Less healthy, but more active: opposing selection biases when recruiting older people to a physical activity study...

Physical activity studies in older people experience poor recruitment. We wished to assess the influence of activity levels and health status on recruitment to a physical activity study in older people.

Comparison of participants and non-participants to a physical activity study using accelerometers in patients aged > or = 65 years registered with a UK primary care centre. Logistic regression was used to calculate odds ratios (OR) of participants in the accelerometer study with various adjustments. Analyses were initially adjusted for age, sex and household clustering; the health variables were then adjusted for physical activity levels and vice versa to look for independent effects.

43%(240/560) participated in the physical activity study. Age had no effect but males were more likely to participate than females OR 1.4(1.1-1.8). 46% (76/164) of non-participants sent the questionnaire returned it. The 240 participants reported greater physical activity than the 76 non-participants on all measures, eg faster walking OR 3.2(1.4-7.7), or 10.4(3.2-33.3) after adjustment for health variables. Participants reported more health problems; this effect became statistically significant after controlling for physical activity, eg disability OR 2.4(1.1-5.1).

Physical activity studies on older primary care patients may experience both a strong bias towards participants being more active and a weaker bias towards participants having more health problems and therefore primary care contact. The latter bias could be advantageous for physical activity intervention studies, where those with health problems need targeting.