The tendon of flexor pollicis longus: its effects on the muscular control of force and position at the human thumb.

Human flexor pollicis longus tendons obtained at autopsy were subjected to repeated sinusoidal stretching movements. The associated force changes were almost in phase with the movement (force led position by less than 4 degrees), and alterations in the frequency of movement between 2 and 16 Hz had little effect on them. Examination of the thumbs of formalin-fixed cadavers demonstrated that the tendon exerts its force 7.4-8.0 mm in front of the axis of movement of the interphalangeal joint. From a knowledge of the tendon properties and the joint anatomy, one can calculate the changes in tendon length that would accompany any force change at the joint. Equipped with this information one can re-examine the responses to sinusoidal movements of the thumb interphalangeal joints of normal subjects. When the subject exerts a steady flexing force in which stretch reflexes play no important part, measurements of joint stiffness indicate that only a portion of the imposed movement reaches the muscle fibres. The extension of those (visco-elastic) muscle fibres lags behind the extension of the (elastic) tendon. Stretch reflexes contribute to the mechanical resistance of muscle fibres to low frequency (3-5 Hz) displacements, but in the presence of a compliant tendon the length of the muscle fibres does not determine the angle of the joint in any positive way. It is suggested that the compliant tendons of many thumb and finger muscles simplify the neuromuscular control of forces during gripping and handling movements.