Achilles tendon stiffness minimizes the energy cost in simulations of walking in older but not in young adults

Abstract

Both Achilles tendon stiffness and walking patterns influence the energy cost of walking, but their relative contributions remain unclear. These independent contributions can only be investigated using simulations. We created models for 16 young (24±2 years) and 15 older (75±4 years) subjects, with individualized (using optimal parameter estimations) and generic triceps surae muscle-tendon parameters. We varied Achilles tendon stiffness and calculated the energy cost of walking. Both in young and older adults, Achilles tendon stiffness independently contributed to the energy cost of walking. However, overall, a 25% increase in Achilles tendon stiffness increased the triceps surae and whole-body energy cost of walking with approximately 7% and 1.5%, respectively. Therefore, the influence of Achilles tendon stiffness is rather limited. Walking patterns also independently contributed to the energy cost of walking because the plantarflexor (including, but not limited to the triceps surae) energy cost of walking was lower in older than in young adults. Hence, training interventions should probably rather target specific walking patterns than Achilles tendon stiffness to decrease the energy cost of walking. However, based on the results of previous experimental studies, we expected that the calculated hip extensor and whole-body energy cost of walking would be higher in older than in young adults. This was not confirmed in our results. Future research might therefore assess the contribution of the walking pattern to the energy cost of walking by individualizing maximal isometric muscle force and by using three-dimensional models of muscle contraction.Summary statementAchilles tendon stiffness and walking patterns independently contribute to the energy cost in simulations of walking in young and older adults. The influence of Achilles tendon stiffness is rather small.

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

“…In a similar study to ours, increasing Achilles tendon stiffness did not significantly decrease walking energy cost in older adults [ 23 ]. Biological ranges of k T incorporate the local cost minima of walking [ 23 , 39 ]. We generally interpret reduced k T as pathophysiological change that would benefit from intervention.…”

“…Altering only the Achilles k T in older and younger adults yields relatively small effects (~1.5% change from baseline) on whole-body metabolic cost across a range of walking speeds [ 23 ]. Although the Achilles tendon and triceps surae musculature may be most impacted by changing k T , we cannot assume that only the Achilles tendon would be affected by altered k T .…”

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

“…Our walking simulations and metabolic outcomes are consistent with the majority of outcomes from other studies, finding that individual muscle actions and metabolic cost during walking are highly sensitive to changes in k T . (18,23,35,36)…”

“…(6) Altering only the Achilles k T in older and younger adults yields relatively small effects (~1.5% change from baseline) on whole-body metabolic cost across a range of walking speeds. (23) Although the Achilles tendon and triceps surae musculature may be most impacted by changing k T , we cannot assume that only the Achilles tendon would be affected by altered k T . Thus, we allowed for uniform changes in k T across all MTUs in our musculoskeletal models.…”

“…Our walking simulations and metabolic outcomes are consistent with the majority of outcomes from other studies, finding that individual muscle actions and metabolic cost during walking are highly sensitive to changes in k T . (18,23,35,36) . CC-BY 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.…”