A Comparison of Motor Unit Control Strategies between Two Different Isometric Tasks

Abstract

Background: This study examined the motor unit (MU) control strategies for non-fatiguing isometric elbow flexion tasks at 40% and 70% maximal voluntary isometric contraction. Methods: Nineteen healthy individuals performed two submaximal tasks with similar torque levels: contracting against an immovable object (force task), and maintaining the elbow joint angle against an external load (position task). Surface electromyographic (EMG) signals were collected from the agonist and antagonist muscles. The signals from the agonist were decomposed into individual action potential trains. The linear regression analysis was used to examine the MU recruitment threshold (RT) versus mean firing rates (MFR), and RT versus derecruitment threshold (DT) relationships. Results: Both agonist and antagonist muscles’ EMG amplitudes did not differ between two tasks. The linear slopes of the MU RT versus MFR and RT versus DT relationships during the position task were more negative (p = 0.010) and more positive (p = 0.023), respectively, when compared to the force task. Conclusions: To produce a similar force output, the position task may rely less on the recruitment of relatively high-threshold MUs. Additionally, as the force output decreases, MUs tend to derecruit at a higher force level during the position task.

Unique case study: Impact of single‐session neuromuscular biofeedback on motor unit properties following 12 days of Achilles tendon surgical repair

“…In this case report, we compared MG motor unit population properties (number of recruited motor units, peak and average firing rate, coefficient of variation, decomposition accuracy, peak and average motor unit action potentials (MUAP), inter‐pulse interval, and motor unit recruitment and derecruitment thresholds) (De Luca et al., 2006 ; Jeon et al., 2020 ; LeFever et al., 1982 ), sEMG amplitude, force paradigm, and AT length before and after single‐session of neuromuscular biofeedback (US and sEMG biofeedbacks). The intervention included submaximal muscle contractions (30% of maximal voluntary contraction of the non‐injured limb) in the repaired AT and non‐injured limb of a patient on day 12 after the AT repair surgery (Table 1 ).…”

Electromyographic activity of pelvic floor muscles in different positions during the use an innovative vaginal educator: Cross-sectional study

“…For the sample size calculation, the interaction parameter related to the variable "intensity" was applied to the EMG amplitude, as described by Jeon et al (2020) [16]. Partial eta-squared values (ηp2 = 0.640) and a minimal correlation between repeated measurements (r = 0.6) were used in this study [16].…”

“…For the sample size calculation, the interaction parameter related to the variable "intensity" was applied to the EMG amplitude, as described by Jeon et al (2020) [16]. Partial eta-squared values (ηp2 = 0.640) and a minimal correlation between repeated measurements (r = 0.6) were used in this study [16]. Error probability values α of 0.05 and β of 0.2 were set for a number of 8 subgroups (with a non-sphericity correction of 1 for ANOVA) with repeated samples and with interaction between paired samples data.…”

Motor unit firing patterns on increasing force during force and position tasks

“…In the elbow flexor muscles, the increase in the EMG amplitude in two tasks was similar, but in the position task, there was a significant increase in motor unit (MU) firing rate variability and a decrease in the mean firing rate (8). During nonfatiguing conditions, different cerebral cortex regions control each task (9) and a relatively high-threshold MU recruited in the force task may not be excited for the position task, implying that the number of motor units recruited in the position task is small (10). Moreover, presynaptic inhibition reduces during the position task, suggesting that the significant influence of inputs from muscle spindles' primary afferents (11).…”