Box jumps have always been a risky exercise. Scraped shins, twisted knees, and patellar tendinitis can happen even to the best athletes. Achilles tendon ruptures are especially devastating because a torn Achilles likely requires surgery and a lengthy five to nine month recovery. A number of trainers and Crossfit coaches now tell athletes to only perform the step-down variation of the box jump due to increasing concern over Achilles tendon injuries.
High Rep Box Jump Achilles Rupture
Earlier this week I came across a video posted on Facebook showing an athlete rupturing his Achilles during a box jump. The injury happened after the jump-down part of the exercise, just as he was about to take off for the next jump. The instant that it happened he quickly turns his head, presumably because he thought someone had hit him in the back of the leg, and then fell to the floor clutching the back of his ankle and letting out a few choice words.
There is a tremendous amount of force placed on the Achilles tendon when landing from a height. Box jumps are a form of plyometric exercise. Plyometrics are explosive movements used by athletes as a training tool to increase speed and power. During a plyometric exercise there is first a forceful eccentric contraction (lengthening) of the muscle followed by a rapid transition into a concentric (shortening) contraction.
The landing portion of the box jump is the eccentric phase of the movement. During this phase the Achilles tendon is being stretched at the same time the calf muscles are contracting to absorb the impact of landing. The lengthening of the Achilles stores energy to be used to start the next jump.
During the concentric phase, the calf muscle contracts again, but now shortening. This pulls on the tendon which then flexes the ankle and generates the force necessary to propel the body back into the air.
This rapid alteration of lengthening and shortening of muscles under tension is what makes plyometric type exercises effective at developing strength. This is also what makes them risky since the muscle and tendon are being tensed to a high degree.
Forces on Achilles During Box Jumps
The Achilles rupture in the video I watched happened during the transition from the eccentric to concentric phase of the box jump. The tendon was under stretch from the landing and then was pulled even more from the contracting calf muscles initiating the next jump. The action of the Achilles during a box jump can be compared to a rubber band. When stretched it can store energy that it uses to return back to a shortened state. Pull too hard and it snaps.
The force on the Achilles tendon during a box jump may be as high as 12 times a person’s body weight. A healthy tendon however is incredibly strong and resilient. Much stronger than a rubber band and unlikely to completely tear even under that much tension.
Most exercise by its nature creates small tears inside tendons. Exercise is a form a stress on the body and will cause tissue to breakdown. We benefit from exercise because the body repairs the damaged tissue to make it stronger and better able to withstand the stresses placed on it.
High rep box jumps can result in Achilles injuries because the repeated cycles of eccentric loading may cause enough micro-tearing of the tendon that it no longer has the strength to absorb the forces. This can lead to a macro-tear or complete rupture of the entire tendon.
Preexisting Achilles Tendon Problems
The current understanding of Achilles injuries is that, in the majority of cases, the tendon was weakening over a period of weeks or months before the actual rupture happened. Over time this incomplete healing will lead to weakening of the tendon, possibly to a point where it is no longer able to withstand the same amount of load as when healthy. Sometimes this process may go on even without pain or other noticeable symptoms.
Since no one is sure why the Achilles starts to degenerate, even well-trained athletes can be at risk for rupture. Someone who already has tendon or calf pain, or tenderness in the back of their ankle should use caution with any type of plyometric exercise.
Proper training will help the Achilles adapt to the stresses placed on it during box jumps. Initially using a box with a low height will reduce the load on the tendon, allowing it to strengthen while reducing the risk of injury. Training sessions should be separated by rest days to give the tendon a chance to repair and get stronger. Training frequently with explosive movements and not allowing enough rest time in between is a recipe for tendon problems. Also note that even with proper training progression and adequate rest it’s difficult to know if the Achilles is adapting well.
Poor Movement Patterns
There are two movement problems that I can see contributing to Achilles injuries: ankle inflexibility and pronation.
If a person has stiff ankles or limited calf flexibility, the Achilles is required to perform the work of absorbing and transmitting forces in a smaller range of motion and in a shorter amount of time than someone with good flexibility.
Pronation causes an inward collapse of the ankle and a bending of the Achilles tendon. Some degree of pronation is beneficial for shock absorption but a large amount may change the alignment of the tendon. Instead of working in a straight line, in a pronated position the tendon has to work at an angle. This reduces the amount of load the tendon can withstand since the forces will be traveling at an angle to the direction of the fibers inside the tendon.
Crossfit often uses workouts where the athlete must do “as many reps as you can” (AMRAP) circuits with different exercises. The 11.2 Crossfit Games Open workout included 15 box jumps on a 24″ box in a circuit with push-ups and dead lifts repeated for 15 minutes. This is the workout that reportedly resulted in a lot of torn Achilles.
When the body becomes fatigued movement patterns change. Fatigue changes muscle activation in the leg that may transfer more of work of executing the jump and landing to the tendon. This may be the major reason high rep box jumps put the Achilles at risk.
Safety Concerns with Box Jumps
Not a lot is known about why a tendon as big and strong as the Achilles ruptures so often. Achilles tears are scary because they often happen unexpectedly without any warning that something was wrong.
The risk of an Achilles injury with box jumps is probably not that different from the risk in other sports that require quick, explosive movements. Box jumps done in high repetitions however increase that risk because tendon overload, bad biomechanics, and fatigue all become factors.
Using the “jump-up step-down” method of doing box jumps seems like the best way to reduce the risk of an Achilles injury. The jump-down part of the exercise places the greatest load on the tendon. Jumping down off the box may not be the safest choice for high repetition work and should probably only be used by athletes looking to be competitive in doing high-rep box jumps for time.
Anyone interested in watching the video of the guy tearing his Achilles can find it on this page.