In the gym everything is controlled. The number of sets, amount of weight, speed of contraction/extension, number of exercises and so on. But on the playing field – it’s a different ball game. Erratic changes in direction, speed and play dictate that players must be ready to apply their power efficiently in a range of different ways. Take the quick change of direction required to compete for the ball in a game of soccer for instance. It’s not just the athlete’s absolute power that determines the speed with which they can change direction. Rather, it’s the rate of power development during the 100 milliseconds of ground contact time that determines how effectively they will be able to compete for the ball.
Explosive power and speed are the hallmarks of superior elite athletes engaged in ball sports like soccer, rugby league and Australian Rules Football. Anyone looking to improve their explosiveness can’t rely solely on the strength development that comes from traditional bodybuilding routines or resistance exercises.
Plyometric exercises are king when it comes to developing explosive strength. This article will provide an overview of the basic theory behind plyometric exercises as well as some examples of plyometric exercises that any athlete can incorporate to improve the rate at which they can develop power.
Plyometric exercises increase explosive strength by means of a phenomenon called the stretch-shortening cycle (SSC). As its name suggests the SSC is defined as an active stretch (eccentric contraction) of a muscle followed by an immediate shortening (concentric contraction) of that same muscle. The key difference between SSC in plyometric exercises versus conventional resistance exercises (i.e. squat) is the speed and length of the contractions. More specifically, the lengthening cycle (eccentric) is much shorter, while the shortening contraction (concentric) is performed as rapidly as possible.
Moreover, studies show the coupling of the eccentric and concentric phases produces a more powerful contraction than either alone1, 2. This is due to the elastic energy stored in muscles and tendons during the eccentric (stretching) phase, which is then released during the following concentric contraction. This phenomena is referred to as the muscle spindle reflex (or myotatic reflex).
Fast & Slow Stretch-Shortening Cycles
SSC exercises can be further broken down into fast and slow based on the length of the stretch phase. Fast SSC exercises are characterised by very short contraction times of less than 0.25s while slow SSC exercises involve longer contraction times. Or put another way, fast SSC are characterised by a short transition between eccentric and concentric phases. As such, fast SSC exercises typically involve less angular displacement of joints than slow SSC. Lastly, fast SSC exercises have shorter ground contact times than slow SSC exercises. An example of a fast SSC exercise is the popular drop jump, while a common example of a slow SSC exercise is the countermovement jump.
Benefits of Plyometric Exercise
A key benefit plyometric exercise is thought to offer over traditional exercise training (be it anaerobic or aerobic) is it provides much higher neuromuscular stimulus. This is concerned with the neural potentiation of contracting muscles, which is an important limiting factor in power development in muscles. The other major plus with plyometrics are they requrie less physical space, time, and equipment to complete the training session(s).
Plyometric exercises have traditionally been used for sprint and power-based sports such as track and field or ball sports. However, in recent years, there is emerging research that plyometrics can also help with performance in endurance sports such as running, cycling, skiing and rowing.
Benefits of Plyometric Exercise for Running
For example, studies in elite middle- and long-distance runners have shown that a short-term (6 wk) plyometric training program involving drop jumps and countermovement jumps with arms was able to improve 20-m sprint time and 2.4-km endurance run time3.
Benefits of Plyometric Exercise for Soccer
A representative example of the power of plyometrics to improve performance in soccer players comes from a study published in the May 2014 issue of the Journal of Strength and Conditioning Research. In this study, young soccer players performing two sessions of plyometric training over a 7-week period were found to have improved explosive power and endurance when compared to an equally matched group who performed the same training with the exception of plyometrics4.
Examples of Plyometric Exercises
There is an endless variety to the range of plyometric exercise one can perform, however, most invariably involve some form of:
- Bounding; or
Two of most common types of plyometric exercises are the drop jump (sometimes also referred to as a depth jump) and the counter movement jump.
The drop jump (DJ) is typically performed by an individual stepping off a raised platform (most usually a box or step) at a height between 20 and 60cm. The individual then attempts to rebound off the floor with both feet as soon as they make contact with ground. In cases where the focus is on executing a fast stretch-shortening cycle (i.e. minimal ground contact time) the drop jump is performed without arm swing (i.e. arms placed on hips) and with minimal bending of knees.
A variation of the classic drop jump is called a depth jump, which involves the same basic movement, but instead emphasises slow stretch-shortening cycle. The depth jump allows for individuals to use arm swing and a greater ground contact time (with a corresponding increase in knee flexion angle) with the aim of rebounding as high as possible. What’s more, depth jumps are typically performed from a greater height (i.e. 95-115cm) than the drop jump. This allows for greater generation of force when rebounding. Unlike the drop jump, the focus is not so much on minimizing ground contact time as it is on maximising rebound/jump height.
