Characteristics of effective jumping

The use of the RSI as the measure of effectiveness considers both the output of the athlete’s effort (jump height) and duration over which it was achieved (ground contact time). Environmentally this reflects the nature of the challenge which athletes typically use plyometric exercise to prepare for. For example, a team sport player changing direction to elude an opponent or a high jumper seeking to convert horizontal velocity into vertical. It is only logical that when RSI is used as a measure of effectiveness, a technique which produces the same or greater impulse over a shorter timeframe will appear superior to one performed over a longer time frame. However, in addition to environmental relevance, there is also a strong physiological rationale for favouring a more rapid ground contact in order to maximise the potential for augmented force production via the SSC. The terms “CMJ-DJ” and “bounce-DJ” have been used within the literature to distinguish between distinct techniques which favour either a long contact time and greater depth of descent (CMJ-DJ) or a short ground contact with a smaller depth of descent (bounce-DJ) (Ball and Scurr, 2009, Ball et al., 2010, Byrne et al., 2010, Marshall and Moran, 2013, Struzik et al., 2016, Bobbert, 1990). The WORST jumps within the present study may be considered more

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reflective of a CMJ-DJ due to the large ground contact times and greater depth of descent than BEST. However, previously it has been demonstrated that the CMJ-DJ may enable achievements of greater jump heights due to the increased opportunity to apply force during the ground contact (Struzik et al., 2016). This was not the case within the present study as jump heights were inferior for this group. It is noteworthy that subjects self- selected this technique despite instruction to minimize ground contact whilst achieving optimal jump height. This may therefore be a reflection of a lack of technical competency or a deficiency of neuromuscular qualities to achieve such an outcome. It should also be considered that the elite athletes within the present study may possess physical attributes which do not necessitate such a long ground contact time.

Chapter 4 demonstrated that elite athletes achieve superior performance over recreational counterparts during plyometric exercise by producing a greater amount of work over a shorter time frame. The present study demonstrated that the BEST jumps achieved a greater RSI by producing greater impulse to WORST over a shorter duration. This is unsurprising due to the inclusion of ground contact in the RSI calculation. However it is noteworthy that the superior performance was achieved through mechanical characteristics which were not only augmented but also utilised a different pattern of force application. Critically the reduced ground contact time was achieved chiefly through large reductions in both the eccentric and concentric phases although the difference was greater in the eccentric phase. Eccentric phase duration was 57% shorter in the BEST jumps whereas the concentric phase was 29% shorter.

The utilisation of a jump technique which is characterised by a shortened eccentric phase without a loss of impulse is critical to effective jumping. The picture of this technique is further described by a number of other variables. CoM displacement during descent in BEST was around half that of WORST (see Table 2). This is accompanied by a more than two-fold increase in stiffness. These data illustrate a jump technique characterised by a stiff landing with a small amount of displacement performed over a brief window of time. BEST jumps exhibited a greater Ecc-RFD which supports the stiffer technique as braking force is applied more rapidly to counter yielding to the impact forces. Clearly such a strategy can only be

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successfully employed provided the athlete has the neuromuscular capacity to tolerate these greater forces.

The movement pattern seen in BEST jumps underpins a number of subsequent kinetic factors which enable an enhanced jump performance. Cormie et al. (2010b) demonstrated that during ballistic jumping, training induced changes in the eccentric phase were correlated with an augmented concentric phase. In the present study this was also evident as force at zero velocity was significantly greater in BEST vs WORST, thus demonstrating that jumpers entered the concentric phase in an advantageous state. Previously, Bobbert et al. (1996a) has described the importance of achieving high active state prior to the onset of the concentric phase when jumping. The rapid nature of the concentric movement in jumping represents a significant time constraint for the production of maximal force. Therefore strategies which enable the generation of high levels of active state and subsequent force generation prior to concentric movement are likely to lead to enhanced performance. This was evident in the present study with a 75% greater relative force being produced at the onset of the concentric phase in BEST. This advantage is continued throughout the movement and results in greater impulse being applied during this phase. Critically, this leads to greater positive movement velocity throughout the concentric phase and at take- off. This represents the conclusion of a chain of events which begins with a brief, stiff landing with a small amount of displacement leading to high levels of isometric force being generated prior to the concentric phase (as reflected in force at zero velocity) and a subsequent augmented peak concentric power output, velocity at take-off and jump height.

