Influences of premotoneuronal command statistics on the scaling of motor output variability during isometric plantar flexion

Abstract

This study focuses on neuromuscular mechanisms behind ankle torque and EMG variability during a maintained isometric plantar flexion contraction. Experimentally obtained torque standard deviation (SD) and soleus, medial gastrocnemius, and lateral gastrocnemius EMG envelope mean and SD increased with mean torque for a wide range of torque levels. Computer simulations were performed on a biophysically-based neuromuscular model of the triceps surae consisting of premotoneuronal spike trains (the global input, GI) driving the motoneuron pools of the soleus, medial gastrocnemius, and lateral gastrocnemius muscles, which activate their respective muscle units. Two types of point processes were adopted to represent the statistics of the GI: Poisson and Gamma. Simulations showed a better agreement with experimental results when the GI was modeled by Gamma point processes having lower orders (higher variability) for higher target torques. At the same time, the simulations reproduced well the experimental data of EMG envelope mean and SD as a function of mean plantar flexion torque, for the three muscles. These results suggest that the experimentally found relations between torque-EMG variability as a function of mean plantar flexion torque level depend not only on the intrinsic properties of the motoneuron pools and the muscle units innervated, but also on the increasing variability of the premotoneuronal GI spike trains when their mean rates increase to command a higher plantar flexion torque level. The simulations also provided information on spike train statistics of several hundred motoneurons that compose the triceps surae, providing a wide picture of the associated mechanisms behind torque and EMG variability.

Multiscale modeling of the neuromuscular system: Coupling neurophysiology and skeletal muscle mechanics

“…and the musculotendon unit (proprioception, joint dynamics, etc.). Recent applications of these biophysical neuromuscular models include the analysis of the force variability during steady isometric contractions (Dideriksen et al, ; Farina et al, ; Farina & Negro, ; Negro et al, ; Negro & Farina, ; Watanabe et al, ), postural sway (Elias, Watanabe, & Kohn, ) and nonlinear control of force oscillations (Watanabe & Kohn, , ).…”

Variabilidade De Força E Relação Força/Emg De Músculos Mastigatórios Na Tarefa De Mordida Incisiva

“…Por exemplo, Jones et al [11] mostraram que durante a contração isométrica do músculo primeiro interósseo dorsal da mão, há um aumento linear do desvio-padrão da força em função do aumento da força média. Já Watanabe et al [10] observaram uma relação proporcional entre o desviopadrão do torque de flexão plantar e o torque médio produzido pelo tríceps sural. Estes padrões da variabilidade da força indicam a estratégia adotada pelo sistema neuromuscular para o controle da força.…”

“…Os valores de média, desvio-padrão e coeficiente de variação dos dados de força de mordida incisiva foram calculados para cada repetição de todas as tarefas realizadas [10]. A amplitude dos sinais de EMG foram estimadas utilizando o valor RMS (Root-MeanSquare) calculado em uma janela móvel com 250 ms de duração.…”

Dissecting mechanisms behind force control in humans by a mixture of experimentation, mathematical analysis and computer simulations of neuronal models

“…In a related study, also combining human experiments with large‐scale modelling approaches (Watanabe et al . ), the presynaptic inputs acting on a given MN pool were represented by gamma point processes with appropriate parameters. The MNs (more than a thousand) and muscle fibres were modelled from biophysical and biomechanical standpoints.…”