Meeting Abstract
P3.50 Tuesday, Jan. 6 Motor Patterns of Distal Hindlimb Muscles in Walking Turtles: Implications for Models of Limb Bone Loading SCHOENFUSS, HL; ROOS, JD; RIVERA, ARV; BLOB, RW*; St. Cloud State Univ, MN; St. Cloud State Univ, MN; Clemson Univ, SC; Clemson Univ, SC hschoenfuss@stcloudstate.edu
Previous studies indicate that the ground reaction force (GRF) in walking turtles exerts a flexor moment at the ankle during stance. As a result, extensor muscles must be active to counter the GRF. Of four proposed ankle extensors in turtles, two (gastrocnemius medialis and pronator profundus) originate on the tibia and fibula respectively, while the other two (flexor digitorum longus, gastrocnemius lateralis) originate from the distal femur, crossing the flexor side of the knee and potentially eliciting antagonist forces from muscles on the extensor surface of the femur that could contribute to femoral stress. Current bone stress models assume that all four muscles are active. However, if only ankle extensors crossing the knee were active during stance then femoral stresses might be higher than predicted, whereas if only those not crossing the knee were active then stresses might be lower. We recorded EMGs from the four turtle ankle extensors during treadmill walking to test which need to be incorporated into bone stress models. Analysis of synchronized footfall and EMG patterns revealed that all four muscles were active during at least part of the stance phase, corroborating previous bone stress calculations. However, functional differentiation among these muscles is evident with, for example, the two heads of the gastrocnemius exhibiting second activity bursts during swing phase, and flexor digitiorum longus activity persisting through stance phase into the beginning of swing phase. These subtle distinctions in muscle contraction patterns among these four ankle extensor muscles indicate the potential for considerable flexibility in the control of distal hindlimb motion in turtles. Supported by NSF IOB-0517340.
