1. A method is described for driving the interphalangeal joint of the thumb through repeated sinusoidal flexion-extension movements, while immobilizing other joints of the wrist and hand.2. The joint met the sinusoidal movement with a force that fluctuated in an approximately sinusoidal manner. This paper is concerned with the relationship between the position and force sinusoids.3. When the thumb was relaxed the forces were small, but when the flexor pollicis longus was tetanically stimulated there was a large force change in response to each cycle of the movement. In either case, the maximum resistance to extension occurred during the later part of the extension movement, some 20 degrees -45 degrees in advance of maximum extension. A similar result was obtained when the subject exerted a maximal voluntary flexing force.4. The resistance to movement can conveniently be displayed as a stiffness vector, the amplitude of which is the force/displacement, and the phase is the angle by which the force sinusoid leads the position sinusoid.5. When the subject maintained a continuous, though sub-maximal, flexing effort the timing of the force fluctuations changed with changing frequency of movement in a characteristic way, and the stiffness vectors described a C-shaped or spiral path. With increasing frequency the stiffness vectors moved round this path in a clockwise direction.6. For descriptive purposes the resistance to movement can usefully be regarded as the vector sum of a reflex and a non-reflex component. Since the reflex pathway involves significant conduction delays, the reflex force can be expected to appear later in the cycle of a higher frequency movement, and give rise to a vector which moves round in a clockwise direction as the frequency increases. The non-reflex stiffness, however, changes much less with frequency.7. It is concluded that the size of the C-shaped or spiral vector path gives an indication of the strength of the reflex activity, while the position of the high frequency points gives an indication of the non-reflex resistance to the movement.