Background Mobility-related fatigue and low socioeconomic position predicts mobility limitations and disability in old age, but the interplay between these two factors is unknown. To evaluate whether mobility-related fatigue is a stronger risk factor for mobility limitations in certain socioeconomic groups, the aim of this study was to examine the combined effect of mobility-related fatigue and socioeconomic position on mobility limitations in a prospective study among older Danish men and women.

Methods Multivariate linear regression models with combined exposure variables using generalised estimating equations were performed using four waves of data on 2874 individuals without mobility limitations at baseline from The Danish Intervention Study on Preventive Home Visits.

Results Low socioeconomic position and mobility-related fatigue are risk factors for mobility limitations in old age. The combined exposure to both factors additionally increased the risk, but there was no synergy effect between the two. Notably, fatigue predicted mobility decline at 3-year follow-up among those aged 80 years at baseline with a mean difference in number of mobility limitations from the joint reference category (high socioeconomic position and no fatigue) of −0.52, p<0.0001 among those in high socioeconomic position and −0.96, p<0.0001 among those in low socioeconomic position.

Conclusions Mobility-related fatigue is not a significantly stronger risk factor for subsequent mobility limitations among those with concomitant exposure to low socioeconomic position, compared with those with high socioeconomic position. Preventive strategies must focus on vulnerable groups of older people with low socioeconomic position as well as on individuals with fatigue.