Figure 1.

The relative orientation of the 2D principal axes of strain rate (SR_fiber, SR_in-plane) compared to that of the muscle fiber (normal strain along the fiber is denoted as SR_ff) and cross-section (normal strain in the fiber cross section is denoted as SR_cc) in isometric (1a). Shear strain is the off-diagonal term obtained by rotating the tensor from the principal basis to that of the muscle fiber basis and its origin is schematically shown in 1b (muscle and endomysium not drawn to scale). In the current shear strain model one end of the endomysium is fixed while the other end moves with the muscle fiber (muscle fiber contracts); the dotted outline is that of the undeformed element in the endomysium and the solid line is the deformed element due to muscle contraction (1b).