A stretching period, in conjunction with a warm up, has long been an accepted method of preparing an athlete's musculoskeletal system for physical activity. Stretching is often prescribed for rehabilitation, injury prevention, imbalance correction and improved athletic performance. Yet, despite its popularity and prevalence, there is limited scientific knowledge regarding the mechanisms and effects of stretching on flexibility in general and on human musculotendinous units in particular.
Flexibility is considered to be one of the key functional elements for complicated coordinated movement. The skeletal muscles are primarily required for the generation, dissipation and recuperation of mechanical energy. The neuromusculoskeletal system's integrity is associated with the length, stiffness and adaptability of each individual muscle in the kinetic chain. Inadequacies in any of these areas may lead to other muscles becoming involved in movements to a greater or lesser extent than would normally be required, leading to muscular imbalances, pain or even injury.
According to Armiger, "...flexibility is an integral part of any training program and is essential in the pursuit of optimum performance, as well as injury prevention and rehabilitation." The suggested reasoning for this is that adequate flexibility permits the required joint range of motion (ROM) during activity, in addition to injury prevention should a limb be subjected to greater than normal range. By comparison, limited flexibility may lead to muscular or joint compensations (as referred to above), direct injury within the musculotendinous unit or sub-optimal muscular performance.
A loss of extensibility in soft tissues may lead to restricted range of motion of a joint. This loss of extensibility may be the result of either microtrauma or simply inadequate flexibility. Stretch interventions are therefore recommended to both competitive and recreational athletes as a means of increasing muscle extensibility and joint ROM. Stretching exercises can be self administered or applied manually by therapists or exercise professionals. Interventions applied in this manner would typically consist of developing a stretch over a period of up to a few minutes each. Alternatively, musculotendinous units can be stretched for prolonged periods of time (up to 24 hours per day) with orthoses, casts, splints or other similar means.
To date, despite the existence of a number of articles in the scientific literature, there are few high quality papers showing consistent methodologies and results. This lack of consistency is then compounded by a number of studies with similar methodologies that are assessing for different intervention outcomes. The result is that the mechanisms for both acute and chronic changes in joint range of motion, the efficacy of stretching for injury prevention and sports performance remain to be clarified.
There are two basic types of flexibility that concern the exercise professional. The first type is static flexibility, which refers to the angle that a limb can be moved to at each joint and is often measured in degrees for a particular joint and limb. The second type is dynamic flexibility, which refers to the ease to which a limb moves through the joint's range of motion, and angles can be measured as muscular force is generated or by using the damped oscillation technique. Due to the ease and accuracy of measuring static flexibility, it is this type that is referred to most often in the scientific literature.
Static stretching of muscles is often incorporated into both warm up and cool down periods of exercise sessions. However, commonly accepted protocols for the application of stretching techniques are still lacking. This is also true with regards to stretching for rehabilitation, injury rehabilitation and postural correction. In short, there is little scientifically based evidence for the correct application of stretching techniques for any outcome. In particular, there is no real consensus as to the best stretching exercises, the optimal frequency, number of sets and repetitions, duration of stretches held or number of stretches per muscle or joint. Furthermore, outcomes of stretching techniques are often difficult to qualify due to confusion with joint or ligamentous laxity, either of which may be the precursor to or result of injury. If studies into flexibility training only consider muscle length, then the exercise scientist has missed all the connective tissues, nerve fibres, muscle spindles and joint complexes that should be involved.
Taking this point further, if someone has used stretching to lengthen a muscle, surely this will affect proprioception? There are many purported benefits and costs of stretching prior to exercise. If someone has an area of tightness within a muscle or a muscular imbalance, then they have an increased susceptibility to exercise induced injury. Stretching could be effectively used to release the tightness in the muscle and return any comparatively shortened muscles to a near normal length. It would not be appropriate to begin exercise immediately following a stretching session as the body's proprioception will have been affected. Proprioception could be restored or "retrained" following an effective mobility phase lasting a minimum of 10 minutes. This mobility phase might include a walking and light jogging period on level ground prior to a running session. For a resistance training session, the mobility phase might include exercising with light weights across similar planes of motion to those to be included in the resistance program.
Different techniques to improve static flexibility range from single set static stretching to multiple set hold/relax techniques (including proprioceptive neuromuscular facilitation or PNF). Static stretching typically involves moving a limb to the biting point of the shortest muscle involved and holding the limb in position for a period of at least 15 seconds. A hold/relax technique could then be a repeat of the static stretch, incorporating a short rest for as many repetitions as required, as dictated by the client or exercise specialist. One representative study by Gribble et al found that both static stretch and hold/relax techniques were equally effective in increasing hip flexion ROM over a six week period. In this study, the static stretch was held for a single 30 second period, while the hold/relax method consisted of an eight second hold followed by a contraction of the agonist muscle for seven seconds, a five second rest and then a 10 second repeat of the static stretch at the new biting point.
