Plyometric Training: how to jump higher to improve performance

Why plyometrics can enhance sport performance over and above simple resistance training

Plyometrics is a topic that has been covered before in Peak Performance, but a number of questions remain about which exercises are best, how much, how often, and to which population should they be given. In this two-part article, James Marshall looks at the latest evidence, and comes up with some practical answers

What are plyometrics?

Plyometrics are any exercises that help develop the stretch shortening cycle (SSC) of movement. They start with the stretching of a muscle, an amortisation phase (the period of time from the beginning of the lengthening phase to the beginning of the take-off phase) and then a muscle contraction phase. The faster the stretching phase, the faster the contraction phase. It’s akin to an elastic band being stretched and then released, rather than just thrown. The actions should be explosive in order to train the SSC, rather than heavy and slow. The exercises can involve some sort of jumping or landing for the lower body, or some sort of throwing for the upper body. The three main jumps are:

Drop (or depth) jump (DJ)

Here the athlete steps or jumps off a bench or step, lands on one or two feet and then jumps up as high as possible. This is a fast jump and one of the fundamental plyometric exercises.

Countermovement jump (CMJ)

The athlete starts in a vertical position, lowers their body by bending the knees and then jumps up, usually with an arm swing. This is a slower jump, although it does result in greater height than the squat jump (see below).

Squat jump (SJ)

Here the athlete jumps up from a squatting position. The depth of the squat does not matter and is related to personal preference. This is a concentric-only jump and therefore not strictly speaking a plyometric action as there is no pre-stretching involved. However, some gains can be made from increasing maximal strength but these should be done in conjunction with the jump training.

Once the basic landing and jumping techniques have been learned and can be performed effectively, the correct mechanics for that sport should be taught and rehearsed in situations that require decision making. This requires sound coaching, but it should make it more interesting and effective for the athlete.

Why do plyometrics?

During the early stages of an athlete’s career, running, jumping and throwing can all be improved by improvements in strength. A weak athlete who is good technically may simply improve their jump by increasing leg strength. However, there is a law of diminishing returns and this is due to the limits of recruiting muscle fibres at speed.

Most sporting actions occur in fractions of a second, and the ability to reproduce maximal strength may take longer than this. Zatsiorsky uses the example of a shot putter who is weak compared to other shot putters (1). The delivery part of the shot put lasts between 0.15 and 0.18 seconds, and elite shot putters can produce a force of 60kg in that time. If a shot putter can bench press a maximum of 220-240kg, this equates to 110-120kg per arm; in the put described above therefore, our shot putter is only using 50% of maximum strength.

By contrast, a beginner shot putter who could only bench press 50kg would definitely improve performance by being able to bench press 150kg, because he or she would be able to produce more force during the put. However as the putter’s maximal strength increases, so does the time it takes to recruit all these muscle fibres. The clear implication is that after a certain point, it’s explosive strength that needs to be developed, and this is where plyometrics comes in. After all, what good is maximal strength if it can’t be used within the short time frame of an explosive action?

Another example is the high jump where the force production to propel the body upwards occurs in just 0.2 seconds on take off. Once a certain level of strength has been reached, doing slow squats will not help the ability to produce force in the legs in that short time. Instead, faster, lighter squats or squat jumps plus unloaded plyometric activities will help train the reactivity needed to increase the speed of the takeoff.

Plyometrics help train explosive strength by shortening the time taken to switch from an eccentric action (such as landing) to the concentric action (such as jumping). You are basically training the muscle fibres and motor neurons to work more quickly and in synchronisation with each other. It follows therefore that the jumps and drills you should use are those that train the actions and timings required in your sport.

However, research in individual sports is limited and study design varies, so analysing what is the most effective jumping programme is difficult. Most studies seem to look at outcomes of 10m speed and vertical jump height, or similar measurements. These are quite useful, but both are linear in nature, not multidirectional (the sort of movements that characterise sport actions) and do not require reactive ability. With those limitations in mind we can look at the best way to improve vertical jump.

How to jump higher

A key question is how to make the jump training  specific for your sport. A recent meta-analysis (a study that pools the data from a number of previous studies) of many different plyometric studies has looked at this issue and provides some interesting answers (2). The Spanish researchers looked at 56 different plyometric jump programmes to see which type best increased vertical jump (VJ) height.

The programmes included in the meta-analysis varied in their design and length, but all incorporated lower body plyometrics of some description (see box). A meta-analysis is useful in this respect because it eliminates some of the minor variables that can cause variability in results between different studies. This can help to paint a more accurate overall picture than when just one individual study is used.

The meta-study looked at studies that were varied in their approach, and used both experienced and inexperienced subjects from different sporting backgrounds such as track and field, volleyball, basketball, bodybuilding, rowing, swimming and football. Some of the major findings were as follows:

  • The greatest gains were found in athletes who were at international level (ie elite) rather than at regional or national level;
  • Men demonstrated greater gains than women;
  • The gains were similar for groups from different sporting backgrounds;
  • VJ height improvement was no greater when plyometrics were combined with different types of exercise compared with when plyometrics were used alone (eg combining resistance training with the plyometric programme didn’t yield greater gains).

Although the international athletes showed the greatest gains, all groups showed improvements, including beginners. Plyometric training improves both the elastic tendencies of the muscles and tendons involved in jumping, together with neurological response. It is not surprising therefore that more advanced athletes who have better coordination patterns to begin with improved more, but the study shows that there does not need to be a training base before starting a programme.

When looking at what type of training was done, many of the individual studies did not detail the jumping and landing technique used, so it is not known how much of a factor the technical and coaching aspects were. The variety of jumps incorporated did make a difference, with a combination of squat jumps, depth jumps and countermovement jumps proving most beneficial.

The depth jump height did not appear to be a factor in how effective the depth jump was, and the authors recommended a maximum height of 20cm to jump from; the reduced intensity of a modest jump height allows more repetitions to be performed with reduced risk of injury and a potentially greater neurological response. Some programmes used ‘weighted jumps’ (wearing a weighted vest or holding dumbbells) but they did not appear to add any extra benefit to the programme.

Although some shorter-term studies did show improvements, overall it was the duration and volume of work that seemed to improve jump height the most. A programme lasting 10 weeks minimum, with two sessions a week and 50 jumps a session was recommended. It’s worth adding that if you’re conducting a session, 50 repetitions don’t last long, so this can easily be incorporated into a field or technical session. However, if you’re doing a line out session in rugby or another sport drill that requires jumping, this may already include repeated jumps, so you need to take this into account.

References
1. Zatiorsky, VM, Science and Practice of Strength Training, Champaign, Il: Human Kinetics. 1995
2. JSCR 23 (2), p 495-506 2009

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