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We will give the levers a rest for a while and look at another physical process involved in the motion of the foreleg. This is the concept of Moment of Inertia.

Essentially moment of inertia relates to the rotation of a body around the centre of rotation. The two diagrams at the side have come from my old physics text book. The top configuration with the weights close to the centre of rotation requires little effort to get the apparatus rotating.

The second configuration with the weights at a distance from the centre of rotation requires much more effort to get it rotating.
Conversely it is easier to stop the rotation of the top arrangement than the bottom with the weights away from the centre of rotation.

The formula for moment of inertia for those interested is

What this means is that the total moment of inertia (I) is the sum of all the little masses times the square of their distance from the centre of rotation. You might ask what this has to do with the motion of the foreleg of a horse.

In the previous articles we have only looked at the motion in terms of what affects the length of stride. What we are interested in here is how much energy is required to get the leg to make that stride.
The diagram at the left represents the position of the leg at maximum extension (solid line). To complete the stride the leg must rotate about the top of the shoulder to reach the position as shown by the dashed line and this requires energy. A light hoof and pastern would require less energy than a heavy hoof and pastern.

If only it were that simple. The horse’s leg is not like a plank, there are several articulated joints such as the shoulder/humerus, humerus/forearm, forearm/cannon, cannon/pastern and so on. So let’s simplify the situation. We will disregard the shoulder and humerus as these are close to the centre of rotation and do not influence the moment of inertia as much as the more distant components of the leg.

Our new diagram on the left represents the top of the forearm as the centre of rotation for the cannon, pastern and hoof from the maximum extension to the the completion of the stride.

The muscles rotate the forearm as well as the cannon,fetlock and hoof in a backward motion. Because the latter units are some distance from the top of the forearm and have considerable mass they have a sizable

moment of inertia. They will tend to lag behind the forearm as a result of the articulation of the knee joint. This is not an ideal situation as can be seen from the following images.

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