Impact of Achilles Tendon and Ankle Plantar Flexor Stiffness on the Net Metabolic Cost of Running

Objectives: This study aimed to report (1) the correlation between net metabolic cost (NMC) of running and Achilles tendon (AT) and ankle plantar flexor passive stiffness and (2) explore the relationship between passive stiffness and key running biomechanics. Methods: Twenty-two male recreational runners participated in this study. The runners performed a 5-minute run at 50% of their maximal aerobic speed as a warm-up on an instrumented treadmill, followed by a 6-minute run at 65% of their maximal aerobic speed wherein NMC was recorded. Passive stiffness was measured using a myotonometry device both before and after the run. Results: There was a negative linear relationship between NMC and AT stiffness. NMC and prerun AT stiffness demonstrated a significant negative correlation between leg stiffness and a significant positive correlation with braking peak force and step length. Moreover, we observed an increase in stiffness between prerun and postrun measurements at rest for the AT and both gastrocnemius muscles. Conclusion: Greater AT stiffness measured at rest is correlated with lower NMC.