Purpose—Determine if the leveling off (‘plateau/inverted-U’ phenomenon) of vertical ActiGraph activity counts during running at higher speeds is attributable to the monitor’s signal filtering and acceleration detection characteristics. Methods—Ten endurance-trained male participants [mean (SD) age= 28.2 (4.7) yrs] walked at 3, 5 and 7 km˙hr−1, and ran at 8, 10, 12, 14, 16, 18 and 20 km˙hr−1 on a force treadmill while wearing an ActiGraph GT3X monitor at the waist. Triaxial accelerations of the body’s center of mass (CoM) and frequency content of these accelerations were computed from the force treadmill data. Results—GT3X vertical activity counts demonstrated the expected ‘plateau/inverted-U’ phenomenon. In contrast, vertical CoM accelerations increased with increasing speed (1.32 ± 0.26 g at 10 km˙hr−1 and 1.68 ± 0.24 g at 20 km˙hr−1). The dominant frequency in the CoM acceleration signals increased with running speed (14.8 ± 3.2 Hz at 10 km˙hr−1 and 24.8 ± 3.2 Hz at 20 km˙hr−1) and lay beyond the ActiGraph bandpass filter (0.25 to 2.5 Hz) limits. Conclusion—CoM acceleration magnitudes during walking and running lie within the ActiGraph monitor’s dynamic acceleration detecting capability. Acceleration signals of higher frequencies that are eliminated by the ActiGraph bandpass filter may be necessary to distinguish among exercise intensity at higher running speeds.