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Biomechanical analysis of factors influencing agility performance

Introduction

Many sports require participants to change direction quickly and suddenly, in an efficient and accurate manner. This has been previously defined as ‘agility’ Johnson and Nelson (1969), Barrow and Mcgee (1971), cited in Sheppard and Young (2006). Currently there is no definitive agreed term for agility defined within the literature; the definition “a rapid whole-body movement with a change of velocity or direction in response to a stimulus” as proposed by Sheppard and Young (2006) encompasses all aspects of agility. In this assignment the decision making component in reaction to a stimulus will not be examined.

Successful performance of a task requiring ‘agility’ requires the execution of multiple technical skills combined with physical attributes which can be applied to biomechanical principles.

In team sports such as football, players are required to make multiple twists, turns, and cutting manoeuvres during a game. Withers et al. (1982) found that footballers perform an average of 49.9 turns per game each. Hawkins et al. (2001) found that 58% of all lower limb injuries sustained over two seasons were in non-contact situations, and twisting/ turning injuries accounted for a significant proportion of all injuries reported. A more recent study by Woods et al. (2003) found that of all ankle ligament injuries, 39% occurred during non-contact of which 77% of those injuries involved a mechanism of twisting, turning and landing.

So this information has clear implications relating to our study, superior agility can benefit performance which could directly contribute or influence the outcome of the game, and it can reduce the likelihood of lower limb injury incidence such as ACL and ankle ligament sprains (Hawkins et al., 2001; Woods et al., 2003).

For the purpose of analysis, a schematic technique movement analysis table was created (see appendices), (Bartlett, 1999; Graham-Smith and Pearson, 2005). The technical aspects of the agility turn were broken down into distinct phases in order  to aid successful performance analysis, any limiting physical attributes to technique could then be located when comparing subject test and normative data (see appendices), (Graham-Smith and Pearson, 2005).    

Physical attributes such as speed, strength and power are related to improved agility, interventions using plyometric drills and resistance training have been shown to increase agility performance (Kraemer et al., 2003; Kyrolainen et al., 2005). Holm et al. (2004) found that functional exercises with balance and proprioceptive aspects improved power and neuromuscular control.

The aims of this assignment are to evaluate physical and technical attributes associated with ‘agility’ and compare physical attributes of the subject with normative data in order to devise an appropriate intervention strategy to improve technique.

Methodology

Subject: 30 yr old male student volunteer. Subject screened for suitability by health questionnaire. Tests were carried out to measure the key aspects of agility. All tests were performed in the order described. The tests for balance, proprioception and power were all performed on a Kistler 9286AA portable force platform. Sampling frequencies and duration of sampling are as per stated for each variable.

Balance: ‘Target sway’ was measured by performing a static single leg balance test. The subject was instructed to place hands on hips whilst standing on their right leg only. Two 10 second measures were taken, sampling at 100 Hz. The mean result was used for analysis. Target sway can be defined as the ability of the subject to control their centre of mass thus improving stability. Studies by Wedderkopp et al. (1999), Myklebust et al. (2003), show balance training reduced the incidence of lower limb injuries.

Proprioception: The drop and hold test was used to assess the subjects’ time taken to achieve neuromuscular control (Holm et al., 2004). The subject was instructed to place hands on hips and then perform a drop landing hold test onto the right leg only (Fig 1).The drop landing was performed from a 28cm step which was offset 25cm back from the edge of the force platform, sampling taken for 4 seconds at 500 Hz (Fig 2). Two repetitions performed, the mean was taken.

Fig 1.        Drop landing test.                                 Fig 2. Time taken to achieve neuromuscular control.

Power: A bilateral countermovement jump was performed on the force platform. Subject was instructed to keep their hands on their hips then bend their knees into a squat position and then jump as fast and as high as possible. Sampling was for 4 seconds at 500 Hz, two repetitions performed, the mean was taken. (fig.3).

Subject was allowed to use the arms to aid drive and to help them explode into the jump. Two jumps performed, mean taken. This test measures the subjects’ maximal power production via the stretch-shorten cycle (Kyrolainen et al., 2005).

fig. 3 force platform data from counter movement jump.

Speed: Measured by the time taken to perform a 15 metre maximal sprint test. Subjects started the sprint half a metre behind the start line to allow for any forward lean. Three sprints were performed, mean result taken. Speed was measured with a Laveg speed gun type (LDM300C). Speed gun positioned 1.5 metres behind the start line sampling at 100 Hz. Although studies report a poor correlation between straight line sprint speed and agility, it can be rationalised that a certain amount of speed is needed for successful agility (Young, Hawkin and McDonald, 1996; Young, McDonald and Scarlett, 2001).

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Strength: ‘Peak torque’ measured from eccentric/concentric quadriceps/hamstrings using a Kincom isokinetic dynamometer (fig.4). The subjects’ right leg was used. Angular velocity was set at 60 degrees/sec. seat angle set at 15 degrees, start angle knee 90 degrees of knee flexion, the moment arm was 37 cm, and the subjects’ leg weighed 77N.

 The ability to generate and accept maximal forces have been suggested to be key components of agility (Newman, et. al., 2004).

Fig.4  Isokinetic strength testing using Kincom.

Technique: A 5 metre agility sprint was performed; the subject was instructed to plant their right leg straight at ...

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