STRETCHING

Angle

Torque

evident that the changes associated with acute stretches are transient. If the joint is returned to the starting position and left for 5-10 minutes, and then stretched again the response is often identical to that recorded at baseline. Thus in a sports scenario, where the athlete is waiting to participate, it is important that they keep their joints moving through the range required in their sport.

Time Time

Figure 2: The force relaxation response. In the left figure, the joint moves to a point in the range of motion and is held there. The torque response is shown in the right figure. Here the torque rises as the joint is moved through its range, but once the joint is stationary, the torque decreases, a phenomenon termed the torque relaxation response.

The findings have been similar to those of Taylor et al with the resistive torque decreasing over time in a non-linear manner depending upon the point where the stretch is being held and also the individual characteristics of the subject’s tissues. An example of such a curve is shown is Figure 2. Researchers (10,13,14) have also noted that in-vivo the decline in torque is greatest in the first 20-30 seconds and thereafter the torque response flattens out providing evidence to suggest this duration to be the most ‘economical time’ to hold a stretch. This expression reflects the greatest decrease in resistive torque in the least time. Subsequently, these times have been used in a number of studies examining longer term stretching effects. Certainly, improvements of over 20% in range of motion have been shown to occur as a result of just a single stretch of 30 seconds each day for five days over a six week period (15). It should be noted though that the torque-angle curve continues to show decreases for much longer periods than 30 seconds. We have observed declines which although very small in magnitude are still occurring 20 minutes after the stretch is held, and it may be that these longer stretch times are beneficial for people with more chronic conditions that involve contractures.

EFFECTS OF REPETITIONS The ideal number of repetitions to stretch in a single session is not known. However, it is evident that the greatest changes in resistive torque and range of motion occur in the first repetition of stretching, and thereafter the responses are much less with each repetition performed. In a recent study (10) that examined 4 times 15 second stretches, the first stretch led to a 14% decline in force while the fourth stretch generated less than 2% change. The mechanisms associated with these acute effects of stretching have included the breakage of actin and myosin cross bridges, fluid movement and molecular motion within the tissues, and strain and slippage of collagen fibrils. These mechanisms are quite different to those that are associated with longer term stretching programs. How much a stimulus from acute stretches transfers to longer term adaptations, whether they be from sarcomeres being added to the muscle fibres or a stretch tolerance effect is not known. It is

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Recent studies (16,17) have suggested that stretching a muscle immediately before a maximum effort contraction by those muscles may be detrimental to the performance generated by the stretched muscles. The findings have shown EMG and torque measures to be decreased by between 3-6%. However, it should be noted that the procedures have generally involved long (large volume) stretching regimes that would not normally be undertaken prior to sporting activities. The mechanisms advanced for these changes in active force production have included decreases in passive mechanical parameters, such as stiffness in the parallel and series connective tissues elements of the muscles, these contributing significantly to the transmission of forces from the contractile elements to the tendon. Furthermore, changes in muscle activation have been attributed to neuromuscular factors such as muscle fibre recruitment, motor control strategies and reflex events. Related to these changes in peak torque is whether the angle at which peak torque occurs is changed by a stretching session. The results of studies examining this point are equivocal.

Given the above thoughts, when therefore will stretching be useful in acute scenarios? Firstly, where the individual has insufficient range of motion for the activities involved in the sport in which they are participating, an acute stretching session with holds will be beneficial. Secondly, if the individual has increased resistance through the range of motion, that is increased stiffness, then a series of dynamic exercises that stretch the tissues will be useful. Related to these comments is evidence (18) that individuals who have limited range of motion suffer increased discomfort and decreased performance in the days following unaccustomed exercise. Where range of motion is sufficient for the activities that are part of a sport, then the situation becomes more complex, and consideration can be given to the predominance of the stretch- shorten cycle. Stretch-shorten cycle activity is used to absorb energy and generate extra power. This activity is important in sports like volleyball and basketball. When muscle is very active, for instance during landing and jumping activities, if the tendon has appropriate amounts of stiffness, it can absorb energy during eccentric muscle action and enhance performance though the use of that stored energy. If the tendon is too stiff, then the energy may be transferred to the contractile elements, which may not be able to absorb it effectively, and therefore muscle injury may occur. Stretching would therefore be useful to decrease the stiffness of the system, primarily the tendon. It seems likely though that there is an ideal level of stiffness that is needed for reducing the likelihood of injury and at the same time not compromising performance. That is, too little stiffness may limit performance. Establishing the right

sportEX medicine 2007:34(Oct):6-9