Quantification of Inter-Limb Symmetries With Rate of Force Development and Relaxation Scaling Factor

The inter-limb (a)symmetries have been most often assessed with the tests that quantify the maximal muscle capacity. However, the rapid force production and relaxation during submaximal tasks is equally important for successful sports performance. This can be evaluated with an established rate of force development and relaxation scaling factor (RFD-SF/RFR-SF). The aims of our study were (1) to assess the intra-session reliability of shortened RFD-SF/RFR-SF protocol and its absolute and symmetry outcome measures, (2) to compare the main absolute RFD-SF/RFR-SF outcome measures (slopes of RFD-SF and RFR-SF: k_RTD-SF and k_RFR-SF, theoretical peak RFD/RFR: TP_RFD and TP_RFR) across gender and sports groups, and (3) to compare inter-limb symmetries across gender and sports groups for main outcome measures (k_RFD-SF, k_RFR-SF, TP_RFD, and TP_RFR). A cross-sectional study was conducted on a group of young health participants (basketball and tennis players, and students): 30 in the reliability study and 248 in the comparison study. Our results showed good to excellent relative and excellent absolute reliability for the selected absolute and symmetry outcome measures (k_RFD-SF, k_RFR-SF, TP_RFD, and TP_RFR). We found significantly higher absolute values for k_RFD-SF and TP_RFD in males compared to females for the preferred (k_RFD-SF: 9.1 ± 0.9 vs. 8.6 ± 0.9/s) and the non-preferred leg (k_RFD-SF: 9.1 ± 0.9 vs. 8.5 ± 0.8/s), while there was no effect of sport. Significantly lower symmetry values for k_RFR-SF (88.4 ± 8.6 vs. 90.4 ± 8.0%) and TP_RFR (90.9 ± 6.8 vs. 92.5 ± 6.0%) were found in males compared to females. Moreover, tennis players had significantly higher symmetry values for k_RFR-SF (91.1 ± 7.7%) and TP_RFR (93.1 ± 6.0%) compared to basketball players (k_RFR-SF: 88.4 ± 8.7% and TP_RFR: 90.9 ± 6.7%) and students (k_RFR-SF: 87.6 ± 8.7% and TP_RFR: 90.5 ± 6.7%). Our results suggest that the reduced RFD-SF/RFR-SF protocol is a valuable and useful tool for inter-limb (a)symmetry evaluation. Differences in symmetry values in k_RFR-SF and TP_RFR (relaxation phase) were found between different sports groups. These may be explained by different mechanisms underlying the muscle contraction and relaxation. We suggest that muscle contraction and relaxation should be assessed for in-depth inter-limb symmetry investigation.