Isometric thumb extension (extensor pollicis brevis) (Thumb radial abduction (extension) strengthening isometric, palm down - extensor pollicis brevis)

Sit upright in a chair with your forearm and palm flat on the table and the tip of your thumb fully bent.
Place a finger from your opposite hand on the back of your thumb between the knuckle and end joint of your thumb.
Try to lift your thumb back from the table, using your opposite hand to resist the movement.
Try to hold this position without moving, making sure that you keep the tip of your thumb bent.