Long-latency (40-80 ms) electromyographic (e.m.g.) responses of the contracting flexor pollicis longus to stretches applied at the thumb-tip, were studied in normal human subjects. Stretches were applied during four classes of contraction: (i) isometric 'hold', in which the subject held a steady isometric contraction; (ii) isometric tracking, in which the subject tracked a steadily rising force target; (iii) isotonic tracking, in which the subject flexed against a constant torque to track a position target; (iv) weight-lifting, in which the subject lifted a weight hung at one end of a lever by pressing the thumb-tip on the other end of the lever. The effects on the responses of prior instructions to 'resist' or to 'let go', and of local anaesthesia of the thumb, were studied. The ability to modify the size of the long-latency e.m.g. response in accordance with prior instruction was variable. All subjects tested could do so during isometric holding contractions, but many could not do so during the other forms of contraction. Local anaesthesia of the thumb significantly reduced the long-latency e.m.g. response in only some subjects, and abolished it in none. The reduction was most reliably seen for isometric force tracking contractions. During thumb anaesthesia 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 e.m.g. response to muscle stretch.