Stretching is a form of physical exercise in which a specific skeletal muscle (or muscle group) is deliberately elongated, often by abduction from the torso, in order to improve the muscle's felt elasticity and reaffirm comfortable muscle tone. The result is a feeling of increased muscle control, flexibility and range of motion. Stretching is also used therapeutically to alleviate cramps.
In its most basic form, stretching is a natural and instinctive activity; it is performed by humans and many animals. It can be accompanied by yawning. Stretching often occurs instinctively after waking from sleep, after long periods of inactivity, or after exiting confined spaces and areas.
Increasing flexibility through stretching is one of the basic tenets of physical fitness. It is common for athletes to stretch before and after exercise in order to reduce injury and increase performance. Yoga involves the stretching of major muscle groups, some of which require a high level of flexibility to perform, for example the lotus position. Stretching can strengthen muscles, and in turn strong muscles are important to stretching safely and effectively.
Stretching can be dangerous when performed incorrectly. There are many techniques for stretching in general, but depending on which muscle group is being stretched, some techniques may be ineffective or detrimental, even to the point of causing permanent damage to the tendons, ligaments and muscle fiber. The physiological nature of stretching and theories about the effect of various techniques are therefore subject to heavy inquiry.
Studies have shed light on a large protein within skeletal muscles named titin. A study performed by Magid and Law demonstrated that the origin of passive muscle tension (which occurs during stretching) is actually within the myofibrils, not extracellular as had previously been supposed. Due to neurological safeguards against injury, it is normally impossible for adults to stretch most muscle groups to their fullest length without training due to the activation of muscle antagonists as the muscle reaches its normal range of motion.
One review suggests that there are many beneficial stretches that can improve range of motion (ROM) in athletes, especially runners. Another study found that classic "static stretching" did not prevent injuries for runners.
Also, certain stretching techniques and protocols prevent injuries when performed (within 15 minutes) prior to exercise.
However, stretching does not prevent delayed onset muscle soreness, either when performed before or after exercise, according to a Cochrane review in 2006.
It is also suggested that one stretching exercise may not be enough to prevent all types of injury, and therefore, multiple stretching exercises should be used to gain the full effects of stretching.
It has also been suggested that "proprioceptive neuromuscular facilitation (PNF) stretching yield the greatest change in range of motion, especially short-term benefits. Reasoning behind the biomechanical benefit of PNF stretching points to muscular reflex relaxation found in the musculotendinous unit being stretched. Others suggest that PNF benefits are due to influence on the joint where the stretch is felt.
Over-stretching or stretching to a point where pain is felt may be inappropriate and detrimental. Effects on performance, both short- and long-term, may include predisposition to injury and possible nerve damage. Other research concludes that active stretching routines will reduce muscle-tendon viscosity and increase muscle compliancy and elasticity. In sports activities where there are little or no short-stretching cycles, (bicycling, jogging, etc.) stretching routines may be detrimental to athletic performance and have no effect on reducing injuries.
Stretching may also cause ischemia in muscles, which reduces oxygen levels and the ability to remove metabolic waste. Higher levels of metabolic waste create a catalyst that contracts muscles. This may cause muscle injury in individual performance. Other theories included claim active static stretching increases inflow of Ca2+ from extra cellular spaces into the muscles being stretched. The increase of Ca2+ reduced the muscle twitch tension by up to sixty percent. Reasoning behind this claim is that increased levels of Ca2+ in resting muscles predisposes individuals to fatigue quicker than individuals who did not stretch.
Some people are more flexible than others as defined by individual body flexibility score; this includes sex differences where females are generally more flexible than males. Stretching may not increase range of motion, but rather increase individual stretch tolerance, becoming detrimental to athletic performance. Among the factors these studies measure are capsular mobility, FlexiScore and joint-muscle compliance.
American football player stretching before a game with assistance from an athletic trainer.
Results of research by Witrouw et al. found that: each of which has a different consideration based on individual activity:
The reason behind conflicting data is claimed to be due to the different levels of observed sports activity.
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