ECSS Paris 2023: OP-BM14
INTRODUCTION: Running generates impacts when the foot contacts the ground, transmitted to the body as transient vibrations. While exposure to shocks and vibrations can have beneficial effects, their multimodal characteristics induce resonance in soft tissues vibration (STV) at their natural frequencies, leading to amplification and an increased risk of deleterious effects. The muscle tuning paradigm (MTP) proposes that muscles adapt to reduce vibrations by actively adjusting their mechanical properties (1). Although MTP is well established for continuous vibrations, further research is needed to confirm its applicability to transient vibrations. This study aimed to objectify the occurrence and the evolution of MTP under fatigue, using an instrumented ergometer capable of reproducing the shocks encountered when running, while eliminating the interference of voluntary contractions during running (2). METHODS: Sixteen active volunteers, running at least once per week, underwent nine hundred impacts corresponding to running at 14 to 15 km/h. Ground reaction forces (GRF) were acquired to control the impacts intensity. Participants were fitted with surface electromyographic sensors (EMG) on the Vastus Lateralis (VL), tri-axial accelerometers on the heel cup of the shoe and the VL. All signals were recorded during 30-sec every 50 impacts. Continuous wavelet transform was used to compute peak acceleration total magnitude of vibration (TMV) and settling time (ST) from acceleration data. The same approach was applied to EMG signal to calculate preactivation. Maximal voluntary contraction torque (MVC) and perceived fatigue were assessed before and after the exercise. Statistical analyses were performed on continuous and discrete data. RESULTS: The GRF recording confirmed that running impacts were accurately reproduced. Both discrete and continuous data statistical analyses indicated an increase in the TMV of heel impacts (+20%, p < 0.05) especially between 20 to 130 Hz, with no change in TMV at the VL. ST decreased at the heel (-10 %, p < 0.01) and increased at the VL. The mean preactivation tend to increase along the exercise (p = 0.05). This was associated with a reduction of MVC (-9%, p = 0.001) and elevation of perceived fatigue (p < 0.001). CONCLUSION: In line with existing literature, heel impact increased with the development of fatigue, especially in the high-frequency domain (3). Furthermore, the total magnitude of vibration did not change at the VL despite a higher impact at the heel. It is interesting to note that the ST at the VL gradually lengthened, indicating prolonged vibration, contrary to the observation of Khassetarash et al. (4). This ability to limit STV amplification even when impact increases, combined with increased preactivation, provides further evidence of the existence of an MTP in transient vibrations, even in cases of fatigue. 1) Nigg and Wakeling, 2001 2) Gassier et al., 2025 3) Gassier et al., 2025 4) Khassetarash et al., 2019
Read CV Christophe HAUTIERECSS Paris 2023: OP-BM14
INTRODUCTION: The current literature suggests that prepubertal children experience less fatigue than adults following neuromuscular exercises. This is particularly supported by studies reporting a smaller decrease in maximal force production in children compared to adults (1). However, many sport-specific actions rely on the stretch-shortening cycle rather than solely on maximal force production. (2) Therefore, the main aim of this study was to examine how an ecological fatiguing exercise composed of repeated jumps affects the function of the stretch-shortening cycle during a plyometric jump in prepubertal children. METHODS: Eight children and eight adults participated in a single experimental session, including a fatiguing exercise composed of five sets of 20 maximal repeated vertical jumps. Pre tests included maximal voluntary isometric contraction of the knee extensors (MVIC) with measurements of the torque and the electromyographic activity of vastus lateralis (RMS VL). Two drop jumps were also performed in tests, with measurements of the reactive strength index (RSI), jump height, contact time (CT), and braking and propulsion phases durations. These tests were repeated immediately after the exercise. All these variables were analyzed using a three-way analysis of variances, with main effects of time (Pre