I’m gonna start this post right by saying we have a problem. There’s a problem with most standard progression schemes for plyometrics. They’re too linear. They’re too rigid. They’re too categorically based to take in to account the nuances and diversity of the exercises that fall under the umbrella term plyometrics. They’re not based on any hard scientific principles. These progression schemes work in the majority of situations but lull us in to thinking they work in every scenario and this can cause problems. The reality is that these standard progression schemes are like Chemistry 101. They provide you with hard and fast rules that give you a framework for your coaching. The categories and rules of Chemistry 101 help you make sense of the subject until you take Organic Chemistry 201 and you learn about all the exceptions to those hard and fast rules. In the world of the physical preparation coach, this can lead us to misclassify exercises because of a rigid categorical based system that isn’t based on scientific principles. It relies on memorization of categories rather than thinking and understanding concepts.
What I aim to do here is present an alternative to the rigid and categorical progression schemes with a more flexible model based on Newtonian mechanics. Much like my Chemistry example above there will be many cases where the old progression schemes will completely correspond with the new model. But this new model (Organic Chemistry 201 if you will) can also explain the exceptions when the original model (Chemistry 101) doesn’t work.
My academic and research background is in sports science with a specific focus in biomechanics. As a result, I tend to look at the physical world around us on a mechanical level before seeing more superficial characteristics. It’s geeky but one of the useful byproducts is that it allows me to create physics based criterion to progress all aspects of training, from sprinting to resistance training, based on the mechanical load on the body. While this is useful for training in general, it is particularly helpful for a training activity like plyometrics. The diversity of exercises that fall under the umbrella of the plyometric classification can make developing sound and sensical progressions challenging. This is where a model based on Newtonian mechanics can help to sort things out.
By definition, plyometrics are any activity that involve the rapid stretching and contraction of a muscle to increase power output via enhanced elasticity and motor recruitment. By this definition, sprinting is a plyometric activity. But so is jumping rope. This is why it’s a huge mistake to think all plyometrics are equal. Similarly, we can quickly tear a hole in the idea of using classic plyometric categorizations to assess intensity. By many classical progression schemes a depth jump is considered a high intensity exercise. And this is certainly true for a depth jump from a 1m (~39″) box. But what if we do a depth jump from a 10cm box (4″)? Can we still call it a high intensity exercise just because depth jumps appear at the end of some rigid, category-based, linear progression? I’d say not. So just as we don’t want to consider all plyometric exercises equal, it would similarly be a mistake to become category-happy and mindlessly classify an exercise as ‘intense’ or ‘easy’ based on its name alone rather than looking at the variables that are essential to understanding physiological demand.
As I’ve just shown using a category-based approach can be problematic because it’s so easy to juxtapose the true intensity of an exercise against it’s classical categorization simply by switching one or two variables. It’s this shortcoming that completely invalidates the use of a categorical-based progression model.
Whether we’re talking about low or high intensity, plyometrics involve a collision between the body and the ground. The mechanical inputs that influence the magnitude of this collision are what I use to underpin my plyometric progressions. The forces at impact are the best estimation of the stress placed on the athlete. This is true regardless of whether we’re speaking of the demand on the muscular system, the nervous system or even, to a large extent, the technical demand.
When looking at plyometrics, impact is largely dictated by 3 factors relevant to coaches: impact velocity, collision time, and distribution of load at impact. While these may seem like obscure or hard-to-measure metrics for the average coach, what I hope to do with this post is simplify the Newtonian mechanics in to basic concepts that will provide a rational and simplified framework for progressing plyometric exercises.
