Stretch-Shortening Cycle

(Plyometrics)

By Tom Taylor

Definitions

Concentric Contraction – The shortening of the muscle during a contraction.

Contractile Component (CC) – The muscle fibers. Can produce tension.

Eccentric Contraction – The lengthening of the muscle during a contraction

Muscle Spindle – A specialized muscle fibers that provide information about the length and rate of change of length of a muscle.

Parallel Elastic Component (PEC) – The connective tissue that surrounds the muscle components including the epimysium, perimysium, endoymysium and sarcolemma. Exerts a passive force with unstimulated muscle stretch.

Plyometric – Powerful movement that utilizes a prestretch or counter-movement involving the stretch shortening cycle (SSC)

Sacromere -

Series Elastic Component (SEC) - The connective tissue surrounding each muscle fiber, groups of fibers and the whole of the muscle. They lengthen and respond like a spring.

Stretch Reflex – The body’s involuntary response to an external stimulus.

Muscle Cross Section

Mechanical Model of Skeletal Muscle - From Albert 1995

The stretch-shortening cycle (SSC) utilizes the elasticity of the muscle fiber and it’s storage of potential energy, and the recruitment of the stretch reflex to produce a quick powerful movement (SEC). During the SSC a muscle is placed in a pre-stretch position prior to execution of a movement. The pre-stretch position stores elastic energy and stimulates muscle spindles. As the movement begins the elastic energy that is stored in the pre-stretched muscle is transferred to the concentric contraction of the muscle increasing the force output. The there distinct phases of the SSC are:

1. Eccentric Phase – Agonist muscle lengthens. Initiation of the SEC and muscle spindle stimulation

2. Amortization Phase – No movement. The longer time in this phase the more elastic energy that is loss due to dissipation by heat.

3. Concentric Phase – Response to the eccentric phase and amortization phase. Stored elastic energy increases the force production. The alpha motor neurons stimulate the agonist muscle resulting in the stretch reflex.

Relationship to Exercise

Most sport movement requires that an eccentric contraction be followed by a concentric contraction. Without these components ballistic sports movements such as jumping and throwing could not take place. The more power an athlete has the more successful they will be.

The variables for power are strength and speed. Power is mathematically described as the rate of doing work; and is measured in watts (= joules/second). Work is mathematically defined as force times distance; and is measured in joules. If one decreases the time required to do work they will be more powerful. In addition, if the force or distance is increased and the time is held constant they will be more powerful. Therefore, the faster that a muscle undergoes the eccentric phase the more powerful the movement will be.

Training of the stretch-shortening cycle has recently been termed plyometrics or “jump training”. Training using plyometrics places the muscle in a rapid prestretch (eccentric phase) followed by a rapid contraction (concentric phase). The training movements must be rapid. The quicker that we can perform a movement the more powerful we will be.

References: