THE PROPORTIONAL WORK OF LIFTING THE CENTER-OF-MASS DURING WALKING

The present study determined the proportion of energy consumption used to lift the center of mass vertically during walking. By assuming tha t the vertical oscillations of the trunk were identical to those of th e center of mass, the complexity of studying the human gait was simpli fied. The work performed on the center of mass, defined as lift work, was calculated using the subject's mass and the vertical displacement of the trunk. Compensating for the fact that human muscles are approxi mately 25% efficient in performing external work, the mechanical work was compared with the total energy expenditure, determined using an ex pired air analysis. At 1.34 and 1.79 m/s, 53.2 +/- 9.3% and 62.8 +/- 8 .6%, respectively, of the total energy can be explained by lift work. At 0.45 and 0.90 m/s, the proportions are considerably less, 21.6 +/- 6.6% and 37.6 +/- 8.2%, respectively. These findings provide a rationa le for why the metabolic energy cost per unit distance is greater at s lower speeds than comfortable or fast speeds of walking. The total ene rgy consumption can be accounted for by changes in the potential and k inetic energies and internal muscular work, defined as the less readil y observable work probably needed for balance. The present study demon strated that the metabolic energy cost of changing the potential energ y of the center of mass during walking is significantly less relative to the total energy consumption at slower speeds than at higher speeds . Because kinetic energy is proportional to the square of the velocity , the metabolic energy cost of changing this energy is relatively smal l at slower speeds. Internal muscular work must therefore play a large role in total energy consumption at slow speeds of walking.