Joint angle and muscle length manipulation reveal regional uniformity of sarcomere lengths in the passive rat medial gastrocnemius

The elongation of musculotendinous substructures attained during muscle stretching determines acute and long-term effects of stretching. We assessed how elongation is distributed across substructures of the rat medial gastrocnemius (MG) and how MG sarcomere elongation relates to plantarflexion resistance when the MG is stretched through joint rotation. We fixated 48 rat hindlimbs in 16 combinations of knee and ankle joint angles (55°, 90°, 125° and 160°), with three limbs per combination. We measured the MG muscle-belly length and the pennation angle in the proximal and distal region. We dissected four to five muscle-fibers from proximal, intermediate and distal muscle-belly regions and measured fiber and sarcomere length. We compared fiber and sarcomere lengths across regions and assessed how muscle-belly lengthening induced by knee or ankle rotation affected fiber length, sarcomere length and pennation angle. Lastly we measured plantarflexion torque of 12 rats in a range of ankle angles from 120° to 70° with the knee fixed in 90°. We found that pennation angles decreased and that fibers and sarcomeres lengthened uniformly across MG regions in response to muscle-belly lengthening. The onset of sarcomere lengthening (beyond 2.1 µm) preceded the onset of passive plantarflexion torque by aproximately 20°. Muscle-belly lengthening induced by ankle rotation had a greater impact on mean sarcomere length, fiber length, and pennation angle compared to knee rotation. In conclusion, sarcomeres lengthen uniformly across MG regions.