Such a technique as that described above is consistent with common coaching methods which place an emphasis on stiff landings and brief ground contact times. However it should be noted that such a performance can only be achieved when supported by the requisite neuromuscular qualities (Beattie et al., 2016). These will include the ability to achieve high levels of pre-activity prior to ground contact, eccentric strength to resist yielding, and the ability to produce a high eccentric-RFD. When utilised during drop jumping these neuromuscular qualities enable a more rapid stretching of tendon tissue during landing and a greater subsequent recoil during propulsion as has been demonstrated previously in drop jumping (Ishikawa and Komi, 2004, Ishikawa et al., 2006, Ishikawa et al., 2005). This is

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essentially a classic demonstration of the SSC model and illustrates the importance of neuromuscular capacity to take advantage of tendon recoil in high force activities such as drop jumping.

Cormie et al. (2009) found that following a power training programme subjects improved jump performance and exhibited a modified technique which utilised a greater depth of descent without extending the duration of the phase. The power training programme was composed of a loaded jump squat regime. It could be hypothesised that such a training regime may lead to fascicle lengthening and therefore optimal fibre length may have altered and influenced optimal depth of descent when jumping. Without data to support such a contention it is merely speculation. However the discussion highlights the need to consider the physical qualities of an athlete when evaluating jump technique. Muscle-tendon architecture may influence optimal muscle length and therefore the most effective joint angles and associated descent. Equally a less stiff landing strategy which allows greater yielding may be used to increase the duration of the concentric phase as an alternative strategy for increasing concentric impulse (McMahon et al., 2017a, Jidovtseff et al., 2014a). Whilst such a strategy would be likely to have a diminished contribution from elastic energy following tendon recoil, such benefits are only available when an athlete has the strength qualities to resist high forces as discussed within Chapter 4.

6.6.2 Practical Applications.

Gross measures of plyometric performance, such as peak power and jump height, have been used effectively as a measure of performance and neuromuscular status in plyometric exercise. Such measures enable quantification of performance outcome and distinction between better or worse performances. The RSI can be considered more insightful than measuring jump height alone as the timeframe of force application is also considered and therefore understanding of how jump height was achieved is increased. The use of TPA to assess plyometric exercise continues this interrogation of the question “how” and provides valuable insight into the mechanical characteristics of a jump. Until fairly recently, plyometric performance gains following training have generally been regarded as representing an upregulation of neuromuscular qualities. The present study extends the

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findings of Cormie et al. (2009) and Cormie et al. (2010b) from ballistic to plyometric exercise and demonstrates that enhanced performances are achieved through a combination of an altered mechanical strategy and the neuromuscular qualities required to support it. Consequently TPA has significant potential as a diagnostic tool to enable coaches and sports science practitioners to evaluate whether training should be directed toward altered technique to make better use of existing neuromuscular qualities or developing these qualities where technique is deemed to be optimal. Further research is required to identify population norms to enable such judgements. Whilst these findings support the use of a detailed interrogation of ground reaction forces to gain insight into plyometric performance they also validate the use of RSI in a practical setting. This represents a simple measure which relates to a number of kinetic variables underpinning jump performance.

6.6.3 Limitations

The use of a drop jump represents one of the most commonly used plyometric exercises in training. The evaluation of this exercise alone is a limitation when considering plyometric exercise more broadly. Alternative plyometric challenges such as unilateral, horizontal and multi-directional should also be evaluated before extrapolating the findings of the present study. The use of varied sample frequencies should be addressed in future studies with a recommendation of a minimum of 2000 Hz for ground contacts of 250 ms and above although higher frequencies are required for shorter contacts in order to ensure at least 500 samples during the TPA.

Finally, the peak values attained during analysis were based on dividing the movement into concentric and eccentric phases. This is a less detailed approach than that applied in the previous chapter whereby the eccentric phase was subdivided into impact and eccentric components. This was due to the opportunity to gain greater insight through TPA rather than a detailed analysis by phase. However use of such an approach may be applied in future studies comparing performances between groups.

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6.6.4 Summary

This study demonstrates for the first time that TPA offers novel insight into the mechanics underpinning plyometric exercise and may be used to distinguish between jumps within an elite athletic group. This analysis describes a technique characterised by a brief and stiff landing phase leading to enhanced force, velocity and power during propulsion.

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Chapter 7

- Discussion

7.1

Overview

This chapter presents a summary discussion of the key findings of this thesis. From a scientific perspective this will include discussing the limitations of the present research as well as suggesting areas of future enquiry. Significant consideration is also given to the practical application of these findings as jumping can be considered an inherently applied topic of study.