A plyometric exercise is any movement that incorporates the stretch-shortening cycle (SSC) to increase power output (Cambridge Dictionary, 2020). The SSC has three general phases: eccentric (downward motion/force absorption), amortization (short pause between eccentric and concentric), and concentric (upward/force production). Some form of plyometrics should be a major part of any strength and conditioning program once a base of strength has been built. They help build elasticity, tendon stiffness, and power output which translates to most sports in the form of jumping, running, and changing direction. There are not many sports where training the stretch-shortening cycle is not a must with some possible exceptions, one being water-based sports such as water polo (but even that could be argued in terms of upper body SSC). Just like any strength exercise, there must be progressive overload to plyometrics. There are different ways to do this but this article will discuss a very simple progression that is useable with all levels of athletes, for lower body plyos. That being said, before an athlete progressed to maximal plyometrics there needs to be a baseline level of strength.
Force Absorption - Eccentric
The first step in introducing plyometrics is making sure the athlete can effectively absorb the forces their own body weight puts on their joints when they land from a jump or change direction. Most injuries occur during the eccentric or deceleration phase of plyometrics. These first exercises will predominately be just landing from different heights. Assuming the athlete either plays sports already or just plays outside, they should have some tolerance to force absorption. However, they may not have the high tolerance they will need as their athletic career advances and becomes more intense.
Athletes move in different planes of motion so they need to be able to absorb force both horizontally and vertically in all of these planes of motion. What this would look like in the vertical, sagittal plane would be landing on the ground off of a box. This is the most basic of force absorption exercises. Start with a box that is approximately the height the athlete can jump and have them "fall" off and land in a stable position. Progress this by either getting a taller box or have them hop off the box instead of "fall". Then progress to a single leg. Horizontally, a simple exercise is to loop a band around the waist of the athlete and have them face where the band attaches. Next, have them hop forward and land on both feet. The next progression would be to scoot back and jump farther forward. Then restart the process with a single leg.
This initial stage will also be where we build some elastic endurance to prepare for the later stages and sport in general. Elastic endurance is being able to generate a consistent amount of force over a longer period of time or a higher number of jumps. These should start out at a relatively low intensity and progressively become more intense just like progressively overloading any strength movement. An example that I frequently use is pogo hops. We start at about 10 reps at 50-60% and steadily build up in the following weeks until we reach near-maximal intensity.
Force Production - Concentric
The next progression is force production or the concentric part of the stretch-shortening cycle. This would be the upward phase of a vertical jump. Force production is a reflection of how fast you can exert force to produce the desired movement and is a byproduct of both strength and speed (Silver, 2013). This is not necessarily done for injury reduction as training the eccentric phase is, but more for sport performance. The ability to produce force concentrically is partly dependent on absorbing and redirecting eccentric force but can be isolated and worked on by itself.
During this phase, eccentric forces can more or less be eliminated if desired to isolate the concentric movement. An example would be performing a box jump from a seated position. There will be some eccentric load on the back end of the landing but it is greatly reduced by jumping up to a box. This example may also help to shorten the amortization phase and ground contact time which are very important to increase reactivity and explosiveness.
Putting Them Together
The third progression is where all three phases (eccentric, amortization, concentric) come together to be trained. This is the most sports-specific of the progressions as full movement jumping is part of almost all athletics. Here we will simultaneously train the three phases with many different types of submaximal and maximal vertical, horizontal, and lateral jumps. A simple example of this would be just a maximal countermovement vertical jump where an athlete eccentrically loads and then transfers that force vertically jumping as high as they can. A more intense example would be depth or drop jumps (not the same) where an athlete starts with a larger eccentric load then performs a maximal jump.
When and where you program plyometrics for athletes is dependent on what the desired outcome is but most likely should be done at or near the beginning of a workout. Maximal plyos should always be done at the beginning of a workout so the athlete can achieve max effort with little to no fatigue. Submax and extensive plyos should also be done relatively early in workouts but may be able to be done later if the desired outcome is submax power repeatability with fatigue.
Regardless of the timing of plyometrics in a program, volume has to be taken into account. This includes the plyometric volume in a training session but also in their sport. For instance, if a basketball player is practicing 5 days a week in the offseason the number of times that athlete impacts the ground has to be taken into account when programming. Overtraining plyos, like overtraining any other movement, can lead to overuse injuries and larger problems down the road.
Single leg plyometrics are just as or more important than double leg plyos. In most sports, athletes find themselves on one leg during most of the competition including running, jumping, landing, or moving in any way possible. Single leg plyos can be progressed in the same way as double leg plyos and should be done simultaneously with double leg.
There are some pre-requisites for single-leg plyometric training just as there are for double leg. Before training plyometrics of any form, a base of strength must be built and be able to absorb the force of the future impacts on their joints. Before single-leg plyos are trained the athlete should have exposure to double leg plyos and have some elastic endurance built up. Just as with double leg plyos volume and intensity must be carefully tracked to limit overtraining and overuse injuries.
Examples of sinlge-leg plyos are the same as double leg. Start with force absorption and elastic endurance by jumping off of things and repeat submax jumps (pogos). Transition to jumping onto things like a short box. Then jump over things like mini hurdles followed by off of and over things (think drop jump over a mini hurdle.
Before any plyometrics are trained a base of strength needs to be built in the athlete and basic movements need to be taught. The first step in the plyometric progression is jumping off of things or force absorption (eccentric) which can be done simultaneously with building a base of strength. This is all about absorbing the horizontal and vertical forces put on the joints during high impact activities in sport like jumping and sprinting. The second step is jumping onto things or increasing force production (concentric). We want the athlete to not only be able to produce a large amount of force but also do it as quickly as possible. The third step is putting both previous steps together and jump over things. This combines eccentric loading (dip before jump), vertical force production (jumping over the thing), and then eccentric force absorption again (landing). Finally, single-leg plyometrics are to be done following the same progression and done simultaneously as double leg. Although, the athlete should have some exposure to double leg plyos before jumping into single leg work as they are more intense. All plyometrics should be progressively overloaded just as all strength exercises are to reduce the risk of overtraining.
Cambridge Dictionary. (2020). PLYOMETRIC: definition in the Cambridge English Dictionary. https://dictionary.cambridge.org/us/dictionary/english/plyometric.
Silver, J. (2020, June 9). Power training provides special benefits for muscles and function. https://www.health.harvard.edu/blog/power-training-provides-special-benefits-for-muscles-and-function-201304226097.