THE TRANSITION BETWEEN WALKING AND RUNNING IN HUMANS - METABOLIC AND MECHANICAL ASPECTS AT DIFFERENT GRADIENTS

Five subjects walked and ran at overlapping speeds and different gradi ents on a motorized treadmill. At each gradient the speed was obtained at which walking and running have the same metabolic cost (Sm) and th e speed of spontaneous (Ss) transition between the two gaits was measu red. Ss was found to be statistically lower than Sm at all gradients, the difference being in the range of 0.5-0.9 km h(-1). The motion anal ysis of walking reveals that at all gradients and at increasing speed: (1) the percentage of recovery, an index of mechanical energy saving related to the pendulum-like characteristic of walking, decreases; (2) the lower limb spread reaches a limit in walking; and consequently (3 ) both the stride frequency and the internal mechanical work, due to l imb acceleration in relation to the body centre of mass, increase much more in walking than in running. Switching to a run, although implyin g a higher frequency, makes the internal work decrease as a result of the lower limb spread. In this paper several influences, such as the ' ratings of perceived exertion' (RPE), on the choice of beginning to ru n when it is more economical to walk, are discussed. A tentative hypot hesis on the determinants of Ss, which is emphasized to be a speed whi ch has to be studied in detail but is generally avoided in locomotion, is based on a comfort criterion from peripheric afferences and is ref lected by the fact that at Ss a running stride costs as much as a walk ing stride. A preliminary measure of the subjects' behaviour during sp ontaneous overground locomotion, where the progression speed can be ch anged freely, reveals that the running speed immediately following gai t transition is approximately 2 km h(-1) higher than the 'last' walkin g speed, supporting the hypothesis of metabolic energy minimization.