AHN, A.N.; BIEWENER, A.A.; Harvard University; Harvard University: IN VIVO LENGTH CHANGES IN RELATION TO IN VITRO FORCE-LENGTH PROPERTIES IN THE TOAD SEMIMEMBRANOSUS MUSCLE.

There is evidence that adjacent muscle segments can contract heterogeneously under in vivo conditions implying that the two segments (& their sarcomeres) may operate on different regions of their force-length relationships simultaneously. To examine this issue, we measured segment strain patterns in vivo using 3 sonomicrometry crystals implanted along the length of the semimembranosus muscle in the American Toad (Bufo americanus; n = 4) to record adjacent segment strains in vivo during hopping. On the same day, we then compared the lengths measured in vivo to their respective in vitro force-length properties at four stimulation levels (tetanic, 200Hz for 50ms; 200Hz for25 ms, and twitch). In vivo during hopping, the muscle segments always operated on the ascending portion of their respective force-length relationships of all stimulation levels. Moreover, the angles of the ascending limbs differed between the two segments. Two adjacent segments can operate on different regions of their force-length relationships simultaneously both in vivo and in vitro. Understanding regional differences within muscles in vivo will allow us to link our understanding of sarcomere behavior with whole muscle behavior during locomotion.