Stretching: an activity wherein a person purposefully attempts to increase ROM by applying a longitudinal force to a muscle

Elastic effects: increase in tissue length that immediately returns to original length when stress is removed

Viscous effects: increase in tissue length that is dependent on time and returns to the original length at a slow rate (i.e., it is reversible); viscous effects occur because molecules move when force is applied over time, and thus, the return to original length is not immediate

Viscoelastic effects: a combination of viscous and elastic effects

Plastic effects: a permanent change in the molecular structure of a tissue as that which occurs when force is applied to a plastic sheet without completely tearing it. Plastic deformation indicates that damage has occurred to a tissue—it does not occur with appropriate stretching (i.e., the ROM returns to normal within a reasonable time frame after appropriate stretching).

Flexibility training: program of stretching exercises designed to increase ROM of targeted joints to a desired level or to maintain that level once it is attained

The immediate gain in ROM with stretching is mostly limited to the muscle being stretched, but there is some increase in ROM in the contralateral limb as well. This suggests that a neurologic reflex is one component of the mechanism for the effects of an acute stretch.

If one stops moving a joint, one loses flexibility. It is unknown how much movement is necessary to maintain flexibility, but it would seem logical that any muscle that is not moved through a particular ROM for a long period of time will lose that ROM.

Most studies have suggested that the effectiveness of stretching increases when the tissue is warmer.

The most effective way to increase muscle temperature is with muscle activity, although deep heating methods (e.g., ultrasound) can be effective as well.

Superficial heat is not an effective method to warm up deep muscles.

Flexibility decreases during the first year of life and then begins to increase until approximately 5–8 years of age. This is followed by a steady decline in flexibility in both boys and girls until 12–14 years of age (near puberty). Subsequently, flexibility increases again until approximately 20–24 years of age, which is followed by a slow but steady decline. These changes appear to coincide with changes in levels of activity, but whether or not this relationship is causal remains to be determined. For example, one begins to walk at approximately 1 year (flexibility begins to increase), and one starts school around 5–8 years of age, which is associated with increased sitting (decreases in flexibility). Flexibility may increase again at the age of 12–14 years as general activity increases with walking to secondary school and social relationships but may decrease by the age of 20–24 years as people enter the workforce and again become less active.

Although females are generally more flexible than males, differences within the general population are small. The general perception that females are much more flexible may occur because females participate in activities and sports that include a large amount of flexibility training (e.g., dancing or gymnastics). There are two additional reasons why the increased flexibility in females may not be caused by sex hormones: (i) it is seen before puberty and (ii) the increase in flexibility that occurs around the time of adolescence occurs earlier in boys than in girls (but girls enter puberty earlier than boys).

The stretch reflex is a protective reflex mediated by muscle spindles; it causes stretched muscles to contract and thus prevents excessive ROM.

The stretch reflex is the primary reason certain people advise against ballistic stretching (bounce stretching). As stretch is released during the bounce, muscles contracts and then subsequent stretch occurs against an eccentrically contracting muscle. This theoretically increases the risk of injury, but there are no studies that actually compare rates of injury with ballistic stretching versus other types of stretching. An alternative view is that a force involved with appropriate ballistic stretching is very small and much less than what occurs during regular sport. Therefore, if an injury occurs during a ballistic stretch, some argue that it would likely have occurred during the sport as well.

A graph of the relationship between flexibility and the risk of injury would be U shaped. Both individuals who are inflexible and those who are extremely flexible are at a higher risk of injury than are those with an intermediate level of flexibility. Individuals with increased flexibility may be representative of a hypermobility group (because of their ligaments) rather than a hyperflexible group (because of their muscle–tendon unit) because these two features are sometimes difficult to differentiate.

The U -shaped curve is based on cross-sectional data and does not mean that inflexible individuals are at a reduced risk of injury if they begin to stretch before exercise. First, there may be some other factor associated with inflexibility that is responsible for the risk of injury, and this other factor might not be affected by stretching. Second, the immediate effects of stretching are opposite to the long-term effects of stretching (see following text). In general, cross-sectional data on flexibility refer to long-term effects and thus provides minimal information on effects of stretching immediately before exercise.

Stretching immediately before exercise: Immediately after an acute bout of weightlifting, muscles are fatigued and weaker. Similarly, but perhaps through a different mechanism, muscles are also weaker after stretching. This would not be expected to reduce the risk of injury, and most studies have shown no change in the risk of injury when a pre-exercise stretching intervention is initiated.

Regular stretching: If a person does weightlifting over weeks to months, the muscles become stronger. Similarly, but perhaps through a different mechanism, muscles are also stronger after weeks to months of stretching. This is expected to reduce the risk of injury. Three studies examining the effects of regular stretching on the risk of injury have reported beneficial effects, but only one was statistically significant. Beneficial effects of yoga may work through this mechanism, although certain types of yoga also include strengthening and balancing exercises in addition to stretching exercises. The potential psychological benefits of yoga (and stretching in general) could be explained through other possible mechanisms.

This area warrants a lot of additional research. For example, most studies have examined lower-intensity activities such as jogging. Whether these results can be generalized to higher-intensity sports such as basketball remains to be determined.