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Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve 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 other 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.
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 extracellularly as 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. If people stretch daily, they will increase their flexibility, elasticity, range of motion, and production of synovial fluid. Stretching improves balance, physical performance, and blood circulation.
Muscle pain is caused by tissue damages and excessive blood accumulation. This can be prevented if one stretches on a regular basis. When stretching one should not pull the muscle too quickly because it will cause a strain or tear. The muscles become relaxed after they are stretched which decreases the likelihood of a person getting a stress fracture. It is important to stretch to increase blood flow to prevent the hardening of arteries.
There are four different types of stretching: ballistic, dynamic, proprioceptive neuromuscular facilitation, and static stretching. Ballistic stretching is a rapid bouncing stretch in which a body part is moving with momentum that stretches the muscles to a maximum. Muscles respond to this type of stretching by contracting to protect itself from over extending. Dynamic stretching is a walking or movement stretch. By performing slow controlled movements through full range of motion, a person reduces risk of injury. Proprioceptive neuromuscular facilitation (PNF) is a type of stretch for a particular muscle and its specific job, so resistance should be applied, then the muscle should be relaxed. Static stretching is a type of stretch whereby a person stretches the muscle until a gentle tension is felt and then holds the stretch for thirty seconds without any movement or bouncing.
A study of soccer players showed a group who did dynamic warm up exercises and static stretches had fewer knee injuries than one that did neither. 
One review suggests that there are many beneficial stretches that can improve range of motion (ROM) in athletes, especially runners. It is also suggested that one stretching exercise may not be enough to prevent all types of injury, and that, multiple stretching exercises should be used to gain the full effects of stretching. It has also been suggested that proprioceptive neuromuscular facilitation (PNF) stretching yields 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.[clarification needed] Others[who?] suggest that PNF benefits are due to influence on the joint where the stretch is felt.[clarification needed]
If done properly, stretching can prevent injury, relax the muscles, increase range of motion and flexibility, and better one's performance, especially athletes. Although stretching does not prevent injury, it can reduce the risk greatly; especially if one stretches properly and on a regular basis. Stretching is more beneficial to those who stretch regularly, as opposed to those people who stretch occasionally. Stretching increases blood flow which prevents hardening of the arteries and it also produces synovial fluid, which lubricates the joints that are surrounded by the muscles; which in turn helps prevent arthritis. Stretching stabilizes the body's natural balance and posture, and aligns the joints leading to better coordination. After any physical activity, there is a build up of lactic acid in one's body and by stretching the lactic acid is removed, therefore alleviating any muscle pain or cramps. It is important for a person to perform each of the four types of stretching properly to gain the benefits.[unreliable medical source?] It has been shown for example that intensive stretching has a synergistic effect with Plyometric training by protecting the joint and making it more receptive to the benefits of the plyometric drills.
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.
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 60%. Reasoning behind this claim is that increased levels of Ca2+ in resting muscles predisposes individuals to fatigue quicker than individuals who did not stretch.
Static stretching in general reduces strength and power. It has also been shown to reduce stability in squats. Static stretching did not help reduce lower leg injuries in a study of military recruits.
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.
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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