Skeletal muscle stiffness is altered by age and fatigue at the cellular level

Advanced age is associated with increased skeletal muscle stiffness at the cellular and whole-tissue levels. In older adults, fatigue increases falls risk. Our lab has previously observed fatigue-induced reductions in skeletal muscle cellular stiffness in younger adults, prompting the question of whether the same occurs in older adults. If so, it is possible that fatigue-induced reduction in cellular stiffness of older adults would reduce the rate of tendon loading and/or increase tissue compliance, ultimately contributing to increased falls risk. Therefore, the OBJECTIVE of the present study is to measure cellular passive and active stiffness in younger and older adults across fatigue conditions to test the hypothesis that fatigue induces tissue compliance that predisposes older adults to falls. Methods: 11 untrained males and females (7 younger, 20±1 yrs.; 4 older, 74±4 yrs.) performed unilateral knee extensions to fatigue, followed by bilateral biopsy of the Vastus Lateralis providing fatigued (F) and non-fatigued (NF) samples. Single fibers (146 younger, 130 older) were passively stretched to 156% initial length and passive modulus was calculated as the slope of the resulting stress-strain curve. Separately, Residual Force Enhancement (RFE) was measured in a subset of fibers (9 younger, 40 older) to assess passive contributions (titin) to active (actin-myosin) stretch. Main effects (age/fatigue) on passive modulus and RFE were tested via linear mixed model and t-tests, respectively. Results: Passive modulus was increased with age (p=0.04) and reduced by fatigue in both the younger (p<0.01) and older (p<0.01) participants. In a subset of fibers, RFE was increased in older women compared to younger (p<0.01) and older men (p<0.01). No main effect of fatigue on RFE was detected (p=0.49). However, RFE trended towards reduction with fatigue in a younger male (p=0.13) and was significantly reduced by fatigue in myosin heavy chain (MHC) I fibers from an older male (p=0.02). CONCLUSION: In agreement with existing literature, cellular passive modulus was higher in older versus younger adults. Additionally, fatigue reduced passive modulus in skeletal muscle fibers from both older and younger adults. Though no main effect of fatigue on RFE was detected, these results suggest that cellular RFE measures are age- and fiber-type specific. This work was supported by funds from the Wu Tsai Human Performance Alliance. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.