By Carl Valle
Science of Vertical Plyometrics
One of the most demanding plyometrics exercises is depth jumps, usually performed by stepping off a box and jumping immediately after ground contact. In sports performance, it’s usually the most poorly applied exercise in all of training, especially at the youth level. Perhaps the most obvious question is not just why one should do depth jumps, how to apply or do them properly? Several experts have promoted the use of depth jumps for improving athletic performance, but where are they improving athletic ability is still a mystery. The common belief is that depth jumps help with maximal speed, and because so little research is on elite sprinters longitudinally, it’s not crystal clear what is happening adaptation wise. In this article an exhaustive investigation into the requirements of top speed and how potentially the use of depth jumps may help improve maximal velocity.
What is a Depth Jump?
Depth jumps are an intensive plyometric exercise that uses an elevated drop to overload the stretch reflex. When an athlete steps or leaps off of the box, the most common technique is to jump with two legs vertically up, immediately after impact. For 50 years coaches and athlete have purposely used the training to improve power, and Russian lore is still seducing coaches to believing that it’s near magic for performance. We know that jumping performance is improved by depth jumps due to the specificity of the exercise, but sprinting is still gray because of the complexity of design requirements in studies.
Vertical Forces and Maximal Velocity
The debate continues on the value of plyometrics and maximal speed, and earlier the question of the value of depth jumps. As of today, we still don’t know how maximal speed can be developed but we know how valuable it is. Hundreds of world class 100m performances have been researched since the 1980s and all of them share one thread, those who have the greatest maximal velocity win nearly 99% of the time. Why? One has to have a great ability to accelerate, and speed endurance int he 100m is 3-4 seconds. Even “drop dead” sprinters are carrying so much speed that endurance is rarely an issue as many technical areas can mask poor fitness. It seems that those that have great abilities to sprint fast likely have great developments in reciprocal inhibition, a characteristic likely to transfer over to points of the race when fatigue hits. Fatigue should not be seen as a fuel limiting phenomena, but more of a central and peripheral threshold that the body must be prepared for. How a rapid contraction at 4-5 times a second can be augmented by a usually a bilateral contraction far longer than ground contact is still a mystery to even the best sport scientists. To date, many world renown scientists and coaches don’t agree on if maximal speed can be enhanced by plyometrics, and some question its value even in the acceleration phase, where contractions are slower and similar to jump training. The case for plyometrics and improving top speed is a difficult one, but coaches have sworn by depth jumps as a ticket to greater performance.
Paradoxical Power of Depth Jumps
The oscillation pattern of the body, sometimes called lift by coaches in various circles is extremely difficult to measure, even with the most sophisticated lab tools. The reason the body is bouncing up and down is part visual illusion and part actual vertical displacement. The amount of vertical displacement necessary to reposition the recovery leg for proper landing and loading is a matter of centimeters, so why are coaches jumping down from boxes that are meters high? A possible answer is not so much the drop distance of the body begin a limiting factor, it’s how fast the body can create propulsive forces forward and up. Braking forces are necessary to create a stretch reflex, but unless the body is able to use them and provide a greater resultant then braking values, one is unable to attain faster speeds.
A theoretical idea is that speed is about rate of force production and rate of relaxation. Several other neurological adaptations exist, but speed needs speed, since maximal strength is a classic diminishing return variable and empirically few elite sprinters demonstrate exceptional ability in absolute lifting. In theory, a depth jump can overload the tendons, muscles, and nervous system enough to reduce the protective mechanism of the brain. Self-preservation is the natural response of the body, and perhaps better athletes have “ a few screws loose” and are able to overcome not only protective mechanisms better than lesser talents. A glimmer of hope exists with athletes that don’t have the same gifts with depth jumps, since the alarm response of a jump from greater heights may induce a shift to higher thresholds, thus raising the speed ability slightly. If the hip, knee, lower leg, and foot can contribute in force production earlier, the theoretically more time can allow greater speeds to be expressed. A modern concept in sport science is that depth jumps are trying to hack the brain, not create hypertrophy or more biochemical changes. In closing, the paradox of running faster horizontally is about earlier rising of the center of mass vertically, provided that the extension of the hip is providing sufficient outcomes of speed.
