THE EVIDENCE BASE

Baseline Response 1 Response 2 Response 3

Range of motion

stretches), changes in the shape of the torque-angle curve have been noted. The changes are not consistent across individuals/study findings. Generally however, three responses are noted (see Figure 1):

1) The curve may shift to the right, indicating that the resistance to motion has decreased for most if not all angles in the range of motion. This response is indicative of a change in muscle length occurring.

2) The curve extends further up its original path. That is, both torque and angle are increased but the relationship between these variables is not changing. Authors (3) have argued that this response is indicative of a change in our tolerance to stretch.

3) The curve is shifted to the right in the early to mid part of the range of motion, but toward the end of range there is an increased resistance to motion. This response can be ascribed to viscous/dissipative changes occurring in the tissues (2).

STRETCHING EFFECTS ON TISSUE TYPES Studies using diagnostic ultrasound have shown that different amounts of stretch occur in muscle, the aponeurosis and tendon. In such studies (4-6), which have most often examined lower leg muscles, the ankle joint is placed in a neutral position and the sub- ject undertakes an isometric contraction. Stretch in the tissues is examined during the contraction. The findings of such studies have not always been consistent, some studies showing the aponeurosis to be stretched more than the tendon while others finding the opposite effect, or no difference in the amount of strain across these different tissues.

The speed at which one stretches influences the above responses, most notably the peak resistance and the stiffness of the musculo- tendinous structures. Fast stretches elicit higher levels of stiffness and also higher peak resistive torque, and hence are not recommended at the start of a stretching session (7). These responses generally reflect passive mechanical events (eg. strain and slippage of collagen) occurring in the muscles and tendon. EMG responses which are indicative of the muscle’s contractile elements becoming active and hence resisting the stretch, are unlikely to be seen until joint motion is at an angular velocity above 80o

per second (8), which is much faster than one would

normally stretch a patient in a clinical situation. However, when an individual is using a dynamic stretching technique where they repeatedly move their limb through range (eg. leg swinging type activity to stretch hamstrings, caution would need to be exercised, particularly during the first 1-3 swings as the joint angu- lar velocities will be much higher and may elicit an EMG response, which together with the greater passive resistance in the tissues that is seen in initial stretches may make the muscle more prone to damage.

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It should be noted that during an initial stretch, particularly in individuals who have not stretched for some time, EMG responses are apparent from about 80% into the range of motion and these increase as the person’s joint is moved to the maximum range of motion. The magnitude of this activity can vary between 2-15% of the activity recorded during a maximum contraction (MVC) of the muscle being stretched and is more often seen in the older adult. With repeated stretching repetitions, this activity usually decreases considerably (less than 2% MVC). To facilitate this decrease in activation, biofeedback equipment is useful, particularly as the patient is often not aware that they have activated their muscles during the stretch. In our studies, we find that approximately 5% of subjects are not able to decrease muscle activation even with feedback.

The level of activation of the muscle will determine the amount of stretch that occurs in the muscle and tendon. In a passive scenario, when muscle and tendon of the same length are stretched, the passive contractile elements lengthen to a greater degree. However, in many muscles the amount of tendon is considerably longer than that of the muscle contractile elements and hence a greater overall length change occurs in the tendon of that muscle in vivo (9). In many sports, particularly those involving jumping activities, energy is stored in the tendon and this can subsequently be used to generate notably more power in the muscle than that generated solely by muscle activation. The caveat though is that the concentric activation must occur almost immediately following the eccentric muscle activation and the gains in power are most apparent in the first few 100ms of the concentric activation.

EFFECTS OF DIFFERENT STRETCHES It is apparent that there are different responses in the tissues according to whether the tissues are being stretched by repeated passive movement or are being held at the end of the range of motion. In a study (10) that examined one minute of passive motion at a joint angular velocity of 5 deg/sec versus a 60 second hold, stiffness through the range of motion was significantly less as a result of the passive repeated movements. In contrast, the hold type stretch was more effective at decreasing the resistive torque in the tissues at the end of range. This work is related to previous research (11) that examined the effects of stretching and jogging on stiffness of the plantar flexors and maximum range of dorsiflex- ion. The findings showed stiffness was decreased more by jogging, while dorsiflexion range of motion was increased more by either stretching or the combination of stretching and jogging, compared to jogging alone.

Researchers have used dynamometers to explore what might be the appropriate time to hold a stretch. These experiments have followed the classic work of Taylor et al (12) whose study investigated the stress relaxation response in the lower leg muscles of animals. In this work, muscles were exposed and stretched to a specific length and thereafter force was monitored, and shown to decrease in a nonlinear manner. Taylor et al. noted that the greatest falls in r esistive forces occurred in the first 18 seconds and argued that this might be an appropriate time over which to hold a stretch. The force relaxation response has subsequently been explored in-vivo by a number of researchers (10,13,14), the procedures generally involving a joint being moved to a particular point in the range of motion (80% of total range of motion), and then being held there.

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Torque