INGEN SCHENAU, G.J. VAN. Some fundamental aspects of the biomechanics of overground versus treadmill locomotion.Med. Sci. Sporis Exercise, Vol. 12, No. 4, pp. 257–261, 1980. The literature shows a wide difference of opinion about the mechanical equality of, or difference between treadmill and overground locomotion. This difference in opinion is often related to the coordinate system which implicitly or explicitly is used. With help of a few theoretical examples of energy calculations this paper shows that the description of treadmill locomotion with respect to a fixed coordinate system can lead to faulty conclusions. It is concluded that as long as the beltspeed is constant a coordinate system should be used which moves with the belt. In such a system no mechanical difference exists in comparison with overground locomotion with respect to a fixed coordinate system. All differences found in locomotion patterns must therefore originate from other than mechanical causes.