Quantitative analysis of the intrinsic muscles of the foot

Background: Understanding muscle architecture of the foot may assist in the design of surgical procedures such as tendon transfer, biomechanical modeling of the foot, prosthesis design, and analysis of foot function. There is limited published information regarding foot intrinsic muscle architecture. Methods: Eleven fresh-frozen cadaveric feet were studied from eight males and three females. Twenty-eight intrinsic muscles were dissected in each foot, and measurements of fiber length, muscle length, and muscle volume obtained using calipers and water displacement technique. The physiologic cross-sectional area, fiber/muscle length ratio, muscle mass fraction, and tension fraction were then calculated. Results: Intrinsic muscle length was related to foot size. The mean fiber length ranged from 13.6 mm (first plantar interosseous) to 28.0 mm (second extensor digitorum brevis). The mean muscle length ranged from 24.8 mm (adductor hallucis transverse) to 115.8 mm (abductor hallucis). The mean muscle volume ranged from 0.4 cc (fifth lumbrical) to 15.2 cc (abductor hallucis). The physiologic cross-sectional area ranged from 0.28 cm² (second and third lumbrical) to 6.68 cm² (abductor hallucis). The fiber/muscle ratio ranged from 0.20 (abductor hallucis) to 0.82 (adductor hallucis transverse). The mass fraction ranged from 0.33% (fifth lumbrical) to 16.59% (abductor hallucis). The tension fraction ranged from 0.34% (fifth lumbrical) to 15.37% (abductor hallucis). Conclusions: The abductor hallucis and adductor hallucis oblique had much greater physiologic cross-sectional areas compared to those of the other intrinsic muscles. The lumbrical muscles had relatively low physiologic cross-sectional areas. These observations illustrate the underlying structural basis for the functional capacities of the intrinsic muscles of the foot.