The relationship of muscle and bone structure to limb weakness was examined in 60 Duroc pigs from 3 lines divergently selected for thoracic limb weakness. The lines were designated high, control, or low, with the low line having inferior thoracic limb structure. At approximately 100 kg, 10 pigs of each line and gender were scored for thoracic limb structure and movement. Right and left thoracic limbs were collected at slaughter. A computerized morphometric image analysis system was used to determine cross-sectional areas of muscles, bones, and soft tissues at levels through the brachium, antebrachium, metacarpus, and digits. The statistical model that was used to analyze the data included the effects of line, sire, gender, and side (left vs right), with weight as a covariate. Total bone area was similar for all 3 lines of pigs at all cross-sectional levels, but significant differences in muscle and other soft tissue areas were observed, including significantly greater extensor area for the antebrachium (P less than 0.001) in low-line pigs than in control- and high-line pigs, smaller total area (P less than 0.05) of the metacarpus in low-line pigs than in control and high-line pigs, and less total area of the medial digit (P less than 0.01) in low-line pigs than in control- or high-line pigs. Total area of bone and soft tissue for each cross-sectional region was significantly greater (P less than 0.05) in boars than in gilts. Side differences also were observed in total cross-sectional areas of bone and soft tissue of the antebrachium, metacarpus, and digits. Pigs selected for inferior thoracic limb structure had less total soft tissue cross-sectional areas in distal limb regions than did control- or high-line pigs. Limb weakness may be related to altered distribution of soft tissue supporting structures of the thoracic limb.