| Popis: |
Limb muscles have important roles during locomotion, such as counteracting ground reaction forces (GRF) and generating propulsive mechanical work and power. Depending on the magnitude and direction of the GRF or the performance demands of locomotion, limb muscles may produce high forces that impose substantial loads on limb bones. While bone loading has been studied over a relatively broad phylogenetic and functional range of tetrapod lineages, much less is known about how muscle contractile function directly influences patterns and magnitudes of bone loading. To better understand mechanisms of limb bone loading in terrestrial locomotion, we correlated direct measurements of in vivo bone strain with muscle strain (via sonomicrometry) and EMG activation in a major hip extensor/knee flexor muscle (m. flexor tibialis internus) of river cooter turtles (Pseudemys concinna) during treadmill walking. EMG recordings indicate activity prior to footfall that continues through approximately 50% of the stance phase. The muscle fascicles reach their maximum length just after footfall and actively shorten to their minimum length at 35% of stance. At the time of peak bone strains (both principal and axial), the muscle fascicles are active, but are lengthening as the knee joint begins to extend. On average, the time difference between peak bone strain and muscle strain was 5% of stance. Thus, due to the coincidence between bone strains muscle shortening, bone loading patterns can be correlated directly with the action of limb muscles, refining models of femoral loading derived from force platform studies, and indicating a significant role for FTI in femoral loading in turtles that may explain differences in safety factor estimates between force plate and in vivo strain analyses. |
