An experimental method is presented to investigate the complex kinesiology of the extensor apparatus.

Microstrain gauges were applied to the tendinous components of the extensor apparatus of five fingers—one chimpanzee and four human adult. The strain variations produced in these tendons during basic finger motions were recorded on a Beckman-type RM direct writing oscillographic recorder.

During extension of the finger without hyperextension at the metacarpophalangeal, significant tension was present in the middle slip and the terminal tendon of the extensor apparatus. During clawing of the finger with hyperextension at the metacarpophalangeal joint the tension was absent in both the middle slip and the terminal tendon at the end of the motion.

The extensor apparatus was loaded in a distal direction by the pull of the flexor profundus. The concept of distal differential loading of the extensor apparatus was developed by Landsmeer. During flexion of the distal phalanx there is a relaxation of the middle slip with gradual increase of tension in the same tendon beyond 60 to 70 degrees of flexion at the middle joint. At 90 degrees of flexion in the middle joint of the finger there is relaxation of the terminal tendon. The data obtained in these experiments support Landsmeer's concept.

The tension in the transverse lamina of the extensor appratus was studied during clawing and extension of the finger without hyperextension of the metacarpophalangeal joint. Increased tension was recorded in the transverse lamina only at hyperextension and flexion of the metacarpophalangeal joint.