Optimum Athletic Surfaces for Horses
A Theoretical Study

by
James Rooney, D.V.M.

This essay attempts to develop a coherent schema for the optimum interaction of several components of the soil for the elasticity/resiliency of the soil. My original work relating the type of soil surface to wearing of the unshod hoof (in the essay on this site on hoof shape) has evolved to this search for a theoretical model for optimum soil conditions for athletic activity.

The closest to an optimum surface for horses that I have encountered (and my experience is not infinite) was the so-called invalid gallop on the training grounds at Newmarket, England. The difference in springiness of that turf surface compared to nearby turf gallops was striking. That surface was achieved, I understand, by the repeated application of peat over many years.

There is good reason to ask: why bother? It is true that horse athletic activity on controllable surfaces is largely limited to race tracks, training tracks, and arenas for dressage, showing, jumping. Work such as trail riding, hacking, and hunting of necessity involve a variety of surfaces with only some of those surfaces to some degree controllable. It is impossible to develop a schema, even a theoretical one, for every possible surface contingency. We shall do what we can with what we have.

A casual review will show, there is an enormous amount of material on proper footing for horses (Canadian Horseman 1999, for example). Much of that material is commercial, and none of it to my knowledge, commercial or otherwise, uses more than opinion as the basis for judgment.

We are seeking an optimum for the soil surface with that optimum being a definite composition and interaction of water, organic material, and grass. Soil per se is composed of silt, sand, and clay particles, the type of soil depending on their relative proportions. All are derived from the breakdown of rock into smaller and smaller grains with sand having the largest grain, followed by silt, and clay the finest.

A standard soil classification (Sachs 1993) is given in Figure 1. I have not attempted to include variations of these three components as such in the action surface since I believe water^[1], organic matter, and grass are more important for elasticity. One can note, however, that organic matter and sand/silt vary inversely in my schema, so that both water and the sand/silt components are included, at least implicitly, in the action surface to be described below.

Figure 1: Soil composition, inorganic. (Sachs 1993).

Interaction of Water, Organic, and Grass

The heart of the interaction is represented in Figures 2 and 3, two views of a three dimensional action surface generated by assigning a series of numbers for water, organic matter, and grass ^[2].

Figure 2: A 3-D action surface in gray with contours in color below. These figures are explained in the text.

Figure 3: The action surface from above.

The water numbers steadily increase (the arrow). The numbers for both grass and organic matter increase and then decrease(double-headed arrows). This was done since the grass increases as water increases and then stops increasing and decreases again as the water continues to increase, i.e., the soil becomes water-logged and the grass stops growing and is drowned out. In the limit the grass would be beneath the water but, in fact, disappears to be replaced perhaps by submerged underwater vegetation. The organic matter in the soil increases with water and grass but decreases again once the grass has decreased or disappeared.

Grass entails coverage of the soil surface together with cohesiveness and mat-like quality or texture of the grass mat. At least part of this mat-like character is caused by thatch – the accumulation of dead grass on the surface. One cannot draw too fine a line of distinction between the cohesive, elastic quality of the grass on the surface as opposed to the cohesive, elastic quality of the deeper soil, plant roots, and decaying vegetation. Although the two sources of elasticity/resilience are treated separately here, they are in fact continuous at an ill-defined boundary.

The contours of the surface are shown beneath the action surface; they indicate areas of identical elevation of that surface. In this particular case the contours represent the interaction of water, grass, and organic matter with maximum interaction at the high point on the action surface and on the corresponding central red area of the contour plot. The peak on the action surface and the central red area are taken to be the optimum for elasticity/resiliency of the surface as will be discussed below.

Evaluation of the action surface

We now look at the action surface and what it represents. We can envision the surface either at a moment in time or as an evolution from one state to another. We shall do the latter first. That process may be helped by looking at a 3-D trajectory curve of the same data, Figure 4.

Figure 4: A 3-D curve – trajectory - of the same data as Figures 2 and 3.

We trace the evolution of the surface from desert to open water: Start at the lower left front corner (Figure 4): There is no organic matter in the soil, no grass, and no water - desert. The soil has little or no elasticity/resilience.

Rain comes, and the trajectory moves from the lower left front toward the left rear corner: water increasing first followed by awakened and growing grass. As the grass grows, it produces roots and, so, an increase of organic matter in the soil. The water and organic matter continue to increase, and the trajectory attains a maximum (optimum). As the process continues, the grass begins to die out because of excess water and the trajectory bends downward. Finally, the organic matter decreases, and the trajectory continues downward and bends back to the left.

