Plyometrics 101 - Mechanisms Basics

Jump science basics

Author: Kosta Krunić

Video version:

If we want to improve our jumping skills we need to understand what actually happens within our bodies when we jump. What mechanisms are at play when you combine several jumps together? This video will cover just the basics, to lay a foundation on which we can build in the following videos.

What is plyometrics?

Plyometrics nowadays describe almost every jumping event, from rope skipping to high drop jumps. The term was coined as a western translation of “Shock Method” developed by Yurii Verhoshensky (1). It was very specific jump training where sets of really high intensity exercises were used to increase the explosiveness of track and field athletes. The idea was to introduce a set of exercises that would bridge a gap between the weight room and the actual events. However, the term changed over time, and plyometrics today became almost synonymous with any activity involving “stretch-shortening cycle” (SSC for short). Well, what is Stretch shortening cycle?

In various movements like running, jumping, throwing, etc. muscle shortening (propulsion phase) is preceded by muscle lengthening (wind up phase). When trying to jump, everyone naturally does a quick squat just before. If you stayed at the bottom of that squat for let’s say 5 seconds, you would jump less. The mechanism called “stretch shortening cycle” is responsible for that sudden increase in power after a fast “countermovement” (2). The stretch shortening cycle has 3 phases (Image 1), the first where the muscles and tendons are lengthening (eccentric), then there’s the transitional static (isometric) phase (tendons still continue to lengthen and then begin to shorten at this phase), and finally the muscle-tendon shortening (concentric) phase.

Image 1 - The eccentric, static (isometric), and the concentric phase of stetch-shortening cycle

During this lengthening, tendons store potential elastic energy. They act as rubber bands, so if you stretch them they will store elastic energy which will be released as soon as you let go (3). Here, tendons will release elastic energy during the concentric phase, given that the transitional (isometric) phase is short enough.

The elasticity of rubber bands, like with most elastic materials, is not time-dependent. That is why you can stretch a rubber band and it will still recoil fast even after two days. However tendons have different properties, they act as a fluid in certain situations having properties like viscocity, hence they are considered viscoelastic. This means they are time dependent (amongst other things), losing elasticity as the time they spend under stretch passes. (4) This all means that the faster the SSC overall, the more elastic energy will be utilized. The amount of energy stored and released by the tendon depends of its elasticity. (5,6) The more elastic the tendon, the more it stretches and more energy it can store but it can’t withstand high forces as they would stretch it too much to a point of failure (injury). (7)

To maximize energy usage muscles need to be really stiff in isometric position during the transitional phase to enable tendon lengthening. (8) If we go back to the rubber band example, the hand that is holding the band needs to be still, because if it moves back with the force that stretches the band not a lot of stretching will happen and energy will be lost.

That is one of the reasons muscle strength is highly important, it helps keep the static position to enable tendons to fully stretch and utilize elastic energy.

It goes without saying that strength also plays a huge role in jumping overall as muscles are responsible for the movement in the first place, so the higher the strength the greater the jump can be. To be more precise, the speed at which that strength (force) is generated is what’s the most important. (9,10) The standing jump occurs in less than 0.2 seconds (11), which is definitely not enough time to develop maximal force. So what force can we produce in that small timeframe is what actually matters. Imagine a really short drag race, like a 100 meters. The top speed of a car won’t matter all that much since it’s impossible to reach it in 100m, and acceleration is the most important. The same goes with muscle force.

However, strength is not everything. We’ve all seen those examples where a super strong person comes along and their standing jump is great, but when they try to perform two jumps in a row (a plyo) everything crumbles. What happens there? Apart from the development of tendons, this is predominantly due to low coordination. (12)

During the flight phase, we expect a certain fixed ground reaction force, based on the height of the jump, distance, ground material, shoe material, our own intentions of how we want to perform the landing and so on. Even with the right anticipation, executing the series of contractions perfectly is a huge challenge, because the timing needs to be right down to a millisecond. This type of coordination is developed through specific training (13,14), and this is why specific training should be a priority, and everything else can be considered complementary. A strong person would have to spend a lot of time practicing movement at slow speeds to develop strength, and that is usually why coordination is lost. Just a lack of specific training. So training plyometrics first, and strength training secondary is the best bet for maximizing plyometric performance.

A few more factors that can possibly influence stretch-shortening cycle are the stretch reflex and the Golgi tendon organ. These two work in opposition to each other. Stretch reflex activates when a muscle is stretched very fast, or when that stretch is too great, and it’s causing that muscle to contract. (15) An example would be when you fall asleep sitting down and your neck stretches too much so this reflex snaps you back into place, and wakes you up in the process. On the other hand, Golgi tendon organ is inhibiting muscle contraction when tendon load becomes too great. This can be seen when you try to do a huge stride but your leg buckles under too much load, it’s the GTO that’s restricting the muscle contraction.

The reflex itself doesn’t seem to be increasing the overall force production in the SSC, but rather reduces the inhibiting effects of the GTO. Reportedly this can be improved as well with plyometric training. (12)

Summary:

We distinguished here 3 different components of plyometric training. -Tendons -Muscles -Nervous system (of course other authors may include 20 different components, this is just my reasoning).

Tendons enhance power output during jumps by storing and releasing elastic energy. You can train this by gradually decreasing ground contact time in smaller plyometric jumps. Ground contact time speed depends a lot on your own movement intentions, but this is general advice.

Muscles are our primary force producers, they are the ones propelling us. However, they also enable tendons to manifest their fullest potential by providing enough stiffness so that tendons can stretch. Training should be directed towards generating a lot of strength in a small timeframe, however, increasing the overall strength pool (maximal strength) should be one of the goals as well.

While I appointed several different factors to nervous system component, the most prominent is coordination. While stretch reflex and Golgi tendon organ have their roles to play, coordination by far has the greatest effect. It’s a factor that can even be changed in almost an instant; a certain technique can just “click” at some point and suddenly “unlock” a whole new level of performance.

I feel obligated to mention that there are way too many factors that influence stretch-shortening cycle. Physiological like titin, fiber type, biochemical factors, preactivation, hysteresis, and the list goes on. This covers the basics, and a deep dive into all these factors is expected in the next articles

Bibliography:

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