Investigation of Inter-Limb Symmetry in Knee Extensors Using Different Strength Outcome Measures

Muscle performance between contra-lateral knee extensors is most often assessed using maximal test for isometric/isokinetic torque evaluation. Recently, the rate of force development scaling factor (RFD-SF) has been used to evaluate neuromuscular capacity with a range of submaximal target peak torques, which could highlight other aspects of inter-limb (a)symmetry. The aim of our study was to investigate the differences, associations, and agreement between inter-limb symmetries of knee extensors using maximal torque (T_max) rate of torque development (RTD), slope of the RFD-SF regression line (k), and theoretical peak of RTD (TP_RTD). A total of 236 young, healthy athletes participated in the cross-sectional study. All participants performed unilateral knee extension (maximal voluntary contraction protocol and RFD-SF protocol) with both legs in the isometric knee dynamometer. Inter-limb symmetries were calculated for each outcome measure. Our results showed significant differences between all symmetry values (T_max (91.7%), RTD (85.2%), k (94.2%), TP_RTD (95.9%)). Significant strong correlations were found between symmetry values calculated from k and TP_RTD (r = 0.88, p < 0.001), while weak correlation was found between T_max and RTD (r = 0.17, p < 0.01. Fair agreement regarding leg dominance was found between T_max and RTD values. Our results suggest that inter-limb (a)symmetries are metric- and task-specific.