Cross-education of the repeated bout effect after unilateral eccentric exercise with and without mirror visual feedback

Abstract

Previous research has demonstrated evidence of the cross education of the repeated bout effect (CL-RBE) to the untrained contralateral limb, however, it has been shown to be weaker in magnitude when compared to the ipsilateral limb. Mirror visual feedback or mirror therapy has been shown to potentially enhance the effectiveness of cross education, but this intervention has not been investigated when looking at the cross education of the repeated bout effect. PURPOSE: The purpose of this study was to investigate if mirror visual feedback could enhance the CL-RBE in the untrained contralateral limb after a single bout of unilateral eccentric exercise. METHODS: 28 participants (14 men & 14 women ages 18-35) were placed into 1 of 3 groups (control, no mirror, and mirror) and completed 10 visits which consisted of 2 familiarization visits, 2 eccentric exercise testing visits (1 or 2 weeks apart), and 6 follow-up visits. This study utilized a pre-test/post-test controlled design where participants would perform pre-exercise measurements of maximal isometric strength (MVC), range of motion (ROM), and muscle soreness rating (DOMS). For the 1st maximal eccentric exercise visit, 3 sets of 8 repetitions of maximal effort eccentric bicep curls with their dominant arm using the KinCom isokinetic dynamometer. During this 1st bout, the Mirror group had a mirror placed on the axilla of their non-dominant arm to create the mirror image illusion effect of bilateral exercise of the elbow flexors. The Control group used their dominant arm again for the 2nd eccentric exercise visit while the two experimental groups (No Mirror and Mirror) performed the bout with their non-dominant arm. After each eccentric exercise bout, they would perform the post-exercise MVC, ROM, and DOMS measurements. During these visits, surface EMG data of the biceps and triceps were recorded. These 2 eccentric exercise testing visits had 3 follow up visits (24h, 48h, and 72h) collecting data for MVC, ROM, and DOMS. RESULTS: The Control group and No Mirror group exhibited a repeated bout effect with a significant difference between bouts when looking at MVC and DOMS (p < 0.05) with the control group exhibiting the largest magnitude of protection. The No Mirror group did not show any significant differences between bouts when looking at these measures (p > 0.05). All groups did not show a significant difference when looking at ROM. All groups did not show a significant difference when looking at EMG RMS between bouts, demonstrating similar muscle activation in all groups between bouts (p > 0.05). CONCLUSIONS: In conclusion, this study demonstrates the supported findings of the RBE in the control and the CL-RBE in the no mirror group, however, there was no evidence of the cross education of the RBE in the Mirror group when looking at any of the dependent variables. Therefore, mirror visual feedback had no influence on the CL-RBE. Future studies should continue to investigate the use of a mirror to augment the CL-RBE with larger sample sizes and up to 5 follow-up visits. Investigation into mirror placement and the validity of the effectiveness of different positions for the mirror when used during various exercises should be considered as well.