The Stretch-Shortening Cycle (SSC)
Movement is caused by voluntary contractions of skeletal muscle which are categorised as either: Concentric (shortening), Isometric (static) or Eccentric (lengthening). The SSC occurs within muscle-tendon units (MTUs) during activities or tasks which need rapid movement or economy of movement. As the name suggests, the SSC involves a stretch of the MTUs followed by an immediate shortening. This can be broken down into three phases:
- Pre-stretch – Eccentric loading of the muscle. This can be the landing phase during sprinting or the windup phase of a throw. It is also recognisable as the downward phase of a counter-movement jump (CMJ). Some research shows evidence of a pre-activation of the MTUs prior to this eccentric phase.
- Controlling – often called the amortization phase. This can be looked at as the transition phase and needs to be minimised in order to maximise concentric performance. It is commonly accepted that this time should be less than 200 milliseconds to be recognised as a true Plyometric action.
- Concentric – Immediate shortening of the MTUs which result in an explosive concentric action. Research has shown an improvement of about 8% in jump height when comparing a CMJ to a concentric-only squat jump.