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 1. The right forefoot lifting off.

Fig 2. This is the fly or open air phase with all legs cramped under the body.

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).

Fig 4. Both hind hooves sharing weight bearing a moment before the left pushes off.

Fig 5. The left fore has made ground contact while its opposite hind leg, the left, pushes off resulting in weight bearing shared by right hind and left fore.

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.

Fig 8. It was found that some thoroughbreds actually have all legs off the ground in between the above hind and fore hoof impacts. In other words, the hind will have pushed off before the fore reaches the ground resulting in a mini-fly period or also known as double air P2.

Fig 9. This photo shows the second possible position in a single stride where an extended thoroughbred's run can have a mini-fly period with all hooves off the ground. It is between the fore leg impacts within the single stride. It is also known as double air P3.

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:


     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.


Secondary Lameness
by James Rooney DVM

     It has long been known that lameness, pain in a leg, can and often does lead to lameness and pain in another leg. As an adage: lameness is a cause of lameness. In the 1960s I called this second site of pain "complementary" lameness. That was not original with me, I'm sure, and is not really correct in any case. Perhaps secondary or consequent lameness would be better; at any rate I shall choose consequent
     Of more importance, however, is defining what we are talking about, systematizing the observations, and seeing if the system we develop stands up to empirical test.

    1.   If a horse is lame in one foreleg, it tends to shift weight bearing to the other foreleg and to the contralateral (diagonal) hindleg. A clinical sign of lameness of, say, the left foreleg (LF) is that the head and neck move up when that lame left leg is bearing weight and move down when the right foreleg (RF) is bearing weight. This is known as nodding. The reason for the nodding is as follows: When the neck is raised (dorsiflexes) there is reflex extension of the leg in support at that time. Thus, with the LF lame and in support - bearing weight - the horse raises the neck, which induces extension of the LF. With extension there is less movement of the joints of the leg and, so, less pain since much of the pain of most lameness is caused by movement. In your own case, if you hurt an arm or leg, you try not to move it because that causes greater pain. Simply put: if it hurts don't move it.An additional sign, usually less obvious, is that the horse will carry the head and neck slightly curved around toward the lame leg.

     2.  The neck is curved around toward the lame foreleg because, again, this induces reflex extension of that foreleg and, so, reduces pain. I hasten to note that these reflex extensions (and many more) are all part of the normal locomotion of the horse. We are here considering them only in relation to lameness.

     3.  As mentioned above, if a foreleg is painful enough, the horse shifts some of the weight-bearing to the contralateral (diagonal) hindleg as well as to the contralateral foreleg. Say the horse is lame in the left fore (LF). We start the horse walking with movement of the LH. The LF moves next, then RH, then RF and back to LH again, Fig.1. Thus, RH is always the next leg to bear weight and move after LF. (Start with RH and you will see the same thing.) One may propose that since RH is next after LF and LH after RF, that when either fore comes to the ground and pain is experienced, the horse shifts weight as quickly as possible to the next leg in the normal sequence - the contralateral hind.

That sequence is, of course, true for the walk and related slow gaits. At the trot if the LF is sore, weight can only be shifted to the RH since that is the only other foot on the ground. Similarly for RF sore and shift of weight to the LH. The gallop sequence, diagonal or round, is different, and we shall try to deal with that later. In any case lameness is evaluated at the walk and trot, and we concentrate on those gaits.

    I must, however, take a moment to consider the lateral gaits, so-called: rack, pace, single-foot, etc. At the pace, for example, if LF is sore, the next leg in sequence is not LH but RH, and the contralateral principle holds.

     It would be nice, very nice, to be able to extend these observations to primary lameness of the hindleg and secondary or consequent lameness of a foreleg(s). Going back to 1. we saw that reflex extension helps to alleviate pain in a leg. If LH is sore, reflex extension should help, and such reflex extension is achieved by the horse lowering the neck (ventroflexion). In the real world, unfortunately, it is difficult and usually impossible to perceive such movement. There is no real and easy explanation for that. One factor is that lameness of the hindleg (e.g., spavin, sacroiliac arthrosis) is almost invariably bilateral - both hindlegs affected. With pain shifting back and forth between the two hindlegs it is virtually impossible to distinguish significant ventroflexion of the head and neck related to one leg or the other. It is generally true that horses lame behind tend to carry the head and neck lower than usual but, again, that is very difficult to appreciate in most cases.

     We have been considering this from the viewpoint of diagnosing the site of lameness. Of equal, and perhaps greater importance, is understanding why a horse lame in one leg becomes lame in another leg. In brief, in summary, all one has to do is to examine the gait sequence for walk and trot as in Fig.1; the next leg in the sequence after the lame leg will be the site of consequent lameness.

The gait sequence for the diagonal gallop is shown in Fig. 2. We take the horse on the left (LF) lead with LF sore. The sequence is: RH-LH-RF-LF. That is obviously not a good situation for alleviating pain, the lame LF being in sole support. The horse shifts to the RF lead with the sequence: LH-RH-LF-RF. LF is now in the diagonal with RH and, therefore, sharing the weight with RH rather than being in sole support. IF there is to be a consequent lameness, it should be in RF and/or contralateral in the RH. In the real world, once again, one more often sees consequent lameness in the contralateral foreleg and not (apparently) in the contralateral hindleg. Consequent lameness of RF occurs because RH leaves the ground a bit before LF, so that alleviating pain in LF requires a quick shift of body weight to RF - the next in line in the gait sequence just as at the slow gaits.

All these bets are off, of course, if one considers the round gallop. I shall leave that to the reader, if interested, since it is an uncommon situation but can be worked out using the gait diagram.