Author(s): HSU, F., YU, C., HSIEH, C., CHEN, J., FU, S., Institution: TAIPEI CITY HOSPITAL-YANGMING BRANCH, Country: TAIWAN, Abstract-ID: 695

INTRODUCTION:
Unaccustomed high-intensity eccentric contractions typically induce exercise-induced muscle damage (EIMD) in sedentary individuals. The Repeated Bout Effect (RBE) describes a protective adaptation where a prior bout attenuates damage from a subsequent bout. While ipsilateral and homologous contralateral RBE (cross-education) are well-documented, evidence regarding heterologous contralateral RBE (e.g., lower limb training protecting upper limb) remains limited. This study aimed to investigate whether an initial maximal eccentric exercise (MaxEc) in the lower limbs could reduce muscle fatigue and enhance performance in the contralateral upper limbs during a subsequent bout two weeks later.
METHODS:
Twenty-four sedentary females (18–30 years) were randomized into an experimental group (KE-CEF, n=12) and a control group (CEF, n=12). The KE-CEF group performed MaxEc on non-dominant knee extensors (60 reps), followed 14 days later by MaxEc on non-dominant elbow flexors (30 reps), while the CEF group performed only the elbow flexor MaxEc. Dependent variables including Maximal Voluntary Isometric Contraction (MVC) and muscle stiffness via Acoustic Radiation Force Impulse (ARFI) were assessed at baseline, immediately post-exercise (D0), and daily for 5 days (D1-D5).
RESULTS:
The KE-CEF group demonstrated significantly superior recovery compared to the CEF group across multiple indices. For muscle strength, the KE-CEF group exhibited greater MVC at D0 (20.26 ± 5.84 N·m/kg) compared to the CEF group (16.26 ± 4.11 N·m/kg, p = 0.029). Regarding concentric strength, the KE-CEF group maintained stability with no significant decline from baseline (26.05 ± 5.38 N·m/kg) across all time points (p > 0.05), whereas the CEF group significantly decreased at D0 (11.05 ± 4.45 N·m/kg) compared to baseline (18.29 ± 4.88 N·m/kg, p < 0.0001). In terms of muscle stiffness (ARFI), the CEF group showed a significant increase at D1 (1.14 ± 0.06 m/s) compared to baseline (0.96 ± 0.03 m/s, p = 0.001), while the KE-CEF group stabilized quickly after D0.
CONCLUSION:
This study confirms a "heterologous contralateral repeated bout effect," where lower limb training protects the upper limb. This aligns with recent findings that eccentric conditioning of knee extensors enhances elbow flexor recovery, suggesting systemic adaptations beyond local tissues [1]. The preserved strength supports the concept of skeletal muscle "cellular memory" and cross-adaptation, where initial stress activates broad protective pathways benefiting remote muscles [2]. Furthermore, the superior functional stability observed is consistent with evidence that contralateral protection is driven by central neural adaptations rather than peripheral changes, optimizing motor unit recruitment [3]. Clinically, these findings advocate for heterologous contralateral training as a viable strategy to preserve neuromuscular function in immobilized or injured upper limbs.