At the right and toward the back, the action surface dips downward out of sight in one projection but is visible in the other as the grass disappears, organic matter decreases, and water continues to increase until at the limit of the action surface there is open water.

On the action surface one can change any of the three variables and obtain an idea of the resulting surface condition. For example, construct a training surface of peat moss and loam such that the surface is springy when “adequate” water is added. We are at the right corner with maximum organic, no grass, and adequate water. Since the optimum, the peak area, has maximum grass, however, the pure peat and loam surface is not optimum. The action surface indicates that grass is needed.

Why should an optimum on this action surface require grass? It would appear that the peat/loam surface with excellent resiliency would be the optimum. The action surface interpretation could be that a matted grass surface would serve to add some resiliency per se and - more significantly – provide cohesion: holding the resilient soil in place without the excessive divoting, cutting up, plowing, etc. which would occur on a grassless surface. (This, in fact, is the invalid gallop in Newmarket mentioned earlier.) If such cutting up of the surface occurred, experience suggests that the surface would no longer be optimum. That is, the drop off on the action surface – and of the 3-D trajectory - would be the result of the loss of grass surface cohesion no matter the status of the water content of the soil.

It is evident that the model developed here could pertain to any surface where springiness, give, elasticity, cohesion are desirable, e.g., athletic activity of many types. Such generality is characteristic of many mathematical/geometrical models which, being abstract, may apply to a variety of real world situations.

Elasticity/Resiliency

This is the ability of something to rebound, return, to its original configuration after being deformed by an external force. Squeeze a rubber ball, and it returns quickly to its original shape when the pressure is released. Kinetic energy is applied by the fingers to the ball and is converted to potential energy (elastic strain energy) in the deformed ball. As soon as the fingers relax, the stored potential energy is reconverted to kinetic energy as the ball regains its original shape.

In horse terms we are seeking an optimal elasticity/resiliency surface which does what the rubber ball does. As the foot impacts and applies kinetic energy, the surface stores that energy, releasing it again as the foot leaves the surface - like a trampoline^[3]. The optimum, it appears, would be a surface which stored as much of the applied energy as possible and returned as much as possible to aid the forward movement of the horse. Complete storage and retrieval are impossible since there are frictional losses; we cannot have a perpetual motion horse/surface system.

Tuning

McMahon (1984 ) defined optimal surfaces for human runners by ”tuning” an artificial surface to the frequency of the runner. He showed that significant increases in speed could be achieved by such tuning. I suggest that such tuning would be desirable for horses as well as humans, providing a less fatiguing safer, working surface if not a faster surface.

Horses stand still and move in the frequency range 0-2.5 Hz^[4]. We should be looking for a surface, then, which vibrates in this range when subjected to the sudden brief impact of a horse’s foot striking the surface. Technically, we are looking for transient vibration in this frequency range. This transient vibration range would correspond to the peak area of the action surface, the dark red oval of the contour map.

If an object impacts a surface, there are three basic responses, summarized in Figure 5. If the surface is yielding, the object buries itself (the cushion of a race track), and we have the overdamped case, a. If the surface is very hard, the object bounces up and down before coming to rest (harness racetrack), the underdamped case, b. If the object and the surface are in tune (some turf courses perhaps, the invalid gallop), we have the critically damped case, c.

Figure 5: Please see the text for explanation.

What we desire, then, is a surface which, when impacted by about 1.8 times the body weight (the dynamic weight of the average horse^[5]), will oscillate somewhere between 0 and 2.5 Hz, within the peak area of the action surface and contour map.

Bibliography

McMahon, T A (1984) Muscles, Reflexes, and Locomotion. Princeton University Press. Princeton, NJ

Rooney, J R (1998) The Lame Horse. Meerdink. Neenah, WI [www.horseinfo.com]

Sachs, P D (1999) Edaphos. Edaphic Press. Newbury. Vermont.

Foot Notes

^[1]The water holding ability of soil depends to a considerable extent on the clay present and not just organic matter.

^[2]For those unfamiliar with such depictions: there are 2-D graphs with, say, time as a variable on the horizontal axis and a second variable such as amount of rainfall on the vertical axis. This action surface is a 3-D graph with the three variables, one on each axis.

^[3]Kinetic energy is also stored in the foot and leg of the horse itself in several different ways.

^[4]Hertz is a measure of frequency, i.e., number of cycles per second.

^[5]It has been shown theoretically that dynamic force is about 2 times the static force. Actual measurements with horses have shown that the dynamic force is about 1. 8 times the static.

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