Effects of elbow angle and forearm rotation on elbow flexion torque and elbow-flexor surface electromyographic amplitudes

Background The coordination of elbow flexor muscles during multi-joint tasks is influenced by joint configuration. However, the combined effects of elbow angle and forearm rotation on maximal isometric torque and corresponding muscle activation are not fully elucidated. We hypothesized a priori that elbow angle and forearm position would each affect torque and the recorded surface electromyography (sEMG) amplitudes of the biceps brachii (BB) and brachioradialis (BR), and that these factors would interact. Methods Forty healthy young adults performed maximal voluntary isometric elbow flexions across 12 posture combinations comprising four elbow angles (60°, 75°, 90°, and 105°) and three forearm positions (pronation, neutral, and supination). Peak torque (PT), average torque (AT), and BB and BR sEMG root mean square (RMS) amplitudes were recorded. Outcomes were analyzed using heteroscedastic linear mixed-effects models with participant included as a random effect; RMS outcomes and the BR-to-BB RMS ratio were natural-log transformed. Marginal F tests were followed by model-based two-sided t tests of estimated marginal means. Holm-adjusted p values, Bonferroni simultaneous 95% confidence intervals for pairwise contrasts, and approximate partial eta squared (η² p ) with 95% confidence intervals were reported. Results PT and AT showed similar effects of elbow angle (η² p = 0.143, 95% CI [0.067, 0.224], and η² p = 0.141, 95% CI [0.065, 0.221], respectively), forearm rotation (η² p = 0.148, 95% CI [0.073, 0.231], and η² p = 0.146, 95% CI [0.071, 0.229]), and their interaction (η² p = 0.055, 95% CI [0.005, 0.110], and η² p = 0.059, 95% CI [0.006, 0.115]; all adjusted p < 0.001). Torque at 60° was generally lower than at 75°–105°. At each angle, both torque measures were highest in neutral, intermediate in supination, and lowest in pronation. Forearm rotation affected BB and BR RMS (η² p = 0.324, 95% CI [0.226, 0.414], and η² p = 0.236, 95% CI [0.143, 0.327], respectively). BB RMS in pronation was lower than in neutral and supination. BB RMS showed no angle effect, whereas BR RMS showed effects of angle (η² p = 0.036, 95% CI [0.001, 0.093]) and the angle-by-rotation interaction (η² p = 0.093, 95% CI [0.024, 0.164]). The BR-to-BB RMS ratio showed a forearm-rotation effect (η² p = 0.070, 95% CI [0.017, 0.143]) and an angle-by-rotation interaction (η² p = 0.045, 95% CI [0.000, 0.099]). The ratio was higher in pronation at 60° and 75°, and at 90° relative to neutral, whereas no rotation differences were detected at 105°. Conclusion Maximal voluntary isometric elbow-flexion torque varied with elbow angle and forearm rotation, including angle-by-rotation interactions, whereas BB and BR sEMG RMS amplitudes showed muscle- and posture-specific responses. The findings across 75°–105° were consistent with a broad high-torque region rather than a uniquely optimal angle. The non-parallel torque and RMS patterns may partly reflect unmeasured force–length and moment-arm effects. The BR-to-BB RMS ratio indicates only a posture-dependent change in the relative recorded amplitudes. These findings characterize posture-dependent responses in healthy young adults but do not establish optimal clinical or training positions.