Meeting Abstract
50.1 Sunday, Jan. 5 10:15 Passive Properties of Anuran Hindlimb Muscles DANOS, N*; AZIZI, E; Univ. of California Irvine; Univ. of California Irvine ndanos@uci.edu
Frog muscles are unusually compliant compared to mammalian muscles. An important implication of this muscle property is that frog muscles can be stretched to long lengths and can operate on the descending limb of the active force-length curve during jumping. In addition, increased muscle compliance may allow for larger range of motion at joints during jumping. However, it is unclear whether the compliance observed in the hindlimb muscles of anurans is a general pattern within the group or whether these passive properties are a jumping specialization. We examined the correlation between locomotor mode and muscle compliance within a single phylogenetic group, the Anura, which exhibits variation in both locomotor modes and hindlimb muscle stiffness. Four species representing four locomotor specialists were chosen: Rana catesbeiana (jumper), Bufo marinus (hopper), Xenopus laevis (swimmer) and Kassina senegalensis (runner/walker). We hypothesized that smaller angles at the knee and ankle joints during locomotion will be associated with more compliant extensor muscles. We measured passive muscle tension in fiber bundles isolated from the cruralis and plantaris muscles of each species. Bundles were subjected to cyclical stretching (5 Hz) and laser diffraction was used to determine sarcomere lengths and define L0. We find that ankle joint range of motion is a good predictor of plantaris passive stiffness, but that all cruralis muscles showed similar passive properties. The differences are likely due to other factors affecting passive moments at joints, such as relative muscle mass, muscle moment arms and total joint excursion. The ankle undergoes larger joint excursions than the knee and the plantaris, being the dominant ankle extensor and a much bigger muscle than the cruralis, appears to play a more significant role in constraining joint angles.
