Torque Regulation Is Influenced by the Nature of the Isometric Contraction

Changes in torque complexity with fatigue are related to motor unit behaviour

Physiological complexity is believed to reflect a system's adaptability to environmental challenges having been proposed as an indirect indicator of the functional capacity of the neuromuscular system. This study aimed to investigate the association between torque complexity's changes with neuromuscular fatigue and motor unit parameters. Twenty-one healthy and young adults visited the laboratory on one occasion. Knee extension maximum voluntary isometric contractions and isometric contractions at 30% of maximum were collected at baseline and immediately after a fatiguing knee extension protocol, which consisted of a series of concentric and eccentric knee extensions at 90°/s until exhaustion. Torque signals were sampled continuously, and torque complexity was assessed through an entropy measure. Motor unit-related parameters were extracted from the submaximal trials and further analysed. Our findings demonstrate that torque complexity's alteration pre-to-post neuromuscular fatigue is highly correlated with vastus lateralis and medialis average firing rate (r = - 0.618 and r = - 0.659, respectively) and peak motor unit action potential amplitude (rs = - 0.801 and rs = - 0.703, respectively) pre-fatigue. Moreover, alterations in torque complexity were observed, indicating a loss of adaptability within the neuromuscular system with neuromuscular fatigue. Overall, our findings supported our hypothesis by demonstrating alterations in torque complexity with neuromuscular fatigue, rendering the system less adaptable. Moreover, our results added to the current knowledge by highlighting the association between torque complexity's changes with neuromuscular fatigue and motor unit parameters.

Torque Regularity is not Affected by the Nature of Visual Feedback During Hold Type of Isometric Contractions in Adults

The literature that investigated the influence of visual feedback properties on the magnitude and temporal structure of variability suggests that increases in the precision of visual information of the torque output (through manipulation of visual gain and its intermittency) lead the neuromuscular system to produce torque in a more steady and complex manner. However, less is known about the influence of the nature of visual feedback on torque variability. The present study aimed to investigate the effect of two different feedback natures, one from the angular position and the other from the torque produced, on the magnitude and temporal structure of torque variability during submaximal hold isometric tasks. Twenty healthy and young participants performed a knee extension isometric task, consisting in sustaining an applied resistance equivalent to 40% of their Maximal Voluntary Isometric Contraction for 30 s with visual feedback from angular position (Hangle) and with visual feedback from torque (Htorque). The magnitude of torque variability was calculated through the coefficient of variation. Sample Entropy was used to analyze the temporal structure of torque fluctuations, i.e., torque complexity. We observed no significant differences between conditions (Hangle vs Htorque) in both magnitude (p = 0.117) and the temporal structure of torque fluctuations (p = 0.940). Our results demonstrated that torque regulation seems not to be affected by the nature of the visual feedback during hold submaximal isometric tasks, suggesting a participation of sensorimotor system due to the nature of the task. Researchers should take this into account to take methodological decisions when using hold submaximal isometric tasks to investigate possible changes in motor control.

Calculating sample entropy from isometric torque signals: methodological considerations and recommendations

We investigated the effect of different sampling frequencies, input parameters and observation times for sample entropy (SaEn) calculated on torque data recorded from a submaximal isometric contraction. Forty-six participants performed sustained isometric knee flexion at 20% of their maximal contraction level and torque data was sampled at 1,000 Hz for 180 s. Power spectral analysis was used to determine the appropriate sampling frequency. The time series were downsampled to 750, 500, 250, 100, 50, and 25 Hz to investigate the effect of different sampling frequency. Relative parameter consistency was investigated using combinations of vector lengths of two and three and tolerance limits of 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, and 0.4, and data lengths between 500 and 18,000 data points. The effect of different observations times was evaluated using Bland-Altman plot for observations times between 5 and 90 s. SaEn increased at sampling frequencies below 100 Hz and was unaltered above 250 Hz. In agreement with the power spectral analysis, this advocates for a sampling frequency between 100 and 250 Hz. Relative consistency was observed across the tested parameters and at least 30 s of observation time was required for a valid calculation of SaEn from torque data.