The Motor Unit Mean Firing Rate versus Recruitment Threshold Relationship is Unaffected by Short-Term Disuse

Abstract

The loss of muscle strength during periods of disuse is rapid. Some investigators have postulated that these changes are due to neural, rather than muscular, adaptations. It is unclear, however, if short-term immobilization of the knee joint affects the voluntary control of motor units. PURPOSE: To determine whether the slope and y-intercept of the motor unit mean firing rate versus recruitment threshold relationship is altered by 72 hours of disuse. METHODS: Fifteen healthy females (mean ± SD age = 21 ± 2 years, body mass index [BMI] = 23.1 ± 2.3 kg/m2) voluntarily underwent left knee joint unloading via ambulating on crutches and use of a brace. The brace was worn at all times except during sleep, and compliance was confirmed via accelerometers secured around both ankles. Following two extensive familiarization sessions at the laboratory, testing was performed immediately prior to immobilization (PRE) and 72 hours later (POST). During both testing sessions, participants performed trapezoidal isometric contractions at a torque level corresponding to 50% of their maximal voluntary contraction (MVC). Participants were instructed to increase torque from 0 to 50% in five seconds, maintain 50% MVC for 15 seconds, and decrease torque from 50% to 0 in five seconds. Bipolar surface electromyographic (EMG) signals were recorded from the vastus lateralis. A surface EMG signal decomposition algorithm was used to calculate the mean firing rate (pulses per second [pps]) and recruitment threshold (% MVC) of each detected motor unit. Motor units with decomposition accuracy levels < 90% were discarded. Linear regression was then used to quantify the slope (pps/% MVC) and y-intercept (pps) of each relationship. RESULTS: Immobilization had no influence on the linear slope coefficient for the mean firing rate versus recruitment threshold relationship (PRE = -0.362 ± 0.127, POST = -0.399 ± 0.108 pps/%MVC; p = 0.413, d = 0.218). Similarly, no change in the y-intercept was observed (PRE = 23.2 ± 2.8, POST = 23.3 ± 4.0 pps; p = 0.972, d = 0.009) CONCLUSION: Our findings demonstrated that 72 hours of knee joint immobilization in healthy females had little influence on vastus lateralis motor unit control during submaximal contractions. Funding: The De Luca Foundation and the UCF Office of Research’s Advancement of Early Career Researchers program