Effects of prior instruction and anaesthesia on long-latency responses to stretch in the long flexor of the human thumb.

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Abstract
Long-latency (40-80 ms) electromyographic (EMG) responses of the contracting flexor pollicis longus to stretches applied at the thumb-tip, were studied in normal human subjects. Stretches were applied during 4 classes of contraction: isometric hold, in which the subject held a steady isometric contraction; isometric tracking, in which the subject tracked a steadily rising force target; isotonic tracking, in which the subject flexed against a constant torque to track a position target; weight-lifting, in which the subject lifted a weight hung at 1 end of a lever by pressing the thumb-tip on the other end of the liver. The effects on the responses of prior instructions to resist or to let go, and of local anesthesia of the thumb, were studied. The ability to modify the size of the long-latency EMG response in accordance with prior instruction was viable. All subjects tested could do so during isometric holding contractions, but many could not do so during the other forms of contraction. Local anesthesia of the thumb significantly reduced the long-latency EMG response in only some subjects, and abolished it in none. The reduction was most reliably seen for isometric force tracking contractions. During thumb anesthesia in different subjects, there was a significant correlation between the proportional increase in apparent heaviness of an object lifted by thumb flexion and the proportional reduction in the size of the long-latency EMG response to muscle stretch.