38 CFR § 4.40 - Functional loss.
Disability of the musculoskeletal system is primarily the inability, due to damage or infection in parts of the system, to perform the normal working movements of the body with normal excursion, strength, speed, coordination and endurance. It is essential that the examination on which ratings are based adequately portray the anatomical damage, and the functional loss, with respect to all these elements. The functional loss may be due to absence of part, or all, of the necessary bones, joints and muscles, or associated structures, or to deformity, adhesions, defective innervation, or other pathology, or it may be due to pain, supported by adequate pathology and evidenced by the visible behavior of the claimant undertaking the motion. Weakness is as important as limitation of motion, and a part which becomes painful on use must be regarded as seriously disabled. A little used part of the musculoskeletal system may be expected to show evidence of disuse, either through atrophy, the condition of the skin, absence of normal callosity or the like.