An Approach for Simulation of the Muscle Force Modeling It by Summation of Motor Unit Contraction Forces

Rositsa Raikova,Piotr Krutki,Jan Celichowski,Hristo Aladjov

doi:10.1155/2013/625427

Abstract

Muscle force is due to the cumulative effect of repetitively contracting motor units (MUs). To simulate the contribution of each MU to whole muscle force, an approach implemented in a novel computer program is proposed. The individual contraction of an MU (the twitch) is modeled by a 6-parameter analytical function previously proposed; the force of one MU is a sum of its contractions due to an applied stimulation pattern, and the muscle force is the sum of the active MUs. The number of MUs, the number of slow, fast-fatigue-resistant, and fast-fatigable MUs, and their six parameters as well as a file with stimulation patterns for each MU are inputs for the developed software. Different muscles and different firing patterns can be simulated changing the input data. The functionality of the program is illustrated with a model consisting of 30 MUs of rat medial gastrocnemius muscle. The twitches of these MUs were experimentally measured and modeled. The forces of the MUs and of the whole muscle were simulated using different stimulation patterns that included different regular, irregular, synchronous, and asynchronous firing patterns of MUs. The size principle of MUs for recruitment and derecruitment was also demonstrated using different stimulation paradigms.

Highlights

The force of a skeletal muscle is an accumulation of forces generated by active motor units belonging to this muscle
The individual contraction of an motor units (MUs) is modeled by a 6-parameter analytical function previously proposed; the force of one MU is a sum of its contractions due to an applied stimulation pattern, and the muscle force is the sum of the active MUs
The muscle is composed of three types of MUs (S, fatigue resistant (FR), and fast fatigable (FF)), and the numbers of each type of

Summary

Introduction

The force of a skeletal muscle is an accumulation of forces generated by active motor units belonging to this muscle. A motor unit (MU) is a motor neuron and all the muscle fibers innervated by its axon. Motor units develop forces in response to trains of motoneuronal action potentials transmitted to the muscle fibers by motor axons. The central nervous system controls the muscle force by two basic mechanisms: (1) rate coding alters interpulse intervals (IPIs) between successive action potentials, which is measured as discharge rate and (2) recruitment-derecruitment processes regulate the number of active MUs [1,2,3,4,5]. Since it is very difficult to study these processes using in vivo experiments, the modeling of muscle force as a result of different types of MUs’ activity patterns can enhance our understanding of force control processes. The most complex and frequently used model in various modifications appears to be the one proposed by the group of Fuglevand [6, 11]

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