Discharge Rate Variability Influences the Variation in Force Fluctuations Across the Working Range of a Hand Muscle

Abstract

The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units ( P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC ( P < 0.01) and remained constant at higher forces (1.2–1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability ( P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.

Effect of fatigue on force fluctuations in knee extensors in young adults

“…Low intersession reliability in the frequency domain relative to the time domain would therefore suggest that, although the accuracy and steadiness levels of the generated force are repeatable between sessions, the periodic structure of the generated force is more variable. This variability seems to be associated with the physiological organization of the motor unit pool (Missenard et al 2009), including recruitment of the motor units (Adam & De Luca 2005;Moritz et al 2005;Tracy et al 2005) and synchronization of the motor unit pool (McAuley & Marsden 2000). From the results of this study, it seems that, during submaximal voluntary contraction, motor units are recruited in a pattern that ensures effective task performance (as demonstrated by measures of accuracy and steadiness in the time domain), but that the periodic structure of the generated force is affected by an inherent shakiness, or physiological tremor (Freund 1983;Löscher et al 1996;McAuley & Marsden 2000).…”

Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

“…On the other hand, normalized measures of motor output variability (e.g. CV of force) are sensitive to motor unit recruitment and discharge behaviour (Taylor et al 2003;Moritz et al 2005), and the increased CV of force support the involvement of changes in the motor unit population during pain. Since the CoP time-series is inversely proportional to the task-related force, the total excursion calculated over the CoP time-series can also be considered a normalized parameter.…”

Percepção de posicionamento articular e do nível de força em sujeitos idosos e jovens

“…Alterações nas características e no comportamento das unidades motoras, assim como alterações na habilidade de coordenar contrações musculares, podem explicar a variabilidade de força em idosos (MORITZ et al, 2005;TRACY;ENOKA, 2002). Os padrões de flutuação de força dependem do número de unidades motoras recrutadas para a execução da tarefa e da interação das múltiplas características de ativação da unidade motora (TAYLOR, et al, 2003;MELLO;MIQUELETI;KOHN, 2008;MORAES, et al, 2000).…”