Postural responses to combined vestibular and hip proprioceptive stimulation in man.

Vestibular-proprioceptive interaction in human postural control in the frontal plane was studied by analysing the lateral body sway evoked in a standing subject by a weak, near-threshold galvanic vestibular stimulation combined with a balanced, bilateral vibration of the medial gluteus muscles. The intensities of the stimuli were adjusted so that none of them produced a consistent postural response when delivered alone. The pattern of the lateral body sway evoked by the combined stimulation was compared with postural responses to suprathreshold vestibular stimulation and asymmetric (unilateral) vibration of the hip abductors.

Motor unit discharge during muscular after-contraction.

Discharges of single motor units (MUs) in human triceps brachii and deltoid muscle were recorded using needle electromyography during after-contraction and voluntary contraction performed either against a small elastic load or under isometry. The steady-state firing rate of the MUs was lower under after-contraction than during voluntary movement of comparable amplitude and time course (or isometric force level), whereas variability of interspike intervals was similar under the two conditions. In the tibialis anterior muscle (where after-contraction was lacking), a weak voluntary contraction preceded by sustained strong voluntary effort also showed lower firing rate of MUs as compared to similar voluntary movement performed after a rest period.