Unilateral High-Load Resistance Training Increases Absolute but Not Relative Muscular Endurance in the Contralateral Untrained Limb

To investigate whether unilateral resistance training can enhance absolute and relative muscular endurance in a trained arm and an untrained arm (cross-education of muscular endurance), and if this is mediated by increases in strength. 160 participants were randomly assigned to one of four groups: 1) training on the dominant arm followed by non-dominant arm (D + ND), 2) training on the dominant arm (D-Only), 3) training on the non-dominant arm (ND-Only), and 4) a time-matched non-exercise control (Control). Training involved 6-week (x3/week) of elbow flexion exercise. Each training session started with 1RM training (≤five attempts), followed by four sets of high-load exercise (8-12RM). Changes in muscular endurance and 1RM strength of the non-dominant arm were compared among groups. Mediation analysis was used to investigate the influence of 1RM strength changes on muscular endurance. Changes in absolute muscular endurance of the non-dominant arm were greatest in D + ND (10.7 reps) and ND-Only (10.1 reps), while the changes were greater in D-Only (4.9 reps) when compared to Control (1.2 reps). Changes in relative muscular endurance of the non-dominant arm were only observed in D + ND (1.6 reps) and ND-Only (2.0 reps). Increased 1RM strength positively mediated the changes in absolute muscular endurance, whereas it negatively mediated the changes in relative muscular endurance. Unilateral high-load resistance training increased absolute muscular endurance in both a trained and untrained arm, likely driven by increased strength. In contrast, no cross-education of relative muscular endurance was observed.