Collection done well is beautiful and powerful. But what is collection? What physically happens in the horse’s body to create a collected gait?
The most common answers about what collection is come from the FEI rulebook. Collection is supposed to involve a shortening of the stride with a maintenance of energy in movement, increased flexion of the joints of the hindquarters, resulting in a lowering of the horse’s croup, and hind limbs that step farther forward under the horse, all of which are said to cause a weight shift from the front legs to the back legs.
Some of these claims are supported by studies of elite horse and rider movement in the collected gaits, but others are not. So what actually happens when a horse collects?
Shortening of the Stride
In examining collection of the walk (Clayton, 1995), trot (Clayton, 1994a), and canter (Clayton, 1994b), Clayton found that as collection increased, stride length decreased. The frequency of strides also decreased in walk and trot, but not as much. The stride length of piaffe is smaller than that of passage, and passage is smaller than that of the collected trot.
Flexion in the Joints of the Hindquarters
Increased flexion of the joints in the hind limbs, resulting in an almost crouching posture, has been found in some studies (Rhodin et al, 2018) and not found in others (Clayton, 1994a). It is likely that this factor of collection depends greatly on the degree of collection, with the greatest degree (piaffe) displaying the most flexion of the hindlimb joints.
It is said that this increased flexion causes the horse to lower the croup relative to the withers, so the back is sloped down and back. However, studies have not seen this happen in either the collected walk or trot (Rhodin et al, 2018), though again in a higher degree of collection such as the piaffe, lowering of the croup can occur.
Interestingly, an increased posterior tilt of the horse’s pelvis at the lumbosacral junction does occur in the collected walk and trot, which lowers the dock (Rhodin et al, 2018). This could create the impression of a lowered croup for an observer.
Protraction of the Hind Limbs
Protraction is the reaching forward of the limb, and retraction is the pushing out behind. Increased protraction as an element of collection may be part myth. The horse does not actually step the hind leg any farther underneath the body, towards the forelimbs, than in a free gait. However, there is another effect of collection that may give the appearance of increased protraction of the hind limbs.
In the collected gaits, the horse down not retract the hindlimb as far as he would in a free or extended gait. This means the hind limb pushes off sooner relative to how far the limb has traveled, it does not stretch out behind the horse when pushing off for swing phase. The average protraction of the limb from start to finish of stance phase, then, does increase, but it is because of decreased retraction, not increased protraction.
Weight Shift to the Hind Limbs
Ground Reaction Force (GRF) is measured to determine whether the above biomechanical effects of collection actually cause the hind limbs to bear more weight than they would in a free gait.
There have been very few studies regarding this, however it seems that there is indeed some small shift of weight, also on a sliding scale with the degree of collection. From collected trot to passage, Clayton et al (2017) found that within a diagonal pair of legs, the hind leg bore proportionally more weight. In the collected trot, the forelimb of a pair bore about 58% of the weight, which was reduced to about 55% in the passage.
Three ways were found whereby a horse could shift weight into the hindlimb: I.) having the hindlimb in a diagonal pair contact the ground prior to the front limb, II.) adjusting how far forward and back each limb moves in swing and stance phases respectively, and III.) increasing muscular effort through the hindlimb to create more vertical movement.
Conclusion: What is Collection?
Collection involves a change in how the horse moves himself. It can be hard for the observer to see exactly what is going on in a collected gait, however. The major discrepancies between common equestrian thought and evidence from studies include: no actual increase in protraction of the hindlimbs, though average protraction is increased through less retraction; the suspension phase of collected gaits does not increase in duration, contrary to popular belief; and finally, the horse’s croup does not lower, rather a posterior tilt of the horse’s pelvis could create this impression by lowering the dock.
The commonly cited weight shift from forehand to the haunches does in fact occur, partially (but not to a great extent) aided by an elevation of the horse’s poll. The stride length of the collected gaits is also much shorter than that of the free gaits.
Understanding what actually occurs in the horse’s body during collection is helpful for riders to influence their horse’s movement, knowing more accurately what changes they want the horse to make.
Byström, A. (2019) The movement pattern of horse and rider in different degrees of collection. Swedish University of Agricultural Sciences, Uppsala. ISBN: 978-91-7760-383-2
Clayton, H. (1994a).Comparison of the stride kinematics of the collected, working, medium and extended trot in horses. Equine Veterinary Journal 26, 230-234.
Clayton, H. (1994b) Comparison of the collected, working, medium and extended canters. Equine Veterinary Journal Suppl 17, 16-19.
Clayton, H. (1995) Comparison of the stride kinematics of the collected, medium, and extended walks in horses. American Journal of Veterinary Research 56, 849-852.
Clayton, H; Schamhardt, H; and Hobbs, S. (2017) Ground reaction forces of elite dressage horses in collected trot and passage. The Veterinary Journal 221, 30-33.