Purpose:

The aim of this study was to determine the effect of age on the psoas and posterior paraspinal muscles (PPM; multifidus muscle and erector spinae) and to evaluate potential sex-related differences.

Methods:

MRI-based quantitative assessments of the cross-sectional area (CSA), the functional cross-sectional area (fCSA), the fat area (FAT) and the proportion of intramuscular fat (FI) were conducted on patients undergoing lumbar fusion surgery between 2014 and 2021. The regions of interest were the psoas muscle and the PPM at the superior endplate of L4. The left and right sides of the muscle groups were summarized and normalized by the patient’s height (cm²/m²). The relationships between age and muscular parameters were analyzed stratified by sex.

Results:

A total of 195 patients (57.9%female) with a median age of 64.2 years and a body mass index of 28.3 kg/m² were analyzed. The CSA_Psoas was 7.7 cm²/m² and differed significantly between females and males (p < 0.001); likewise, the fCSA_Psoas differed significantly between the sexes. The CSA_PPM was 18.8 cm²/m² with no sex-specific differences. Significant differences were found in the FI_PPM (males: 41.1% vs. females: 47.9%; p < 0.001), but not in the FI_Psoas (males: 3.7% vs. females: 4.5%; p = 0.276). Considering the effect of age on FI, a significant positive correlation was observed for the PPMs for both sexes. Only in women, there was a negative correlation between age and CSA_Psoas (ρ = – 0.248; p = 0.008), FAT_Psoas (ρ = – 0.421; p < 0.001) and FI_Psoas (ρ = – 0.371; p < 0.001).

Conclusion:

This study demonstrated sex-specific differences in spinal muscle morphology in relation to patient age. With increasing age there was a decrease in FI_Psoas in women only, unlike in the PPMs in which there was increased FI that was significantly higher in women compared to men.