Motor patterns of distal hind limb muscles in walking turtles: Implications for models of limb bone loading

Previous studies of limb bone loading in walking turtles indicate that the ground reaction force exerts a flexor moment at the ankle during stance, requiring extensor muscle activity to maintain joint equilibrium. Of four proposed ankle extensors in turtles, two (gastrocnemius medialis, 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 aspect of the knee and potentially eliciting compensatory forces from antagonist knee extensor muscles that could contribute to femoral stress. Published bone stress models assume all four proposed ankle extensors are active during stance in turtles. However, if only the ankle extensors that cross the knee were active then femoral stresses might be higher than predicted by published models, whereas if only extensors that do not cross the knee were active then femoral stresses might be lower than predicted. We analyzed synchronized footfall and electromyographic activity patterns in slider turtles (Trachemys scripta) and found that all four proposed ankle extensors were active during at least part of stance phase in most individuals, corroborating bone stress models. However, activation patterns were complex, with multiple bursts in many ankle extensors that frequently persisted into swing phase. In addition, two hypothesized ankle flexors (tibialis anterior, extensor digitorum communis) were frequently active during stance. This might increase the joint moment that ankle extensors must counter, elevating the forces they transfer across the knee joint and, thereby, raising femoral stress. Recognition of these activity patterns may help reconcile differences between evaluations of loads on turtle limbs based on force platform versus in vivo strain studies. Moreover, while some variation in motor patterns for the distal hind limbs of turtles may reflect functional compartmentalization of muscles, it may also indicate flexibility in the control of their limb movements.