“Motion is created by the destruction of balance, that is, of equality of weight, for nothing can move by itself which does not leave its state of balance, and that thing moves most rapidly which is furthest from its balance” (Leonardo Da Vinci, 1452).
Part 2 of this series closed with a statement that gravity is the motive force in terrestrial locomotion. Parts 1 and 2 provided evidence disproving the existence of an active push in accelerating the center of mass forwards. In this final part of the series, gravity, or more specifically, torque created by gravity, will be shown to provide forward acceleration of the center of mass according to the undisputed-fundamental laws of physics.
Destruction of balance creates acceleration by gravitational torque.
The object in A is in balance / stationary, with its center of mass (the cross) vertically aligned over its stationary point of support. In B, the center of mass of the object is out of alignment with the stationary point of support. Gravity acts on the center of mass, creating a turning force (torque) about the point of support as an axis. Left alone, the center of mass will continue to accelerate at 9.81 m/s every second (i.e. constantly increasing speed), until the object hits the ground. The further the center of mass of the object from its point of support, the greater the gravitational torque and the faster the center of mass will fall. This is GRAVITATIONAL TORQUE.
Picturing gravitational torque in locomotion.
The figure left is adapted from the work of Morton (1935). He labeled the tilted (away from the gravitational vector) line drawn from the point of support through the center of mass the ‘angle of instability’. From left to right, the figure shows a state of balance, moderate instability in walking and maximum instability in running. The forward acceleration in walking and running is provided by gravitational torque, acting on the center of mass around the stationary-supporting foot.
While the figure is useful to conceptualize how forward movement can result purely from gravity, without an active push, it is a gross oversimplification and has some critical problems that do not agree with observation. Nevertheless, this conception of how gravity can be a motive force has been used as the cornerstone of some schools of running, in particular, Pose technique (Romanov and Fletcher, 2007). While the general idea of gravitational torque as the motive force is correct, the mechanisms by which it acts have been incorrectly described.
The problem of constant acceleration.
As stated earlier, gravity accelerates mass continuously, so the idea of holding a fixed angle of lean at a constant velocity and simply increasing or decreasing the angle to accelerate or decelerate is flawed. Suppose a runner adopts an angle of instability of 15 degrees. As soon as this happens, his center of mass is accelerating under gravitational torque at an ever-increasing rate. Unless he is able to accelerate the recovery of his feet to match the rate at which his center of mass is falling away from his feet, the angle of instability will constantly increase and an imminent face plant is inevitable!
So near, yet so far, but the truth is out there.
The simple model of gravitational torque outlined above cannot account for constant-velocity running. By now, it should be clear that a theory at odds with evidence is, by definition, wrong. The ‘concept’ of gravitational torque must be true based on fundamental laws of physics. It is simply the case that the mechanism by which it operates in locomotion has been oversimplified. The true mechanism by which gravity acts as the motive force in locomotion is well accepted, but apparently not well known. It is the ‘Virtual-Pivot-Point model’. It explains all observations, has been used by engineers to create human-like running robots, and also explains why inherently-unstable bipeds are not as unstable as predicted.
For an explanation of the TRUE application of gravity as the motive force in locomotion, watch out for the ‘Science of Falling’ post coming along soon.
Maus, H.M., Lipfert, S.W., Gross, M., Rimmel, J and Seyfarth, A. (2010). Upright human gait did not provide a major mechanical challenge for our ancestors. Nature Communications, 1(70), 1-6.
Morton, D.J. (1935). The Human Foot: its evolution, physiology and functional disorders. New York: Columbia University Press.
Romanov, N and Fletcher, G. (2007). Runners do not push off the ground but fall forwards via a gravitational torque. Sports Biomechanics, 6(3), 434-452.