Here are the factors I consider when progressing plyometrics:
- Impact Velocity. This may be the biggest influencer of all our variables on the physiological demand on the athlete. Now I know what you’re thinking….”how the hell am I supposed to measure velocity?” Thankfully, you don’t have to. You just have to answer some very simple questions. Answering these questions allow us to use corollaries to estimate velocity at impact. But before we get to that, let’s take a quick look at velocity from a mechanical perspective. Many use the term synonymously with speed but they’re not quite the same thing. The key difference is that velocity is what’s known as a vector quantity. This means that we must consider both it’s magnitude and direction. Speed on the other hand is a scalar quantity that only deals with magnitude. This key distinction sets the groundwork for us to be able to estimate impact velocity with 2 simple questions: how far did they fall from and to what extent were they moving forward when they landed? Let’s take a look at what we can gather from answering these questions…
- How high did they fall from? If you had to use just one variable to determine the intensity of a plyometric exercise the height that an athlete falls relative to their point of impact would be it. This is because the height that someone falls is a DIRECT indicator of the vertical velocity of the athlete’s body at impact. We can thank gravity’s constant acceleration for this nifty relationship. And since vertical forces will exceed horizontal forces for 90+% of plyometric exercises, simply knowing the change in position of the athlete’s center of mass (COM) between its apex in flight and its position at impact gives us an incredibly clear estimate of the magnitude of impact. In the absence of true 3D motion analysis equipment, the easiest way to gauge this is to look at the vertical displacement of the belly button (the best indicator of the COM) between its highest point in flight to the point when contact is first made with the landing surface. Note that it’s possible to manipulate this metric greatly by manipulating the position of the landing surface relative to the takeoff surface. When landing on a surface that is higher than the point of takeoff (such as jumping up stadium stairs) you greatly reduce the vertical velocity at impact. Conversely, when landing on a surface below the point of takeoff (such bounding down hill) you greatly increase the vertical velocity at impact. Knowing this can have important implications for progressing a plyometric exercise based on mechanical loading.
To what extent were they moving forward? Although the vertical velocity at ground contact will likely have the biggest effect on the impact the athlete experiences, the effect of horizontal velocity cannot be discounted. In many plyometric activities, horizontal velocity at impact will be negligible. For example, on an exercise like in-place tuck jumps, the physiological load the athlete experiences will be overwhelming related to their vertical velocity at ground contact. However, if we were to do those same tuck jumps but progress forward on each jump we’d add a horizontal component that would add to the total impact (if all else stayed the same). And if we look at bounding activities of any sort (whether that be baby bounds, speed bounds, bounds for height, etc) the horizontal velocity will be a significant contributor to the resultant impact velocity. The faster an athlete is moving forward, the greater the horizontal velocity at impact and the more physiologically demanding the exercise will be.
- Collision Time. The time course of the actual collision is something few coaches take in to account when determining the intensity and impact of a plyometric exercise. This is unfortunate because it has a huge effect. Even if everything we’ve looked at above remains constant, we can increase or decrease the physical demands of a plyometric exercise by modulating the timespan that
it takes for the collision to occur. When taking this variable in to account there are two primary things coaches should consider: the hardness of the landing surface and the stiffness of the athlete at impact. In both cases, what we’re really doing is applying the concept of coefficient of restitution to a plyometric activity. The coefficient of restitution is a measure of how much kinetic energy remains after impact and how much is lost to heat or deformation of the ground or athlete’s body. A softer surface will deform and slow down the athlete over an extended period of time. During this time energy will be ‘absorbed’ by the ground to cause deformation. A harder surface will have less deformation and more of the kinetic energy will be transferred to the athlete. This is why running and jumping on grass feels so much less jarring than on concrete. Similarly, when an athlete lands ‘soft’ they lengthen the duration over which they experience the shock of impact and reduce the physiological demand. Soft landings where the athlete attenuates the impact of ground contact are characterized by greater amortization or flexion of the ankle, knee and hip. ‘Stiff’ landings are characterized by minimal ankle, knee and hip flexion at contact. They typically feel more ‘jarring’ and are far more intense than ‘soft’ landings because the full force of impact is absorbed very quickly.
- Distribution of Load at Impact. The final variable that I consider when classifying plyometric exercises is the distribution of load at the moment of impact. The greater the surface over which the load is borne at impact, the less the physiological demand will be. In the case of plyometrics this is going to play itself out in whether the activity is bilateral, unilateral, or something in between. Plyometric exercises can be placed in to one of 4 sub-categories that also nicely correspond with their physical demand:
- Bilateral with a Temporal Offset. In this type of plyometric activity, the shock of landing is primarily taken on one leg and the forces necessary to takeoff are primarily generated by the other leg. Classically these activities can be identified as galloping or skipping movements (skips for height, skips for distance, etc) but other variations like step-ups with a jump exist. As above, many mistakenly classify these exercises as unilateral but this isn’t a fair representation of what is actually happening or the imposed physical demands. Due to the incredibly short time frame between one foot contact and the next and the inability to generate significant velocities in either the vertical or horizontal direction from this type of touchdown-takeoff sequence, these exercises actually tend to be quite low in physical demand…far lower than true unilateral activities. Athletes can try hard but because of the movement constraints they can’t jump high, move fast horizontally and are forced to spend a long time on the ground (increasing collision time = decrease impact).
