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BRAGE, S., N. WEDDERKOPP, P. W. FRANKS, L. B. ANDERSEN, and K. FROBERG. Reexamination of Validity and Reliability of the CSA Monitor in Walking and Running. Med. Sci. Sports Exerc., Vol. 35, No. 8, pp. 1447–1454, 2003.To evaluate the reliability and validity of the CSA (model 7164) accelerometer (MTI) in a wide walking-running speed range in laboratory and field.Twelve male subjects performed three treadmill walking/running sessions and one field trial with the same continuous protocol involving progressively increasing velocities at 5 min per interval from 3 to 6 km·h−1 (walking) and 8 to 20 km·h−1 (running). In the field trial, this protocol was terminated after 35 min (14 km·h−1), but the trial then extended with 5-km running at a freely chosen velocity. In both scenarios, two CSAs were mounted on each hip and the step frequency measured at each velocity. Oxygen uptake (V̇O2·kg−1) was measured on the last two treadmill sessions. Correlation analyses were performed for mean CSA output relationship with speed, V̇O2 per kilogram, and step frequency.In all trials, CSA output rose linearly (R2 = 0.92, P < 0.001) with increasing speed until 9 km·h−1 but remained at ∼10,000 counts·min−1 during running, thus underestimating V̇O2 per kilogram at speeds > 9 km·h−1. Estimation errors increased with speed from 11% (P < 0.01) at 10 km·h−1 to 48% (P < 0.001) at 16 km·h−1, when assuming a linear relationship. Freely chosen velocities in the field trial ranged from 10.9 to 16.3 km·h−1. No difference in the CSA-speed relationship was observed between the two scenarios. Differences in CSA output between subjects could partially be attributed to differences in step frequency (R = −0.34 (P = 0.02) for walking and R = −0.63 (P < 0.001) for running).CSA output increases linearly with speed in the walking range but not in running, presumably due to relatively constant vertical acceleration in running. Between-subject reliability was related to step frequency because CSA data are filtered most at higher movement frequencies. Epidemiological CSA data should thus be interpreted with these limitations in mind.