In the January 2010 issue of EQUUS magazine, Deb Bennett, PhD, discusses the importance of proportion in equine conformation. After you read "A Sense of Proportion" in that issue, follow the instructions below to take your own horse's measurements to see how well the stoutness of his legs match his overall size.
Measuring the Bone-Tendon Circumference.
In the living horse, it has become traditional to use a standard measurement (bone-tendon circumference or "B-T") as the indicator of the "stoutness" or "amount of substance" in the limbs. As shown in Fig. 1A, the measurement is taken at the point, just below the knee, where B-T circumference is least--thus, the part of the animal's limb that most closely approximates the actual circumference of the cannon bone.
How do we relate this circumferential measurement to the cross-sectional area of the bone? The formula for calculating the area of a circle is A = ? r2, while that for calculating circumference is C = 2 ? r. To find a solution to either equation requires that we know the value of ? (3.1415) and r, which is the width of the cannon bone at its narrowest point divided by 2. Since both equations utilize the same factors, they are "inter-convertible" for engineering purposes; in other words, we can express a cylindrical cannon bone's capability to support weight equally well whether we say "this bone will support a maximum load of 12 pounds per square inch of cross-sectional area" or "this limb with a B-T circumference of 8 inches will support a maximum bodyweight of 1000 lbs.").
Because what we really want to know is not merely how big around a horse's legs are, but whether they are stout enough to support his mass, I express a horse's "limb substance" in terms of inches of bone-tendon circumference per thousand pounds of weight (Fig. 2). This is the most useful and revealing information we can have concerning a horse's limbs. There is an assumption hidden in taping horses' legs. The reason the measurement is called "bone-tendon circumference" is that the tape passes around not only the horse's cannon and splint bones but also all the soft tissues in that area, the largest of which are the tendons of the deep and superficial flexor muscles (Fig. 2B). The tendons and other soft tissues are not static-load support structures, yet they more than double the B-T measurement over what it would be if the cannon bone alone were all that was being taped. Variation--which is usually small enough to be ignored--exists from horse to horse in the percentage of the B-T measurement represented by the tendons. I am pointing this out so that when you measure your own horse, you will have a perfectly clear idea of what you are taping as well as the assumptions implicit in the procedure.
Measuring Your Horse's Feet
Maximum hoof width, measured from medial to lateral wall, is another very useful indicator of the weight bearing capacity of a horse's limbs (Fig. 3). Since the shape of a normal horse's fore hoof is almost perfectly circular, the area of the bearing surface of the foot is easy to calculate.
A hoof width measurement should go from the outside edge of the wall on the medial side to the outside edge of the wall on the lateral side. If the horse's feet differ in size, use the smaller foot.?Many horses' feet are distorted in shape (flares, wall separations, etc.), and in these cases a procedure of estimation must be used.