The biomechanics of standing start and initial acceleration: reliability of the key determining kinematics.

Author:Standing, Regan J.
Position:Research article - Report
 
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Introduction

Sprinting is an athletic event that requires individuals to cover a set distance as fast as possible, predominantly comprising of an acceleration, transitional and maximal velocity phases (Cronin and Hansen, 2006; Delecluse, 1997). In numerous team sports such as football, rugby league, netball and hockey, short sprint speed is crucial to achieve gameplay tasks, especially when trying to do so against opponents (Leblanc and Gervais, 2004). The ability to improve initial acceleration will provide the individuals the opportunity to be more successful during these movements and therefore improve their overall gameplay (Leblanc and Gervais, 2004). Sprint distances at Olympic events include 100m, 200m and also the 400m, with competitors constantly seeking individual improvement. It is common to see winning margins of less than 0.01s in many of these sprint events; therefore small alterations in performance can be the defining factor between gold and silver medal placings (Docherty and Hodgson, 2007; Young, 2006). Research has investigated the effects of acute interventions and longitudinal training programmes, in an attempt to identify if these can aid in improving an individual's sprint times, with numerous concepts proving beneficial (Cronin and Hansen, 2006; Kotzamanidis, 2006; Matthews et al., 2004; Needham et al., 2009; Yetter and Moir, 2008). However, for these interventions to be used with confidence throughout the sprinting community, research must prove that the corresponding effect on performance is actually due to the intervention or training protocol and not caused by random variation. To do this, the reliability of sprint performance (sprint times) must be investigated, allowing concise conclusions to be made about the intervention strategies being implemented (Hopkins et al., 2001).

Reliability is a term used to describe the consistency of research to produce the same, or similar results on different occasions (Bannigan and Watson, 2009; Levinger et al., 2009). A measurement that is less reliable produces data which has a larger degree of variation; therefore making systematic changes caused by interventions much harder to recognise (Hopkins et al., 2001). Moreover, reliability provides a quantitative description of the spread of the error involved with a particular movement or performance, allowing accurate conclusions to be drawn based on these values (Hopkins et al., 2001).

The reliability of sprint performance is a concept that provides a numerical reference to the variability in sprint times, indicating how much these values differ between trials, protocols and testing procedures (Hopkins, 2000). As previously stated, the reliability of this performance is important in determining the degree of influence that interventions or training strategies may have. Acute interventions such as post-activation potentiation (Matthews et al., 2004; Smith, 2012), ergogenic aids (Forbes et al., 2007; Paton et al., 2010) and variations in equipment (Squadrone and Gallozzi, 2009), have been shown to improve performance; with longitudinal training strategies also contributing to improvements in this area (Cronin and Hansen, 2006; Kotzamanidis, 2006). The influence of these interventions is determined by the change they induce in the measurements. For a worthwhile change in performance to be attributed to an intervention, the change incurred must be greater than the typical variance in the measured performance (Pearson et al., 2007). If the reliability of sprint performance is low (high variability), then it is less likely that any changes seen can be attributed to the inclusion of an intervention. The problem with sprint performance is that it is a product of many other contributing factors such as acceleration, top speed and deceleration characteristics (Cronin and Hansen, 2006; Delecluse, 1997), which are themselves influenced by each individuals physical sprinting mechanics (Hunter et al., 2004a). These mechanics play a large role in raw sprint performance and therefore need to be recognised in order to understand why changes may occur with the inclusion of intervention strategies or training programmes.

