Figure 1.

Fiber strain heterogeneity in a pennate muscle. (A) Schematic of pennate muscle fiber arrangement during relaxed and contracted conditions. The schematic is based on the anatomy of turkey gastrocnemius muscle. m denotes the relaxed length of the muscle. During contraction there is a change in muscle length (Δm) along the line of action of the muscle. L_fp is the length of the proximal fiber, which has a pennation angle of about 30°. L_fd is the length of the distal fiber, which has a pennation angle of about 20°. Although the initial thickness of the muscle varies along the proximal to distal axis, the thickness does not change dynamically during the contraction. There is a change in the pennation angle from angle α to β during the contraction. This change in pennation angle alters the relationship between shortening of the fibers and the shortening of the whole muscle. (B) A simple geometric model predicts variation in proximal and distal muscle fiber strain as whole muscle strain increases. The model predicts that higher fiber strains are associated with muscle fibers with lower angles of pennation. Closed and open circles are empirically measured mean fiber strain values from the proximal and distal fibers in the turkey lateral gastrocnemius muscle (n = 5). Error bars represent the standard error of the mean.