For every action there is an equal and opposite reaction. It's simple physics. This principle is true whether its a car slamming into a tree or a barbell crashing onto your shoulders.
Anytime force production is trained we also train force absorption. They are as complementary as ying and yang. This ability protects us both in our everyday lives and while we compete as athletes.
Force Absorption As A Skill
As you may have heard a thousand times before, CrossFit is training us for the unknown and the unknowable. To be truly prepared for what life throws at us we must a strong and wide foundation of basic human movements such as running, throwing, jumping, lifting and so on. This is also known as general physical preparedness (GPP).
Force absorption is a integral piece of this GPP model of fitness. We couldn't live satisfying lives if we feared dropping down 30 inches from the bed of a truck. The concept of force absorption is present in almost every functional movement, from clean and jerks to running.
Use The Force
If you paid attention in high school physics you remember that energy cannot be destroyed, only transferred. Your body can use energy it absorbs to aide in more powerful movements.
There are two ways that the eccentric phase of the stretch shortening cycle (SSC) helps produce more powerful concentric muscle actions. The first is using the elastic property of muscles.
Mechanical Model of the SSC
Within the muscle tendon complex there is various elastic tissue that is broken up into three categories, parellel elastic component (PEC), series elastic component (SEC) and contractile component (CC). When a muscle is lengthened these elastic components get stretched similar to a spring, and store energy.
Of these three, the SEC stores the majority of the energy. During the eccentric phase of the SSC the SEC gets stretched and, when it is released in a timely manner, contributes to a powerful muscle shortening.
If the stretch phase is too long the majority of the stored elastic energy will be dissipated as heat. Holding a stretch for any longer than a single second will reduce peak force. This is why if you have heard me coach the jerk I will cue athletes to be fast down and fast up. Pausing the dip causes energy to be lost and missed lifts.
Neurophysiological model of the Ssc
The second way to get the most out of the SSC is by hacking the nervous system. Within muscles the are proprioceptive organs called muscle spindles. These organs sense rate and magnitude of tension within muscle cells. Whenever a quick stretch is detected the muscle activity increases drastically for the agonist muscle.
Lets use the example from before, the dip and drive of the jerk. During the down phase the muscle spindles within the leg extensors are stimulated. After a quick trip to the spine, the stimulate the muscle fibers to fire rapidly. Similar to the mechanical model if the stretch is held for too long the benefit of the stretch reflex is reduced.
Just like any other skill the stretch shortening cycle can be trained. By developing a power development program with a qualified coach you can develop explosive movements.
Dangers of Ballistic Training
Before you grab your plyo boxes and barbells be aware that training the SSC comes with some inherent risks. The peak force absorbed by the tissues during active lengthening of the muscle fibers is much greater than during shortening.
You can prevent injury through smart programming and building prerequisite strength.
For example your body takes much more mechanical stress dropping 30 inches than it does jumping 30 vertical inches. This is why when I introduce box jumps or other plyometric movements, I instruct athletes to step down rather than to drop.
Using lower weights relative to the athletes can prevent injury during the olympic lifts. Being able to properly catch a barbell safely is a prerequisite before any athlete moves up in weight.
Programming should take into consideration the stress that these movements add to the body. High intensity power development training shouldn't be done more than twice a week for novice athletes.
Proper rest must also be give between sets. Not only does this limit risk of injury but also increases the training benefits. Rests between olympic lifts, plyometric work and speed training should last between 90 seconds and 3 minutes. This increased rest period always the nervous system and metabolic system being used to recover fully.