The suggestion that this study was representative of the scientific literature on flexibility highlights the vast number of variables involved in such a study. In the study by Gribble et al, both techniques were equally effective, although many practitioners would argue that the biting point of the muscle increases more in a hold/relax stretch. The reason may be due to a less than optimal progression of the stretch, the short time periods involved in the hold/relax technique in this study or a combination of factors. To properly test effectiveness of stretches, the different techniques should probably be held for a longer duration. Liken flexibility training to resistance training. An exercise professional will advocate a number of exercises per body part, across a number of planes of movement, with each exercise for a specific number of sets and repetitions (following warm up sets), with the total duration on each muscle equaling a minimum of 15 minutes. Yet the same exercise professional will probably recommend only one stretch, in one plane of movement, for one set, lasting a total of 15 to 30 seconds. Should it be considered a "no brainer" that studies are inconclusive and stretching practices are equivocal with regards to effects on injury prevention, rehabilitation and sports performance? The best way to stretch has not yet received consensus, and steps to find such a consensus are being inhibited through poor studies based on poor trends in practice.
In the study by Gribble et al, we are left with a comparison of muscle stretches in a static only technique lasting for 30 seconds, compared with a hold/relax stretch with a static component lasting only 18 seconds. How can we use this to draw real useable conclusions? If there is no consensus as to the correct application of static stretching, is it wise to now be recommending dynamic flexibility techniques, which have even less base of evidence to work from? The problem here is that the industry trends are based on scant evidence, and while the scientists are working to improve our base of knowledge, industry "leaders" are teaching flexibility practices that are without support.
PNF Stretching Techniques and Application
During normal stretching of a muscle, a myotatic (stretch) reflex is activated, which prevents further muscle lengthening. This reflex is useful to prevent musculotendinous injury by preventing the muscle-tendon unit from being stretched passed its normal passive range of movement. During stretching techniques, it is often the goal to diminish this myotatic response, so as to permit the muscle to be lengthened further. PNF techniques in particular are believed to effectively diminish the myotatic reflex and are consequently believed to be the most effective for increasing muscle length and therefore joint range of motion.
In a study by Carter et al, the authors concluded that PNF stretching techniques effectively diminished muscular activity of the biceps femoris, which was apparently due to an inhibition of the myotatic reflex and would therefore permit greater muscle stretching. Chan et al found that PNF hamstring stretching increased hip flexion by 11.2 and 8.9 degrees for an eight week and four week training program, respectively. The eight week group stretched for one set of 30 seconds, three times a week. The four week group stretched for two sets, with a one minute rest interval between sets, three times a week. Prior to the static stretch component, each subject held an isometric contraction in the agonist muscle lasting three to six seconds. The increase in range of motion was the result of a total of 3,600 seconds of stretching for both groups. This study suggests that gains in ROM become most significant, not with increased frequency or duration of stretch or the number of repetitions or sets, but with the number of weeks that stretching is included in the program. This is not to say that the other variables are not involved, but specifically that changes in the muscle itself occur with recovery from stretching sessions, just as the effects of resistance training and cardiovascular training also occur with recovery time following overload.
Despite an increase in ROM, associated with the effectiveness of a flexibility training intervention, the underlying mechanisms and physiology are poorly understood. The exercise professional and scientist alike must surely find it an arduous task to make recommendations for improving flexibility, if they do not understand how flexibility is actually improved. So what can be taken from this article and the scientific literature in general? Well, it can be said that there are a number of reasons for improving flexibility, and some examples have hereby been given that detail how it may be achieved. The literature suggests that there is little long term difference between static and PNF techniques. Furthermore, the literature shows that it is necessary to include a stretching program three times a week, with stretches held for minimum of 30 seconds, for at least one set. As the study by Chan et al shows, 3,600 seconds of stretching improves muscle length, whether that be 3,600 seconds spread out over eight weeks or four weeks. The most significant gains, however, came with the eight week program, suggesting that recovery from exercise sessions plays an important role. Muscles may also become more efficient at adapting to flexibility training as a long term effect of a stretching program. This may be in a similar manner to how strength training becomes more efficient as more neuromuscular pathways are introduced.
Addressing the various issues of using flexibility training to decrease injury risk, rehabilitate muscle damage or improve sports performance is outside of the scope of this article. The first step for the exercise professional is to start considering flexibility training in a similar manner to how other resistance and cardiovascular training is considered. To improve flexibility around a particular joint, the muscles involved should be stretched through a number of planes of movement, for at least one set of 30 seconds, in regular stretching programs. These programs should be adhered to for at least four weeks for long term improvements to be imposed on the trained muscles.
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