Activation of human arm muscles during flexion/extension and supination/pronation tasks: a theory on muscle coordination.

Generally the number of muscles acting across a joint exceeds the number of degrees of freedom available to the joint. This redundancy raises a problem regarding the ratio in which these muscles are activated during a particular motor task. In this paper we present a theory to explain the activation patterns of muscles used during voluntary and reflex induced contractions. The basic assumptions underlying the theory are that 1) coordination of muscles is based on synergistic muscle activities, 2) the synergisms involved satisfy certain transformations of muscle spindle signals to muscle activation signals and 3) muscle spindle output is proportional to the ratio of muscle stretch and muscle length in lengthening muscles, and is zero in shortening muscles. The theory is used to predict the recruitment threshold of motor units in six arm muscles during voluntary isometric contractions. All theoretical predictions are in reasonable agreement with the experimentally observed behavior of a large population of motor units within each muscle. However, within a single muscle sometimes motor-unit populations have been found to have different types of recruitment behavior. This deviating behavior is discussed in the light of the theory presented here.

Inhomogeneous activation of motoneurone pools as revealed by co-contraction of antagonistic human arm muscles.

There is now considerable evidence that motoneurone pools in several human arm muscles are activated inhomogeneously during isometric and reflex-induced contractions in different directions (van Zuylen et al. 1988; Gielen et al. 1988; ter Haar Romeny et al. 1984). In this paper we investigate the activation of antagonist muscles (m. brachialis, m. brachioradialis, m. biceps and m. triceps) during co-contraction of the upper arm muscles. The results show that there is a marked difference between the distribution of the activities of synergistic flexor muscles, or even within these muscles, in co-contraction tasks and in flexion tasks. This discrepancy may be attributed to the existence of inhibitory mechanisms between motoneurone pools of antagonist muscles. These mechanisms can also account for different types of recruitment behaviour of motor unit populations in a single muscle.

A general model for the absorption of ultrasound by biological tissues and experimental verification.

In this paper, a closed-form expression is derived for the absorption of ultrasound by biological tissues. In this expression, the viscothermal and viscoelastic theories of relaxation processes are combined. Three relaxation time distribution functions are introduced, and it is assumed that each of these distributions can be described by an identical and simple hyperbolic function. Several simplifying assumptions had to be made to enable the experimental verification of the derived closed-form expression of the absorption coefficient. The simplified expression leaves two degrees of freedom and it was fitted to the experimental data obtained from homogenized beef liver. The model produced a considerably better fit to the data than other, more pragmatic models for the absorption coefficient as a function of frequency that could be found in the literature. Scattering in beef liver was estimated indirectly from the difference between attenuation in in vitro liver tissue as compared to absorption in a homogenate. The frequency dependence of the scattering coefficient could be described by a power law with a power of the order of 2. A comparable figure was found in direct backscattering measurements, performed at our laboratory with the same liver samples [Van den Aarssen et al., J. Acoust. Soc. Am. (to be published)]. A model for scattering recently proposed by Sehgal and Greenleaf [Ultrason. Imag. 6, 60-80 (1984)] was fitted to the scattering data as well. This latter model enabled the estimation of a maximum scatterer distance, which appeared to be of the order of 25 micron.