Self-Reported Physical Activity is Not a Valid Method for Measuring Physical Activity in 15-Year-Old South African Boys and Girls.

Physical activity plays an important role in the prevention of chronic lifestyle-related diseases. The development of valid instruments for the assessment of physical activity remains a challenge in field studies. The purpose of the present study was therefore to determine the level of agreement between physical activity objectively measured by the ActiHeart&reg; (Cambridge Neurotechnology Ltd, Cambridge, UK) device and subjectively reported physical activity by means of the International Physical Activity Questionnaire Short Form (IPAQ-SF) among adolescents attending schools in the Tlokwe Local Municipality, South Africa. A cross-sectional study design was used with a total of 63 boys and 45 girls aged 15 years who took part in the Physical Activity and Health Longitudinal Study (PHALS). Stature and weight were measured according to standard International Society for the Advancement of Kinanthropometry (ISAK) protocols. Objective physical activity (PA) was measured by a combined heart rate and accelerometer device (ActiHeart&reg; ) for seven consecutive days. Time spent in moderate-to-vigorous intensity physical activity (MVPA) was assessed. Subjective physical activity was assessed with the self-reported IPAQ-SF. Objective PA indicated that 93% of the participants were inactive and only 6% were highly active. The IPAQ-SF showed that 24% were inactive, with 57% active. A non-significant correlation ( r = 0.11; p = 0.29) between the ActiHeart&reg; measure of activity energy expenditure (AEE) and total physical activity (IPAQ-SF) was observed. The Bland⁻Altman plot showed no agreement between the two measurement instruments and also a variation in the level of equivalence. When Cohen&rsquo;s kappa ( &kappa; ) was run to determine the agreement between the two measurement instruments for estimated physical activity, a poor agreement (&kappa; = 0.011, p < 0.005) between the two was found. The poor level of agreement between the objective measure of physical activity (ActiHeart&reg; ) and the IPAQ-SF questionnaire should be interpreted cautiously. Future physical activity research using a combination of subjective and objective assessment methods in a large-scale cohort in adolescents is recommended.