Association between education and walking speed: counterfactual mediation analysis in favor of a motor reserve hypothesis

We quantified the extent to which the association between education and fast walking speed (FWS) is explained by 17 mediators (cardiovascular risk factors/diseases, comorbidities, health behaviors, socioprofessional characteristics, cognition), and examined whether mediators interact with education, in favor of a reserve hypothesis. Cross-sectional analyses are based on Constances (a population-based study of French adults 45-69 years). Three-meter FWS was measured using photoelectric cells. Education was categorized as lower vs higher. After multiple imputation of missing values, we used counterfactual mediation models for multiple mediators allowing for education x mediator interactions, to estimate the total effect (TE), total indirect effect (TIE), and mediated interaction (IMD) of lower education on FWS. Analyses are based on 71222 participants (52.6% women; mean age = 57.2 years; 27.2% higher education; mean FWS = 180.2 cm/s). In joint mediation analyses, the TE of lower education was -8.19 cm/s (95% CI, -8.87 to -7.51), with a TIE of -5.76 cm/s (95% CI, -6.10 to -5.41; proportion mediated = 70.3%; 95% CI, 65.6-75.0). The IMD was negative (-2.52; 95% CI, -3.31 to -1.72); 30.8% of the TE and 43.8% of the TIE were attributable to the IMD. Several mediators explain a large part of the association between lower education and slower FWS. The detrimental effect of mediators was more pronounced in participants with lower than in those with higher education, in agreement with a reserve hypothesis.