The Physiology of Flexibility

A number of anatomical and physiological factors influence an athlete's flexibility...

While some we are stuck with (such as age, gender, and joint structure), others are under our control. These include activity level, muscle bulk and stretching exercises.

1. Joint Structure
There are several different types of joint in the human body. Some intrinsically have a greater range of motion (ROM) than others. The ball and socket joint of the shoulder for example, has the greatest range of motion of all the joints and can move in each of the anatomical planes (1).

Compare the shoulder joint to the ellipsoidal joint of the wrist. It moves primarily in the sagittal and frontal planes. The hinge joint of the ankle is similar while the modified hinge joint of the knee allows on ROM in the sagittal plane.

2. Age & Gender
ROM and flexibility decreases with age. This is due, in part to the fibrous connective tissue that takes the place of muscle fibres through a process called fibrosis (1). Females tend to be more flexible than males. Older individuals should take encouragement that, just as with strength and endurance, flexibility can be increased at any age with training (2,3,4)

3. Connective Tissue
Deep connective tissue such as fascia and tendons can limit ROM. In particular, two characteristics of connective tissue, elasticity and plasticity are related to ROM. Elasticity is defined as the ability to return to the original resting length after a passive stretch (5). Plasticity can be defined as the tendency to assume a new and greater length after a passive stretch (5).

Ligaments do not seem to display any elastic properties. However, with exposure to stretching they may extend to a new length (9). The strength and conditioning coach must remember that increased mobility in the ligaments reduces the stability of the joint - often an unfavorable adaptation, particularly in contact sports.

4. Muscle Bulk & Weight Training
Hypertrophy of skeletal muscle can adversely affect ROM. It may be difficult for very bulky athletes to complete certain stretches such as an overhead triceps stretch. However, in these athletes, significant muscles mass is usually move favorable to their sport than extreme ROM.

Resistance training can increase flexibility (6) although when heavy loads are used within a limited ROM, weight training can reduce flexibility (7).

5. Proprioceptors
The capacity of the neuromuscular system to inhibit the antagonists (those muscles being stretched) influences flexibility (9).

There are two important proprioceptors involved in the mechanics of stretching and flexibility. The first is the muscle spindles. Located within the muscle fibres they monitor changes in muscle length. The stretch reflex is the body’s involuntary response to an external stimulus that stretches the muscle (5) and causes a reflexive increase in muscular activity. It is the muscle spindles that activate this response.

When stretching, it is best to avoid this activating the muscle spindles and the stretch-reflex response, as it will limit motion.

Static stretching does not elicit the muscle spindles, allowing muscles to relax and achieve a greater stretch.

The other important proprioceptors are the golgi tendon organs (GTO). These are located near to the musculotendinous junctions and are sensitive to increase in muscle tension. When the GTO is stimulated it causes a reflexive relaxation in the muscle. When this relaxation occurs in the same muscle that is being stretched, it is referred to as autogenic inhibition and can facilitate the stretch (8).

Autogenic inhibition can be induced by contracting a muscle immediately before it is passively stretched – a technique used in PNF stretching.

Reciprocal inhibition occurs when the GTO is stimulated in the muscle opposite to that being stretched (i.e. so the opposing muscle relaxes) (8). This can be achieved by simultaneously contracting the opposing muscle group to the one being passively stretched.

6. Internal Environment
The athlete's internal environment affects ROM. For example, mobility is decreased immediately upon waking after a night's sleep (9). Ten minutes in a warm (40oC) bath increases body temperature and ROM (9).

7. Previous Injury
Injuries to muscles and connective tissue can lead to a thickening, or fibrosing on the affected area. Fibrous tissue is less elastic and can lead to limb shortening and reduced ROM. Fibrous nodules in connective tissue and muscle are often called trigger points. A technique called myofascial release may be able to alleviate pain and restriction casued by trigger points.