Longitudinal field comparison of the RT3 and an activity diary with cardiac patients.

This study investigated reliability and validity of two methods of measuring patients' physical activity following coronary artery bypass graft (CABG) surgery. Sixty-five patients wore an RT3 accelerometer and recorded activity in a diary at 3 weeks, 6 weeks, and 3 months postsurgery. Generalizability coefficients greater than .80 required approximately 2 to 4 days of data, fewer than reported elsewhere. Energy expenditure estimates correlated .77, .72, and .57 (p < .01), with mean RT3 estimates higher, especially when overall energy expenditure was low. Changes from 3 to 6 weeks correlated only moderately (r = .37-.46) across methods. The methods produce reliable but differing estimates of physical activity in this population. Although no method bias is evident in assessing change over time, correlations support the importance of using multiple methods.