Association between physical fitness tests and neuromuscular properties.

Purpose: While various fitness tests have been developed to assess physical performances, it is unclear how these tests are affected by differences, such as, in morphological and neural factors. This study was aimed to investigate associations between individual differences in physical fitness tests and neuromuscular properties.
Methods: One hundred and thirty-three young adults participated in various general physical fitness tests and neuromuscular measurements. The appendicular skeletal muscle mass (ASM) was estimated by bioelectrical impedance analysis. Echo intensity (EI) was evaluated from the vastus lateralis. During submaximal knee extension force, high-density surface electromyography of the vastus lateralis was recorded and individual motor unit firings were detected. Y-intercept (i-MU) and slope (s-MU) from the regression line between the recruitment threshold and motor unit firing rate were calculated.
Results: Stepwise multiple regression analyses revealed that knee extension strength could be explained (adjusted R ²  = 0.712) by ASM (β = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R ²  = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R ²  = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R ²  = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R ²  = 0.631). On the other hand, i-MU and s-MU could be explained by five-sec stepping and counter-movement jump, respectively, but the coefficients of determination were low (adjusted R ²  = 0.100 and 0.045).
Conclusion: Generally developed physical fitness tests were mainly explained by morphological factors, but were weakly affected by neural factors involved in performance.
(© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)