Ipsilateral Repeated Bout Effect Across Heterologous Muscle Groups: Eccentric Knee Extensor Conditioning Enhances Elbow Flexor Recovery in Young Women

This study investigated whether prior eccentric exercise of knee extensors could attenuate muscle damage in ipsilateral elbow flexors, supporting the presence of an ipsilateral repeated bout effect (IL-RBE) across heterogeneous muscle groups. Sixteen young women were randomized into an intervention group (NL/NU) and a control group (C/NU). The NL/NU group performed eccentric knee extensor exercise 14 days before elbow flexor eccentric loading. Compared to the C/NU group, the NL/NU group exhibited an earlier return to baseline in muscle stiffness (D3: NL/NU = 1.14 ± 0.05 vs. Pre = 0.96 ± 0.03 m/s), joint release angle at 30° (D3: NL/NU = 22.79 ± 1.02 vs. Pre = 24.46 ± 0.87°), and joint release angle at 45° (D2: NL/NU = 37.75 ± 1.38 vs. Pre = 38.83 ± 0.87°), indicating a faster recovery trend in these specific neuromuscular and morphological measures. These results suggest that prior remote eccentric loading induces systemic and neuromuscular adaptations, facilitating improved functional recovery. The findings expand IL-RBE applicability to heterologous muscles within the same limb and support its integration into training and rehabilitation protocols.