The time that strength and conditioning professionals have with their athletes is often limited, so maximizing the effects of training activities is important. One area of research that has changed exercise prescription is the biomechanical effects of stretching. The prescription of stretching by strength coaches should be guided by these results demonstrating the differences in short-term and long-term mechanical effects and the expected effects of stretching on performance and risk of injury.
RECENT STRETCHING RESEARCH
About 10 years ago, the results of basic science research began to cast doubt on the belief that stretching in the warm-up would improve subsequent maximum muscular performance (8). Since then, consistent and conclusive evidence from dozens of subsequent studies has shown that stretching in the warm-up creates a short-term decrease (5-28%) in most kinds of maximal muscular performance (15,17). About 20-30 seconds of static stretching is the minimum dose to create an immediate 5% reduction in strength, and the greater the total duration of stretching, the greater the inhibition of performance (9). This inhibition of muscular performance can last up to 60 minutes for very large dosages of stretching and is related to neuromuscular inhibition and decreased contractile force (4).
Given the lack of evidence for a positive effect of preactivity stretching and the numerous studies reporting negative effects, static stretching in the warm-up is contraindicated for most activities involving maximal muscular performance for athletes of all ages and skill levels. For example, a strength coach who has athletes stretch before fitness assessments would only improve flexibility scores and likely decrease scores on strength, endurance, and running tests. A coach using a vigorous static stretching routine with athletes before a competition could easily decrease strength that not only would decrease maximal performance for about 30 minutes but also could reduce an athlete's ability to resist overload, possibly contributing to a higher risk of musculoskeletal injury.
This evidence-based conclusion, interestingly, has not been universally accepted because some are not willing to make broad stretching recommendations or assume the efficacy of stretching interventions until there is conclusive evidence of performance inhibition and higher injury rates in all possible situations. This has lead to rather confusing conclusions in some articles that there is not enough evidence to either discontinue or continue with static stretching before vigorous physical activity (16). This position is not defendable for a National Strength and Conditioning Association professional who should prescribe stretching when there is experimental or professional evidence of the efficacy, not just a likely absence of harm.
There is also evidence that stretching before vigorous physical activity will not affect muscular injury risk. Several large prospective trials using recruits in military basic training reported no differences in injury rates when warming up for physical training with and without static stretching (1,12,13). In theory, joint stability might also decrease after the weakening from preactivity stretching, which would tend to decrease the margin of safety for a muscular injury. Taken in conjunction with the lack of evidence for the positive effects of stretching on performance, this lack of an effect on muscular injury argues strongly for the elimination of stretching before most physical training and competition.
Because the high passive tension in static stretching does not create beneficial short-term effects on the muscle and tendons or injury rate, strength coaches should recommend active warm-up routines rather than stretching before training and competition. Some recent literature has referred to new active warm-up exercises as “dynamic stretching” protocols, but this terminology should be avoided because it has not been clearly defined and it creates confusion with well-established terminology like ballistic stretching and static stretching that are not beneficial as warm-up activities. Dynamic stretching is essentially activity-specific active warm-up, and preliminary evidence supports warm-up-related mechanisms for its benefits (3).
Recent research has begun to explore customizing active warm-up routines for specific kinds of muscular performance. For example, an active warm-up with weighted (2% body weight) vests significantly improves jumping performance in athletes compared with stretching and other active warm-ups (2). This kind of research is important to determine optimal warm-up protocols for specific activities. Some preliminary work has also been conducted in an attempt to optimize intensity of warm-ups for individual athletes (11). In the future, the strength professionals may have more specific active warm-up data to maximize performance in different athletes or events. Current evidence supports the use of a progressive, active warm-up routine that mimics the sport, or training exercises to follow, to prepare for maximal muscular performance and minimizes the risk of muscular injury.
LONG-TERM EFFECTS OF STRETCHING ARE DIFFERENT
Many of the long-term training effects of stretching, however, are different from the short-term effects. Stretch training over several weeks can create an increase in flexibility and lower passive muscular tension at specific joint angles. This effect is important because genetics, repetitive sport movements, or lack of activity can result in poor levels of flexibility in some athletes. Stretching, therefore, is still important for most athletes to maintain normal levels of flexibility.
Stretching overload not only increases range of motion but also, in some studies, resulted in significant increases in strength above strength training alone in low fitness individuals (10) or athletes (14,15). The passive tension in a muscle group that is stretched consistently in conditioning programs may be high enough to create some training effects beyond the benefits of increased flexibility. Strength coaches should emphasize that static stretching is an important long-term training stimulus and fitness routine for all athletes.
The basic science and clinical research are remarkably consistent on the conditions for effective static stretching. The literature supports slow, static stretches held for 20-30 seconds maximum because most of the stress relaxation occurs in the first 15-20 seconds of a stretch (5). Consistency of stretching over days and weeks is more important than a large dosage (more than 5 repetitions of a stretch) per session in creating increases in range of motion.
The speed and force level of static stretches should be minimized because of the viscoelastic response of muscles and tendons. Viscoelastic means that the mechanical response of the tissue is both rate and length dependent, so a fast stretch will build up a larger passive tension in the muscle-tendon than a slow stretch to the same length. Strength coaches should teach that static stretches be performed slowly and held at each athlete's perception of tightness without discomfort. This will create the safest and least stressful stretching overload to contribute to improved flexibility. More structured stretching routines like proprioceptive neuromuscular facilitation can be used, although evidence indicates that their effects on flexibility are similar to static stretching.
General recommendations for the safe development and maintenance of flexibility are listed in the Table below. Although there are a few prospective studies providing information on how much flexibility is associated with lower injury risk, there is some support for the hypothesis that the maintenance of normal flexibility should be the goal of a stretching program. Retrospective studies on athletes and military recruits have reported a higher rate of injury in both extremes (very flexible and inflexible) of the distribution of flexibility (6,7). Inflexible muscles could be easily overstretched, whereas too compliant muscles might not be able to quickly limit excessive accessory joint motion. This is also consistent with biomechanical theory that joint stability and mobility are inversely related.
Strength coaches should then focus on programming adequate stretching exercises at the end of training bouts to maintain normal ranges of motion in athletes. Periodic flexibility assessment will help the strength coach address flexibility imbalances and adjust the volume of stretching to avoid both extremes of the flexibility distribution.
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