The stretch-shortening cycle (SSC) describes a muscle-tendon unit being actively stretched in an eccentric action and then immediately shortening in a concentric action. Compared with a concentric action performed from a static start, an SSC often allows greater force, power, or efficiency.
Several mechanisms are thought to contribute. Tendons and other elastic tissues store energy as they stretch and return part of it during recoil. Muscle activation that is already present before the concentric phase means less time is spent building force, and stretch reflexes may also contribute. The relative importance of each mechanism is still debated in research.
Common Examples
- The countermovement before a vertical jump
- Ground contact in sprinting, hopping, and bounding
- Drop jumps and other plyometric exercises
- The backswing or arm cocking before a throw or strike
SSC actions are sometimes divided into slow and fast types. Slow SSCs, such as a countermovement jump, involve larger joint movements and longer ground contact, often described as above roughly 250 milliseconds. Fast SSCs, such as sprint contacts or rebound jumps, involve shorter contacts and smaller joint movements. The benefit depends on a quick transition: a long pause between the stretch and the shortening reduces it.
Observing It in Video
Frame-by-frame review can identify the eccentric phase, the turnaround point, and the concentric phase, and can show whether there is a pause between them. Counting frames gives an estimate of ground contact time and transition duration. Comparing a countermovement jump with a squat jump recorded from the same position can illustrate the effect of the pre-stretch.
Limits and Pitfalls
- Video cannot show elastic energy storage, reflex activity, or force; these require force plates, electromyography, or laboratory methods
- Timing estimates depend on frame rate; at 30 frames per second each frame spans about 33 milliseconds
- A deeper countermovement is not automatically better if it slows the transition
- Fatigue can lengthen contact times and change SSC behavior within a session
The SSC is a useful framework for coaching rhythm and timing in jumping, sprinting, and throwing.