Probably the first order of affairs for a logical analysis of gait and the anatomical mechanics of the racehorse would best be served by examining the gait itself. The Thoroughbred may be considered to have two gaits of speed: the slower  transverse gallop as seen in morning gallops and the transverse run seen in morning speed works or actual races. One will also have a rotary gallop which will be seen occasionally in the racehorse, mostly in the first few strides out of the starting gate or in lead transitions. This distinction in the speed gaits were first studied and analyzed by number of men, but Eadweard Muybridge's photographic studies provided the foundation for all. John Henry Walsh (pseudonym: Stonehenge) suggested that the canter was a distinct gait from the gallop and so it tis, but probably not for the reasons he suggested. Edward Anderson in his 1883 text, The Gallop, went further to suggest that the Muybridge photographs commissioned by Governor Stanford showed the canter, gallop, and run to be three distinct gaits. He wrote that the run is so dissimilar from the gallop that they could never be observed as identical. He viewed the differences in rhythm, action, and sensation of those two gaits as prove of this premise. He describes one stride of the run as:

The Transverse (diagonal) Gallop and Run:

The horse leaves the ground from one of his forefeet (fig 1), resulting in all legs cramped underneath the body in open air suspension (fig 2), then receives the weight upon the diagonally disposed hind-foot (hence the name, the transverse gallop) which is planted 4' or more beyond the initial forefoot impact print (fig 3).

Fig 3.   The diagonal (opposite), left hind    making ground contact as the preceding right fore pushes off.

The diagonal hind foot receives the entire body mass (fig 3) and then it is shared with the opposing right hind hoof shortly there after (fig 4). The left fore then makes ground contact while the left hind leaves resulting in the right hind and left fore sharing weight bearing assignments for a moment (fig 5).

The right fore hoof shares weight with the newly contacting left fore, before it lifts-off, completing the stride (fig 6).

Fig 6.  The left fore is joined in support by the right fore resulting in both hooves sharing weight.

     Each leg has in-turn received the whole weight of the horse with the burden of the horse's weight shared only in three instances: (1) by two hind legs, (2) by a hind leg and a fore leg, and (3) by two fore legs. After the horse leaves the ground with the push-off of the left fore, a new stride is initiated causing the horse to fold all limbs under the body in a very cramped position as pictured below (fig 7).

Fig 7. The one "fly" period in the stride resulting in all hooves off the ground occurs after the front hooves leaves the ground.

    The animated run as depicted by Muybridge's photographs of the mare Annie G. can be seen below. It shows the above dissected one stride frames in an animated 16 phases on her right lead. Time of one stride is .46 of a second with a mile time of 1:47 or a furlong equivalent of 13.4 seconds. Considering that Muybridge's work was done in the late 1800s, it was a considerable break-through in understanding locomotion and step sequenses. Hoof contact sequence:  RF—(fly)—LH-RH-LF-RF—(fly)—

   The above analysis of the extended gallop was all that was known of the Thoroughbred's running gait until Seder & Vickery completed a motion study in 2003 on over 6500 racehorses. They surprisingly found that sometimes, the extended Thoroughbred not only has the one commonly agreed upon "fly" period (Fig. 7) of open air as denoted above between the last striding fore hoof and the next planting hind, but the race run can also have one or even two, mini-fly suspensions within that one stride in addition to this major fly, open air suspension period. These mini-suspension periods are extremely short in duration (less than 1/300th of a second) and within the one stride could occur between the changeover from the last rear leg impacting the track to the next fore leg impaction phase (Fig. 8) and/or between the successive impact of both front hooves (Fig. 9). Muybridge's cameras were not precise enough to catch these open air phases because of such a short time duration, if they had occurred in his subjects. Not all racing thoroughbreds will exhibit these additional airborne phases within its one stride. All will certainly have the one major fly phase (fig 7). However, some will have one or even two additional airborne phases as labeled in Fig 8 and 9 below.

     In summary, the racehorse can have a normal stride of one fly period or they may also have the double-air P2 and the double air P3 in addition for a "triple air" running stride. Seder & Vickery found that out of their 6500 pool that 25% of horses had a double air phase (either P2 or P3) inserted in their racing run when speed increased beyond a 12 second furlong. They also found that horses started to exhibit a double-air P2 phase in their gait at the 12.9 second furlong rate while the double-air P3 could be seen at around the 14.7 second furlong rate. Only 1.9% of the studied racehorses showed a triple air (a stride consisting of the main fly period and the two other mini-air suspensions) and this occurred at around a 11 second furlong rate. They found with horses exceeding the 11 second furlong rate, 17% exhibited the triple air style of running. At first glance, these seems like rather low percentages, but then again, good fast racehorses are a rarety. It has yet to be proven that most of the fastest horses are the triple air horses.

The hoof ground contact sequence for a double air horse in one stride:

—(fly)—LH-RH—(mini-fly)—LF-RF—(fly)—    (double air P2)            or 

—(fly)—LH-RH-LF—(mini-fly)—RF—(fly)—    (double air P3)

The triple air horse has both the double air P2 and P3 in its stride:

—(fly)—LH-RH—(mini-fly)—LF—(mini-fly)—RF—(fly)—

James R. Rooney, DVM writes of one interesting observation in regard to complimentary lameness as observed in the thoroughbred. He writes that it is a well known fact that lameness causes lameness, that a minor lesion can cause a major lameness. I agree whole heartedly. He also writes that in the running horse, one will mostly see the following:

1) Lameness in one foreleg leads to lameness in the other foreleg.

2) Lameness in one hindleg leads to lameness in the foreleg on the same side.

3) Lameness in foreleg does not lead to lameness of the hindleg and the lameness in one hindleg does not lead to lameness in the other hindleg.

The Rotary Gallop or Run

     Unlike the previous transverse gallop, this form of gallop is rather rare in the equine and can only be seen in certain instances, primarily at the first few strides when a horse breaks from the starting gate or in lead changes. The rotary gallop is commonly seen in other animal species, i.e. the dog, deer and others.

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