Sprint mechanics refers to the kinetic and kinematic variables associated with human running movements (Morin et al., 2012). The interaction of these variables ultimately determines sprint velocity and therefore sprint performance, making them very important when identifying variations in sprint times (Hunter et al., 2004a). The kinetics of sprinting are typically measured via force platforms which are expensive and burdensome as they are often fixed in place. This makes them unpractical for your typical track coach, or team sport strength and conditioner, who don't have access to a research facility. Kinetic determinants are very important to sprint performance, as measures such as maximal power, average power and horizontal and vertical ground reaction forces have been shown to have correlations of 0.59 - 0.87 with maximal speed (Hunter et al., 2004a). Due to this correlation with maximal speed, it is important to acknowledge the relationships between kinetic determinants and kinematic variables associated with sprinting, because many of these kinematic variables have an influential relationship with sprint kinetics, and therefore may play an important role in maximal speed also (Hunter et al., 2004a). There are numerous studies investigating the kinematic determinants of sprint performance, with common trends witnessed throughout. Murphy et al., (2003), compared kinematic variables between a 'fast' group and a 'slow' group of individuals, differentiated by horizontal velocity measures. The 'fast' group displayed significantly shorter (11-13%) ground contact times (p

To this author's knowledge, information on the reliability of sprint kinematics is scarce. Two articles by Hunter et al., (2004b), and Salo et al., (1996), report reliability for several of the key sprint variables, as outlined by Hunter et al., (2004a). These measurements were taken during late acceleration (16m), and nearing max speed, whereas many interventions aim to improve short sprint performance due to its importance in a larger range of sports (Cronin and Hansen, 2006; Randell et al., 2010).

The primary aim of this study was to investigate the reliability of sprint acceleration performance and the reliability of the key kinematic determinants involved during the first three steps of the sprint. The secondary aim was to utilise a practical method of kinematic analysis to help explain why changes may occur in sprint performance via the use of correlative statistics and to provide reference values for intervention research to make conclusions about their change scores.

It is hypothesised that short sprint performance will provide reliability scores which indicate a low level of variability, with the kinematic variables having a lower level of reliability in comparison to performance. Third step frequency and step length will show small levels of association with sprint performance, with variables such as stance time and flight time having a lesser level of association.

Methods

Study design

This study utilised a test-retest design to determine intersession reliability of both short sprint performance and the kinematic variables, where each change score was measured in respect to each individual over two sessions (Hopkins, 2008). These two sessions were separated by a minimum of 48 hours, with the participants not partaking in any other form of moderate to high intensity exercise between sessions. A cross-sectional design was performed utilising Pearson correlation analyses to identify the associations between 5m sprint times and third step sprint kinematics. Cronin et al., (2007), and Frost et al., (2008), were studies that established significant reliability over a 5m distance; therefore this distance was also chosen for this study so comparisons can be made. The third step was chosen to allow comparable data with other kinematic based research (Maulder et al., 2008; Moir et al., 2007; Salo, 2005).

Participants

A total of 10 physically active individuals (mean [+ or -] SD: age 22.4 [+ or -] 3.4 yrs; height 1.80 [+ or -] 0.06 m; weight 87.3 [+ or -] 11.8 kg; training years 3.4 [+ or -] 2.7 yrs) volunteered for this study, all recruited through various sporting communities. Each individual was required to be involved in a current training regime (minimum of three trainings per week for at least six weeks prior to testing) in a sport that contained a running component of maximal or near maximal intensity. Participants were within the age of 17 and 27 and had a minimum of one year resistance training experience with no injuries within the two months prior to testing (Chiu et al., 2003). Before being included in the study, protocol information and participant requirements were provided and explained to the volunteers, before medical questionnaires and consent forms were issued and signed. Ethical approval was granted for all procedures from the institutes' ethics committee.

Experimental procedures

Subjects were required to attend two separate testing sessions, each involving familiarisation and data collection. Participants were requested to wear their preferred style of training shoe and short sleeved/legged training attire. The initial testing session began with individual height and weight taken (shoes off). Testing sessions commenced with a 15min self-selected warm-up, typically consisting of cycling, jogging, dynamic stretching and acceleration based run-outs. Familiarisation followed with a verbal and visual demonstration of the required sprint start position. This consisted of a standing split stance with their preferred foot placed on the starting line. They were to begin each trial with a forward movement of the torso, as opposed to a...

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