Stretch-Shorting Cycle (SSC)
Human movement, such as throwing, jumping, and even walking, is characterized by an eccentric phase immediately followed by a concentric phase, called the stretch-shortening cycle (SSC). During the eccentric phase, the tendons develop and stores potential kinetic energy, similar to a stretched elastic band. Then, during the concentric phase, this potential kinetic energy is returned, resulting in greater force output than if the movement had begun concentrically. During many movements, such as jumping rope, the muscle maintains static contraction, with movement being provided by the storing and release of elastic energy through the tendons. Since static muscular activity uses up less energy than concentric activity, SSC is an extremely energy-efficient way of moving. The efficiency of the SSC is easy to test:
- Perform a vertical jump in a normal manner, where you first crouch, and then jump upwards as explosively as possible.
- Next, crouch, but pause for five seconds, and then jump upward.
- Measure each attempt.
- You will see that the jump where the crouch (or eccentric phase) was IMMEDIATELY followed by the jump was more successful. The key to preserving as much potential kinetic energy as possible is to switch from eccentric to concentric as rapidly as possible.
SSC may be seen in Taekwondo by watching how fighters camber, or cock their kicks before they are executed. Punches all start from the guard, or chambered position.
Strength training methods should reflect the SSC nature of athletic skills. The best forms of resistance training to use are constant resistance, or "free weights," and variable resistance machines that attempt to "match" the resistance values to the strength curve of the muscle being trained. Free weights are preferred because machines tend to rob the synergists and stabilizers of adaptive stress.