Physiology of Flexibility
Muscle
The ability of muscle to relax is essential for optimal health and movement. Relaxation is completely passive and
happens when muscle fibres no longer receive nerve impulses.
Muscular fibres are incapable of lengthening or stretching themselves. A force needs to be received from outside the
muscle. Among these forces are gravity, momentum, antagonistic muscles on the opposite side of the joint.
Muscles can lengthen up to 50% above their resting status. However, connective tissues and the nervous system
interact with muscle to limit the range of movement.
Healthy muscles maintain a structural homeostasis. A key to this balance is an equal pull by antagonistic, or
opposing muscles on the opposite of a joint. Muscle imbalance can be due to several factors including hypertonic
(contracted) or weak muscles. Treatment in such cases is to strengthen the weak and stretch the shortened muscle.
Connective Tissue
Collagen is the most abundant protein in the mammalian body and is the primary structural component of living
tissue. Collagen fibres are capable of only slightly extensibility, but are very resistant to tensile stress. Therefore they
are the main constituents of ligaments and tendons. Collagen can also have an elastic component which is composed
of crimped fibrils aggregated into fibres, like springs. When a fibre is pulled, its crimp straightens, and on release it
returns to its original length. As collagen ages, specific physical and biochemical changes take place: Reduction in
extensibility, increased rigidity, and decreased lubrication due to loss of water in the tissues (from 85% in babies to
70% in adults).
Elastic Tissue is a primary structural component of living tissue. Large amounts are found in the ligaments of the
vertebral column. It therefore is a major determiner of range of movement. Elastic fibres are almost always found in
close association with collagenous tissues. The elastic component provides the ability of a stretched material to
return to its original resting state, while the collagen component provides rigid constraints which limit the
deformations of the elastic elements.
Structures composed of Connective Tissue
Tendons – connects muscles to bones. Primary function is to transmit tension from muscles to bones. Tendons are
not extensible because they are required to transmit tiny muscular movement into fine movements of for example,
the fingers. (stretch max 4% of passive length)
Ligaments – bind bone to bone. Function is to support a joint where two or more bones meet. They are pliant and
flexible to allow movement, but strong, and inextensible so as not to yield to applied forces. Stretching ligaments
can destabilize joints. Ligaments can contribute up to 47% of the total resistance to movement and are therefore very
significant in determining the range of movement of a joint.
Fascia – consists of three general divisions or types.
1.
The superficial fascia lies directly below the skin, and contains an accumulation of fat in its outer layer,
and a thin membrane as its inner layer. In many parts of the body the superficial fascia glides freely
over the deep fascia producing the characteristic movability of skin.
2.
Deep fascia lies directly beneath the superficial fascia and is tougher, tighter and more compact. It
covers, and is fused with, muscles, bones, nerves, blood vessels and organs. It also compartmentalizes
the body by separating such things as muscles and the internal visceral organs
3.
Subserous fascia is around body cavities – the pleura around the lungs, the pericardium around the heart
and the peritoneum around the abdominal cavity.
The fascial connective tissue makes up as much as 30% of muscle mass. It is this tissue that allows muscle to change
its length. The sum of the muscles’ fascia accounts for 41% of the total resistance to movement. Therefore fascia
represents the second most important factor limiting the range of movement. A programme of stretching should
therefore be directed primarily toward elongating the fascia. This has been shown to be most effective when
stretching is achieved by low force, long duration stretches at elevated temperatures. When joints are immobilized
for any length of time, the connective tissue elements of the capsules, ligaments, tendons, muscles, and fascia lose
their extensibility. The fascia may thicken, shorten, calcify and erode. Immobilisation is associated with a change in
chemical structure – particularly a 40% decrease in hyaluronic acid (with attached water the principal connective
lubricant). Consequently connective tissue fibres come into closer contact and eventually stick, encouraging the
formation of cross-linking and a loss of extensibility and increase in tissue stiffness.
Neurophysiology of Flexibility
The nervous system constitutes one of the main communication systems for the body and plays a significant role in
determining the quality and quantity of movement available to the body. The nervous system comprises the central
nervous system (CNS) – brain and spinal cord, and the peripheral nervous system (PNS) – the cranial and peripheral
nerves, which include spinal nerves and those of the autonomic nervous system.
Sensory Receptors related to stretching
1.
2.
3.
Muscle spindles – located in muscles – activated by stretching
Golgi Tendon Organs (GTOs) – located on muscles – tendon junctions – regulate tension in the muscle
– decreasing muscle contraction
Joint receptors – sense mechanical forces on joints and can signal the direction, amplitude and velocity
of joint movements produced actively or passively
Reflexes
Stretch reflex – two types known as phasic and tonic. Phasic reflex – like the knee jerk – is a rapid rise in muscle
tension. Tonic reflex lasts the entire duration of the stretch and is proportional to the amount of stretch. If a sudden
stretch is applied to a muscle, a reflex action results that causes the muscle to contract. Muscular tension increases,
defeating the purpose of the desired stretch.
