Rooker, Kathryn

Sarcopenia is the age-associated muscle weakness and atrophy. As this muscle weakens, so does the capacity for an animal to perform its associated behaviors. The tibialis anterior (TA) muscle is a lower leg muscle that is important for dorsiflexion during actions such as jumping and running. In humans and Fischer 344 rats, behavioral and pathophysiological assessments show spinal cord motor unit dysfunctions with age are primarily of fast fatigueable motor units, comprising larger MN and the type IIx/b fibers they innervate. In addition, mitochondrial dysfunctions are implicated in aging neuromotor dysfunction and muscle weakness, however, little fiber type specific knowledge exists regarding the function of aged mitochondria in type-identified muscle fibers. Based on the specific atrophy and reduced oxidative capacity (succinate dehydrogenase [SDH] enzymatic function) of type IIx/b fibers in respiratory muscle, we expected to observe type-specific atrophy and reduced SDH activity in type IIx/b TA fibers. In young (6-months-old, n=12) and old (24-months-old, n=12) female and male F344 rats, cut serial TA sections (8 µm) on a cryostat to evaluate type specific morphology and SDH. To determine CSA TA fibers were labelled with MyHCSlow (Novus NBP2 50299), MyHC2A (Novus N1551) and anti-Laminin antibodies, with alexa488, alexa555 and alexa408 secondaries, respectively. SDH was determined using 10 min incubation n 80 mM succinate with 1.5mM nitro blue tetrazolium, with precipitate imaged using a Nikon AR1 inverted microscope with a 579nm high pass filter. We observed a fiber type-specific atrophy of IIx/b fibers in old TA fibers. We also observed a marked reduction in TA SDH activity in old age, confined to type IIx/b fibers. These findings illustrate consistent fast fatigue resistant motor unit degeneration in old age. Strategies specifically targeted to these motor units are imperative in interventions of sarcopenia. Funding: This study was supported by R56 HL166204 (MJF), R01 AG086136 (MJF) and AG44615 (GCS) from the NIH. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.