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I was reading the Conformation article on the club web site and came across the reference to the length of rein being an important consideration when inspecting a horse. The article then went on to say that a good length of rein confers balance to the horse to counter the power produced by the hind quarters. So far so good but there other factors involved where a good length of rein adds to the performance of the athletic racehorse.

To fully understand why a good length of rein is important we must go back to basics : How does the horse move?

Locomotion requires the use of muscles, tendons and bones. These are arranged in a complex manner of levers and pulleys.
There are three classes of levers depending where the load, effort and fulcrum are located as in a seesaw, wheel barrow and BBQ tongs.
In nearly all cases with the horse class 3 levers apply. Here a small movement of the effort (contraction of a muscle) produces a greater movement of the load (the leg). The negative side of the equation means that a greater effort is needed to move the load.

The reference to pulleys in horse locomotion is not your usual block and tackle but rather as a snatch block used to change direction of the force. The most obvious of these are the sesamoids which convert the upward pull of the flexor tendon to a backward pull on the hoof thus causing locomotion.

The locomotion muscles are the effort, the bone joints are the fulcrum and the bone itself and other bony attachments are the load..

The muscles are attached to the bone by tendons either close up or at a distance. The motive power is in the muscle.

Muscles work by contracting. Muscles are great at pulling, not much good at pushing.

There is a limit as to how much they can contract. The longer the muscle the greater the overall contraction. The bigger the muscle the more force it can deliver. This is the important property of the length of rein.
In essence the longer the length of rein, the greater the contracted length and thus the greater movement of the bone.

So back to the horse and in particular the front leg.

The front legs of a horse have no bone connection with the trunk.

However the shoulder is firmly held in place by muscular attachments to the vertebrae, ribs etc. so for all intents and purposes we can say that the top of the shoulder can act like a fulcrum allowing rotation of the rest of the shoulder.

In the diagram opposite the attachment of the shoulder to the vertebrae is simplified. There is not just one muscle attachment but others running down the neck line. The arrow line represents the muscle and so becomes the effort.

Also for simplicity the humerus, forearm and so on down the leg represents the load for the lever. We can see that this system is a class 3 lever.

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LENGTH of REIN