Normalizing expressions of strength in elite rugby league players

Physical strength has often been expressed per kilogram of body mass. Research suggests that strength increases in proportion to a body mass raised to a power between 0.6 and 0.7 rather than the ratio held exponent of 1. The current study was designed to identify differences in the strength of elite-level rugby league players and to identify whether ratio (per kg(-1)) expressions would penalize heavier subjects. Fifty-four elite rugby league players were recruited to the study during the preseason. Subjects were classified according to their highest playing level. Players performed 3 maximal lifts, using a dynamometer, to determine leg strength. Body mass and muscle mass estimations were also recorded. Results showed that absolute expressions of strength revealed differences by playing level (p < 0.05). These differences were removed when a ratio scaling technique was applied (p > 0.05). Mass and muscle mass exponents of 0.62 and 0.63 were derived and applied to the strength data. Differences in strength by playing level were reestablished following this adjustment (p < 0.05). These findings emphasize that ratio (per kg(-1)) expressions, despite being commonly used, can penalize heavier athletes and mask differences in performance. Coaches and sports scientists should reconsider using the ratio expression due to its potential for error when describing physical strength.