Acute neuromuscular and metabolic responses to a novel low-load combined isometric and isotonic resistance training protocol

Abstract Purpose: To investigate the acute physiological responses to a novel combined resistance training (CB) protocol—integrating isometric pre-exhaustion with low-load, high-velocity, short-arc isotonic contractions—compared with traditional low-intensity slow training (LST) and high-load (HL) training. Methods: Forty-eight healthy men were randomized into three groups: LST (50% one-repetition maximum [1RM], slow movement), HL (80% 1RM, normal speed), or CB (40% 1RM; a 20-s isometric contraction immediately followed by eight high-velocity, short-arc repetitions). Surface electromyography (EMG) and near-infrared spectroscopy (NIRS) of the vastus lateralis, alongside serum IGF-1 concentrations, were evaluated during and after three leg-press sets. Results: Despite a 40% 1RM load, CB elicited mean EMG amplitudes similar to HL and significantly higher than LST in the final set (p < 0.05). NIRS revealed significantly lower tissue oxygen saturation in CB compared to HL from the first set (p < 0.05), indicating rapid intramuscular hypoxia. Furthermore, third-set integrated EMG was significantly higher in CB than in other groups (p < 0.05). Serum IGF-1 levels increased similarly post-exercise across all groups. Conclusion: By simultaneously imposing high neural demands and profound metabolic stress, the time-efficient CB protocol provides a robust physiological stimulus despite very low mechanical loads. This presents a practical, joint-friendly alternative for populations requiring minimized mechanical stress. Trial registration number: UMIN-CTR (UMIN000060486)