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 (kT) and push-off propulsive force (FP) 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 FP) may be adopted to mitigate the metabolic consequences arising from changes in structure (reduced kT). We recruited 12 young adults and asked them to walk on a force-sensing treadmill while prompting them to change FP ({+/-}20% & {+/-}40% of typical) using targeted biofeedback. In models driven by experimental data from each of those conditions, we altered the kT of personalized musculoskeletal models across a physiological range (2-8% strain) and simulated individual-muscle metabolic costs for each kT and FP combination. We found that kT and FP independently affect walking metabolic cost, increasing with higher kT or as participants deviated from their typical FP. Our results show no evidence for an interaction between kT and FP in younger adults walking at fixed speeds. Individual lower body muscles showed unique effects across the kT and FP landscape. Our simulations suggest that reducing FP during walking would not mitigate the metabolic consequences of lower kT. Wearable devices and rehabilitative strategies can focus on either kT or FP to reduce age-related increases in walking metabolic cost. Author SummaryOur 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.