If you run with a tight illio-tibial band, you’ll eventually end up with knee pain. If you do 1000 jumping pull-ups after a six month break from training, you’ll get rhabdomyloysis. If you go overhead without sufficient scapula stability, you’ll develop impingements in the shoulder. If you palm a pull-up bar for high rep chest to bar pull-ups, you’ll lose your palms. If you do wall ball with your eyes closed you will hurt your face.
If you do high rep box jumps with poor movement preparation and in an untrained state, you’ll tear your achilles tendon or end up with patella-tendinitis.
Management is key. And knowledge of how to manage is vital.
Exercise is a form of stress – it’s actually termed ‘Eustress’. Eustress is positive stress, stress causing some favourable effect to your body. None-the-less, it is stress, and stress has the potential to cause damage and degeneration. The beneficial effects of exercise far outweigh the potential negatives, but some forms of exercise need an extra degree of care to ensure that the specific exercise puts the ‘EU’ in eustress.
Box jumps are one such exercise. Their potential positive effect is considerable, but there also exists the potential for injury. The two most common culprits are patella tendinitis (aka jumper’s knee) and achilles tendon issues – the most severe of which is a complete tendon rupture.
From a biomechanical point of view, a structure ruptures when the forces being applied to it exceed the strength of the structure. No surprise there. We know that exercise strengthens muscles, but it also strengthens tendons. Untrained individuals (with weaker soft tissues – muscles and tendons) are at higher risk of injury. This applies to anything, and exemplifies the importance of scaling and gradual progression of intensity for any exercise. Exercise, almost by definition is hard. To cause the body to adapt via an increase in strength, cardiovascular endurance, flexibility, or any other facet; a stimulus must be applied. Not only must this stimulus be applied, but to facilitate this improvement it must be of a greater magnitude than previous stimuli. Box jumps are no exception. Leppilahti and Orava state that “…maintenance of a continuous level of activity counteracts the structural changes within the musculotendinous unit induced by inactivity and ageing.”
The question then becomes what magnitude of a previously unapplied stimulus can we justify applying to maximise both results and safety? How do we maximise results without compromising safety?
The answer lies in controlling the environment in which we exercise – coupling an unstable and unfamiliar degree of exercise with a stable and familiar environment. In this way we can manage the risks. Note the aim is to manage the risks, not remove them.
So what are the best strategies to manage the risks of high repetition box jumps? The risk of injuring HEALTHY achilles tendons is miniscule (McMaster, P. E.: Tendon and muscle ruptures. Clinical and experimental studies on the causes and location of subcutaneous ruptures. J. Bone and Joint Surg.,15: 705-722, July 1933.15705 1933). So how can we prepare the achilles to be as healthy as possible?
Research has shown that the area of the achilles tendon most susceptible to injury is that with the smallest amount of blood supply. Ruptures have also been shown to occur at a higher frequency in older people – again, due to reduced blood supply. To this end, the importance of increasing blood flow to the achilles tendon is paramount. Warm-up! Skip. Start with low box jumps and work your way up.
Increases in stiffness of the achilles tendon increase injury rates (Hess) – lending support the the importance of PNF stretching.
Research indicates that “Proper warm-up and stretching are essential for preventing musculotendinous injuries, but improper or excessive stretching or warming-up can predispose to these injuries.” (Leppilahti and Orava).
As much as there are risks (as with any exercise), the benefits of plyometric based movements like box jumps are considerable.
Bouncing at the bottom of a rep gives you more power. But why?
It’s due to something called a ‘eccentric-concentric movement pattern’. To understand this, you need a basic knowledge of neuromuscular biomechanics.
A concentric contraction occurs when the muscle fibres are shortening. If you stand with one arm hanging at your side and bend your elbow and bring your hand towards your shoulder, your bicep is performing a concentric contraction. If you then lower it slowly, your bicep is performing an eccentric contraction. The muscle is lengthening, but it is a controlled lengthening. It is effectively ‘breaking’ and slowing the movement.
Most movements involve an eccentric contraction followed rapidly by a concentric contraction. For example. Lowering into the bottom of a squat involves the quadriceps, hamstrings and gluteals working eccentrically, while standing back up uses them concentrically. Lowering into the bottom of a push-up involves the pecs, deltoids and triceps working eccentrically, while pushing back up uses them concentrically. Lowering a bar in a deadlift involves the lower back, gluteals and hamstrings working eccentrically, while lifting back up uses them concentrically.
But how and why does this work?
There are several reasons, but for ease of explanation, the muscle and tendons store energy during an eccentric contraction. This can then be used in the concentric (or lifting) phase. Think of a rubber band being flicked across the room. If you pull back on the rubber band and release it straight away it will fly further than if you pull back on it, pause, then release it. If you’re interested in further reading, research pre-load, muscle potentiation and stretch reflex potentiation as other reasons that eccentric-concentric movement patterns create higher force.
This explains why hand release push-ups (taking your hands off the ground at the bottom of a push-up) are so much harder than the regular kind, and ‘touch and go’ deadlifts are easier than re-gripping every rep.
Time is a big factor here. You lose 50% of the stored energy every second. So if you stop at the bottom of a hand release push-up for one second, you lose half the stored energy. Stopping for four seconds will result in all the stored energy being lost.
Try this. Lower into the bottom of a squat. Hold at the bottom for four seconds, then explode upwards, jumping as high as possible. Now repeat, but explode or ‘bounce’ out of the bottom of the squat, repeating the jump. You should feel a lot more power without the pause. Thank you eccentric-concentric movement pattern.
So how can we use this knowledge to improve performance? By moving quickly from the lower phase to the lifting phase of a movement (the bottom of a squat/dip/push-up/press/pull-up, the back of a backswing in striking sports, the back of a throw in throwing sports, the bottom of a take-off in jumping sports etc) we are able to take advantage of this stored energy to maximise performance. ‘Explode out of the bottom of a rep’ is a good internal dialogue to maximise this effect.
