RESUMO
OBJECTIVE. We investigated the sources of variance and reliability in an objective measure of physical activity for a 14-hour and 4-day monitoring period. Design. A convenience sample of rural (N=31) and urban (N=30) adult; Northern Sotho-speaking blacks was recruited. Physical activity was assessed for 8 consecutive days usinga uni-axial accelerometer. Physical activity indices were total counts; average counts; inactivity (500 counts) moderate-1 activity (500 - 1 951 counts); moderate-2+vigorous activity (.1 952 counts); and were expressed per hour or per day as required. RESULT. Accelerometry data from 41 subjects (23 males; 18 females) complied with selection requirements and were analysed for variance distribution and reliability (intraclass correlation coefficients (ICCs)). For the 14-hour monitoring period variance was distributed as follows: intra-individual (71- 82); inter-individual (3 - 18) and hour-of-day (2 - 14). Attenuated ICCs ranged from 0.31 to 0.75 (median: 0.70). Variance for the 4-day monitoring period differed from the 14-hour monitoring period: inter-individual (47 - 58); intraindividual (43 - 51) and day-of-week (0 - 6.5). Attenuated ICCs ranged from 0.27 to 0.84 (median: 0.79). Irrespective of the monitoring period; total counts; average counts and moderate-2+vigorous activity tended to be the most reliable measures requiring the fewest number of monitoring periods. CONCLUSIONS. These findings provide an insight for understanding how variance is distributed in objectively measuredactivity patterns of a South African sample and show that reliable measures of adult physical activity behaviours require 18 - 128 hours and 3 - 44 days; depending on the monitoring period; physical activity index; residence status and sex