Mechanochemical constants (elastic coefficients of the series and parallel elastic components, heat constants and rate constants of energy liberation) were estimated in human thumb muscles (flexor pollicis longus and extensor pollicis longus muscles). These constants were determined by comparing model responses with measured force caused by stretching or shortening of the thumb, over the interval when the contractile force was maintained constant; the interval was from the onset of length perturbation to 47ms after that onset. Close agreement between measured responses and calculated model responses was obtained. Some of the estimated mechanochemical constants depended on the contractile force, and changes in the constants were almost the same in the flexor and extensor muscles. (1) Both series and parallel elastic coefficients increased almost linearly with the contractile force. (2) The heat constant and the rate constant of energy liberation were constant, not dependent on the contractile force. This implied that the viscous coefficient increased almost linearly with the contractile force. An experimental setup was devised to measure the mechanical characteristics around the distal joint of the thumb. The displacement of the thumb in response to the sudden change in load was smaller and of higher frequency, when both flexor and extensor muscles generated larger contractile forces. A mathematical model consisting of both flexor and extensor muscles was presented, based on the above-mentioned muscle properties. Computer simulation showed the same characteristics as the experimental ones.