Unilateral High-load Training Does Not Augment Strength In The Contralateral Arm Undergoing The Same Training

Unilateral resistance training improves strength of the trained limb and the contralateral untrained limb via the cross-education effect. Recent work found that training one limb with high loads augmented the strength changes of the contralateral limb training with low loads via the cross-education effect. However, it is unknown whether this would also occur when the contralateral limb is undergoing the same high-load training. PURPOSE: To determine if the cross-education effect of unilateral high-load training can augment the strength changes in the contralateral arm that is also engaging in the same high-load training. METHODS: 160 participants were randomized to one of four intervention groups: 1) training on both the dominant and non-dominant arm (D + ND, n = 38), 2) training on the dominant arm only (D-Only, n = 40), 3) training on the non-dominant arm only (ND-Only, n = 39), and 4) a time-matched non-exercise control (CON, n = 43). All exercise groups performed 18 sessions of unilateral elbow flexion exercise over six weeks. Training always started with one-repetition maximum (1RM) test (≤ five attempts), followed by four sets of high-load exercise at an 8-12RM. The main outcome variable was the 1RM strength change in the non-dominant arm. Data are represented as mean and 95% credible intervals. RESULTS: Our results found that the cross-education effect was not additive to a limb training with high loads (posterior probability of 0.76). The D + ND [∆ 2.6 (2.2, 3) kg] and ND-Only [∆ 2.6 (2.2, 2.9) kg] groups both observed changes in strength over CON [∆ -0.2 (-0.5, 0.1) kg]. Notably, the D-Only group produced greater changes in strength in the non-dominant arm [∆ 1.5 (1.1, 1.8) kg] compared to CON but this was less than that observed from D + ND and ND-Only groups. CONCLUSIONS: Unilateral high-load training has repeatedly been shown to result in a cross-education effect. This cross-education effect has also been shown to explain the strength changes in a contralateral arm undergoing low-load training. However, the current study found that this cross-education effect was not able to augment strength when the contralateral arm performed high-load training. These results should not necessarily be interpreted that within-subject models are methodologically sound to study strength change if both limbs are trained with high loads.