Physiological Aspects of Legged Terrestrial Locomotion by Giovanni Cavagna

Author:Giovanni Cavagna
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

6.3 Total Work

This was measured by Cavagna and Kaneko (1977) as the sum of the absolute values of external and internal work neglecting energy transfer between the kinetic energy of the center of mass and that of the limbs. There has been uncertainty concerning a possible transfer between the mechanical energy of the center of mass and that of the limbs (Cavagna and Kaneko 1977; Aleshinsky 1986; Caldwell and Forrester 1992; Minetti et al. 1993; Willems et al. 1995). This external-internal energy transfer may possibly occur when the center of mass of the body accelerates or decelerates during ground contact. For example, if the center of mass decelerates forwards, the limbs tend to maintain their forward velocity by inertia and, therefore, to accelerate forwards relative to the center of mass. Consequently, in this case, the increase in kinetic energy of the limbs relatively to the center of mass is not only due to positive work done by the muscles. This example shows that, while the body is in contact with the ground, external forces may change the movement of the limbs relative to the center of mass, and such movement, therefore, cannot be attributed solely to internal forces.

A maximum transfer between the mechanical energy of the center of mass and that of the limbs is obtained by summing at each instant the kinetic energy of center of mass relative to the surrounding with that of the limbs relative to the center of mass. This procedure however is not correct because, for a given change in velocity of the center of mass relative to the surrounding, the corresponding change in kinetic energy depends on the average speed of locomotion, which per se cannot affect the transfer between kinetic energy of the center of mass relative to the surrounding and kinetic energy of the limbs relative to the center of mass. However, the acceleration of the center of mass relative to the surroundings may indeed affect the calculation of internal work from the kinetic energy changes relative to the center of mass. This possibility was tested by comparing the internal work calculated taking into account the described effect of inertia with the internal work calculated as the sum of the increments in kinetic energy of the limbs relative to the center of mass: the ratio between the two values was 0.97 ± 0.03 (mean ± S.D., N = 106) in walking and 1.03 ± 0.03 (mean ± S.D., N = 123) in running (Willems et al. 1995).

In conclusion: the total work is given by the sum of the absolute values of W ext and W int, this last measured from the kinetic energy changes of the limbs relative to the center of mass, assuming possible kinetic energy transfer only between segments of the same limb, but not among the limbs or between the limbs and the center of mass of the whole body (Willems et al. 1995).

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