Dynamics of neuromuscular adjustments to the progressive reduction in external forces. Dynamique des ajustements neuromusculaires à la réduction progressive des forces externes

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Fazzari, Camille | Macchi, Robin | Casanova, Rémy | Nicol, Caroline | Chavet, Pascale

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International audience. INTRODUCTION:Previous studies have investigated the running pattern adjustments to sudden perturbations, such as unexpected changes in terrain stiffness or height (1). Interestingly, lower body positive pressure treadmills (LBPPTs) allow for progressive unweighting. During the transition phase, positive correlations have been reported between changes in braking force and changes in vasti and soleus muscle activity (2). However, these correlations were not examined separately for each participant, likely masking inter-individual variability. The current study investigated the dynamics of neuromuscular adjustments to the progressive reduction of external forces, using an individualised analysis. METHODS:Forty men (19±1yr) ran on a LBPPT in 3 consecutive conditions performed at 100, 60 and 100% body weight. The analysis focused on the unweighing transition which included 18±3 right strides. Normal ground reaction force and surface EMG activity of 11 right lower limb muscles were averaged over the braking phase and normalised to their mean value recorded at 100% body weight. Spearman’s correlation tests were used to quantify the relationship between the normalized mean braking force and mean EMG activity. Chi-squared tests were used to compare the proportions of positive and negative significant correlations. RESULTS:Braking force was positively correlated with VM (ρ=0.67±0.11), VL (0.66±0.10) and RF (0.68±0.10) activity in 65, 60 and 63% of the participants, respectively. It was negatively correlated with STSM (-0.67±0.10) and BF (-0.68±0.10) activity in 50 and 28% of them. Braking force was positively correlated with shank muscle activity in few participants (<13%), but negatively correlated with SOL (-0.59±0.13), GaM (-0.58±0.15), GaL (-0.70±0.15), TA (-0.59±0.10) and PL (-0.63±0.10) activity in 8, 13, 28, 23 and 25% of them. Positive correlations predominated for the quadriceps (VM: X²=26, VL: 21.2, RF:25, p<.001), whereas negative correlations predominated for the hamstrings (STSM: X²=20, BF:20, p<.001) and three shank muscles (GaL: X²=6.4, TA: 6.2, PL: 3.8, p<.05).CONCLUSION:The progressive reduction of external forces mainly affected thigh muscle activity. It showed a decrease in quadriceps activity, as previously reported (2), but an increase in hamstring activity in most participants. Operating in the optimal range of their force-length relationship, the hamstrings may have contributed to the reduced peak knee flexion during the stance phase (3). On the other hand, unweighting resulted in opposite adjustments in shank muscle activity between participants. Nevertheless, their activity increased in most of them, suggesting an increased ankle stabilisation. This is attributed to the shift towards a more forefoot strike pattern (3). This adjustment appears to be transient as it is no longer observed after 3 minutes of unweighted running (4).REFERENCES:(1)Daley, in Understanding Mammalian Locomotion, 2016(2)Sainton et al., PloS One, 2016(3)Neal et al., J Orthop Sports Phys Ther, 2016(4)Fazzari et al., Front Physiol, 2023

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