Footballers require a high level of technical expertise and tactical awareness to successfully compete at the professional level, operating within a dynamic, fast-moving and volatile environment (Young et al., 2005; 2010). Their ability to produce a technically proficient performance relies upon a suitable foundation of athletic conditioning and muscular strength, often acquired through-out the systematic developmental pathways of their sport (Hoshikawa et al., 2009; Iga et al., 2009). However, in the absence of sufficient physical development, the differential loading patterns inherent within football sports, under training and competitive contexts, may produce or exacerbate strength imbalances within and between the lower limbs (Kubo et al., 2010; Newton et al., 2006), potentially facilitating or diminishing performance outcomes.
In football sports, kicking is the most important and widely used skill, routinely employed to deliver a ball accurately, over a desired distance, to an intended target or location, under a variety of different situational contexts (Ball, 2011; Young and Rath, 2011). However, due to the volatile nature of competitive play, footballers rarely engage their lower limbs with equal preference within the tactical realm of their sport, selectively utilising the dominant limb for most game-based activities (Ball, 2011; Zakas, 2006). As the kicking skill places considerably different demands on the kicking and support limbs during task execution (Baczkowski et al., 2006; Hides et al., 2010; Nunome et al., 2006; Young and Rath, 2011), the regularity of kicking performance may uniquely and specifically develop each limb preferentially for kicking and support purposes (Gstottner et al., 2009; Hart et al., 2013; Hoshikawa et al., 2009; Kearns et al., 2001; Newton et al., 2006; Sadeghi et al., 2000; Stewart et al., 2010).
Previous research has attempted to identify whether a laterality effect exists within football, specifically profiling footballers at various stages of the development cycle, spanning the full junior-to-senior and recreational-to-professional athletic spectrum (Hides et al., 2010; Hoshikawa et al., 2009; Iga et al., 2009; Kearns et al., 2001; Kubo et al., 2010; Masuda et al., 2003; 2005; Rahnama et al., 2005; Stewart et al., 2010; Thorborg et al., 2011; Veale et al., 2010; Zakas, 2006). However, a lack of consensus remains, with contrasting and contradictory outcomes evident across the research landscape. Given that some senior, elite footballers can have a visible limb imbalance (Hides et al., 2010; Stewart et al., 2010; Thorborg et al., 2011), while others can have no limb imbalance (Kubo et al., 2010; Hoshikawa et al., 2009; Zakas, 2006); there appears to be no definitive understanding as to whether the potential asymmetry of strength and muscularity in the lower limbs is favourable to performance, or undesirable to injury incidence (Hart et al., 2013; Newton et al., 2006). Such discrepancies might be explained by the uncontrollable influence of different strength and conditioning programs underpinning the physical development of each cohort of athletes, in addition to the notable limitations inherent within common research methodologies and assessment modalities employed to measure the aforementioned strength and muscularity profiles (Kubo et al., 2010; Newton et al., 2006; Veale et al., 2010; Zakas, 2006).
While earlier investigations have endeavoured, with limited success, to match strength and muscularity profiles of athletes to their stage of development or level of profession, no studies have yet attempted to describe these profiles, or assess lower limb laterality within athletes of various technical competencies. As kicking accu racy is critically important in football sports (Dichiera et al., 2006; Hart et al., 2013; Young et al., 2010), it seems reasonable to investigate whether lower limb symmetry or lateral dominance enhances or influences technical proficiency of the kicking skill. It is therefore the purpose of this study to assess the unilateral and bilateral leg strength and leg mass characteristics of the kicking and support limbs within accurate and inaccurate kickers.
Thirty-one sub-elite Australian footballers (age: 22.1 [+ or -] 2.8 yrs; mass: 85.1 [+ or -] 13.0 kg; height: 1.81 [+ or -] 0.07 m) were recruited from the Western Australian Football League (WAFL). All athletes were competing in the highest state-level grade, within the same football team and same developmental zone; and therefore received a similar prescription of kicking practice during structured training and skills sessions for at-least five years prior to the current study, with two consecutive years of experience at their current playing level. Athletes were free from injury at the time of testing; and were not permitted to perform any strenuous exercise or lower body resistance training within 48 hours of their assigned testing session. All athletes were notified of the potential risks involved, and provided written informed consent for participation. Ethics approval was provided by Edith Cowan University's Human Research Ethics Committee.
