Population Differences

Relatively little attention has been paid to the differences in performance and adaptation to plyometric exercise in different populations. This is perhaps reflected with the coaching training literature which makes little or no distinctions in plyometric prescription nor the potential for differing adaptation mechanisms and magnitudes across populations. The primary focus of cross population comparisons to date has been placed on training background.

A number of studies have highlighted differences in how people jump. Whilst it does not automatically follow that such differences will require an alternative prescription or result in altered adaptations an awareness of such differences and the potential implications is important. Eloranta (2003) compared the muscle coordination patterns of swimmers and track & field athletes via sEMG during CMJ and drop jumps. Track and field athletes performed according to the proximo-distal model described by Bobbert and van Soest (2001) and displayed a pattern of reciprocal inhibition between agonists and antagonists associated with efficient technique. Authors suggested that the patterns displayed in this group best reflect a stiffness innervation associated with effective elastic recoil of the MTU. In contrast, swimmers exhibited more of a simultaneous model of muscle contraction rather than proximo-distal. Swimmers also tended to co-contract antagonists and agonists of the thigh and shank rather than demonstrating reciprocal inhibition. Such a coordination pattern is consistent with the postural and stiffness demands of swimming and suggests an adaptation of the CNS to prolonged exposure to sport specific stimuli which impacts on non- related movement patterns. The stark differences seen across groups in this study could potentially have significant implications on the biomechanical stimulus of a plyometric training programme with the athletics model being consistent with optimal use of tendon recoil and the SSC and the swimmers potentially relying more on contractile force.

Evaluations of plyometric performance in power versus endurance athletes have also received attention within the literature. Kyrolainen and Komi (1995) compared power and endurance athletes during drop jumping. As would be expected, power athletes produced more power in each drop jump condition. However, analysis of muscle activation patterns

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suggest that this superior performance may be the result of differing patterns of muscular activity rather than simply greater magnitude of work. Power athletes demonstrated a faster rate of preactivity prior to ground contact and a smoother muscle activation pattern during ground contact. As discussed previously, preactivity is a critical element of an effective SSC and plays a significant role in determining the opportunity to achieve mechanical stiffness and optimal tendon loading. Therefore this neuromuscular skill, most likely an adaptation to exposure to this type of training, may differentiate between the nature of force production and the capacity to make use of a SSC between groups.

Power and endurance athletes have also been compared during hopping by Hobara et al. (2008). Kinetic and kinematic data were used during in-place hopping at 1.5 Hz and 3.0 Hz to assess leg and joint stiffness. Surface EMG data were also collected from six leg muscles. The power groups demonstrated significantly greater leg stiffness at both hopping frequencies, at the knee at 1.5 Hz, and at the ankle at 3.0 Hz. Endurance athletes demonstrated significantly greater sEMG activity at both frequencies. These results suggest that power athletes are able to produce greater levels of leg stiffness through specific joint stiffness as a result of intrinsic qualities of the MTU rather than greater levels of muscle activation. The same group performed a similar protocol which compared endurance athletes with recreationally active subjects during hopping at 2.2 Hz (Hobara et al., 2010). A similar effect was seen with endurance athletes demonstrating greater leg stiffness as well as specifically at the ankle and knee. Combined, these two studies illustrate a pattern of greater joint stiffness, most likely through training exposure, with increased stiffness from recreational, to endurance to power athletes. Such findings not only illustrate the adaptations which may accompany these forms of training but highlight to coaches the altered performances which should be expected across groups.

Further evidence of differences in neuromuscular patterning have been shown by Avela et al. (2006) when comparing high jumpers with sprinters. These may typically be regarded as two relatively homogenous groups in that both represent power-based track and field events. However close analysis suggests there may still be important distinctions in jump performance. H-reflex and short latency reflex (M1) sensitivity were assessed during drop jumping. As exercises progressed a fatigue effect appeared to manifest in the sprinters

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whereby both reflex peak-to-peak amplitudes showed a significant reduction towards the end of the exercise. This group also showed significant rises in serum creatine kinase 2 hours post-exercise. The authors hypothesise that the effects seen in the sprinters was a result of presynaptic inhibition as a result of muscle damage which was not evident in high jumpers due to a protective effect from high jump training. The finding of such differences between these similar populations highlights the need for further research across multiple populations.

2.2.5 Summary

Plyometric exercise is an effective tool for augmenting movements which utilise a SSC. This mode of training has been demonstrated to improve vertical jumping, sprint performance and running economy. These gains appear to be achieved through a number of contributory factors including increased MTU stiffness, increased tendon stiffness and neural adaptations, specifically enhanced preactivity and increased activity during the braking phase of the SSC. Furthermore, plyometric programmes may be used to reduce injury risk through adaptations in motor control. Further research is required to explore potential population differences in the performance of and adaptation to plyometric exercise. The focus of this review will now move towards the effective assessment of intensity and volume of this important exercise modality with a view towards enabling more precise prescription.

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