Hamer et al. conducted a 13-year longitudinal study on the predictive ability of self-reported physical activity on physical activity measured by accelerometers (1). Participants were 394 healthy men and women aged 54 years on average. They concluded that the habits of physical activity in adulthood are partly tracked into older age. About their attractive report, I have several concerns on their statistical outcomes.

First, Hamer et al. categorized self-reported physical activity into four groups, and analysis of variance on accelerometer-derived physical activities was made by adjusting several confounders. They adopted general linear models as multivariate analysis in combination with correlation coefficient (CC). Although they concluded their outcome from that statistical significance, the level of association presented by CC was weak and there was no presentation of explanation rate to predict accelerometer-derived physical activities from multivariate analysis.

Second, I appreciate their long term follow-up of the target participants, but their analysis was the trend analysis, presenting that groups who declared high levels of self-reported physical activity subsequently showed high objective physical activity on average. On this point, individual agreement of physical activity over 13 years should be presented such as intra-class CC, concordance CC and the Bland-Altman plot, which would improve the quality of study.

Third, I understand that the self-reported physical activity and objective physical activity by accelerometers have different meaning on activity monitoring (2). If the authors assume the physical activity by accelerometers as a gold standard, please present validation data for the accelerometer around the waist to monitor physical activity. Relating to the selection of indicators on physical activity, predictive ability of self-reported physical activity for the same indicator after 13 years is also a simple and understandable for their study. In this case, (weighted) kappa statistics would become useful statistical indicator for the agreement.

Anyway, caution should be paid on the validity of accelerometer as a monitoring tool for physical activity or sedentary behaviour from mid-life to early old age (3).

References

1. Hamer M, Kivimaki M, Steptoe A. Longitudinal patterns in physical activity and sedentary behaviour from mid-life to early old age: a substudy of the Whitehall II cohort. J Epidemiol Community Health 2012;66:1110-5.

2. Martinez-Gomez D, Gomez-Martinez S, Ruiz JR, et al. Objectively- measured and self-reported physical activity and fitness in relation to inflammatory markers in European adolescents: the HELENA Study. Atherosclerosis 2012;221:260-7.

3. Evenson KR, Buchner DM, Morland KB. Objective measurement of physical activity and sedentary behavior among US adults aged 60 years or older. Prev Chronic Dis 2012;9:E26.

Conflict of Interest:

None declared