Missouri got hit with a pretty decent snowstorm today, which means the facility is closed. What better time to experiment a little bit with our DARI motion capture system? I’ve seen a decent number of posts talking about the negative consequences of performing box jumps on boxes that are way too high for the athlete. One obvious drawback is an increased risk of injury. Having an athlete jump onto a plyo box that is at the limits of their athletic abilities can have some dire consequences, such as the shins taking a beating or worse…
Another drawback to maximal box jumps is the compromised landing position the athlete must achieve in order to clear the box. This results in excessive hip flexion and a kyphotic lumbar spine. Not only is this position inefficient for absorbing and producing force, it doesn’t resemble any of the positions we can expect the athlete to achieve in sport. To learn more about that, click here to see Coach Garrett Buschjost explain (also, give him a follow on his new Instagram account @jc_athletic_performance).
I have a different question though: How does my vertical jump height change as the box height increases. To answer this question, I used our DARI motion capture system to collect a little bit of data. DARI is a markerless motion capture system, which means it is able to track my skeleton in space without having to attach any markers to my body. This system is able to record vertical jump height by subtracting the height of my pelvis while standing from the peak height of my pelvis while jumping. I performed a countermovement jump with no box first, then with an 18-inch box, and finally a 36-inch box. I repeated this circuit 2 additional times to collect a bit more data without fatigue becoming a factor. Below are the results:
Diminishing Returns
Although the data set is small, there was one thing that really stood out to me. My average vertical jump height with no box and with an 18-inch box was within 1% of each other. When I increased the box height to 36-inches, however, my average vertical jump height decreased by almost 10%. While that might not seem like a lot, you’d be hard pressed to find an athlete or performance coach who would be okay with a 10% decline in performance.
So, what happened? To answer this question, I took screenshots of my peak vertical height for the third circuit of jumps and there was a stark difference in the 36-inch box jump.
During the first two jumps, I am able to maintain my triple extended posture (hip extension, knee extension, and ankle plantarflexion). Now look at the third image. When I achieve peak vertical height, I am in the process of flexing my hips and knees in order to clear the box.
I have a couple theories as to why this may have decreased my peak height:
1. Pulling my legs upward to clear the box may be slowing down my momentum. This would be akin to firing the retro thrusters on a rocket while in flight.
2. I may be applying less force into the ground because I know I will need to flex my hips and knees shortly after takeoff to clear the box.
I am definitely open to discussing differing opinions on this topic, as I am not a physicist, but let’s not lose site of the loss in jump height as I moved from an 18-inch box to a 36-inch box.
When Are Box Jumps Appropriate?
Let me be clear, I am a big fan of box jumps. I definitely think that they have a place in physical therapy as well as performance training and I frequently implement them in my own training and with my clients. In fact, I wrote a blog last year that discussed how box jumps can be used as a progression for plyometric training (click here). In this blog, I discussed how to initiate plyometric training following an ACL reconstruction. First, we emphasize landing mechanics, then takeoffs, and finally combine the two together.
Box jumps are a great tool for teaching our athletes takeoffs. Let’s review some physics to better understand why. Absorption forces present at landing can be calculated by multiplying the body mass, acceleration due to gravity, and jump height together.
F = (m) (g) (h)
We can use this equation to calculate the absorption forces I experienced while jumping with no box and while jumping with an 18-inch box. For consistency, we will say that I jumped 30 inches, or 0.76 meters, for each and my body mass was 86 kg. Based on these assumptions, we can infer that, while falling from a height of 0.76 meters, I absorbed 640 Newtons.
F = (86) (9.8) (0.76) = 640 N
While performing an 18-inch box jump, however, I fell from only 0.3 meters (0.76 – 0.46). This resulted in a force of only 253 Newtons.
F = (86) (9.8) (0.3) = 253 N
This shows that, when the appropriate height is chosen, box jumps can reduce the amount of force we have to absorb upon landing. This decrease in force during landing can allow us to focus on takeoffs, rather than having to worry about takeoffs and landings during the same repetition. Once the athlete demonstrates competency with landing and taking off separately, we can then combine the two together.
Summary
Box jumps can be a good adjunct to a training program, but choosing the correct box height is critical. Social media tends to reward those who can jump on a higher plyo box, but there can be negative consequences when doing so. Two obvious draw backs to maximal box jumps are poor landing mechanics and increased risk of injury. One that may surprise most, however, is a decrease in vertical jump height.
The point of this post is not to crucify the box jump. I am a big fan of box jumps and implement them frequently. The point of this post is to emphasize the importance of choosing the correct box height. Choosing a lower box may produce a movement that is more authentic to what we can expect out of our athletes in their sports. While we can debate this too, I tend to choose a box height that is slightly lower than the athlete’s maximal vertical jump height. This ensures that we can give the athlete a chance to achieve triple extension while also maintaining proper landing mechanics and mitigating the risk of injury.
For those interested, I have included the videos of my last two jumps below, when I attempt the 18-inch and 36-inch box jumps. In these videos, pay special attention to my landing mechanics. If you would like to add to this discussion, I encourage you to leave a comment below!