ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

MUSCLE MODEL COMPOSED BY MOTOR UNITS AND SIMULATION
OF THEIR CONTROL BY USING HIERARCHICAL GENETIC
ALGORITHM
1. ABSTRACT
R. Raikova 1 , H. Aladjov 2 , D. Łochyński 3 and D. Kaczmarek 4
There are simplifications in macro-biomechanical models which influence their realism.
Usually each muscle is presented as one force generator. It is assumed that the muscle
force is controlled by a single, continuous "activation" signal. In fact, the smallest
functional muscle part is the motor unit (MU) and its force is controlled by electrical
pulses with variable inter-pulse intervals (IPIs). The force of the muscle is a sum of
forces of all active MUs belonging to this muscle. It is regulated by two mechanisms -
by changing the number and the type of active MUs and by variation in firing
frequencies of active MUs, i.e. recruitment and rate coding. Recently a new muscle
model was developed and it was based on the force which a MU developed as a result
of one single impulse, the twitch. The proposed and implemented in the software
MotCo four-parameter analytical function described precisely experimentally recorded
twitches of rat muscles. Each muscle is composed by many MUs with different
customizable twitch parameters. A hierarchical genetic algorithm is implemented to
manipulate the time moments of MUs impulsation which are design variables. Using
genetic operations initial solutions are modified and new populations are estimated by a
fitness function. Elbow flexion/extension movements in the sagittal plane with different
controlled flexion speeds were simulated with the software and the results were
compared with experimental data. Two muscles were modeled - the flexor biceps
brachii and the extensor triceps brachii. Considerable antagonistic co-contraction was
predicted when the external joint moment changed its direction, which was observed for
the fastest motions. The muscle model composed by MUs predicts better experimental
observations and allows deeper investigation of the motor control. In contrast to static
optimization, the estimation of the fitness function is made for the whole movement
duration. Since the software provides information about all MUs impulsation, it can be
useful for generation of suitable patterns for functional electrical stimulation.
2. INTRODUCTION
It is reported in many papers (for a review see in [1, 2]) that using one degree of
freedom (DOF) macro-biomechanical models antagonistic co-contraction can not be
1
Professor, Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Sofia
1113, Acad. G.Bonchev Str., bl.105, Bulgaria
2
Assoc. Professor, Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences,
Sofia, 1113, Acad. G.Bonchev Str., bl.105, Bulgaria
3
Assistant Professor, Department of Motor Rehabilitation , University School of Physical Education,
27/39 Królowej Jadwigi Str., 61-871, Poznan, Poland
4
Assistant Professor, Department of Biochemistry, University School of Physical Education, 27/39
Królowej Jadwigi Str., 61-871, Poznan, Poland