Isometric thumb MCP joint flexion (flexor pollicis brevis) (Thumb MCP joint flexion strengthening isometric - flexor pollicis brevis)

Sit upright in a chair with your affected arm on the table and your hand on its side.
Keeping your wrist and hand straight, place a finger from your opposite hand between the knuckle and end joints of your thumb.
Try to bend the knuckle of your thumb without bending the tip, using your opposite hand to resist the movement.
Try to hold this position without moving.