Changes in human multifidus muscle size with aging and short-term disuse

Lumbar multifidus (MF) muscle plays a key role in spinal stability, yet its adaptations to aging and disuse, which become increasingly prevalent with advancing age, remain unclear. We conducted two studies to investigate age- and inactivity-induced changes in MF size. In Study 1, we assessed 32 young adults (50% females) and 75 older adults (67% females), categorized as non-sarcopenic (NS) or probable sarcopenic (PS) based on EWGSOP2 criteria. In Study 2, we examined early MF responses to 10-day horizontal bed rest in 10 young males. MF cross-sectional area (CSA) and side-to-side asymmetry were measured using ultrasound imaging and compared with the vastus lateralis (VL) CSA. In Study 1, MF CSA was negatively associated with age ( p < 0.0001) and probable sarcopenia, being lower in PS compared to NS ( p = 0.012). MF showed larger effect sizes than VL Y vs NS: MF g = 1.62, VL g = 1.35; Y vs PS: MF g = 2.48, VL g = 1.35. MF asymmetry was greater in PS compared to Y ( p = 0.003). In Study 2, bed rest induced early reductions in MF CSA (detectable by day 4, p = 0.047) without increasing asymmetry. In conclusion, the MF was smaller in older populations and decreased following short-term disuse, while greater asymmetry was observed only with aging. These findings suggest that the MF is a highly plastic muscle in response to aging and disuse and that its assessment may serve as a potential hallmark of muscle maladaptation in clinical and experimental settings. • The lumbar multifidus, a key spinal stabilizer, showed larger size differences between age groups than the commonly studied vastus lateralis. • Greater multifidus side-to-side size asymmetry was also observed with aging. • Bed rest induced detectable multifidus atrophy within just 4 days, underscoring its rapid responsiveness to disuse and the need to consider trunk muscles in studies of muscle adaptation.