Stretch-Shortening Cycle (SSC)

The stretch-shortening cycle is a natural pattern of muscle function in which an active eccentric (lengthening) action is immediately followed by a concentric (shortening) action, enhancing concentric output. It is what makes a counter-movement jump higher than a static squat jump.

Two main mechanisms are proposed:

1. Elastic (mechanical) contribution — the muscle-tendon unit stores elastic strain energy during the rapid stretch and returns part of it during shortening, like a spring. The energy must be reused quickly or it dissipates as heat.
2. Neurophysiological (reflexive) contribution — the rapid stretch activates muscle spindles, triggering the stretch (myotatic) reflex, which increases motor-unit activation during the concentric phase. Pre-activation and reduced reflex inhibition also contribute.

Effectiveness depends on three things: a short coupling/amortization time (the pause between stretch and shortening), the velocity/magnitude of the stretch, and adequate muscle stiffness. A long delay lets stored elastic energy leak away and reduces the reflex benefit. SSC actions are classified as fast SSC (ground contact <250 ms, e.g., sprinting, depth jumps) or **slow SSC** (>250 ms, e.g., a counter-movement vertical jump, layup). the 250 ms fast/slow threshold is commonly cited but varies.

• SSC = active eccentric immediately followed by concentric, increasing concentric force.
• Two mechanisms: elastic energy storage/return + stretch-reflex potentiation.
• Benefit requires a short amortization (coupling) time; delay dissipates stored energy.
• Fast SSC: ground contact <~250 ms (sprint, depth jump); slow SSC: >~250 ms (CMJ).
• The physiological basis of all plyometric training.