Effect Of Tendon Stiffness And Leg Stiffness On Running Economy In Well-trained Middle Distance Runners

Abstract

PURPOSE: Previous research suggests that a stiff 'leg spring' during running can provide significant energy savings from the elastic properties in the leg‘s soft tissues. Knowing that the absorption and recovery of elastic energy in muscle-tendon units is important in determining the energy cost of running, researchers have used the leg stiffness measured during the ground contact phase in running as a surrogate for muscle-tendon stiffness. However, no research has examined both the leg spring stiffness and the stiffness of the major tendons simultaneously. The aim of the present study was to examine how leg stiffness and the mechanical properties of the primary ankle and knee tendons influence running economy. METHODS: Eleven well-trained male middle distance runners volunteered for the study. Medial gastrocnemius (MG) and vastus lateralis (VL) tendon force-elongation properties were measured during an isometric maximal voluntary contraction. Tendon forces were determined from joint moments measured by isokinetic dynamometry, corrected for agonist-antagonist co-contraction and joint rotation. Tendon elongation was measured using B-mode 2-D ultrasonography. Eccentric leg stiffness was measured during over-ground running using a previously published kinetic model. Total leg stiffness of the entire ground contact phase was derived from kinetic and kinematic measures. Running economy was measured using a standardised incremental treadmill protocol. RESULTS: Analysis revealed no correlation with MG or VL tendon stiffness and either eccentric (r = -0.03 and r = -0.07, respectively) or total leg stiffness (r = -0.06 and r = 0.16, respectively). Leg stiffness was not correlated with running economy; however, MG tendon stiffness and peak ankle joint moment were significantly correlated with economy (r = -0.66 and r = -0.68, respectively). CONCLUSIONS: Leg stiffness does not appear to be a surrogate for tendon stiffness and is not predictive of running economy in well-trained runners. Leg stiffness might be influenced by changes in leg mechanics or muscular co-contraction. Interestingly, Achilles tendon stiffness tends to be greater in more economical runners. Faster tendon recoil, indicative of stiffer tendons, could benefit power generation at the ankle joint during high speed running.