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The hand serves as the origin and/or insertion for a vast number of muscles. The intrinsic muscles of the hand contain the origin and insertions within the carpal and metacarpal bones. The extrinsic muscles of the hand originate outside the hand, commonly the forearm, and insert into hand structures. A rule of thumb is that any muscle tendon that crosses a joint will act on that joint. For example, muscles of the forearm that cross the carpometacarpal joint will produce flexion or extension at the wrist joint.