Plyometric training involves the usage of jumps, hops, bounds, and/or skips and should not be confused with ballistic training. This form of training is governed by the stretch-shortening cycle, otherwise known as the reversible action of muscles. Plyometric activities can be separated into two categories depending upon the duration of the ground contact time: 1) fast plyometric movements (≤250ms); and 2) slow plyometric activities (≥251ms). This training modality appears to be very effective for improving athleticism in both youth and adult populations. Moreover, both land- and aquatic-based plyometric training appears to be a potent stimulus for improving athletic qualities. As plyometric activities are highly-coordinated and skilful movements, they should be coached with full care and attention by qualified personnel. Lastly, though training volume is relatively easy to measure, training intensity is far more complex due to the individual variability of each athlete.
Keywords: plyometric training, plyometrics, stretch-shortening cycle, concentric, eccentric, ground contact time
What is Plyometric Training?
Plyometric training, otherwise referred to as ‘plyometrics’ or ‘shock training’, is a training modality which often requires athletes to jump, hop, bound and/or skip. Plyometrics should not be confused with ‘ballistic’ training, which is ultimately another word for ‘trajectory’ training. Ballistic training involves the trajectory of objects and implements (e.g. barbells and medicine balls), whereas plyometric training uses the previously mentioned movements. Having said this, it is important to understand in some circumstances there is a degree of crossover, where some movements are considered both ballistic and plyometric. Ultimately the differing factor between the two is that plyometric training typically involves rapid reactive contacts with a surface (e.g. foot contacts during sprinting), whilst ballistic training involves the trajectory of objects/implements.
Plyometric training takes advantage of a rapid cyclical muscle action known as the ‘stretch-shortening cycle (SSC)‘, whereby the muscle undergoes an eccentric contraction, followed by a transitional period prior to the concentric contraction (1) (Figure 1). Figure 1 demonstrates an athlete’s ankle moving through the SSC sequence (eccentric, to amortization, to concentric) during a jump. Therefore, this muscle action (i.e. SSC) is often referred to as reversible action of muscles (2) and is existent in all forms of human motion whenever a body segment changes direction.
Duration of the ground contact time
During walking, running, and jumping our feet continuously strike the ground and then leave it again in a reciprocal fashion – meaning when one foot leaves the ground, the other is quick to contact it. The time period in which the foot is in contact with the ground is known as the ‘ground contact time’ (GCT). During sprinting for example, the foot GCT can be anywhere between 80-90 milliseconds (3).
Plyometric movements which are synonymous with the SSC, are classified as either a ‘slow’ or ‘fast’ plyometric activities (2).
- Slow plyometric exercise = GCT ≥251-milliseconds (0.251 seconds)
- Fast plyometric exercise = GCT ≤250-milliseconds (0.25 seconds)
Table 1 provides some clear examples of the GCTs during common movements and their plyometric classifications.
Why is Plyometric Training important for sport?
As the SSC exists in all forms of human motion from changing direction in rugby, to jumping in basketball, and even sprinting in the 100m, it becomes obvious that all of these movements can be deemed as plyometric activities. As all of these movements are classified as plyometric movements/activities/exercises, its importance in sport suddenly becomes transparent.
With the growing interest in plyometric training, many researchers have attempted to identify the potency of this training modality for improving athletic performance. To date, plyometric training has been shown to improve the following physical qualities in both youth and adult populations:
- Strength (12-23)
- Speed (1, 12-16, 18, 20, 21, 25-31)
- Power (16, 18, 28, 31, 32)
- Change of Direction Speed (14, 21, 27, 30, 33-35)
- Balance (12, 15, 16, 23)
- Jumping (14-19, 21, 24, 25, 27-39)
- Throwing (18, 32)
- Kicking (34, 26, 21, 39)
- Bone density (40, 31)
Furthermore, even aquatic plyometrics have been shown to improve:
- Speed (12)
- Change of Direction Speed (33)
- Balance (12)
- Jumping (33)
How does Plyometric Training improve performance?
