THE PRESENCE OF BILATERAL IMBALANCE OF THE LOWER LIMBS IN ELITE YOUTH SOCCER PLAYERS OF DIFFERENT AGES STEPHEN J. ATKINS,IAN BENTLEY,HOWARD T. HURST,JONATHAN K. SINCLAIR, AND CHRISTOPHER HESKETH Centre for Applied Sport and Exercise Sciences, University of Central Lancashire, Preston, England, United Kingdom ABSTRACT Atkins, S, Bentley, I, Hurst, H, Sinclair, J, and Hesketh, C. The presence of bilateral imbalance of the lower limbs in elite youth soccer players of different ages. J Strength Cond Res 30(4): 1007–1013, 2016—The aim of this study was to examine bilat- eral differences in ground reaction forces (GRF), measured during a deep squat (DS) exercise, in a population of elite youth soccer players. Bilateral muscle balance is a key component in promoting musculoskeletal health of performers, yet there is a lim- ited evidence base investigating such imbalances in youth. Seventy-four subjects were assigned to performance groups ac- cording to chronological age (younger than 13, 14, 15, 16, 17 years). Analysis of physical maturity status revealed that very few players were classified as “early” or “late” maturers. Players com- pleted an overhead DS exercise, as part of preseason functional movement screening. Peak GRF were assessed using a twin force plate system. Significant differences (p # 0.05) were iden- tified between right and left side peak GRF for all groups except the youngest (U13) and oldest (U17). Nondominant “sides” showed the highest levels of PGRF across all groups. The mag- nitude of PGRF was not significantly different both within and between groups, except for the left side in the U13 to U15 groups (p = 0.04). Results from this study show that performance asymmetry is marked in adolescence. There seems a “trigger point” during the early stage of adolescence, when bilateral im- balances become marked. These differences do seem to reduce during the later stages of adolescence. Correct attention to focussed training, designed to remediate any imbalance, is war- ranted in adolescent groups. This is important with respect of the key associations between bilateral asymmetry and risk of injury. KEY WORDS adolescence, ground reaction forces, squat, bilateral balance, asymmetry INTRODUCTION B ilateral movements, evidencing symmetry and balance within the human, are the foundation of many simple and complex human movements. In the performance context, it is important to maintain symmetrical balance between the right and left sides. Fluctuating bilateral asymmetry is reportedly common in humans (50,52). Asymmetry across the body can be linked to increased prevalence of injury and impaired sports perfor- mance (3,4,11,20,39,42,43,52). The unilateral demands of sporting performance have been linked with mechanical adap- tations (27). A bilateral imbalance in lower body strength, whether in accordance with handedness, preferred foot, or side dominance measured by ground reaction forces (GRF), have been identified in adults (15,26,28,36). Such imbalances may affect performance and also increase the incidence of injury (2,11,12,36,43). Although the evidence supporting such imbalances is somewhat compelling in adult populations, there is a marked dearth of research investigating such imbalances in youth. Additionally, the contribution of both chronological and biological age has received limited attention in functional performance settings, particularly when considering human movement symmetry. The screening of functional movement has become an integral part of both preseason and inseason assessment. At youth levels, such assessments are regularly undertaken to assess both functional capabilities and the potential for muscu- loskeletal injury. Functional movement is classified as the ability to produce and maintain balance between mobility and stability through the kinetic chain (32,38). Such balance should ensure that fundamental movement patterns are performed with utmost accuracy and efficiency (32). The assessment of func- tional performance can provide an useful insight into the strength, flexibility, balance, and coordination necessary for a variety of movements (32). This may also provide a mecha- nism to assess bilateral imbalances in the upper and lower body. Although quantitative analysis of functional movement is often the preserve of laboratory-based testing, advances have been made with regard to effective field-based methods of assessment. Recently, test batteries such as the Functional Movement Screen (FMS) have been shown to be reliable There are no external funding streams associated with this research. Address correspondence to Dr. Stephen Atkins, satkins@uclan.ac.uk. 30(4)/1007–1013 Journal of Strength and Conditioning Research Ó 2013 National Strength and Conditioning Association VOLUME 30 | NUMBER 4 | APRIL 2016 | 1007 Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.