Functional, anatomical, and neurophysiological study of human upper limb muscles

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1.Functional and anatomical studies of the elbow flexors.Forearm supination produced by electrical activation of two elbow flexors (biceps brachii : BB,brachioradialis : BR) was examined in healthy human subjests. The muscles were activated separately by electrical stimulation through wire electrodes with a computer-controlled multi-channel stimulator. Activation of BB induced flexion and supination and that of BR induced flexion. Activation of BR with adequate stimulation intensity provided holding of elbow with maintaining the forearm prone. In this situation, additional activation of BB accompanied with a decrease of stimulation intensity for BR resulted in supination without flexion. Since the triceps brachii showed no contraction throughout the motion, it supports a theory that compensatory changes of activities among the flexors permit BB to work for supination without flexion.2.Neurophysiological studies of the muscles crossing the elbow joint.Neural projections among the biceps brachii (BB), brachioradialis (BR), and pronator teres (PT) in humans were studied using the method of post-stimulus time histograms of voluntarily activated motor units. Electrical conditioning stimuli to the BB,BR and PT nerves resulted in inhibition in motor units of BR and PT,of BB and PT,and of BB,respectively. The inhibition was induced with the stimulation intensity well below the motor threshold. The latency of the inhibition was 0-2ms longer than that of the homonymous facilitation. These findings suggest that inhibitiory projections between BB and BR,and between BB and PT,and from BR to PR exist and the inhibition is mediated through group I afferents and oligo (di or tri) -synaptic linkage.