Author(s): HAYMAN, O., ANSDELL, P., ANGIUS, L., THOMAS, K., ŠKARABOT, J., GRAY, S., HOWATSON, G., KIDGELL, D., GOODALL, S., Institution: NORTHUMBRIA UNIVERSITY , Country: UNITED KINGDOM, Abstract-ID: 1239

INTRODUCTION:
Unaccustomed eccentric exercise (EE) causes symptoms of exercise-induced muscle damage (EIMD) that are accompanied by changes in contractile properties and nervous system function, which persist for several days post-exercise [1]. Following a repeated bout of EE, the damage response is attenuated (repeated bout phenomenon [2]), but limited understanding exists [3] of how changes in motor unit (MU) behaviour are involved in this response . Accordingly, this study utilised high-density electromyography decomposition to examine single MU properties during the recovery from an initial and repeated bout of EE.
METHODS:
Twenty participants performed 2 bouts of maximal lengthening dorsiflexion contractions (10 × 10) three weeks apart. Maximal isometric voluntary toque (MVT), soreness (DOMS), and pain pressure thresholds (PPT) were measured pre- 24, 48, and 72 h post EE. Single MU properties were recorded pre-EE, 24, 48, and 72 h post-EE during trapezoidal contractions at 40% MVT. Mean MU discharge rate during the plateau of the trapezoidal contractions (DRAP), along with discharge rate at recruitment, derecruitment, and co-efficient of variation of inter-spike interval (CoVISI) were computed. Changes in outcomes variables within and between bouts were assessed with a linear effects model.
RESULTS:
Work done was similar between bouts (1289 ± 507 vs. 1320 ± 523 J; P=0.721). However, a faster recovery in muscle function and soreness was observed after the repeated bout (MVF & DOMS differed at 24 & 48 h, PPT differed at 24 – 72 h; all P<0.05), confirming the repeated bout effect. A similar population of MUs were identified between bouts (738 ± 26 vs. 698 ± 9; P=0.54). Mean discharge rate at recruitment increased following bout 1 at 24 and 48 h post-EE (P<0.001) with increases evident only at 24 h post-EE after bout 2 (P<0.001). Mean DRAP increased from baseline at 24 and 48 h post-EE during bout 1 (P<0.001), with the relative increase attenuated in bout 2 compared to bout 1, evident only at 24 h (18 [16.7-19.4] vs. 17.4 pps [16.0-18.7]; P<0.001). Discharge rate at derecruitment increased at 24 h post-EE in bout 1 (7.82 pps [7.21-8.42]; P=0.03) with no changes following bout 2. The CoVISI increased after bout 1 at 24 h post-EE (17.5 [16.3-18.8] vs. 19.2% [17.9-20.5]; P<0.001), but no changes were evident following bout 2, with lower respective values at the same timepoint (17.1% [15.9-18.4]; P<0.001).
CONCLUSION:
A quicker recovery in muscle function and soreness was evident following the repeated bout of EE and this was accompanied by specific alterations in motor unit behaviour. Specifically, increases in DRAP and CoVISI following the first bout were attenuated. Such an adaptive response provides preliminary evidence demonstrating that the repeated bout phenomenon is partly explained by changes in MU behavior.