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

Abstract

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. We also reveal complex individual muscle responses to the kT and FP landscape. For example, although total metabolic cost increased by 5% on average with combined reductions in kT and FP, the triceps surae muscles experienced a 7% local cost reduction on average. 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.

Reduced Achilles tendon stiffness in aging persists at matched activations and associates with higher metabolic cost of walking

“…From a clinical perspective, our results suggest that interventions designed to augment Achilles tendon stiffness may help to reduce walking metabolic cost for older adults. These results provide experimental credibility to similar recommendations made following the use of musculoskeletal simulations to predict the bioenergetic consequences of walking with reduced Achilles tendon stiffness 7,9,10 . As one example, conventional resistance training exercises (i.e., calf muscle strengthening) have been shown to effectively increase not only muscle strength, but to also elicit tendon remodeling and increased stiffness 29 .…”

“…From a clinical perspective, our results suggest that interventions designed to augment Achilles tendon stiffness may help to reduce walking metabolic cost for older adults. These results provide experimental credibility to similar recommendations made following the use of musculoskeletal simulations to predict the bioenergetic consequences of walking with reduced Achilles tendon stiffness 7,9,10 There are several limitations of this work. One limitation is the lack of mechanistic data during walking to objectively link reduced kAT with increased metabolic cost with more cause-effect precision.…”

“…Thus, for a given activation, the more compliant Achilles tendon of older adults would compel shorter triceps surae muscle operating lengths. These predictions of shorter operating lengths are supported by computational simulations and experimental comparisons between younger and older adults 7,10,19,20 . Ultimately, it is this neuromechanical cascade, arising from reduced kAT, that we have proposed contributes at least in part to a higher metabolic cost of walking for older adults.…”

“…Unfortunately, the mechanisms responsible for the increasing metabolic cost in the aging process are poorly understood. Given their role in powering locomotion, muscle-tendon units (MTUs) spanning the ankle have been shown through experimental and computational modeling studies to play a role in governing metabolic cost [6][7][8][9][10] . In particular, neuromechanical interaction between triceps surae muscle function and Achilles tendon stiffness is critical for tuning positive power generation during push-off which is in turn responsible for as much as much as half of the cost of walking 11,12 .…”