Stretching is the most common method employed for improving flexibility. The forces responsible for stretching a muscle can broadly be categorized into active stretching and passive stretching. Active stretching refers to when an agonist muscle moves a body part through the range of motion, and the force provided by the active contraction stretches the opposing muscles. Passive stretching refers to when outside forces assist in the stretching process; these extra forces contribute to muscular and fascial elongation. Examples include gravity, momentum, force applied by another person, or force applied by the indivdual, as in pulling on the feet to get a deeper hamstring stretch.
Active stretching is a type of stretching that involves assuming a position with no assistance other than using the strength of the agonist muscles. The tension of the agonists in an active stretch helps to relax the muscles being stretched (the antagonists) by reciprocal inhibition. An example would be bringing the leg up high and holding it in place without anything (other than the leg muscles themselves) to keep the leg in that extended position. Active stretching is also referred to as static-active stretching. 
A method of stretching which employs the use of repetitive bouncing movements and is thought to be the oldest stretching method. Many trainers have discontinued use of this method due to safety concerns.
Dynamic stretching consists of a series of exaggerated yet controlled motions similar in nature to the activity that follows. It is typically used as a warm up for athletes in preparation for an event or critical practice. Dynamic stretching is likely a more effective pre-workout routine than other types of stretching, such as active stretching.
The forces responsible for stretching a muscle can broadly be categorized into active stretching and passive stretching. Passive stretching refers to when outside forces assist in the stretching process; these extra forces contribute to muscular and fascial elongation. Examples include gravity, momentum, force applied by another person, or force applied by the indivdual, as in pulling on the feet to get a deeper hamstring stretch.
Proprioceptive Neuromuscular Facilitation
Proprioceptive neuromuscular facilitation (PNF) is a method of stretching. It is capable of facilitating greater gains in flexibility in a shorter amount of time than with other methods, such as static stretching or dynamic stretching. Proprioceptive neuromuscular facilitation involves the use of muscle contraction before the stretch in an attempt to achieve maximum muscle relaxation. PNF also typically involves a partner who assists the stretcher by actively stretching the muscle. Each PNF stretch is done by alternating the contraction and relaxation of both agonist and antagonist muscles. It is usually done by a 10 second holding/contraction phase followed by a 10 second relaxation phase. ACSM suggests 6 second contractions followed by a 10 to 30 second assisted stretch.
Static stretching is perhaps the most popular, or at least the most well-known, method of stretching. It involves passively stretching a muscle to the period of mild discomfort and holding it for an extended length of time. This has been proven an effective means for correcting muscle imbalances as well as preventing long-term injuries. It has also been shown that it may help prevent muscular injuries, but not bone and joint injuries. Some controversy arises from this when debate ensues as to which stretching method is the most effective. It is not recommended for athletes to use prolonged static stretching before competition or an important practice session. This is due to the fact that static stretching slows muscle activation for about an hour afterwards. DROM, or dynamic stretching, is better suited for athletic warm ups. Static stretching, however, is beneficial for increasing flexibility, and should be performed at the appropriate times, such as the beginning of a season, or separate from workouts or practice.
The use of static stretching is also the preferable method to facilitate an increase of range of motion in mature adults, as well as individuals with overly tightened muscles, such as the hamstrings (most common). Static Stretching has also been known to aid the decrease of body fat and reduction of cholesterol as well as the reversal of hardening of the arteries in adults with coronary disease. The optimum length of time to hold a static stretch seems to be 30 seconds. Any longer will still bring about the same results.
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