Ballistic flexion movements of the human thumb.

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Abstract
In response to an auditory stimulus normal subjects made ballistic flexion movements of the top joint of the thumb against a lever attached to the spindle of a low-inertia electric motor. Electromyographic (EMG) activity was recorded from pairs of fine wire electrodes inserted into flexor pollicis longus and extensor pollicis longus, respectively, the sole flexor and extensor of the joint. Movements of 5.degree., 10.degree. and 20.degree. were made from initial angles of 10.degree., 20.degree. and 30.degree. flexion against torques of 0.04, 0.08 and 0.16 Nm. The EMG activity initiating such movements was characterized by a triphasic pattern of sequential bursts of activity in the agonist (flexor pollicis longus), then in the antagonist (extensor pollicis longus), and then in the agonist again. The duration of the 1st agonist and 1st antagonist bursts ranged from about 50-90 ms and there was no significant change of burst length in the different mechanical conditions. Responses of the muscles to perturbations either before or during the ballistic movements were studied. Current in the motor could be altered so to extend the thumb (stretch), to allow it to accelerate (release), or to prevent further movement (halt). Suitably timed stretch increased the EMG activity of the 1st agonist burst while release decreased it. There was a small response of the agonist to stretch or halt timed to act during the interval between the first 2 agonist bursts; the major response was an augmentation of the 2nd agonist burst. Stretch, timed to act between the first 2 agonist bursts which released the antagonist, diminished the activity of the 1st antagonist burst while halt virtually eradicated it in all but 1 subject. Release, at this time, which stretched the antagonist, increased the activity of the 1st antagonist burst.