The purpose of this study was to conduct a comprehensive evaluation of the effect that walking speed, gender, leg length, motion sensor tilt angle, brand, and placement have on motion sensor step-counting error. Fifty-nine participants performed treadmill walking trials at 6 speeds while wearing 5 motion sensor brands placed on the anterior (Digiwalker, DW; Walk4Life, WFL; New Lifestyles, NL; Omron, OM), midaxillary (DW; WFL; NL; ActiGraph, AG), and posterior (DW, WFL, NL) aspects of the waistline. The anterior-placed NL and midaxillary-placed AG were the most accurate motion sensors. Motion sensor step-count error tended to decrease at faster walking speeds, with lesser tilt angles, and with an anterior waistline placement. Gender and leg length had no effect on motion sensor step-count error. We conclude that the NL and AG yielded the most accurate step counts at a range of walking speeds in individuals with different physical characteristics.