Endurance riders love a BIG trot. When looking at your upcoming superstar or admiring how your favourite horse trucks down the trail, nothing excites more Oohs and Aahs amongst endurance riders than that big trot. Many endurance horses, maybe yours, undertake most of their competition miles in that big trot. I would like to suggest there are a number of reasons why the big trot is not the be-all and end-all and that more canter should be included when covering distance, for a number of reasons including:
1. Energy efficiency;
2. Scapular inhibition; and
3. Lumbar/sacral strain.
This horse clearly demonstrates all the hallmarks and pitfalls of a big trot. Although momentum plays some part in his extravagant movement, this degree of limb hyperextension requres a significantly higher degree of muscular effort than would a more modest stride. His scapular (the large bone of the shoulder) is forced through an excessive range of motion with the scapular cartilage (at the top of the shoulder blade) hitting the saddle with each stride. His hind legs are widely separated - one forward, one backward - placing excessive strain on the joints of the pelvis/sacrum and the lumbar region of the back. For these reasons, his neck is short, his back is hollow and his hindlegs show minimal joint flexion at hip, stifle and hock; instead they swing, pendulum-like, outside the path of the forelegs.
1. Energy Efficiency
When your horse moves in hyperextension, that is lengthening more than provided for by momentum, excessive muscular effort is required: extended gaits are not energy efficient. The further forward the hoof lands relative to the body mass, the more braking action occurs. Generally, a hyperextended hoof also stays on the ground for a longer period of time; tempo decreases as stride length increases. Muscular effort is required to overcome both the increased braking action and the inertia of the grounded hoof. From an energetic point of view, for most horses, any work faster than 15km/hr (ca 9mph) should ideally be performed at the canter rather than trot.
Unlike the canter, the trot lacks respiratory coupling. The up and down motion of the canter causes the substantial mass of the digestive system to move backward and forward within the body cavity with each stride. This piston-like motion activates your horse's diaphram, automating the breath with each stride, with almost no effort - an incredibly efficient way to move. The only time cantering is less effecient than trotting at fast speeds is travelling up very steep hills. Here, gravity works to push the digestive tract backward so respiratory coupling is not activated and the tempo of the canter is too slow to provide sufficient oxygen with each breath/stride and your horse quickly moves into an anaerobic work state. Although breathing at the trot requires more energy, your horse can choose to take a breath with different numbers of strides: increasing available oxygen and removing carbon dioxide.
2. Scapular Inhibition
This photo shows what happens to your horse's shoulder when he trots with lengthened strides for extended periods of time. Note the bulging scapular cartilage at the top of the shoulder blade from repeated trauma as it impacts the saddle and the 'hollows' immediately in front of and behind the wither as the horse tries to protect his shoulder from bruising by the saddle.
The shoulder of your horse has no bony attachment to his body: rather, his thorax is supported between his fore-legs within a sling of muscle and connective tissue. As a consequence, the shoulder blade (scapula) is very mobile, with a wide range of motion. As your horse extends his leg, the scapula rotates backward: the greater the extension, the greater the degree of scapular rotation. The top of the scapula consists of a cartilagenous area and this will commonly impact against the front of the saddle.
Over time, repeated impact results in congestion of the scapula cartilage - trauma. In an attempt to protect himself from more trauma, your horse braces the muscles around the top of the shoulder - in particular the trapezius muscles but many of the muscles of his topline are impacted - giving rise to the classic endurance musculature where there is a dip in front of the wither at the base of the neck and a hollow behind the wither. Flexible and semiflexible saddles that allow the scapula to move underneath them without impact greatly reduce this issue and, combined with appropriate body work and body-use reconditioning allow this issue to be addressed and enable normal scapular function.
3. Lumbar/Sacral Strain
In the trot, your horse's hind-legs are widely separated with each stride, with one hind-leg reaching forward and the other hind-leg reaching back. The pelvis and in particular the sacrum come under increasing stress with increasing stride-length at trot. In this configuration, it is difficult for the lumbar region of the back (the area directly under and behind the back of the saddle) to remain supple as one side of the horse is in extension while the other is in flexion. Inevitably this leads to lumbar strain and pain. Again your horse braces his muscles in this area in order to protect himself from lumbar pain, leading to further back stiffness and hollowing.
In the first picture the hind-legs are widely seperated in the lengthened trot, which, over time, leads to sacral strain and lumbar bracing. In the second picture the hind-legs move closely together at the canter enabling the sacrum and lumbar region to tuck freely, mobilising the horse's back.
These three issues inevitably combine - the result is your endurance horse has a hollow back.
The photo shows back and hind-quarter musculature typical of endurance horses, though this is a more extreme example than commonly encountered. For the reasons discussed here, rehabilitation requires that big trots are totally avoided. The thermograph of the same back shows how extensively circulation through the musculature has been disrupted. Note the relatively cold area (green) where the saddle sits as the muscle fascia has thickened. The lumbar area (immediately behind the saddle area) is very restricted with little blood flow whatsoever (green and blue) and has put excessive strain on the sacral tuberosities (the pointy lumps at the top of the croup). A horse with this degree of muscular shut-down will not be able to use the hindquarters in any meaningful way.
By all means, use all the gaits available to you when training and competing your horse. Just dont get bogged down with the idea that a big trot is the be-all and end-all. Break the trot up with canter wherever appropriate and your horse will go much farther for less effort.