Does plyometric training improve strength performance? A meta-analysis

Abstract

Majority of the research suggests plyometric training (PT) improves maximal strength performance as measured by 1RM, isometric MVC or slow velocity isokinetic testing. However, the effectiveness of PT depends upon various factors. A meta-analysis of 15 studies with a total of 31 effect sizes (ES) was carried out to analyse the role of various factors on the effects of PT on strength performance. The inclusion criteria for the analysis were: (a) studies using PT programs for lower limb muscles; (b) studies employing true experimental design and valid and reliable measurements; (c) studies including sufficient data to calculate ES. When subjects can adequately follow plyometric exercises, the training gains are independent of fitness level. Subjects in either good or poor physical condition, benefit equally from plyometric work, also men obtain similar strength results to women following PT. In relation to the variables of program design, training volume of less than 10 weeks and with more than 15 sessions, as well as the implementation of high-intensity programs, with more than 40 jumps per session, were the strategies that seem to maximize the probability to obtain significantly greater improvements in performance (p < 0.05). In order to optimise strength enhancement, the combination of different types of plyometrics with weight-training would be recommended, rather than utilizing only one form (p < 0.05). The responses identified in this analysis are essential and should be considered by the strength and conditioning professional with regard to the most appropriate dose–response trends for PT to optimise strength gains.

Introduction

Muscular strength and power are considered as critical elements for a successful athletic performance, as well as for carrying out daily activities and occupational tasks.1, 2 Much research has been focused on the development of maximal strength performance as this neuromuscular quality appears to underpin most other domains of human physical capacity. Although various training methods, including weight-training,3, 4 explosive and ballistic-type resistance training methods,⁵ electrostimulation training,6, 7 and vibration training⁸ have been effectively used for the enhancement of strength performance, there is solid research evidence that plyometric training (PT) is also effective for improving ballistic and maximal strength.9, 10, 11, 12, 13

Plyometrics refers to exercises that are designed to enhance neuromuscular performance. For the lower body this involves application of jump, hopping and bounding training. Plyometric exercises constitute a natural part of most sport movements as they involve jumping, hopping and skipping (i.e., such as high jumping, throwing or kicking).14, 15, 16 Plyometric exercises are implemented in various forms depending on the purpose of the training program. Typical plyometric exercises include the countermovement jump (CMJ), the drop jump (DJ) and the squat jump (SJ). These exercises can either be combined within a training program or can be applied independently. Furthermore, plyometrics can be performed at various intensity levels, ranging from low-intensity double-leg hops to high-intensity unilateral drills. As far as the lower body is concerned, plyometrics includes the performance of various types of body-weight jumping exercises, such as DJ, CMJ, alternate-leg bounding, hopping, and other stretch-shortening cycle (SSC) exercises.17, 18, 19, 20 These exercises are characterised by SSC actions, that is, they start with a rapid stretch of a muscle (eccentric phase) and are followed immediately by a rapid shortening of the same muscle (concentric phase).17, 19, 20, 21, 22, 23

Research indicates PT improves strength, power output, coordination, and athletic performance.24, 25, 26, 27, 28 Numerous studies on PT have demonstrated improvements in maximal strength,9, 10, 11, 12, 13, 29 ranging from 11 kg to 60 kg (performing exercises such as DJ, CMJ, SJ, combined jumps or combined weights and plyometric training) that could be attributed to the enhanced coordination and the individual's ability to rapidly increase muscle tension resulting in greater maximal rate of force development (RFD).11, 37 In addition, a number of authors determined24, 30, 31, 32, 33, 34, 35 significant positive effects of PT on maximal strength when compared with other training modalities (i.e., weight-training, eccentric training, isometric training). However, several authors have shown that for optimising maximal strength enhancement, the combination of training modalities (i.e., plyometrics and high-intensity resistance training) is recommended rather than using only a single modality.9, 12, 16 However, the characteristics of a training program that achieves better gains are not clear.

The effects of PT may differ depending on the various subject characteristics, such as training level,36, 37, 38 gender,³⁹ age,40, 41, 42 sport activity or familiarity with plyometric training.11, 29 Research studies that combine these variables in different ways sometimes lead to conflicting results.7, 12, 43 Other factors that seem to determine the effectiveness of PT are program duration and training volume. Research studies have used numerous combinations of duration, intensity and volume characteristics4, 10, 12, 13, 44, 45 therefore, the optimal combination of these factors for maximum enhancement remains unclear.

Despite the advantages of PT, the principal issue of determining the optimal elements of a plyometric program remains inconclusive. Identification of the role of the various elements of a PT program with regard to their effectiveness can be achieved with the use of meta-analysis: a method that overcomes the problems both of small sample size and low statistical power. Meta-analysis is a quantitative approach in which individual study findings addressing a common problem are statistically integrated and analysed.⁴⁶ Because meta-analysis can effectively increase the overall sample size, it can also provide a more precise estimate of the effect of PT on strength performance. In addition, meta-analysis can account for the factors partly responsible for the variability in treatment effects observed among different training studies.4, 9, 11, 12, 13 Thus, the purpose of this study was to examine the influence of various factors on the effectiveness of PT using a meta-analysis approach.