vs. Post), age (children vs. adults), and sex (females vs. males). RESULTS: A significant time ́ age interaction was observed for MVIC Torque (p = 0.008), highlighting a decrease for adults (p = 0.002), while no change was observed for children (p = 0.780). No significant time effect was observed for RMS VL (p = 0.444). A significant time effect was observed for DJ RSI (p = 0.006), indicating a decrease, independently of the age group. This decline in RSI was associated with a decrease in DJ height (p = 0.045), and an increase in DJ CT (p = 0.013). Moreover, braking phase duration increased (p = 0.014), whereas no change was observed in propulsion phase duration (p = 0.436). All these modifications were observed independently of age or sex. CONCLUSION: Consistent with previous literature, children exhibited a smaller decrease in maximal force production capacity than adults. However, they showed a decrease in jumping performance, suggesting that in children, the stretch-shortening cycle may be more affected by fatigue induced by repeated jumping exercises than maximal force capacity. REFERENCES: 1 Patikas, Williams, & Ratel, Exercise-induced fatigue in young people: advances and future perspectives, 2018 2 Komi, Stretch-shortening cycle: a powerful model to study normal and fatigued muscle, 2000
Read CV Baptiste CHANELECSS Paris 2023: OP-BM14
INTRODUCTION: Muscle fatigue is a complex, multifactorial phenomenon that is accompanied by adjustments in motor unit (MU) recruitment and firing patterns [1]. Although strength- (ST) and endurance-trained (ET) individuals show distinct neuromuscular adaptations, it remains unclear whether their MU behaviour differs in response to fatigue. To date, no study has examined fatigue-induced changes in both groups with MU tracking. This study therefore investigated fatigue-related changes in MU recruitment threshold (RT), firing rate (FR) and conduction velocity (CV) in ST and ET men using high-density surface electromyography (HDsEMG) decomposition with MU tracking to enable within-unit comparisons. METHODS: Twelve ST (21.6 ± 2.8 yr) and 11 ET (19.6 ± 2.2 yr) men performed maximal voluntary isometric contractions (MVICs) of the knee extensors, and a trapezoidal contraction at 20%MVIC (5%MVIC·s-1 ramp-up & down) before and immediately after a sustained submaximal isometric contraction at 30% MVIC to task failure. HDsEMG was recorded from the vastus lateralis. A total of 355 MUs were decomposed from the trapezoidal contractions. The RT was determined during the ramp-up, while the mean FR and CV were calculated during the plateau. A multi-dimensional tracking algorithm was then used to track MUs in the pre- and post-fatigue trapezoidal contractions. Approximately 30% of the MUs were successfully tracked between pre and post fatigue. RESULTS: Pre-fatigue MVIC was 33% higher in ST than ET (p = 0.026), with no between-group differences observed in post-fatigue MVIC decline or time-to-failure (TTF). Pre-fatigue RT was higher in ST than in ET (14.1 vs. 11.8%MVIC; p = 0.047), with no pre-fatigue differences in FR or CV. At the population level, group × fatigue interactions were observed for RT (p = 0.012) and CV (p = 0.031), with post-fatigue RT (p = 0.012) and CV (p < 0.001) decreasing in ST but not in ET. FR decreased in both groups, with a smaller reduction in ST (8.9%; p = 0.003) than in ET (18.6%; p < 0.001). In tracked MUs, however, only a fatigue main effect was observed, with comparable declines in RT (p = 0.001) and CV (p = 0.01) in both groups, and no change in FR. CONCLUSION: Despite similar global fatigue, the population-level MU results indicated distinct neuromuscular strategies. ST exhibited post-fatigue decreases in RT and CV, whereas ET did not. This suggests a post-fatigue shift towards a greater contribution from lower-threshold MUs in ST. However, tracked MU results showed comparable within-unit declines in RT and CV in both groups, indicating similar intrinsic MU adjustments regardless of training status. Overall, the training-specific effect on population-level MUs probably reflects fatigue-related changes in the recruitment of lower-threshold MUs. This is likely due to the differences in fibre-type composition and recruitment strategy, rather than to fundamentally different fatigue responses within individuals MUs. [1] Contessa et al., (2016)
Read CV Rui WuECSS Paris 2023: OP-BM14