Abstract
The pathway of voluntary joint torque production includes motor neuron recruitment and rate-coding, sarcolemmal depolarization and calcium release by the sarcoplasmic reticulum, force generation by motor proteins within skeletal muscle, and force transmission by tendon across the joint. The direct source of energetic support for this process is ATP hydrolysis. It is possible to examine portions of this physiologic pathway using various in vivo and in vitro techniques, but an integrated view of the multiple processes that ultimately impact joint torque remains elusive. To address this gap, we present a comprehensive computational model of the combined neuromuscular and musculoskeletal systems that includes novel components related to intracellular bioenergetics function. Components representing excitatory drive, muscle activation, force generation, metabolic perturbations, and torque production during voluntary human ankle dorsiflexion were constructed, using a combination of experimentally-derived data and literature values. Simulation results were validated by comparison with torque and metabolic data obtained in vivo. The model successfully predicted peak and submaximal voluntary and electrically-elicited torque output, and accurately simulated the metabolic perturbations associated with voluntary contractions. This novel, comprehensive model could be used to better understand impact of global effectors such as age and disease on various components of the neuromuscular system, and ultimately, voluntary torque output.
Highlights
Muscle cross sectional area is the greatest determinant of maximal isometric joint torque in humans [1], only about twothirds of maximal torque is accounted for by muscle size
By synthesizing existing and de novo models of neuromuscular function and bioenergetics, the work presented here significantly advances our ability to investigate the relationships between individual events in the pathway of voluntary torque production and estimate their relative impact on in vivo function
Several computational models have been developed that provide unique insights into the function of individual components of neuromuscular function [3,4,7,20]; few modeling studies have attempted to integrate across such a wide range of physiological functions
Summary
Muscle cross sectional area is the greatest determinant of maximal isometric joint torque in humans [1], only about twothirds of maximal torque is accounted for by muscle size. There are examples in the literature of considerable variation in maximal voluntary torque per unit area [N?m?cm22]) [2], termed ‘‘specific strength’’ [1]. The generation of voluntary torque, illustrated, begins with neural excitation in the motor cortex, which produces propagation of excitatory potentials down the cortico-spinal tracks to the a motor neurons. These motor neurons innervate muscle cells, causing depolarization of the sarcolemma and release of Ca2+ from the sarcoplasmic reticulum. In this way, cross-bridge cycling is initiated and force is produced, leading to torque generation about a joint. The complex interrelationships among the physiological systems that govern these processes further impact their combined function
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