Cross-education Increases Absolute But Not Relative Muscular Endurance In The Untrained Limb

High-load resistance training in one limb induces strength adaptations not only in the trained limb but also in the opposite untrained limb (i.e., cross-education of strength). Whether high-load training can increase muscular endurance in the untrained limb is less understood. It is also unknown if the cross-education effect can be additive to the training effect when both limbs are trained. PURPOSE: To examine the cross-education of muscular endurance with unilateral high-load resistance training and determine if it differs from training both limbs. METHODS: 160 participants (18-35 years) were randomly assigned to one of four groups: 1) high-load training in the dominant arm (D, n = 40), 2) high-load training in the non-dominant arm (ND, n = 39), 3) high-load training in both arms (D + ND, n = 38), and 4) a time-matched, non-exercise control group (CON, n = 43). All exercise groups completed 18 training sessions across 6 weeks. High-load training consisted of a one-repetition maximum (1RM) test (up to five attempts), followed by four sets at an 8-12 RM. The D + ND group started the training sessions with the dominant arm. Both absolute (i.e., 60% pre-testing 1RM) and relative (i.e., 60% pre- and post-testing 1RM) muscular endurance tests were performed only in the non-dominant arm. Data are presented as mean and 95% credible intervals. RESULTS: We found evidence that training the dominant arm increased absolute endurance (posterior probability of 0.70) in the untrained non-dominant arm [∆4.9 (3.2, 6.6) rep]. The change was greatest in the D + ND [∆10.6 (8.8, 12.4) rep] and ND [∆10.0 (8.3, 11.8) rep] but those groups were not different from each other. All were greater than CON [∆1.2 (-0.4, 2.8) rep]. Relative endurance increased in the D + ND (∆1.5 rep) and ND (∆1.9 rep) groups (posterior probability of 0.74), but there was no evidence of a cross-education in relative endurance in the D group [∆-0.3 rep]. Both the D + ND and ND groups were greater than CON (∆1.2 rep). CONCLUSIONS: A notable finding was that the change in absolute endurance in the untrained arm was about half of what individuals achieved with high-load resistance training. This may be attributed to increased strength in the untrained arm. The cross-education effect was not additive to the training effect when both arms were trained.