The Mechanical Model
This surrounds the storage of elastic energy after a rapid stretching of the muscle and tendon components. When a concentric muscle action follows this movement, the stored energy is utilised which increases the total production of force. The series elastic components (SEC) contains a majority of tendon components, as well as some muscular. When the agonist muscle is eccentrically contracting (stretching), the SEC acts as a spring and lengthens, causing the storage of elastic energy. When this is followed by a concentric contraction of the agonist, the stored energy is released allowing the SEC to contribute to a greater production of force. However if the concentric muscle action does not occur immediately after this prestretch or if this eccentric stretch is too long, the storage of elastic energy will dissipate and be lost as heat.
Neurophysiological Model
This involved the reasoning behind why the subsequent concentric muscle action in potentiated- the stretch reflex. When an external stimulus causes the muscle to stretch, the natural involuntary response of the body is the stretch reflex (much like the characteristic of a stretched elastic band). Muscle spindles are responsible for the responses of this movement as they detect the rate and magnitude of the stretch. The faster the stretch, the greater the muscle activity and reflex. Similarly in the mechanical model, if the time between the prestretch/ eccentric action and concentric muscle action is too long, the potential positive influence of the stretch reflex is lost.
The SSC combines the elastic energy storage of the SEC and potentiation of the stretch reflex to produce a maximal muscle force over a minimal time period which is shown in the figure above when applied to vertical jumping. The SSC can be separated into 3 distinct phases when applied to practical movements.
Phase 1- The eccentric phase occurs initially, this is when the SEC stores elastic energy from the preloading of the agonist muscle groups (the prestretch) during this movement. The time of the foot touchdown to the bottom of the movement is the eccentric phase.
Phase 2- This is the amortization phase which occurs between the eccentric and concentric phases. Once the foot has touched down and the movement stops, the amortization phase has begun, this ends when the movement begins again through the concentric muscle action. This phase is important as the duration must be kept short to allow greater power production, muscle potentiation from the stretch reflex and avoiding stored energy being dissipated as heat.
Phase 3- This is a concentric muscle action which follows the eccentric and amortization phases. The energy stored from the SEC is utilised to increase the force production during this movement. This means that the force production here is greater than that of an isolated concentric muscle action alone. This is practically seen between a squat jump (concentric only) and countermovement jump (eccentric followed by concentric), where the countermovement jump results in greater vertical jump height.
These phases can be understood in practical terms when applied to the long jump in the figure below. At touchdown the gastrocnemius rapidly stretches (eccentric phase), there is a delay in the movement (amortization phase) and the ankle then plantarflexes due to the concentric muscle action to push off the ground (concentric phase).
