One of my favorite subjects to teach students, patients and athletes is plyometric exercise and its use of the Stretch Shortening Cycle (SSC). Plyometric exercise is any movement that involves the SSC. The SSC divides athletic movements into three phases (eccentric, amortization, concentric), with the end result being increased force and power production (potentiation). Stated differently, when the body undergoes the first phase of the SSC eccentric, the body is able to produce more force and power during the third (concentric) phase than can be produced without it.
A recent study from the University of Alabama-Birmingham attempted to determine the point during this concentric phase that the eccentric phase best helps. The researchers evaluated recreational distance runners who performed “ballistic (or rapid) leg presses with 150% of their body mass.” Velocity, acceleration, force and power were all measured. They had an interesting, novel conclusion:(eccentric, amortization, concentric), with the end result being increased force and power production (potentiation). Stated differently, when the body undergoes the first phase of the SSC eccentric, the body is able to produce more force and power during the third (concentric) phase than can be produced without it.
- The more force that occurs during the end of the eccentric phase, the more force and power are generated during the concentric phase
Previously, it was assumed that the quicker the amortization phase (the time between the eccentric and concentric phases), the more force and power were increased. This appears to only occur with lighter loads. Why? When using lighter loads, movements are faster; when movements are faster, the stretch reflex (like when your patellar tendon is tapped and your leg kicks) is engaged. Muscle spindles are responsible for this reflex and they respond best to quick stretches.
This study, however, essentially focuses on the other physiological mechanism underlying the SSC, the body’s natural elasticity. When these elastic structures are stretched, energy is stored; this stored energy can then be used to increase force and power. That is what happened during this study. The large (150% body mass) weight stretches the elastic structures and more force and power are produced.
The message I take home from this study is to employ an approach I have been using for several years now. That is, focus on both the reflexive AND the elastic components of the SSC. Each component responds best to a specific stress (i.e., light resistance and quick movements for the reflexive component and high resistance with large eccentric load for the elastic component) and neither will improve if that specific stress is ignored.