Effects of marathon fatigue on the discharge rates of individual motor units in older adults

Aim: Age-related changes in motor unit properties are essential to understand muscle behavior after exercise in older adults. In this study, we compared the changes in motor unit discharge rates during steady isometric contractions before and after a half-marathon in elderly individuals. Methods: During the event Run4Science, we recorded high-density surface electromyography (HD-sEMG, 128 channels) on the tibialis anterior muscle (TA) in eight old subjects. Subjects executed a 40 seconds static dorsiflexion at 25% MVC. The test was performed in two consecutive days, the first under non-fatigue conditions before half-marathon (BM) and the second immediately after half-marathon (AM). Using a novel decomposition technique for HD-sEMG recordings (Negro at al. 2016), we compared the changes in the discharge properties of the TA motor units before and after the half marathon. We calculated the global average discharge rate value (GADR) of all MUs in the two conditions. Only MU pulse trains with a silhouette measure > 0.9 were used in the study. Results: We extracted the individual contribution of 486 unmatched MUs (~ 30 per contraction). The GADR were 13.43 ± 1.46 (pps) and 14.89 ± 1.67(pps) for the BM and AM conditions respectively. Paired T-test analysis showed a significant difference between conditions (P<0.05). Conclusion: The results showed that the discharge rates of the identified MUs increase after the half-marathon. Future work will focus on the tracking of the same MU in the two conditions (Martinez‐Valdes 2017) in order to increase the sensitivity of the estimation.