ECSS Paris 2023: CP-BM13
INTRODUCTION: Eccentric training, which involves muscle lengthening under load, enhances athletic performance and prevents injuries. While most interventions focus on bilateral exercises, unilateral movements are crucial in real-life activities, aiding balance, coordination, and rehabilitation. However, differences in muscle activation between unilateral and bilateral exercises remain unclear. Therefore, the current study investigated these variations to guide training and rehabilitation programs. METHODS: A cross-sectional laboratory-controlled design was used. Twenty-five male participants performed four self-weighted eccentric tasks: single- and double-leg Nordic hamstring exercise (NHE-S/NHE-D) and eccentric leg extension (ELE-S/ELE-D). Electromyographic (EMG) data were collected using the Delsys Trigno Wireless system (Delsys Inc., Natick, MA, USA) from dominant side muscle groups: rectus abdominis (DRA), lumbar erector spinae (L3ES), multifidus (L5MF), biceps femoris long head (BFlh), semitendinosus (ST) at mid (50%) and distal (30%) regions, and gluteus maximus (Gmax). EMG parameters included RMS (normalized to MVIC), mean frequency (MF), waveform length (WAV), and tension index (TI). A Mann-Whitney U test was used to analyzed EMG features across four tasks. (P<0.05). RESULTS: The BFlh and Gmax exhibited significantly higher RMS values during unilateral tasks compared to bilateral tasks (P<0.001). While the ST showed no significant difference in the NHE, it demonstrated higher RMS values during ELE-S (P<0.01). In terms of trunk muscle (DRA, L3ES, L5MF) responses, most of them showed no significant differences in activation, except for the DRA during NHE-S exhibited increased RMS (P=0.01). The lumbar muscles (L3ES, L5MF) had significantly higher activation levels compared to the DRA (P<0.01, all comparisons), with the L5MF displaying higher levels of MF and TI than the L3ES during ELE (P<0.01, all comparisons). Regarding regional patterns, MF values consistently followed the hierarchy: BFlh50 > ST50 > ST30 > BFlh30 (P<0.05). The RMS of ST50 exceeded that of BFlh50 only during NHE-D (P=0.02), and ST50 demonstrated greater WAV and TI compared to ST30 during NHE (P<0.05, all comparisons). These differences in inter-muscle comparisons were consistent across both unilateral and bilateral patterns. CONCLUSION: Unilateral eccentric training elicited greater activation in BFlh and Gmax than bilateral training, while ST showed increased activation only in ELE. Trunk muscle activation remained largely unchanged, except for higher DRA activation in NHE-S. L5MF exhibited greater fatigue and tension than L3ES in ELE, and muscle fatigue patterns followed a consistent ranking (BFlh50 > ST50 > ST30 > BFlh30). These findings suggest that unilateral training can enhance hamstring activation and may be valuable for athletic performance, rehabilitation, and injury prevention.
Read CV Yanan YOUECSS Paris 2023: CP-BM13
INTRODUCTION: The force output produced during muscle contraction is modulated by activating different numbers of motor units (i.e., recruitment) and modulating their discharge rates (i.e., rate coding) [1]. However, there is still limited knowledge regarding how rate coding modulation across force levels differs between isolated and synergistic muscle contractions. In this study, we examined the first dorsal interosseous (FDI) motor unit (MU) discharge rates across three submaximal force levels during isolated finger flexion and synergistic pinch task. METHODS: Eight healthy adults sat in front of a custom-made manipulandum with their right arm and wrist stabilised and their index finger and thumb fixed to adjustable supports attached to load cells. After performing maximum voluntary isometric contractions (MVC), subjects were asked to perform submaximal contractions at different target forces (10%, 20%, and 30% MVC). Subjects followed trapezoidal feedback with a ramp up phase from 0% MVC to the target force at 3% MVC/s, keep at the target force for 30 s (plateau), and a ramp down phase from the target force to 0% MVC at 3% MVC/s. Each force level was repeated for two conditions: isometric finger flexion and pinch. During the tasks, high-density surface electromyograms (HDsEMG) were recorded from the FDI muscle. MU spike trains were decomposed from HDsEMG signals [2] and matched between 10% and 20% MVC (10-20%), as well as between 20% and 30% MVC (20-30%). MU matching was achieved by reapplying the MU separation vectors [3]. The mean discharge rate (MDR) during recruitment (up), plateau, and de-recruitment (down) phases was calculated and compared using linear mixed models, with force level and task as fixed effects and subject as random effects. The analyses were carried out separately per matching conditions (10-20% and 20-30%). RESULTS: A total of 586 matched MUs were identified across subjects. In both the 10-20% and 20-30% matching conditions, MDR during plateau phase increased at higher force levels (p < 0.001) and was higher in the pinch task compared to the isolated finger flexion (p < 0.001). No significant changes were observed in recruitment or de-recruitment phase. Additionally, no significant interaction was found between force level and task in all phases. CONCLUSION: By reapplying MU separation vectors in the concatenated decomposed files, it was possible to track motor units across force levels and analyse their mean discharge behaviour. Our results confirmed that FDI MU discharge rate increased with force level, but this increase did not depend on whether the task involved isolated or synergistic contraction. Interestingly, pinch task required higher MU discharge rates than isolated finger flexion, revealing a task-dependent modulation in rate coding. References: [1] Enoka & Stuart, 1992 [2] Negro et al., 2016 [3] Goodlich et. al., 2022
Read CV Nijia HuECSS Paris 2023: CP-BM13
INTRODUCTION: Human movement involves a dynamic interplay of isometric, concentric, and eccentric muscle actions. There is a need to understand the contribution of the reticulospinal tract (RST) to human movement control during different muscle actions. This research aimed to determine the excitability of the RST during isometric, concentric and eccentric muscle actions. METHODS: Fourteen neurologically intact participants (Age: 26 ± 7 years; Sex: 3 female, 11 male; Stature: 176 ± 8 cm; Mass: 78.5 ± 10.9 kg) performed isometric, concentric, and eccentric muscle actions with the biceps brachii. Participants performed a submaximal contraction at 25% of their isometric maximum voluntary contraction (MVC) during all muscle actions. Neurophysiological electrical stimulations to indirectly measure RST excitability consisted of conditioned (startling auditory stimulus of ≥110 dB) and unconditioned (no auditory stimulus) cervicomedullary motor evoked potentials (CMEPs). RESULTS: Larger conditioned CMEP responses compared to unconditioned CMEPs were observed for all muscle actions (p = 0.008). However, no differences in RST excitability, inferred from the difference between conditioned and unconditioned CMEP responses, were observed across the three muscle actions (p = 0.319). CONCLUSION: These results suggests that across isometric, concentric, and eccentric muscle actions, there are no differences in RST excitability whilst performing a submaximal contraction at 25% of their isometric MVC. It could therefore be inferred from this that RST input to motoneurons is not different between isometric, concentric, and eccentric muscle actions of the biceps brachii at relatively low contraction intensities.
Read CV Justin AndrushkoECSS Paris 2023: CP-BM13