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We determined the validity of the Nike+® device for estimating speed, distance, and energy expenditure (EE) during walking and running. Twenty trained individuals performed a maximal oxygen uptake test and underwent anthropometric and body composition testing. Each participant was outfitted with a Nike+® sensor inserted into the shoe and an Apple iPod nano®. They performed eight 6-min stages on the treadmill, including level walking at 55, 82, and 107 m·min-1, inclined walking (82 m·min-1) at 5 and 10% grades, and level running at 134, 161, and 188 m·min-1. Speed was measured using a tachometer and EE was measured by indirect calorimetry. Results showed that the Nike+® device overestimated the speed of level walking at 55 m·min-1 by 20%, underestimated the speed of level walking at 107 m·min-1 by 12%, but closely estimated the speed of level walking at 82 m·min-1, and level running at all speeds (p<0.05). Similar results were found for distance. The Nike+® device overestimated the EE of level walking by 18-37%, but closely estimated the EE of level running (p<0.05). In conclusion the Nike+® in-shoe device provided reasonable estimates of speed and distance during level running at the three speeds tested in this study. However, it overestimated EE during level walking and it did not detect the increased cost of inclined locomotion. [ABSTRACT FROM AUTHOR]