Femur bone densities and tibialis anterior muscle properties of type II collagen-induced arthritic mice were determined. Furthermore, voluntary running activities of arthritic mice were compared with those of controls. Arthritis was induced by an intradermal injection of type II collagen in the adjuvant. Body and muscle weights were lower in arthritic mice than in controls. Cortical and trabecular bone densities and muscle fiber cross-sectional areas were decreased by arthritis. After classifying the arthritic severity into slight, intermediate, and severe levels based on the degree of knuckle swelling, cortical and trabecular bone densities, fiber cross-sectional areas, and fiber succinate dehydrogenase activities were lowest when arthritis was most severe. Furthermore, arthritic mice, especially those with intermediate and severe levels, showed lower voluntary running activities. These findings indicate that lower bone density and muscle atrophy of type II collagen-induced arthritic mice are related to arthritic severity and decreased motor activity.