Changes in tendon compliance and muscle energetics of in vivo human skeletal muscle

Recently published reports suggest the role of the muscles and tendons of the lower limbs are an important factor in determining the energy cost of running (Erun). Specifically, there exists a link between the mechanical properties of the Achilles tendon (AT) and Erun but the impact of the muscle’s energy cost is not considered. To date, very little is known regarding the interaction between AT stiffness, muscle energetics and Erun. Further, little is known about the AT stiffnessenergetics relationship in female runners. Therefore, the overall goal of this thesis was to explore the relationship between AT stiffness and muscle energetics in male and female distance runners. The first study revealed AT stiffness of female runners was lower than in males, but Erun was similar to males. Further, the relationship between Erun and Achilles tendon stiffness was not different between the sexes. Results from the second study demonstrated that when reductions in AT stiffness were simulated, the rate of muscle energy use was elevated and the magnitude of muscle activation needed to reach a target force was increased. A novel method of assessing AT moment arm was assessed in study four. A key finding was that moment arm did not change through ankle range of motion. These results were used in the fifth study which demonstrated using estimates of muscle energetics, along with kinematics and kinetics during running that strain energy release from the AT during running was significantly lower than the muscle energy cost required for strain energy storage to occur. Lastly, using a prolonged run as an acute method of reducing AT stiffness, the impact of changes in AT stiffness during running on muscle energetics and Erun was evaluated. Results from this final study suggest that prolonged running reduces AT stiffness, the impact of which is an elevated muscle energy cost and increased whole-body Erun without a significant increase in estimated AT strain energy release. Together these findings support the notion that the role of the AT in running is to accommodate muscle-tendon unit length change, thereby reducing the amount of muscle fascicle shortening and therefore muscle energy cost.