Stretch-Shortening Cycle

The following is an exclusive excerpt from the book NSCA's Guide to Program Design, published by Human Kinetics. All text and images provided by Human Kinetics.

Most powerful activities involve a countermovement during which the muscles involved are first stretched and then shortened to accelerate the body or limb. This action of the muscle is called a stretch-shortening cycle (SSC) (66). It involves many complex and interacting neural and mechanical factors, such as activation of the stretch reflex and muscle–tendon interactions. A great deal of research has been directed toward the study of the SSC (10, 11, 37, 42) because it has been observed that performance is greater in SSC movements than when the activity is performed with a purely concentric action (11). For example, differences in jump height of 18% to 20% have been observed between static or squat jumps (SJ) and countermovement jumps (CMJ) (12). An SJ is a purely concentric jump initiated from a crouching position. The CMJ is initiated from a standing position. The athlete performs a quick countermovement, dipping the hips down and then jumping up.

Although several mechanisms have been proposed (10), it would appear that the difference in CMJ and SJ height is due primarily to the fact that the countermovement allows the athlete to attain greater force output at the start of the upward movement. This results in greater forces being exerted against the ground and, subsequently, an increase in impulse (F x t) and acceleration of the whole body upward. The other proposed mechanisms, such as recovery of stored elastic energy, muscle–tendon interactions, and activation of the stretch reflex, appear to play a secondary role in the enhancement of performance by the SSC (10).

Maximal power performance has been shown to respond to training that involves performing SSC movements more rapidly than the athlete is accustomed to with a stretch load of greater magnitude than usual. These activities, termed plyometrics, have been found in a number of studies to effectively increase jumping ability and power output (1, 21, 91, 102). Plyometric training results in an increase in the overall neural stimulation of the muscle and, thus, an increase in force output. However, qualitative changes in the muscle activity are also apparent (91). In subjects unaccustomed to intense SSC loads, some studies have shown a reduction in electromyographic (EMG) activity, starting 50 to 100 ms before ground contact and lasting for 100 to 200 ms (91). This is attributed to a protective reflex mechanism by the Golgi tendon organ, which acts during sudden, intense stretch loads that typically reduce the tension in the musculotendinous unit during the force peak of the SSC. After periods of plyometric training, these inhibitory effects (and the observed reduction in the EMG) are reduced (termed disinhibition) and SSC performance results are increased (91).

Plyometric training places considerable forces on the musculoskeletal system. Although it has been recommended that athletes have a preliminary strength training base prior to commencing a plyometric training program (e.g., an athlete should be able to squat 1.5 times his or her body weight) (20), low-intensity plyometric drills (e.g., squat jumps, countermovement jumps, lateral jumps, box jumps) can be performed safely without any minimal strength requirement. Keep in mind that plyometrics are often part of the jumping games that children play. The potential for injury is thought to be much higher for depth jumps, which should not be attempted by beginners (89).

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