This study utilised an acute, between groups, cross-sectional design consisting of a single two hour testing session. Testing sessions commenced with anthropometric measures including height and weight, followed by an assessment of whole-body composition and lower body segmental mass characteristics (lean, fat, total) using Dual-energy X-ray Absorptiometry (DXA). Athletes were taken through a standardised general dynamic warm-up spanning ten minutes in duration, prior to a series of lower limb isometric strength measures (unilateral and bilateral). Following this, a mechanically specific warm-up was provided (kicking over variable distances), in order to stabilise kicking performance. The kicking protocol (drop punt over 20 metres) was then completed in the biomechanics laboratory. Subjects were required to wear their club issued football shorts, and were provided with indoor football shoes (Nike5 Bomba, Nike Inc, USA) for use during the testing session. All athletes were thoroughly familiarised with all testing procedures prior to the commencement of testing.
Following the assessment and analysis of kicking performance, subjects were subsequently assigned to accurate (n = 15; age: 22 [+ or -] 3 yrs; height: 1.82 [+ or -] 0.07 m; weight: 81 [+ or -] 8 kg; body fat: 11 [+ or -] 2%; kicking efficiency: 91 [+ or -] 11%) and inaccurate (n = 16; age: 23 [+ or -] 2 yrs; height: 1.81 [+ or -] 0.07 m; weight: 89 [+ or -] 15 kg; body fat: 17 [+ or -] 5%; kicking efficiency: 58 [+ or -] 16%) groups for analysis in accordance with previously established accuracy determination criteria (Hart et al, 2013). Specifically, each kick was scored 1 (accurate), 2 (moderate), and 3 (inaccurate), with all ten kicks totalled (Table 1). Athletes who scored between 10-18 points were classified as accurate, whereas athletes who scored between 19-30 points were classified as inaccurate.
Stature was recorded to the nearest 0.1 centimeter using a wall-mounted stadiometer (Model 222, Seca, Hamburg, DE), with body weight recorded to the nearest 0.1 kilogram using a standard electronic weighing scale (AE Adams CPW Plus-200, Adam Equipment Inc., CT, USA). Stature was measured three times for each subject by the same accredited exercise scientist (ESSA; Exercise & Sport Science Australia), with the average of the three trials retained for analysis (CV [less than or equal to] 0.3%; ICC [greater than or equal to] 0.994).
Unilateral and bilateral lower body strength assessments were performed on both lower limbs using an isometric back squat exercise at pre-set hip and knee joint angles of 140[degrees] (Figure 1). An isometric strength test was chosen in order to safely examine total strength capacity of the lower limbs under multi-joint unilateral and bilateral conditions while also minimising neuromuscular fatigue prior to the performance of the kicking protocol. Joint angles were assessed manually using a handheld goniometer, and were chosen on the basis of positional specificity, to best correspond with maximal force production at zero velocity (Haff et al., 2005; Nuzzo et al., 2008). Particular details concerning subject positioning and exercise instruction for bilateral and unilateral isometric conditions have been previously described and illustrated with very high between-trial reliability established for bilateral (CV [less than or equal to] 3.6%; ICC [greater than or equal to] 0.973) and unilateral (CV [less than or equal to] 4.7%; ICC [greater than or equal to] 0.961) maximal strength assessments (Hart et al., 2012).
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Athletes were provided with sub-maximal familiarisation (10 minutes duration interspersed with recovery) of all three trial types [Bilateral, BL; Unilateral (Kicking), KL; Unilateral (Support), SL], prior to performing a total of nine lower body maximal isometric contractions: three bilateral and six unilateral (kicking and support limb) trials. All subjects identified their right leg as the kicking limb and left leg as the support limb. The order of testing was randomised and counterbalanced to negate the effects of muscular potentiation or fatigue (Rassier and MacIntosh, 2000). Subjects maintained maximal contractions for a period of five seconds, and were provided with two minutes passive recovery between each maximal effort. The investigator, and two research assistants provided verbal encouragement, and visually monitored athlete technique during each trial to ensure postural and mechanical compensatory adjustments did not occur; and ensured athlete safety was maintained.
The best trial for each isometric condition was chosen for analysis, identified by the greatest peak force output. Absolute strength (bilateral and unilateral) was represented...