• Opposition of the small finger towards the thumb.
• Hypothenar eminence muscle (an intrinsic hand muscle).
Primary: Opposes (flexes with slight rotation) the carpo-metacarpal joint of the small finger. True opposition occurs when the thumb and small finger come together in contact.
Secondary: Assists in the cupping posture of the hand.
Palpation: Push radially off the fifth metacarpal in the volar direction to move the abductor digiti minimi over to palpate opponens digiti minimi.
Strength Testing: Position – the hand palm up. Stabilize – the hand and first metacarpal. Resist – against opposition of the metacarpal of the small finger towards the thumb.
Possible Substitutions: Abductor digiti minimi – in order to distinguish this substitution and the differences, it is important to observe whether the small finger rotates or abducts during opposition. Rotation of the small figure is evidence for a working opponens while abduction without rotation of the small finger during opposition is caused by the abductor substituting for the opponens being out.
• Roots: C8, T1.
• Nerve: Ulnar nerve, deep (motor) branch.
• Innervation Route: C8, T1 → ulnar nerve → deep (motor) branch → opponens digiti minimi branch.
Origins: Hook of hamate and flexor retinaculum.
Insertion: Entire length of fifth metacarpal, ulnar aspect.