Plyometric training: load up for greater benefits?

Sports Performance Bulletin looks at new research on plyometric training, and how to manipulate it for greater benefits

As regular readers will be aware, plyometric training is a form of conditioning exercise in which an eccentric muscle contraction is quickly followed by a concentric muscle contraction. Plyometrics involves performing bodyweight jumping-type exercises and/or throwing/catching medicine balls in order to exploit the so-called stretch-shortening cycle (SSC) of muscle action (for a more in-depth discussion of the rationale behind plyometric training, see this article).

The SSC enhances the ability of the nervous systems and muscles/tendons to produce maximal force in the shortest amount of time. Thus the use of plyometric exercise enables a bridge between strength and speed to be established⁽¹⁾. Plyometric strength training is currently very popular in training for many sports where speed and power are important for performance such as soccer, handball, and basketball^(2,3). In this regard, plyometric training has been used extensively as a method of helping improve dynamic athletic actions such as sprinting, change of direction and jumping performance.

Does loading help?

Given the physiology underpinning the use and benefits of plyometrics training, an obvious question to ask is whether increasing the effective bodyweight on athletes – ie increasing the loading experienced during the plyometrics exercise – can produce benefits over and above those of straightforward plyometrics training? This practice is often adopted in rehabilitation⁽⁴⁾, but some previous research has produced mixed results on this topic, finding little or no benefit by adding resistance in the form of wearable weights⁽⁵⁾.

However, these previous studies have investigated lower-limb plyometric exercises using wearable weights on the upper body rather than on the lower limbs. Some researchers have postulated that due to a change in balance and propioception that wearing upper-body weights produces, any benefits are lost due to interference. But what if the extra loading is worn at the sharp end of the action – just above the ankles? Could this produce tangible benefits? A newly published study by a team of international scientists has just been published and has tried to come up with a definitive answer.

The research

In this study, researchers to compare the effects of ten weeks of twice-weekly unloaded versus loaded (2.5% of body mass, placed above the ankle joint) plyometric training on sprinting, jumping, ability to change direction, balance, repeated changes of direction, and aerobic power in elite junior male soccer players⁽⁴⁾. Thirty eight male soccer players (38 players; 3 goalkeepers and 35 field players) were assigned randomly to one of three training interventions:

• Unloaded plyometric training
• Loaded plyometric training (the standard plyometric routine was modified by placing a load equivalent to 2.5% of the player’s body mass above the ankle joint)
• Control group (no plyometrics, continuing with the standard in-season regime

Twice a week for ten weeks, the two experimental groups replaced the initial part of their standard training program (10–15% of regular soccer training time) with plyometric exercises. These exercises included hopping, hurdles and horizontal jumps, finishing with a 20-metre sprint. The inclusion of sprinting during plyometric training sessions is considered a ‘transfer’ exercise, which carries the plyometric training stimulus across to running). Before and after the 10-week intervention period, the players’ performances were assessed by 40-metre sprint times, 9/3/6/3/9-metre sprints with 180° turns and with backward and forward running (see figure 1), 4 × 5-metre sprints, four jump tests, measures of static and dynamic balance, repeated change of direction tests and the Yo-Yo intermittent recovery test.

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Figure 1: The 9/3/6/3/9-metre sprint test with 180° turns

A and E are start and finish lines respectively. B, C and D are lines of change of direction.

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The findings

The key findings were as follows:

• Both the loaded and unloaded plyometrics enhanced sprinting performance relative to controls (unsurprisingly) but the performances in the loaded group increased to a significantly greater degree than the unloaded group (see figure 2).
• Change of direction times were also significantly shortened by the loaded plyometrics relative to unloaded.
• Both the loaded and unloaded plyometrics repeated change of direction scores relative to the controls, but with greater improvements in the loaded players.
• The loaded plyometrics enhanced some balance scores relative to the unloaded plyometrics.

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Figure 2: Training associated changes in sprint performance

S180° = sprint time over 9–3–6–3–9 m with 180° turns; SBF = sprint time over 9–3–6–3–9 m with backward and forward running; S4 × 5 = sprint time over 4 × 5 m.

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What does this mean for athletes?

When it came to sprint speeds, especially those involving changes of direction (a common requirement in many sports), the loaded plyometrics exercises produced a significant additional performance gain over standard plyometrics. As the researchers explained, the apparent superiority of the externally loaded training probably reflects the ‘overload’ principle, where muscles show a greater adaptive response when stressed beyond their normal capacity⁽⁶⁾. Adding the additional loading may have enabled players to apply greater amounts of force against the ground over a longer time, generating higher impulses during the jumps, thus facilitating greater adaptations.

In short, plyometric training with additional loading enhances appears to enhance several attributes important for sports involving sprints and rapid changes of direction (soccer, rugby, basketball, tennis etc). Importantly, this element of training can be introduced by replacing some of the normal training with the loaded plyometrics (rather than adding it), thus helping to ensure that athletes don’t become overtrained. It also makes it perfectly feasible to incorporate loaded plyometrics into traditional in-season technical and tactical training sessions.

Athletes wishing to try this approach should however stick to a maximum loading of 2.5% of bodyweight placed just above the ankles, which this study found was effective. For a 70kg sportsman/woman, this equates to 1.75kgs. Using more may be counterproductive; not only could it increase injury risk, excessive additional loading may interfere with the efficient execution of the plyometrics exercises, thus reducing their efficiency.

References

1. J. Orthop. Sports Phys. Ther. 2006, 36, 308–319
2. J. Strength Cond. Res. 2016, 30, 3312–3320
3. J. Strength Cond. Res. 2019, 33, 662–675
4. Int J Environ Res Public Health. 2020 Oct 27;17(21):E7877
5. Front. Physiol. 2017, 8, 742
6. Sports Med. 2013, 43, 675–694