The purpose of this post is to provide an overview of the countermovement jump (CMJ) and how it relates to testing readiness to return to sport after ACLR.
Here are the things we will cover…
What is the CMJ?
The Countermovement Jump is depicted in the image above. Essentially, the athlete will start standing and then attempt to jump as high as they can. The movement requires the ability to control deceleration as your center of mass drops and then explode concentrically into the air and then control/absorb the landing forces. This can be done on both legs or on one leg. The movement gives an idea of limb to limb contributions to acceleration, deceleration, and landing velocity.
In an ideal environment, a force plate will be used to measure the performance of this movement. The force plate allows a deeper insight into the athlete’s strategy to absorb and produce force. Measuring conventional output variables (ie. jump height or distance) does not reveal anything about movement strategy and can lead to premature return to sport. See below for the output variables to look at with the DL- and SL-CMJ.
Double Leg CMJ (DL-CMJ) vs. Single Leg CMJ (SL-CMJ)
There are two types of CMJ’s: the DL-CMJ and the SL-CMJ. Each test can be valuable, but they do measure different things. The (DL-CMJ) is just as it sounds: jumping off two feet while the SL-CMJ requires jumping off one leg. There are pro’s and con’s to each of these movements. We will go over the DL-CMJ first.
How does the CMJ inform RTS?
The CMJ provides an assessment of the involved limb’s ability and strategy to accelerate, decelerate, and land at a high velocity. It is important to be aware of the different variables that you may monitor when deciding if an athlete’s training program can be progressed or concluded.
You have to look beyond common variables (ie. Jump Height) when utilizing the CMJ. Those variables give you an idea of output, but not the strategy in which the athlete is using. Paying attention to the eccentric and landing phase of the DL-CMJ will give you a better understanding of willingness to load the involved limb.
You can also assess the speed in which the eccentric/concentric phases are performed hoping to see faster movements as the patient improves. This is harder to measure without a force plate, however, it is important to look at if able to. There is a misleading outcome that is related to speed of descent. If someone is performing the CMJ well, but is unable to descend quickly, you will not have insight into their ability to stop at a high rate of velocity (An example analogy by the Sports Scientist Drew Cooper: testing the brakes on a car going 5 mph vs. 70 mph). Sports occur quickly, therefore, you need to get an idea of how the athlete performs at similar speeds.
Positive adaptation to training would be a greater CMJ height and shorter eccentric/concentric phases. Essentially, you want the movements to be faster and you want the athlete to be able to control deceleration in both the eccentric and landing phases. If these are improving, you can be confident that the athlete is positively adapting to their program.
[Don’t completely rule out jump height measurement with the SL-CMJ as that will provide insight into the involved limb’s capacity to produce force, but you do have to be aware of intralimb compensations.]
How do we measure this without a force plate?
Having a force plate in the clinic is a luxury. If you don’t have one, there is a cheaper option in the form of the My Jump 2 app, which only costs $13.99.
The My Jump 2 app allows you to perform various vertical and horizontal jump tests, force-velocity profile, asymmetries between limbs, and repeated jumps. It is a valid and reliable tool for assessing countermovement jump height and provides detailed metrics such as Flight Time (ms), Velocity (m/s), Force (N), and Power (W) to give insight to the athlete’s physical profile. These metrics can be tracked over the course of rehab in order to determine how well the athlete is adapting to their reconditioning program. Their performance can be compared to age, sport, and position normative standards to help determine whether the athlete is prepared to return to training or competition.
The DL- and SL-CMJ can be used to inform progression and return to sport when understood properly. Both of these provide insight into different performance indicators regarding the involved and uninvolved LE.
If you read this whole way, I hope you picked up something useful for your clinical practice and athletes looking to return to sport safely.