Elastic Energy in Stroke Play
Rowden Fullen (1990’s)
Many sporting activities involve a stretch-shorten cycle where the muscles involved in the exercise are first stretched then shortened. This is generally observed in racket sports as a counter-movement during the back-swing or preparation stage of the activity (the stretching phase), that precedes the actual forward or upward movement (the shortening phase). One of the reasons for the use of the stretch-shorten cycle is that it enhances the quality and efficiency of the movement through the utilization of elastic energy.
The mechanical principle underlying the use of elastic energy in stretch-shorten cycle activities is a relatively simple process. During the stretching phase the muscles and tendons are actually stretched and store elastic energy in the same way as an elastic band stores energy when stretched. On movement reversal, during the shortening phase, the stretched muscles and tendons recoil back to their original shape and in so doing a portion of the stored energy is recovered and assists in the movement.
Biomechanical research has shown that, in running for example, the use of elastic energy has been estimated to account for approximately 50% of the total energy requirement. In other similar stretch-shorten cycle activities such as racket sports, (movement and stroke play for example), the use of elastic energy also contributes a significant proportion to the total energy requirement.
Elastic energy is stored in tendons and in muscle itself. The storage of elastic energy within muscle is dependent upon the level of muscular activity present during the stretching phase. The greater the tension in the muscle being stretched, the more elastic energy will be stored. Therefore, to maximize the storage of elastic energy, the stretching phase should be resisted by muscular effort. In a stretching movement of very short duration, such as the foot contact phase in sprinting, the energy can be stored during the entire stretching motion. However, in a movement of longer duration, such as in a forehand topspin, the energy is best stored just prior to the shortening phase. This is achieved by producing a high level of force, (large muscular resistance), towards the end of the stretching phase.
Research indicates also that increasing the speed of the stretching phase from a slow speed to a relatively high speed enhances the storage of elastic energy. This occurs as an increased speed or force of stretch extends the muscles and tendons to a greater extent thus storing even more energy. Therefore the final portion of the back-swing should be performed quickly as the faster the back-swing, the greater the elastic energy recoil will be during the forward swing. In the case of our attacking (or defensive) strokes in table tennis it is important that these stretch-shorten cycle movements be performed with a minimal delay between the stretch and shorten phases.
It has been demonstrated that 93% of stored elastic energy can be recovered. This recovery is largely dependent on the time period between the stretching and shortening movement phases. Elastic energy is reduced if a delay period occurs during the stretch-shorten cycle because during the delay period the stored energy is released as heat. The longer the delay the greater the loss of elastic energy. Research indicates that after a delay period of around one second, 55% of the stored energy is lost — after 2 seconds, 80% and after 4 seconds there is total loss.
Some training practices encourage players to prepare very early for stroke production and this often inadvertently produces a delay period between the back-swing and forward swing of the stroke. As a result stored energy is lost and an inefficient movement strategy results. For maximum efficiency players must practise allowing the back-swing and forward swing to flow naturally from one phase of the movement to the other. This is particularly important when playing defensive players, where there can be some seconds’ time-lag in returning the ball. Try more to move into a good position, but only to pull back the arm in the stretch phase of the topspin or drive movement at the time the ball bounces on your side of the table or even after. In this way you save a higher ratio of elastic energy and utilize it in the stroke.
The recovery of stored elastic energy tends to occur relatively quickly during the shortening phase of the movement. Tests show that all stored energy is released 0.25 seconds into the shortening phase. Thus in drive and topspin strokes the stored energy is used primarily to assist in the early forward swing stage of the movement.
The implications from this research are that the stretching or counter-movement phase should be performed quickly with large muscular resistance exerted over the final 0.2 seconds and that all stretch-shorten cycle movements should be performed with a minimal delay between the stretch and shorten phases.
Other research indicates that plyometric training (depth jumping, bounding etc.) may also enhance an athlete’s ability to utilize elastic energy and may even alter the elasticity of the tendons and muscles enabling them to store greater quantities of energy. Also in such training, the delay time between the stretch-shorten cycle is minimized ensuring maximal recovery of all stored energy. It would appear that plyometric training, as compared to conventional weight training, involves the implementation of those movement strategies which maximize the contribution of elastic energy to stretch-shorten cycle movements.
However although plyometric exercises may represent a more specific form of overload for many athletes, the performance of high impact stretching movements often results in muscle soreness in the days following training. It may therefore be necessary that the implementation of plyometrics in a training routine allows for recovery days between exercise sessions.