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.