Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment?

The recruitment and the rate of discharge of motor units are determinants of muscle force. Within a motoneuron pool, recruitment and rate coding of individual motor units might be controlled independently, depending on the circumstances. In this study, we tested whether, during human quiet standing, the force of the medial gastrocnemius (MG) muscle is predominantly controlled by recruitment or rate coding. If MG control during standing was mainly due to recruitment, then we further asked what the trigger mechanism is. Is it determined internally, or is it related to body kinematics? While seven healthy subjects stood quietly, intramuscular electromyograms were recorded from the MG muscle with three pairs of wire electrodes. The number of active motor units and their mean discharge rate were compared for different sway velocities and positions. Motor unit discharges occurred more frequently when the body swayed faster and forward (Pearson R = 0.63; P < 0.0001). This higher likelihood of observing motor unit potentials was explained chiefly by the recruitment of additional units. During forward body shifts, the median number of units detected increased from 3 to 11 (P < 0.0001), whereas the discharge rate changed from 8 ± 1.1 (mean ± SD) to 10 ± 0.9 pulses/s (P = 0.001). Strikingly, motor units did not discharge continuously throughout standing. They were recruited within individual, forward sways and intermittently, with a modal rate of two recruitments per second. This modal rate is consistent with previous circumstantial evidence relating the control of standing to an intrinsic, higher level planning proces

Dynamics of Simple Balancing Models with State Dependent Switching Control

Time-delayed control in a balancing problem may be a nonsmooth function for a variety of reasons. In this paper we study a simple model of the control of an inverted pendulum by either a connected movable cart or an applied torque for which the control is turned off when the pendulum is located within certain regions of phase space. Without applying a small angle approximation for deviations about the vertical position, we see structurally stable periodic orbits which may be attracting or repelling. Due to the nonsmooth nature of the control, these periodic orbits are born in various discontinuity-induced bifurcations. Also we show that a coincidence of switching events can produce complicated periodic and aperiodic solutions.Comment: 36 pages, 12 figure

A NOVEL SYSTEM OF ELECTRODES TRANSPARENT TO ULTRASOUND FOR SIMULTANEOUS DETECTION OF MYOELECTRIC ACTIVITY AND B-MODE ULTRASOUND IMAGES OF SKELETAL MUSCLES

Application of two dimensional surface electrode arrays can provide a means of mapping motor unit action potentials on the skin surface above a muscle. The resulting muscle tissue displacement can be quantified, in a single plane, using ultrasound imaging (US). Currently however, it is not possible to simultaneously map spatio-temporal propagation of activation and resulting tissue strain. In this manuscript we eveloped and tested a material that will enable concurrent measurement of 2D surface EMGs with US images. Specific protocols were designed to test the compatibility of this new electrode material both with EMG recording and with US analysis. Key results indicate that, for this new electrode material: i) the electrode-skin impedance is similar to that of arrays of electrodes reported in literature; ii) the reflection of ultrasound at the electrode-skin interface is negligible; iii) the likelihood of observing missing contacts, short-circuits and artefacts in EMGs is not affected by the US probe; iv) movement of tissues sampled by US can be tracked accurately. We therefore conclude this approach will facilitate multi-modal imaging of muscle to provide new spatio-temporal information regarding electromechanical function of muscle. This is relevant to basic physiology-biomechanics of active and passive force transmission through and between muscle, of motor unit spatio-temporal activity patterns, of their variation with architecture and task related function, and of their adaptation with ageing, training-exercise-disuse, neurological disease and injur