Collection : a biomechanical approach part 2

« Engagement of the hind quarters is associated to a lowering of the haunches and raising of the forehand, with a raising of the neck in an arched position »

Regarding the arching of the neck :

« It is the closing of the angle of the head flexed from the second vertebra, where the base of the neck is raised and rounded, with the poll remaining the highest point ».

Is there more engagement of the hind limbs when the horse is collected ?

In scientific litterature, there are two definitions of engagement of the hindquarters. The first, (Holmström 1995), presents engagement as being the angle formed between a horizontal line and an imaginary line going from the tip of the haunch (tuber coxae) through to the hoof. The more this angle is closed, the more engagement can be noted (figure 1).

The second definition, (Galloux also in 1995), presents engagement as the distance between the projection of the center of gravity on the ground, and the support phase of the hind limb (figure 2). When jumping, Galloux defines strong engagement as being a distance of less than 15 cms, whereas weak engagement corresponds to a distance of more than 20 cms.

Is there higher engagement in collected gaits ? The answer depends on the definition used ! Holmström notes lesser engagement in collected gaits, whereas Galloux notes an increase.

The pelvis, femur and hock all flex markedly, with a decrease in the angle of up to 10° for the pelvis and the hock, and of up to 20° for the femur. The horse’s anatomical specificities (which will be dealt with in the following part), mean that the entire joint system is involved, therefore the lumbar-sacral junction, the stifle and the fetlock also take part in the movement. There  is also a decrease in the amplitude of the pendular movement of the hind limbs (back and forth movement), the hind limbs therefore move « less forward » and « less backwards » (Holmström, 1995, 1997). The speed of the movement also decreases, compared to working gaits. The hind limbs consequently « move more slowly ».

As the pendular movement is decreased, the angle between the horizontal plane and the tuber-coxae  - foot line is consequently less acute. This is why Holmström concludes that engagement is not greater in collected gaits.

On the other hand, as there is a flexion of the entire hind joint system which leads to the center of gravity moving back, this induces, according to Galloux, an increase in engagement.

Based on Holmström’s findings, in 2009 Rhodin hypothesised that the increased flexion of the sacrum when the hind legs strike the ground, associated to a decreased retraction (movement towards the rear) amplitude in the same hind leg at the beginning of the flight phase form the basis for the impression of greater engagement.

What about the elevation of the the forehand mentionned in equestrian litterature ? Two teams of scientists addressed this issue, interpreting elevation as being the maximum height reached by the front feet. Once again the results differ.

Holmström did not note any difference in elevation in collected gaits, he only noted a change in the movement of the forelimbs. In fact, maximum height is no different, but it is reached earlier in the stride.

Weishaupt on the other hand noted a significantly greater elevation, with a moderating factor to be taken into account : the measures were made with the horse on an equine treadmill, where in these conditions locomotion is markedly modified.

The modification in the movement of the forelimbs is linked to greater flexion in the joint system of the forehand (elbow, knee, fetlock) and could be the reason for the visual impression of lightness.

A team of scientists (Rhodin, Weishaupt) addressed the question of the relationship between the head-neck position, and the implications on the rest of the body, more specifically on the locomotor changes which occur.

What are the effects of the head-neck position on locomotion and on the rest of the body ? What link is there with collection ?

To carry out this study, the team studied 7 head-neck positions  (HNP) in all, of which the 4 major positions represented below :

The first (HNP1) is the attitude of a free attitude, the horse is on a long rein, the second (HNP2) corresponds to the so-called « FEI » attitude, where the neck is arched according to Equestrian books, HNP4 is an overbent attitude and HPN5 is the attitude of a very high poll, and an open angle of the head.

The results show that the HNP influences numerous parameters, such as the amplitude in back movement (flexing and extending, lateral flexion, and axial rotation), amplitude of strides, joint angles, amplitude of pendular movement in the limbs, ground reaction forces ….

The team also attempted to establish a link between the HNP and collection by trying to answer the following question : is there a HNP which fulfills all the conditions inherent to collection ? In other words, is there an attitude which alone induces all the biomechanical characteristics of collection ?

To do this the scientists took all the definitions of collection they could find (in the field definitions, or results of previous scientific studies) and determined which HNPs induced the desired effect. For example : « Collection means putting more weight on the hindquarters », the physical reality which corresponds to this definition, is the increase in ground reaction forces developed by the hindlimbs. The only HNP which increases the force in the hindlimbs is HNP5 (head held high, open head angle).

This method showed up a certain number of contradictions, such as the example above, where HNP5 is not commonly accepted as being an attitude for collection.

They therefore concluded that there is no HNP which can alone induce all the criteria of collection. Collection is a complex attitude, which cannot be « broken down ». It is a whole.

However the HNP which is the most adequate for collection is the FEI position, with an arched neck. This attitude appears to be the best compromise between the locomotor parameters and behavioural reactions.

The authors also take this opportunity to remind riders that extreme positions can cause intense musculo skeletal pain, can impair breathing and cause conflictual behaviour.

An arched neck favours mobility

This scientific approach brings to the fore the idea of COMPROMISE in dealing with the height of the horse’s center of gravity, trajectory and speed… any extreme position should be avoided.

When looked at from the joint point of view, collection is characterised by flexion of the entire hindquarters joint system, with a decrease in the amplitude of pendular movement of the hind lims, as well as greater flexion of the forelimb joint system. The attitude where the neck is arched described in riding books is the most adequate to obtain the different parameters of collection, whilst respecting the horse’s physical integrity. All the authors agree in their conclusion that any extreme position should be avoided.