Force fluctuations in a simulated motoneuron pool

Abstract

Force fluctuations, even for maintained isometric force, have many origins and are attributed to noise, errors and variabilities associated with different elements of the sensory-motor system. Part of the random-like fluctuations of the force is due to the motor pathway per se and much work has been focused in quantifying the fluctuations and in determining their dependence on variables such as number of motor units, discharge rate variability of each motor unit, etc. A recently developed simulator of the neuromuscular control system was utilized here to verify stochastic features of the simulated force signal generated by the soleus muscle for different mean force levels. The mean force level increased in a sigmoidal fashion as the intensity of the descending drive increased. The standard deviation of force as a function of mean force level resulted in an approximate parabola, while the coefficient of variation of force decreased monotonically with mean force level. Force power spectrum was strongly concentrated in the 0-5 Hz frequency range. Individual motor unit force signals showed a tendency for decreased force variability as overall muscle force level increased, helping to explain the relation found between muscle force variability and mean muscle force. Comparisons are made with results from the literature of simulations of other muscles and the differences and similarities are discussed. © 2009 Springer-Verlag.