The countermovement jump (CMJ) is another plyometric exercise that focuses on a slow stretch-shortening cycle (SSC) and maximum force generation. Again, the exercise can be performed with or without swinging of the arms – depending on the desired length of the SSC. The basic goal of the CMJ is to jump as high as possible. Research studies employing the CMJ usually have a specialised piece of equipment called the VerTec (as shown opposite) to objectively measure jump height.
Loosley speaking, DJ are thought to best stimulate power development of muscles, while CMH best improve muscle coordination. While the DJ and CMJ focus increasing explosive muscle strength of the legs, there a few other basic plyometric exercises that help increase explosive strength of the upper torso.
Perhaps the most simple one is the clap push up (pictured opposite). In this exercise, the subject must have a strong enough concentric contraction to push off the ground and clap hands together before commencing the eccentric (downward) phase of the push up.
The other common upper body plyometric exercise is that of medicine ball throws as pictured below.
Examples of Plyometric Routines
Learning how best to incorporate plyometric training into competitive endurance and team sports is arguably the area that has seen the most advances in recent times. The general consensus with incorporating plyometric training into existing regimes is to air on the side of moderation. Two sessions per week is generally all that is needed to get the maximum benefit from plyometrics if one is already engaged in a competitive sport that requires multiple training session a week. In reviewing earlier studies where plyometrics were found to have equivocal effects when added to existing training regimes, scientists found a trend to incorporate more than two plyometric training sessions per week, which when totalled lasted over an hour.
This is compared with more recent studies showing just two sessions a week lasting 20 minutes each were sufficient for competitive soccer players to see improvement in power and performance. A sample set used in the study of young soccer players is shown below.
Sample Plyometric Routine
2 Sets of:
- 10 x Drop Jump Reps (20cm)
- 10 x Drop Jump Reps (40cm)
- 10 x Drop Jump Reps (60cm)
Plyometric Exercise Technique Notes
- 15 seconds rest between repetitions and 90 seconds rest between sets
- Step off platform with supporting leg straight to avoid any initial upward propulsion or sinking
- Aim to jump for maximal height and minimum ground contact time
- Ensure a minimum of 48 hours between sessions
Varying Parameters of Plyometric Exercise
Recently studies have began to look at the effect of varying the parameters of plyometric exercises such as the rest interval between repetition and set. Regimes that employ a short number of repetitions and a higher number of sets are often referred to as cluster sets. For example, a normal plyometric training session might look like this:
Classic Plyometric Set
2 sets of 10 repetitions with 90 seconds rest between sets
Plyometric Cluster Set Variations
Cluster Set 1: 4 sets of 5 repetitions with 30 seconds rest between sets; or
Cluster Set 2: 10 sets of 2 repetitions with 10 seconds rest between sets
A recent study has shown the above cluster set variations to be more effective than the traditional plyometric set5. This suggests cluster sets may allow for better performance of common plyometric measures such as plyometric jump power, take-off velocity, and jump height. Based on these findings some coaches suggest it is best to have athletes perform 2-5 jumps with 27-45 seconds of rest between sets.
Plyometric Training On Unstable Surfaces
The other variation concerns performing plyometric exercise on unstable surfaces. These might consist of:
- Foam rocker boards
- Balance pads
- Inflatable discs
- Balance boards
- Wobble boards
Studies have already been published confirming the effectiveness of performing lower body plyometric training on unstable surfaces for improving physical performance on stable surfaces6.
In closing, its worth highlighting the wealth of positive research concerning the application of plyometric exercise to power development in a range of sports including endurance sports. While there has sometimes been an association with increased risk of injury with plyometric exercise, this is not a substantiated belief and to the contrary, plyometrics training has been shown to confer increased protection from injury when performed according to the demands of an individuals sport. With it's time efficiency and low cost, plyometrics is a type of exericse that can be incorporated easily by a vast range of athletes with good success.
1. Cavagna GA, et al Positive work done by a previously stretched muscle. J Apply Physiol. Jan 1st 1968.
2. Cavagna GA, et al. Effect of negative work on the amount of positive work performed by an isolated muscle. J Apply Physiol. 1965.
3. Ramirez-Campillo R, et al. Effects of plyometric training on endurance and explosive strength performance in competitive middle- and long-distance runners. Journal of Strength and Conditioning Research. 2013;28(1):97-104.
4. Ramirez-Campillo R, et al. Effects of in-season low-volume high-intensity plyometric training on explosive actions and endurance of young soccer players. 2014;28(5):1335-1342.
5. Moreno SD, et al. Effect of cluster sets on plyometric jump power. J Strength Cond Res. 2014;28(9):2424-8.
6. Kibele A, et al. Metastability in plyometric training on unstable surfaces: a pilot study. BMC Sports Science, Medicine, and Rehabilitation. 2014;6:30.