The action of ‘making a fist’ or of gripping an object is primarily one of contracting the long flexor tendons of the fingers and thumb; but synergic contraction of the extensors of the wrist is indispensable to an efficient grip.
As the long flexor tendons contract the interphalangeal joints are flexed. This movement is overlapped and followed by the flexion of the metacarpophalangeal joints. The thumb is opposed and then flexed against the fingers. Simultaneously with the closing of the digits the wrist is moved into extension by contraction of its radial and ulnar extensors. The length of the flexor muscles is thereby increased and their power of contraction greatly enhanced. A powerful grip cannot be exerted with the wrist in full flexion, neither can the fingers be closed into a fist.
Most of the long flexor musculature arises from radius and ulna. The humeral head of flexor superficialis, however, passes across the elbow joint. Its flexing action on the elbow during gripping is counteracted by synergic contraction of the triceps.