intrinsic muscles. Definitions: 1. muscles fully contained (origin, belly, and insertion) within the structure under consideration. For example, the interossei and lumbrical muscles are intrinsic muscles of the hand.The definition information for intrinsic muscles is provided by Stedman's.You can search our medical dictionary here.efinitions: 1. muscles fully contained (origin, belly, and insertion) within the structure under consideration. For example, the interossei and lumbrical muscles are intrinsic muscles of the hand. The definition information for intrinsic muscles is provided by Stedman's.