Bilateral with Asymmetrical Loading. These are plyometric exercises where both feet make contact with the ground simultaneously but in different positions. As such, the demand placed on each leg is different. While these types of activities are often categorized as a unilateral activity this is a misunderstanding. In an activity like alternating lunge jumps both legs make contact with the ground at the same time but the loading is asymmetrical due to the differences in the positions and actions of the limbs needed to absorb the impact and propel the athlete back in to the air. These exercises tend to be greater in intensity than bilateral temporal offset exercises but less than the following two categories.
- Bilateral with Symmetrical Loading. These are jumps where both feet contact the ground at the same time in a symmetrical position. Examples would be exercises like tuck jumps, hurdle hops, consecutive broad jumps, and depth jumps. Because both legs are in a mechanically efficient position to produce vertical forces (and thus increase the height of the jump which in turn increases the vertical velocity on the subsequent impact) these movements have the potential to be very intense. That said, the impact of landing is split between two legs symmetrically which increases safety and reduces the mechanical loading when compared to our final categorization.
Unilateral Loading. These are plyometric exercises characterized by single support during each landing. Practically any plyometric exercise can be performed unilaterally however classic and commonly used variations typically come in the form of bounding exercises (alternating bounds, speed bounds, bounds for height, etc). When all else is equal, unilateral loading can be the most demanding form of plyometric exercise. This is because the shock of impact is taken exclusively by one limb rather than distributed across two limbs. In fact, the forces on the landing limb are actually well in excess of two times the bilateral equivalent exercise. If a 100kg (220 lb) athlete were to perform box jumps and land on both legs the athlete would be required to produce force to overcome the acceleration of approximately 88% of their body mass (since the right and left lower leg and foot make up about 12% of a person’s total body mass and wouldn’t need decelerating after impact). The force of this 88kg would be equally divided between two legs (~44kg body mass on each leg). In contrast, if that same athlete performed box jumps from an equivalent height but landed on only one leg that leg would experience more than double (about 94kg of their body mass) since the mass of the entire free leg would effectively be a dead weight. Furthermore, the asymmetrical nature (legs are on one side of the body) of unilateral plyometric exercises creates postural challenges not seen in our other forms of plyometric exercises.While this may seem to be a lot to remember, from a practical standpoint just know that single leg plyometric variants can be more than twice as intense as their bilateral equivalents.
Final Points to Consider
There is an additional factor to consider when looking at plyometrics and that is the Mass of the System. I didn’t include it in the primary 3 above because its effect on the physical demand of a plyometric exercise is more complex. For the sake of simplicity, we can think of mass as roughly equivalent to the weight of the athlete plus (or minus) anything that may be adding to (or reducing) their total weight. On the surface, it would be easy to assume that more mass means more physiological demand. But that would be looking at plyometric exercises through the biased lens of a coach who never leaves the weight room.
It is true that heavier athletes will experience a greater collision force at impact than lighter athletes. Within normal ranges though, coaches shouldn’t have to make concessions for the weight of an athlete. However, with extremely large athletes (i.e. American Football Players or Throwers) it is definitely something that should be considered as an athletes ability to withstand the increased impact of shock likely does not increase to the same degree as the shock experienced itself. What is perhaps more relevant for physical preparation coaches though are scenarios in which the athlete is using additional loading or unloading. Additional loading can be achieved by performing plyometric exercises with weighted vests, medicine balls, dumbbells, or barbells. In these cases, if all else stays the same, the additional load increases the impact force. While this can be viewed as a form of additional overload, it isn’t always the case. When an external load is used on a plyometric exercise with an athlete that does not have the capacity to be stiff at landing they will typically display greater amortization of the ankle, knee and hip after landing. In such a case, any potential overload is lost (because collision time and ground contact time went up) and the likelihood for technical breakdown and injuries likely increased.
While not as commonly used, it is also possible to unload a jump. This can be achieved using special harness devices or stretch band assisted jumps (see video). While one might think this is an underload, research evidence suggests that it can still provide a beneficial training stimulus. This may be due to it either being a speed overload or perhaps just a novel training stimulus. So from a training progression standpoint, this is a scenario where the mechanical load alone does not sufficiently explain what is relevant for the training overload.
Wrapping things up, I think a model for plyometric progressions based on Newtonian mechanics can be useful for long term athlete development, seasonal and technical progressions, and modulating volumes and intensities appropriately. The next time you incorporate plyometrics in to your plan try stepping outside of the box of category-based progression models and try incorporating some of these science-based principles.