We have established a simple method of measuring joint motion under physiological conditions. For this purpose we use an ultrasound measuring system employing marker points consisting of miniaturized ultrasound transmitters. This device was tested on a simple biomechanical model, the linkage of the proximal and distal interphalangeal joints. The angles of these joints were recorded during opening and closing of the fist in 34 index fingers of 17 healthy persons. The results of the measurements were plotted on a rectangular coordinate system. Analysis showed an approximately linear linkage between the IP joints of the index finger. The curve for extension was the same as that for flexion. The linkage varies greatly. On average 1 degree of PIP joint flexion is equivalent to 0.76 degree of DIP joint flexion. Our study showed no significant difference between the dominant and non-dominant hand. The results showed that there is a linear linkage between the proximal and distal interphalangeal joints, which is equal for flexion and extension.