Does unilateral high-load resistance training influence strength change in the contralateral arm also undergoing high-load training?

Training one limb with a high-load has been shown to augment strength changes in the opposite limb training with a low-load (via cross-education of strength), indicating that within-subject models can be problematic when investigating strength changes. This study examined if the cross-education of strength from unilateral high-load training could augment the strength changes in the opposite arm undergoing the same unilateral high-load training. 160 participants were randomized to one of four groups: (1) training on the dominant arm followed by the non-dominant arm (D?+?ND), (2) training on the dominant arm only (D-Only), (3) training on the non-dominant arm only (ND-Only), and (4) a non-exercise control. All exercise groups performed 18 sessions of unilateral high-load elbow flexion exercise over 6?weeks. Participants were compared for changes in 1RM strength and muscle thickness. Changes in strength of the non-dominant arm were greater in D?+?ND (2.7?kg) and ND-Only (2.6?kg) compared to D-Only (1.5?kg) and control (?0.2?kg), while the changes were greater in D-Only compared to control. The same finding was observed in the dominant arm. Only the arms being directly trained observed increases in muscle thickness. Unilateral high-load resistance training increased strength in the opposite untrained arm, without changes in muscle thickness. This cross-education of strength did not augment the strength changes in the contralateral arm undergoing the same unilateral high-load training. However, it does not necessarily indicate that within-subject models are methodologically sound to investigate strength change if both limbs are trained with a high-load.