At matched loads, aging does not alter ankle, muscle, or tendon stiffness

• Authors: Kristen Leigh Jakubowski, Emory University; Daniel Ludvig, Emory University; Sabrina S.M. Lee, Simon Fraser University; Eric J. Perreault, Emory University
• Journal Title: bioRxiv
• Publisher: NIH | 2023-11-26, Pages 568676-None
• Type of Work: Article | Preprint: Prior to Peer Review
• Publisher DOI: http://doi.org/10.1101/2023.11.25.568676
• Subjects: Biology, Physiology; Health Sciences, Rehabilitation and Therapy; Health Sciences, Public Health
• Keywords: Aging; muscle; tendon; ankle stiffness; system identification

Abstract

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, age-dependent changes in muscle stiffness have yet to be tested. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p>0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p=0.008), muscle (37%; p=0.02), and tendon stiffness (22%; p=0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p=0.004). Together these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.