Though seemingly simple, this is in fact a difficult, and very exhaustive, question to answer. As plyometric training is governed by the SSC, the question “how does plyometric training improve performance” is perhaps better referred to as “how do changes to the SSC improve performance?”
Many neurophysiological mechanisms have been considered to underpin and explain the impact of plyometric training on the SSC. Most of which include:
- Improved storage and utilisation of elastic strain energy (41-43)
- Increased active muscle working range (44, 45)
- Enhanced involuntary nervous reflexes (46, 47)
- Enhanced length-tension characteristics (48)
- Increased muscular pre-activity (49, 50),
- Enhanced motor coordination (44, 45)
Whilst there is still no consensus as to which of these neurophysiological adaptations is primarily responsible for the enhancement of the SSC, research is beginning to highlight the significance of following mechanisms:
- Improved storage and utilisation of elastic strain energy (2, 51)
- Increase in active state due to an increase in the active working range (2, 51)
Improving these qualities will likely lead to an increase in leg stiffness during contact with the ground, and also force production during the concentric contraction. Increases in both leg stiffness and force production will likely lead to improvements in athletic performance.
If you wish to understand these mechanisms in greater detail, we recommend you read our other article on the Stretch-Shortening Cycle (SSC) by clicking here.
Issues with Plyometric Training
Though plyometric training is a very potent training modality for improving athletic performance, there are several important issues practitioners must fully understand and take into consideration before they attempt to deliver any form of training prescription.
Plyometrics are highly-coordinated and skilful movements
Plyometric activities require athletes to produce high-levels of force during very fast movements. They also demand the athletes to produce this force during very short timeframes. Perhaps the best example of this is sprinting. Maximal speed sprinting demands that the athlete moves their body and limbs at the very pinnacle of their ability – making it an extremely fast movement. Athletes have also been shown to produce ground reaction forces during each foot contact of 3-4 times bodyweight (52, 53). Not only that, but they must apply these huge forces in a GCT of just 80-90 milliseconds (3). So during sprinting, athletes are required to move as quickly as possible, produce forces of over 3-4 times bodyweight, and do so in just 80-90 milliseconds.
As a result, plyometrics are not typically seen as just exercises or drills, but more as complex ‘movement skills’ due to their high-complexity. Understanding this is vital and highlights how highly-coordinated these movements are, and why they require a large amount of attention and coaching if optimal, yet safe, performance gains are to be made.
The intensity of plyometrics is difficult to measure
Arguably, volume of plyometric training is relatively easy to measure and prescribe, and is typically done so by counting the number of ground contacts per session, otherwise referred to as simply ‘contacts’. However, measuring and prescribing plyometric intensity is far more complex. To accurately measure plyometric intensity, the following components must be taking into consideration (2):
- Speed of movement
- Amplitude of movement
- Points of contact (i.e. unilateral or bilateral)
- Body mass
- Technical competencies
- Strain yielding competencies
To provide just one example, let’s take a quick look at body mass. If two athletes perform a drop-jump from a 30cm box, but athlete A is 60kg and athlete B is 80kg, then athlete B has to absorb and re-apply more force than athlete A simply because of their weight.
This simple example demonstrates how the intensity of this plyometric activity is different for each of these athletes. Practitioners must ensure they take this information into consideration when planning and prescribing any form of plyometric training.
Plyometric training is primarily used by strength and conditioning coaches to enhance human neuromuscular function and improve the performances of both explosive- and endurance-based athletes’ (54). It is commonly agreed that plyometric training develops the neural and musculotendinous systems of the SSC to generate maximal force in the shortest amount of time. Given this, plyometrics are often used as a method of training to bridge the division between strength and speed (54). Even despite rigorous scientific investigation, plyometric training continues to prove itself as a potent training method for enhancing athletic performance.
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