ECSS Paris 2023: OP-BM14
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.
Read CV Oliver HaymanECSS Paris 2023: OP-BM14
INTRODUCTION: The current literature has shown a lower fatigue following muscular-type exercises for prepubertal boys in comparison with adults. While, isometric contractions are frequently explored, less is known in dynamic conditions. Given that fatigue is contraction mode dependent in adults, one can expect similar findings in prepubertal children. Thus, the aim of this study was to compare the fatigue development between prepubertal boys and men, during maximal concentric and eccentric knee extensors contractions. METHODS: This cross-over, randomized study consisted in three visits at the laboratory (one familiarization session and two experimental sessions). Twelve boys (9-11 years old) and 12 men (20-25 years old) were included. Tests and exercises were conducted on the right knee extensor muscles using an isokinetic ergometer. The fatiguing protocol was composed of three sets of 30 concentric (CONC) or eccentric (ECC) knee extensor maximal contractions realized at 60°/s, separated with 30 seconds rest between each set. Maximal voluntary isometric contractions (MVIC) were conducted before the fatiguing exercise (PRE) and were repeated immediately after each sets (POST S1, POST S2, POST S3), five minutes (POST R5), and one hour (POST R60) after the end of the exercise. Torque (MVIT) and electromyographic activity (EMG) of vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) were recorded during MVIC. MVIT and EMG values were analyzed with a three-way (age x contraction mode x time) analysis of variances (ANOVA). Age corresponded to boys vs. men. Contraction Mode corresponded to the fatiguing exercise (CONC vs. ECC). Time corresponded to differences between all time points. RESULTS: A significant Time x Age interaction was found for MVIT, revealing a decrease after both fatiguing protocols. Post-hoc revealed a signitificant difference with PRE from POST S1 to POST R60 for men (P < 0.05). For boys no difference with PRE values was observed with the post-hoc. When considering relative torque values, a significant Time x Age interaction was also observed. Post-hoc test revealed a significant group effect at POST S2 and POST S3 indicating a significant lower torque decrease (P < 0.05) for boys than for men. A significant Contraction Mode x Age interaction was found for RF EMG. Post-hoc tests revealed a significant higher EMG decrease during MVIC for boys than for men in CONC conditions (P < 0.05). No difference was obtained between boys and men in ECC. CONCLUSION: This study shown a lower MVIT decrease for boys than for adults. During concentric exercise, the EMG revealed a higher decrease for boys, like already shown with isometric fatiguing protocol. However, no difference was observed during the eccentric exercise, suggesting a similar central fatigue development during this exercise modality between prepubertal boys and adults. These observations confirmed that fatigue development between men and prepubertal boys was contraction mode dependent.
Read CV Baptiste CHANELECSS Paris 2023: OP-BM14
INTRODUCTION: Electrical stimulation can be delivered over a nerve trunk or muscle belly, and can generate contractions by activating motor (direct recruitment) and sensory (indirect recruitment) axons. The aim of the present study was to investigate the impact of the stimulation mode (nerve trunk vs muscle belly) on the extent and origin of neuromuscular fatigue of the triceps surae. METHODS: Fifteen participants visited the laboratory for two experimental sessions. Each session consisted of delivering 25 stimulation trains (100 Hz, 20s ON – 20s OFF) at an intensity evoking 10% of maximal voluntary contraction (MVC), either applied over the triceps surae muscles or over the posterior tibial nerve. Before (PRE) and after (POST) the 25 stimulation trains, MVC, muscle contractile properties, as assessed by the twitch torque (Pt) and neural changes, as assessed by the voluntary activation level (VAL), were evaluated. For each stimulation train, the torque-time integral (TTI) was determined. Moreover, three twitches at the intensity of each stimulation train (PtIstim) were delivered before each 25-train session to identify the initial motor unit (MU) recruitment (direct vs indirect, [1]). Finally, the sum of the TTI of the 25 stimulation trains was calculated to determine the total TTI developed by each subject during each of the two sessions (∑TTI). RESULTS: Results showed that MVC, VAL and Pt were significantly decreased (P < 0.05) after the 25 stimulation trains for both muscle and nerve stimulation (-9.5 and -7.6% for muscle and nerve stimulation, respectively). The ∑TTI was not different between the two modalities of stimulation (P = 0.72). When the EMG associated to the PtIstim was considered to discriminate the trials according to the initial MU recruitment, independently of the stimulation modalities, 17 trials out of the 30 were considered as inducing a direct MU recruitment and the rest 13 trials were considered as inducing an indirect MU recruitment. Statistical analysis revealed a significant decrease of MVC after the 25-stimulation trains only after the trials inducing a direct MU recruitment (P < 0.001). Finally, ∑TTI developed was not significantly different between trials inducing direct and indirect MU recruitment (P = 0.21), although mean TTI was higher in trials inducing indirect (6270 ± 4282 Nm) comparatively to those inducing direct MU recruitment (4539 ± 3124 Nm). CONCLUSION: These results indicate that muscle and nerve stimulation induce the same amount of neuromuscular fatigue that can be attributed both to neural and muscle alterations. It appears that the site of stimulation is not a discriminating factor for fatigue development. On the other hand, the loss of MVC reported only for the direct MU recruitment suggests that when an indirect MU recruitment is induced, fatigue development is less pronounced. It appears thus that the level of induced fatigue seems more linked to the initial type of MU recruitment than the site of stimulation. 1 - Vitry et al, 2019
Read CV Florian VITRYECSS Paris 2023: OP-BM14