Reciprocal Inhibition – muscles usually operate in pairs so that when one set of muscles is contracting (the
agonists), the opposing muscles are relaxing (the antagonists). For example bending the arm at the elbow (flexing) is
done by contracting the biceps. The triceps, which normally straighten the arm (extending) at the elbow, must relax.
If not the two muscles would be pulling against each other and no movement would occur.
Autogenic Inhibition (Inverse stretch reflex) – up to a point, the harder a muscle is stretched, the stronger is the
resistance to the movement. This increased resistance is explained by the stretch reflex. But after a certain limit is
reached the resistance suddenly yields, collapsing like the blade of a pocket knife snapping shut – and sometimes
referred to as the clasp-knife response. The physiological name is the ‘lengthening reaction’. The phenomena was
originally thought to be caused GTOs, but this has been disproved. The lengthening reaction is sometimes exploited
is stretching programmes (see later).
Types of Stretching
1
Ballistic (isotonic) ‘Bouncing’ type stretching. Very little evidence of increasing flexibility, and high
risk of injury.
2
Static stretching (isometric) involves holding a position for a long period of time
3
Proprioceptive Neuromuscular Facilitation (PNF). This covers a range of stretching techniques, but it
basically means that is a muscle is strongly contracted, then on release it will relax further.
Theoretically, stretching should only begin when a muscle is completely relaxed. So first strongly
contract, then release, then stretch. At this stage, connective tissue is most efficiently stretched out, and
as we have seen, this plays a major role in extensibility.
Terminology
Concentric shortening – muscle fibres are stimulated by nerve impulses and the entire muscle responds by
shortening, eg when the biceps muscle in the arm shortens concentrically to lift a book
Eccentric lengthening – a muscle increases in length under tension while resisting gravity eg. Putting down a book
by slowly extending the arm is accomplished by allowing the muscle as a whole to become longer while keeping
some of the muscle fibres in a state of contraction.
Isotonic activity – the shortening of a muscle that involves movement (strictly speaking under a constant load) eg.
Raising and lowering a book
Isometric activity – holding still, often under conditions of substantial or maximum resistance (holding the book
extended in front with no movement
Hatha Yoga as an effective stretching technique
Hatha yoga exploits the techniques already discussed:
From the physiological point of view: we have seen that a significant factor limiting muscle extensibility is the
condition of the surrounding tissue sheath. Encouraging a wide range of movement keeps the connective tissue
pliable and helps slow down the ageing process by retaining fluid with the collagen matrix. It has been shown that
when the connective tissue is heated, it becomes more viscous, and adhesions between collagen layers are reduced,
allowing muscle more freedom of movement and removing one of the major restraints to flexibility. The increased
temperatures achieved in astanga yoga act on the collagen sheaths in the same way as heating from an external
source like a hot climate or studio (as in Bikram Choudhury’s popular style of yoga). Slow controlled stretching
prevents over-lengthening ligaments and de-stabilising joints. Properly executed asanas, many of which use
isometric activity, will lead to re-shaping and building of bone mass and density. A yoga practitioner’s body can be
transformed safely using the methods of Hatha Yoga without recourse to body-building exercises which carry the
risk of weakening rather than strengthening bone and tissue.
From the neurological point of view: static stretching as opposed to ballistic stretching allows time for the relaxation
response to establish. Rapid movements reinforce the stretch reflex causing the stretched muscle to contact to
contract and leading to hypertonic muscle. Long slow stretching prevents the stretch reflex from being initiated too
quickly and the muscle can be stretched further within the limits set by other factors.
How long should a stretch be held to allow a significant lengthening of the muscle fibres?
The time will vary according to the individual, but generally a stretch held for between 20 and 30 seconds will be
effective. This is one of the important features of hatha yoga and a major difference to many flexibility programmes.
If you are practicing astanga yoga and hold an asana for five breaths, assuming a breath lasts four seconds, you have
held the stretch for 20 seconds and are just within the required time. Research has shown that 30 second and 60
second stretches are more effective than a 15 second stretch. BUT, significantly, the 60 second stretch gave no
greater gains in flexibility than the 30 second stretch. The conclusion is that there is no advantage in holding a static
stretch for longer times. In addition, repetition of the stretch up to 4 times showed a greater increase in flexibility
than holding just once. However, repetition of more than 4 times showed no further gain in flexibility. There appears
to be an optimal duration/repetition to achieve maximum gain in flexibility.
Additionally, we have seen that methods like PNF can enhance flexibility still further. PNF is employed
automatically in many asanas. For example, Padangustasana can be done until the maximum stretch is reached, and
then the muscle put under contraction simply by pulling on the big toes without any overall movement. If the stretch
is repeated, the muscle will elongate further. It is worth looking at many asanas to see whether they exploit PNF, or
can be adapted to do so.
© Brian Cooper 2012