AHN, A.N.; MONTI, R.J.; BIEWENER, A.A.; Harvard University; Harvard University; Harvard University: In vivo and in vitro heterogeneity of segment length changes in the toad semimembranosus.

Many studies examine sarcomere dynamics in single fibers or whole muscle function, but few studies link the two levels of organization. To link data addressing in vitro sarcomere behavior with in vivo muscle behavior during locomotion, we measured segment strain patterns in vivo using 3 sonomicrometry crystals implanted along the length of the semimembranosus in the American Toad (Bufo americanus; n = 6) during hopping. The center crystal emitted the ultrasonographic signal, while the outer crystals received the signals allowing us to measure the instantaneous muscle lengths from two adjacent segments. Proximal segments shortened 13.2�4.7%, 14.8�2.3%, and 15.6�6.0% when animals hopped 1, 2, and 2.6 body lengths, respectively. The strain of the distal segment was lower and more variable, often lengthening before shortening during the hop. From rest length, the distal segment lengthened 3.4�2.6, 2.6�2.0, and 2.9� 2.2% before shortening 3.9�3.7, 5.9�3.8, and 5.7�1.9%. During cyclical contractions in vitro, the segment strain pattern depended on the phase of stimulation. When stimulated as the whole muscle-tendon lengthened (n = 4), the adjacent segments exhibited heterogeneity where the central segment shortened, while the distal segment lengthened. These differences in strain amplitude imply that the two adjacent segments are operating on different regions of their force-length and force-velocity relationships simultaneously. Understanding regional differences within muscles in vivo will allow us to link our understanding of sarcomere behavior with whole muscle behavior during locomotion.