Power, Stiffness, and Elasticity
While every athlete has a unique stride, most of the best sprinters have a remarkably similar set of running mechanics. Very little style exists, and most of the differences are from body type and athletic abilities. The commonalities are neuromuscular power, the joint stiffness, and the elasticity of the athlete’s tissues. Perhaps a barely perceptible underlying component of training is maximizing the athlete’s abilities while addressing, not totally eliminating, the weaknesses. A chain might be only as strong as it’s weakest link, but performance is more of a chain link fence versus a linear chain. Instead of fighting a weakness or genetics, minimize loss so areas are sufficient and go to areas that are responding easily.
Stiffness — One of the lesser known qualities of athletes is stiffness, or the ability to lock a joint system temporarily at a key time during the movement. Ironically all athletes need to have stiffness at key times during sprinting in order maximize work done in a small amount of time. Remember stiffness is component of stability, and one needs to be anchored extremely quickly to create propulsion. Excessive movement means excessive time, and athletes need quickly to get stiff in order to move fast.
Elasticity — Sprinters who can produce energy from a combination of passive and active sources of the body are usually efficient athletes and elasticity works in conjunction with stiffness. In order to create a stretch reflex, a joint must be in place to allow for an elongation of the tissues. How much tendon, muscle, and fascial extension happens is based on numerous factors, such as eccentric strength and anatomical factors.
Power — The most commonly trained quality is power, usually improved by maximal strength and high velocity loaded movements. Power is slower to improve than strength, but having a large ability to produce power isn’t a guaranteed ticket to running fast. Since change is so visible and easier to make than the above qualities, the popularity of training power is logically going to be high.
When all three of these qualities are optimized and technique and other variables are developments, speed is maximized. What is difficult is knowing precisely how much of each quality is needed for high performance, and what the ideal balance for each athlete. Depth jumps can potentially offer a way generally to increase overall neuromuscular changes while specifically training the anatomical areas that may adapt for greater levels of output.
Science of the Optimal Depth Jump
Depth jumps may just offer a chance for athletes to tap into the neurological side of the body and while the research is dodgy, may be able to increase the tensile strength of tendons, thus creating stronger and more explosive contractions. The fine line between tendon-degeneration and positive adaptation is an enigma, but in general the optimal dose of depth jumps has eluded coaches and is more of a trial and error process. The most likely reason for depth jumps being difficult to prescribe is that most applied options are not easily measured besides the estimated height of the drop and the technique and output during the jump. Most coaches prescribe a box height and number of ground contacts, but the sensitivity and precision for the right contraction are not using an eyeball test. Sport Science and coaches that are early adopters have used applied lab quality equipment to peer into the nervous system and found that the height of the box should be adjusted to just the right level. If the height is not perfect, the athlete misses the opportunity to get better or risks injury.
Coaches are trying to improve the amount of positive work (propulsion) during a very narrow window of time. Performance can be improved by getting the same amount work done in less time or more work done in the same time. Due to the lack of inexpensive and accurate equipment to estimate output, coaches usually rely on the eyeball and past athletes to progress box heights. Force plates and contact mats are usually included in research studies to get metrics of how reactive the athletes are. Those metrics are the following:
- Contact Time — The period of time of initial foot contact through final takeoff.
- Air Time — The period of time from takeoff to landing.
- Displacement — The actual positive distance of the center of mass traveled vertically.
- Muscle Activity — The electrical activity of muscle using surface or needle electromyography.
- Kinematics — The measured motions of the body to represent the pattern of the depth jump.
- Kinetics — The forces describing the activity of the depth jump.
- Technique Used — The selected style of execution of the depth jump.
- Targeting — The use of a visual and goal based aid to enhance the arousal of the jump.
As you can see many variables exist, and how they are all related is extremely important. Four potential outcomes happen with depth jumps, and three are bad. The first potential change is the athlete is successfully completing them with proper form and execution. Down the road the athlete gets better. Second, the athlete fails to do it right and receives no benefit and wastes time and energy that could have been used in something productive. Third the athlete does the depth jumps in a way that it rehearses poor physiological output and this results in performance degradation. Finally, the worst case scenario is injury, when the jump was done poorly or at a level that was too demanding. Given the above probable outcomes, doing depth jumps is not as simple as telling athletes to jump high off of arbitrary box heights.
