Consider, for a moment, how many movements in CrossFit involve ankle, knee and hip extension — that is, coming up out of a squat position. It’s the basis for every kind of squat, wall balls, squat cleans, thrusters, box jumps, bear complexes, burpees and deadlifts.
To progress at CrossFit, therefore, it’s critical to become proficient at this movement. Certainly, developing a stronger squat helps, but what’s more important is explosiveness, the combination of strength and speed. Enter plyometrics.
Plyometrics is a type of training that enables muscles to achieve maximal force development in the shortest possible time. Essentially, plyometrics training consists of a fast, powerful movement involving a pre-stretch (such as a rapid descent in a squat) followed immediately by a countermovement (ascent).
Performance in the countermovement is enhanced (i.e., the athlete jumps higher) as a result of a phenomenon called the “stretch-shortening cycle.” This cycle makes use of four forces within the muscle to create a greater countermovement:
First, the natural stretch reflex is activated by virtue of muscle spindles, special sensors that respond to a rapid and forceful stretch by causing a contraction in the muscle (as a way of protecting it).
Second, the “series elastic component” in muscle tendons stores elastic energy during any rapid stretching movement. That energy then adds to the force of the muscle’s next action as long as it occurs immediately following the pre-stretch.
Third, the contractile components within the muscle — those structures that actually cause contraction, known as actin and myosin — may be in a better position to produce maximal force when they are slightly stretched.
Finally, the “parallel elastic component” also provides passive resistance to a stretch. This means that the lengthening of the muscle tissue itself provides some force as it seeks to return to its normal length. Together, these factors result in an accentuated countermovement — a more powerful muscle action.
One type of plyometric exercise that is, unfortunately, rarely used in WODs is the depth jump. In a depth jump, the athlete steps off a box, lands with both feet and then immediately performs a powerful jump (“countermovement”). That jump can be a vertical jump into the air or a jump onto another box. The idea is to minimize the amount of time the feet are in contact with the ground — to “explode” on landing. If a second box is used in a depth jump, the intensity of the movement can be magnified by making that second box greater in height than the first.
What would that look like? The National Strength and Conditioning Association recommends athletes more than 220 pounds not perform depth jumps from a height greater than 18 inches in order to prevent injuring their joints. However, because there is no scientific agreement (and very little research) on the appropriateness of this recommendation, it seems an overly conservative standard.
Work in a lab at Armstrong Atlantic State University in Georgia supports moving beyond the NSCA’s guidelines. Researchers looked at ground-reaction forces in athletes of different weights depth-jumping from different heights. Ground-reaction forces reflect the acceleration of the center of mass of the body. All things being equal — namely, that the ankle, knee and hip flex consistently to the same degrees as they absorb some of the load upon landing — one might assume ground-reaction forces would increase linearly as bodyweight increases or as the height of the box increases. However, in the study, researchers found no correlations between body mass and ground-reaction forces at depth-jump heights up to 35 inches. (Some elite athletes regularly depth-jump off boxes as high as 48 inches.) The bottom line is that when determining the appropriate box height for depth jumps, common sense should be your guide. Beginners would do well to begin at 18 inches and increase with proficiency and training.
Depth jumps are considered high-intensity exercises, so work them into your training regimen slowly, ensuring and maintaining good technique. If you can do that, you certainly can depth-jump to greater heights.