Simulations suggest walking with reduced propulsive force would not mitigate the energetic consequences of lower tendon stiffness

by Richard E. Pimentel, Gregory S. Sawicki, Jason R. Franz

Aging elicits numerous effects that impact both musculoskeletal structure and walking function. Tendon stiffness (k_T) and push-off propulsive force (F_P) both impact the metabolic cost of walking and are diminished by age, yet their interaction has not been studied. We combined experimental and computational approaches to investigate whether age-related changes in function (adopting smaller F_P) may be adopted to mitigate the metabolic consequences arising from changes in structure (reduced k_T). We recruited 12 young adults and asked them to walk on a force-sensing treadmill while prompting them to change F_P (±20% & ±40% of typical) using targeted biofeedback. In models driven by experimental data from each of those conditions, we altered the k_T of personalized musculoskeletal models across a physiological range (2–8% strain) and simulated individual-muscle metabolic costs for each k_T and F_P combination. We found that k_T and F_P independently affect walking metabolic cost, increasing with higher k_T or as participants deviated from their typical F_P. Our results show no evidence for an interaction between k_T and F_P in younger adults walking at fixed speeds. We also reveal complex individual muscle responses to the k_T and F_P landscape. For example, although total metabolic cost increased by 5% on average with combined reductions in k_T and F_P, the triceps surae muscles experienced a 7% local cost reduction on average. Our simulations suggest that reducing F_P during walking would not mitigate the metabolic consequences of lower k_T. Wearable devices and rehabilitative strategies can focus on either k_T or F_P to reduce age-related increases in walking metabolic cost.