How many motor units is enough? An assessment of the influence of the number of motor units on firing rate calculations

• Published in: Journal of electromyography and kinesiology 2024-04, Vol.75, p.102872-102872, Article 102872
• Main Authors: Marsala, Michael J., Gabriel, David A., Greig Inglis, J., Christie, Anita D.
• Format: Article
• Language: English
• Subjects: Aged; Bootstrapping; Electromyography; Female; Firing rate; Humans; Isometric Contraction - physiology; Male; Motor Neurons - physiology; Motor units; Muscle Contraction - physiology; Muscle, Skeletal - physiology; Recruitment, Neurophysiological - physiology
• ISSN: 1050-6411; 1873-5711
• DOI: 10.1016/j.jelekin.2024.102872

The number of motor units included in calculations of mean firing rates varies widely in the literature. It is unknown how the number of decomposed motor units included in the calculation of firing rate per participant compares to the total number of active motor units in the muscle, and if this is different for males and females. Bootstrapped distributions and confidence intervals (CI) of mean motor unit firing rates decomposed from the tibialis anterior were used to represent the total number of active motor units for individual participants in trials from 20 to 100 % of maximal voluntary contraction. Bootstrapped distributions of mean firing rates were constructed using different numbers of motor units, from one to the maximum number for each participant, and compared to the CIs. A probability measure for each number of motor units involved in firing rate was calculated and then averaged across all individuals. Motor unit numbers required for similar levels of probability increased as contraction intensity increased (p