The purpose of this dissertation was to examine the existence of force production and strength symmetry while concentrating on its role with performance. Specifically, in this dissertation I sought to determine if differences in symmetry exist between male and female athletes as well as between athletes of different strength levels. Various symmetry assessments were used and the amount of carryover between tests was also evaluated. Furthermore, the relationship between asymmetry magnitude and various aspects of jump performance was also assessed. The results showed that female athletes were more asymmetrical than their male counterparts for most assessments. Interestingly, weaker athletes regardless of gender were more asymmetrical than stronger athletes for all assessments. Symmetry characteristics appeared to carryover between all tests and conditions with the exception of the bilateral strength assessment. When separated by strength level, good carryover of explosive strength symmetry characteristics was noted. A trend of negative relationships was noted between jump performance and kinetic asymmetry, measured during jumps and weight distribution. Isometric mid-thigh pull force production asymmetry did not relate to jump performance as it had previously. Kinetic asymmetry was shown to influence the direction of center of mass displacement (COMd). Specifically, asymmetrical rate of force development showed the most ability to differentiate between values of COMd in the mediolateral direction. The finding that female athletes are more likely to produce force asymmetrically may indicate that they are more susceptible to the performance detriments that accompany force production asymmetry. This may also indicate that female athletes are more susceptible to injury if force asymmetry is an injury predictor. However, differences in asymmetry magnitude between the sexes may be due to differences in strength level between the sexes in the current study, as weaker athletes were shown to be more asymmetrical than their stronger counterparts. Thus, it may be inadequate strength level that is a risk factor to performance detriments and possible injury. It also appears that force production asymmetry can both alter and hinder optimal jump performance. In order to optimize performance and possibly reduce injury risk, it is recommended that weaker athletes focus on bilateral strengthening exercises.

Carry-over of force production symmetry in athletes of differing strength levels. J Strength Cond Res 29(11): 3188–3196, 2015—This study sought to determine the level of association between bilateral force production symmetry assessment methods (standing weight distribution [WtD], unloaded and lightly loaded jumps, and isometric strength) and to determine whether the amount of symmetry carry-over between these tasks differs for strong and weak athletes. Subjects for this study included male (n = 31) and female (n = 32) athletes from National Collegiate Athletic Association Division I sports. Athletes performed WtD, unloaded and lightly loaded (20 kg) static and countermovement jumps, and isometric midthigh pull (IMTP) assessments on 2 adjacent force plates. Ground reaction force data were used to calculate symmetry variables and performance-related variables. Using Pearson zero order correlations, evaluations of the amount of symmetry carry-over were made. Weight distribution correlated strongly with jump peak force (PF) (r = 0.628–0.664). Strong relationships were also observed between loading conditions for jump variables (r = 0.568–0.957) as were the relationships between jump types for PF, peak power, and net impulse (r = 0.506–0.834). Based on the pooled sample, there was a lack of association between IMTP and WtD for jump symmetry variables. However, when examining strong and weak groups, rate of force development showed moderate to strong symmetry carry-over in the strongest athletes (r = 0.416–0.589). Stronger athletes appear to display similar explosive strength symmetry characteristics in dynamic and isometric assessments, unlike weaker athletes. Strength seems to influence the amount of force production symmetry carry-over between bilateral assessments. There may be optimal loads and variables for symmetry assessment, but these may differ based on population characteristics.

The purpose of this investigation was to determine the existence of bilateral strength and force-production asymmetry and evaluate possible differences based on sex, as well as strength level. Asymmetry was assessed during weight-distribution (WtD) testing, unloaded and lightly loaded static- (SJ) and countermovement-jump (CMJ) testing, and isometric midthigh-pull (IMTP) strength testing. Subjects included 63 athletes (31 male, 32 female) for WtD, SJ, and CMJ tests, while 129 athletes (64 male, 65 female) participated in IMTP testing. Independent-samples t tests were used to determine possible differences in asymmetry magnitude between males and females, as well as between strong and weak athletes. Cohen d effect-size (ES) estimates were also used to estimate difference magnitudes. Statistically different asymmetry levels with moderate to strong ESs were seen between males and females in WtD, 0-kg SJ (peak force [PF]), 20-kg SJ (peak power [PP]), 0-kg CMJ (PF, PP, net impulse), and 20-kg CMJ (PF), but no statistical differences were observed in IMTP variables. Dividing the sample into strong and weak groups produced statistically significant differences with strong ES estimates in IMTP PF and rate of force development, and many ESs in jump symmetry variables increased. The results of this investigation indicate that females may be more prone to producing forces asymmetrically than males during WtD and jumping tasks. Similarly, weaker athletes displayed more asymmetry than stronger athletes. This may indicate that absolute strength may play a larger role in influencing asymmetry magnitude than sex.