You’ve probably heard of plyometrics. Perhaps you’ve looked up videos of plyometric drills because somebody told you they would increase your running speed. You begin to perform them - sometimes before a running workout, sometimes after, or even on non-running days. However, using plyometric exercises incorrectly will not only fail to bring about the expected benefits, but could leave us prone to injuries.
In order to know WHY we should include plyometrics in our triathlon or running program, let´s start with WHAT is a plyometric exercise.
“Plyometrics exercise refers to those activities that enable a muscle to reach maximal force in the shortest possible time.”1
In practical words, the goal of a plyometric exercise is to increase the power of a subsequent movement using muscles and tendon´s elastic components and the stretch reflex.
We can explain it in a functional way. When we run or jump, the energy produced due to the impact of the foot against the ground is loaded by the musculotendinous components of the foot and leg in form of elastic energy. If this phase is immediately followed by a muscle action like a stride or rebound, the elastic energy stored is released adding up to the total force production (force of muscle contraction + elastic energy stored and released = total force production). This is the mechanical model of the plyometric exercise.
The neurophysiological model involves the stretch reflex that includes the proprioceptive organs of the muscle fiber. The proprioceptive organs are stretch-sensitive and produce a reflexive muscular action after a quick stretch of the muscle – the impact of the foot against the ground is an example. You can also think of the reflexive action when you touch a hot surface. This reflex will increase or potentialize the muscular activity of the muscle in use.
Both the mechanical and neurophysiological models require an immediate muscle contraction after the stimulus in order to be effective. If there is too much time between the stimulus (stretch) and muscular contraction, the potentiating effect will disappear and the elastic energy stored will be dissipated and lost as heat with a potential adverse effects over the adjacent tissues.
This series of events is named the Stretch-Shortening Cycle (SSC) and is enlisted in three phases:
PHASE 1: Eccentric Phase - Preloading of the muscle, storage of the elastic energy and stimulation of the muscle spindles.
PHASE 2: Amortization Phase - This is the elapsed time between the stimulus and the reaction. The time is crucial, so the shortest time elapsed in this phase the greater power production.
Image 1.
Short amortization phase.
Overstride running and heel strike can increase the amortization phase avoiding the potentiate effect of the SSC, because of the excessive time the foot is in contact with the ground. Adding in the absorption of the impact by the joint, there is the heat produced by the elastic energy that is not unloaded. This increasing the probability of injury, not only to the cartilage due to the impact, but to the soft tissues surrounding the joint due to increased heat.
Image 2. Overstriding.- Note the excessive amount of time the foot is in contact with the ground, losing the potentiate effect of SSC.
Phase 3: Concentric Phase - This phase includes the muscle action plus the unloading of the energy plus the action of the stretch reflex. Plyometric exercises or drills produce a high musculotendinous stretch rate which produces a greater muscle recruitment and activity during the SSC concentric phase.
Plyometric training allow us to increase not only the power of the muscular contraction but the muscular recruitment as well, increasing speed and improving efficiency of our movements and other functional capabilities.
References
1 Thomas R. Baechle, Roger W. Earle, Essentials of Strength and Conditioning. National Strength and Conditioning Association. Third Edition.