This study was designed to investigate the relationship between Actigraph counts and running speed; and to describe differences due to accelerometer position on the body and due to exercise modality. Eleven physical education students (age, 25.1 ± 3.7 years; height, 1.73 ± 0.10 m; body mass, 70.8 ± 10.8 kg) completed two exhaustive exercise tests (continuous and intermittent), with MTI accelerometers mounted both at the hip and ankle. Exercise consisted of running for 3-min at incremental speeds until volitional exhaustion. During both exercise tests, the relationship between the ActiGraph outputs worn at the hip and speed was linear in the range 1.1 - 3.3 m·s^-1 (r² = 0.94 and 0.95, p < 0.01 for continuous and intermittent exercise respectively). A coefficient of determination of r² = 0.97 (p < 0.01) was found with ankle wearing from walking, jogging and running at high speeds. There was a body placement effect at all absolute speeds (p < 0.01); but no exercise effect on accelerometer counts and no interaction between placement and exercise (p> 0.05). The ActiGraph seems to be a reliable tool for estimating a wide range of activity or exercise intensities. An ActiGraph worn at the ankle may be more appropriate to reflect normal human movement.