Author(s): OLIVEIRA, J., GOMES, J.S., BAUER, P., PEZARAT-CORREIA, P., VAZ, J.R., Institution: UNIVERSIDADE DE LISBOA FACULDADE DE MOTRICIDADE HUMANA, Country: PORTUGAL, Abstract-ID: 1252

INTRODUCTION:
Torque complexity has been proposed as a key marker of the ability of the neuromuscular system to adapt to external changes and, therefore, is considered as an important measure of motor control. Interestingly, the literature demonstrates an inverted U-shape relationship between the torque complexity and contraction intensity, suggesting that motor control is influenced by the intensity of contraction. Furthermore, it has been suggested that changes in torque complexity are only observed above the critical torque (CT)/end-test torque (ETT) intensity. However, the physiological mechanisms behind this well documented relationship are yet to be determined. This study aimed to investigate whether changes in motor unit behaviour (motor unit recruitment and firing rate) with contraction intensity would accompany and potentially explain the inverted U-shape relationship between force complexity and contraction intensity.
METHODS:
25 participants were asked to perform a knee extensors’ sustained submaximal hold isometric task at the intensities corresponding to their ETT, 50%ETT, 75%ETT, 150%ETT and 175%ETT. Measurements involved recordings of submaximal isometric torque while recording high-density surface EMG signals from the vastus lateralis (VL) and vastus medialis (VM) muscles. Sample Entropy (SampEn) and Coefficient of Variation (CV) were extracted from torque signals as measures of temporal structure (i.e. complexity) and magnitude of torque variability. EMG signals were decomposed into individual Motor Unit Action Potentials (MUAP) and Firing Rate (FR). A one-way ANOVA for repeated measures was performed on SampEn, CV, MUAP and FR to test the effect of contraction intensity on the temporal structure and magnitude of torque variability as well as on motor unit related measures. In addition, a stepwise multiple linear regression analysis was conducted to predict changes in torque SampEn based on changes in motor unit related variables.
RESULTS:
The results showed an effect of contraction intensity on SampEn (p<0.001), with ETT and 150%ETT presenting higher SampEn’s values when compared with 50%ETT (p<0.001) and 75%ETT (p<0.001 and p=0.019, respectively). For motor unit behaviour, we observed a significant effect of contraction intensity in MUAP and FR for both VL and VM (all p<0.001). The results demonstrated that MUAP and FR increased with contraction intensity until 175%ETT. Additionally, the multiple linear regression was statistically significant (R2=0.18, p<0.001), indicating that SampEn is lower when MUAP and FR of VL present higher and lower values, respectively.
CONCLUSION:
The study demonstrated that torque complexity is contraction intensity dependent in an inverted U-shape relationship that could be, potentially, explained by changes in motor unit behaviour with contraction intensity, particularly above the CT/ETT, which could influence the capacity of the neuromuscular system to be flexible and to adapt the motor output to the changes in task demands.