Question: Hand pain felt when shaking someone's hand means there likely is an injury to what structures?

Answer: The interosseus muscles in the hand.

Have you ever had someone shake your hand so hard it hurt? What you felt being squeezed were the small interosseus muscles (or interossei), which run between the metacarpal bones and help control fine motor movements of the fingers. There actually are two different sets of interossei in the hand. The palmar interossei, located on the anterior side of the metacarpals, adduct the fingers (toward the middle finger). The dorsal interossei, located between the metacarpals, abduct the fingers (away from the middle finger). Both sets of muscles also help flex the metacarpophalangeal joints and extend the interphalangeal joints.

The interosseous muscles of the hands get injured when fatigued through overuse, such as many straight hours of typing. When these muscles are inflamed, a moderate squeezing of the metacarpal bones (as in a handshake) produces pain. There also are interosseous muscles in the foot, which perform similar functions. Injuries to these muscles occur more frequently, often through running, and cause a great deal of foot pain.

You can test for injury to the interosseous muscles either by gently squeezing the hand or by performing resisted abduction and adduction of the affected fingers. These small but important muscles easily can be treated with friction or massage therapy.