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

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. Individual lower body muscles showed unique effects across the k T and F P landscape. 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. Author Summary Our muscles and tendons are affected by aging. Tendon stiffness and push-off forces both impact the energy cost of walking, which in turn increases with age. We investigated whether age-related changes in function (less push-off force) may be adopted to mitigate the metabolic consequences arising from structural changes (lower tendon stiffness). Reducing push-off force during walking would not mitigate the metabolic consequences of lower tendon stiffness. Wearable devices and rehabilitative strategies can focus on either tendon stiffness or push off intensity to reduce age-related increases in walking metabolic cost.