have more
elastic fibres, made of the protein elastin, than do tendons. Stretching ligaments leads to
loose-jointedness and can be effectively applied only with children. In adults, an age-related increase in
rigidity of collagen fibres makes any stretches aimed at elongating ligaments hazardous. When children
stretch ballistically or statically, their muscles do not contract as strongly as an adult's and their
ligaments can be stretched. If a ligament is stretched more than six percent of its normal length, it tears.
There is no need to stretch ligaments to perform even the most spectacular karate or gymnastic
techniques. The natural range of motion is sufficient. Stretching ligaments destabilizes joints and thus
may cause osteoarthritis (Beighton Grahame and Bird 1983).
Bone is dynamic, living tissue made of crystals of calcium and phosphorus that are associated with
collagen fibres. Exercises can change the density and shape of bones. The forms of joint surfaces,
covered by a glasslike, smooth and elastic cartilage, also change in the long-term process of exercise,
e.g. dynamic stretching. Depending on the amount of stress (exercise, for example), bones and joints
can adapt to it or be destroyed by it.