Reactive Strength Index and Time to Stabilization
Researchers have spent decades trying to see what happens during a depth jump, and a few years ago a study done by Flanagan and colleagues investigated two key metrics, one was the Reactive Strength Index (RSI) and the other was Time to Stabilization (TTS). The RSI uses ground contact time and jump height to create a summary of the effectiveness of the exercise. While this is valuable from a force production standpoint, it doesn’t show the mechanics of the movement, and how the forces were produced from the neuromuscular system. Different techniques will create different strains on the body locally, so video or motion capture usually helps dive into the details. Time to stabilization isn’t very reliable, but the metric is evolving. Defined by Flanagan:
The TTS can be calculated by measuring the time taken for vertical ground-reaction force to reach and stabilize within 5% of the subject’s body weight after the landing from a jump.
Similar to stiffness, athletes need to stabilize properly to get a proper stretch reflex. Otherwise it’s trying to sprint or jump on a waterbed.
At this point of the article, the reader is likely asking for the nuts and bolts of the application of doing depth jumps. Setting up a good depth jump can range from having a box and trusting the experience of both he coach and athlete to perform it well without equipment, or it can range to something far more. Coaches are known to add hurdles to take advantage of the arousal or potentiation of the depth jump and combine it with less demanding jumps. A popular option is to have a sponge ball hanging (targeting) above to increase and make performance more consistent. If the depth jump is not reactive, the exercise is fruitless and that requires the right height with the right athlete.
Adjusting Heights for the Specific Output
The primary variable is obvious, proper heights of boxes for each athlete. With depth jumps ranging from shallow 25-centimeter platform boxes to tall 1.2-meter towers, what an athlete can do will range tremendously. No coach or sport scientist has the magic metric or guideline that is infallible, but the RSI and some of the other metrics are great references to keeping the heights sane and appropriate.
I find that jumping without arms is fine for testing squat jumps and counter movement jumps, but with intense depth jumps, full arm action is suggested. Something to keep in mind, the longest ground contact time of all plyometric exercises belongs to depth jumps, so more range of motion and longer time to generate forces are needed. How the overload from depth jumps helps with contact times of 80 milliseconds is unknown, one can argue that the shock to the nervous system may have some physiological benefit to the athlete.
Successful depth jumps include the following parameters. One, a ground contact that is brief enough to exploit the stretch reflex. Second, output from the jump to displace the athlete high enough that the forces originate from overcoming protective mechanisms and provide an adaptation with greater motor unit recruitment. Third, and the most important, technique must be of the highest quality, for both transfer and safety. Listed are the final specifics of executing great depth jumps.
Ground Contact Time (CT)- Contact mats, named after the role they play with measuring first touchdown to take off is estimated to be usually between .2-.25 seconds. Contact time doesn’t equate to work done, but it’s a valuable metric when combined with other variables to look at the big picture.
Jump Height (JH)- The flight time sometimes calculated via formula the amount of displacement or jump height of the athlete. Other measures such as force production can be used with force plates, but jump height is enough with contact mats and a little video. Doing this with a team is not easy, and wearable sensors and other technologies is going to make this more consumer friendly.
When both measurements are collected, one can create a Reactive Strength Index or RSI and get a snapshot on the appropriate height of the box.
RSI = JH/CT
The ratio above is hardly perfect, since doing well requires specialized attention to get better scores, but performance improvement usually comes when an athlete is near the best ceiling physiologically. When training improves the ability, rather than practicing the assessment, neurotically the athlete is improving.
Increasing height only makes sense when the ratio is maintained, provided contact time is still in range and jump height is the same. If the RSI parameters are not followed the athlete is adapting to potentially ineffective stimulus. Just having a number improve still has prerequisites such as the technique engaging the hips, since many athletes depend on a knee dominant execution. Depth jumps do overload the lower leg and foot more than traditional jumps that are initiated from the ground. One warning, depth jumps are artificial and I don’t think the human species are designed to land on the ground and artificially try to jump high immediately. Perhaps humans are programmed and designed to land to absorb energy from heights rather than redirect it, so pretension and an active foot may load the lower extremities. Watching a person step down on the ground they tend to plantarflex their foot and ankle, so slight dorsiflexion may be a safer option.
Testing the Depth Jump Influence and Parting Thoughts
Several laboratory assessments can evaluate the foot strike during maximal speed, breaking down forces and other details. Those descriptive factors are nice, but the most straightforward way to see the influence of depth jumps is seen if short max velocity sprinting is faster. Provided the same run up distance is used one can see the flying 20-30m and judge if a change is happening each year. Maximal speed takes a long time to development, and each year constant and significant improvement is a potential sign of depth jump effectiveness. The only value of depth jumps for sprinting is when one improves the speed, and if improving the RSI and other metrics helps do that, coaches may want to invest into them.
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