Passive motion of the neck

Passive motion is important for the horse because it aids in reducing metabolic energy expenditure to maintain balance. The majority of a horse’s metabolic energy expenditure is dedicated to the digestive system, which makes sense when you think about it, horses spend more time eating than they do moving. Therefore strong passive connections are vital for saving energy and being able to move freely (also very important for a prey animal who needs to move away quickly).

During walk the head and neck moves in a horizontal figure 8 pattern in time with the limbs (as shown in picture one), this in part occurs due to extrinsic muscular connections between the neck and forelimb (extrinsic means muscles connecting between different body parts) which work through active contractions (under neurological control) and passively through soft tissue connection, just like strings on a puppet.

When the forelimb swing forward it creates a passive motion of the head and neck downwards, and as the limb swings back the head and neck moves upwards (as shown in picture two), this has been demonstrated in Passive dynamics of the head, neck and forelimb in equine by C.Luisi and H.Davies (2023) (again there is a lot more to this but I’m simplifying!). However this passive relationship is only in one direction, when moving the head and neck passively (in a cadaver) there is no effect on the limbs. I can delve into why this is the case in a part 2 on this topic, which will explain the important role of the serratus ventralis muscle.

Because no neural input is required for this movement you could think of this as the pure mechanical movement of the limbs and the neck when in motion. What we then do as riders and handlers is we greatly interfere with this passive movement by restricting it with reins, the bit, gear and gadgets.

The secondary problem is that if you stop movement in one place it needs to end up somewhere. If you imagine the horse’s body like a stream and the water moves passively through it at a relatively constant rate, but now we place a log across the stream, it won’t stop the water but now there will be areas where water is stagnant and areas where the pressure builds up…this also happens in the body and we may see that as “stiffness” or “hypermobility”, I’ve tried to demonstrate this is picture three. THEN on top of all this we have the reflex actions and conscious muscular control…but I’ll explain this another time.

To summarise, the subtle passive motions need to work synchronously with active muscular activation for optimal movement. Therefore, when we interfere with it or this passive motion is not working optimally, there will be a chain of events through the whole system that will alter the mechanical movement pattern, which is incredibly important for the horse’s balance and energy expenditure. Horses that trip, knock poles, lack stability, have inconsistent rhythm, can all be signs that this system is not functioning coherently. I hope that makes sense! I’ve literally written pages and pages about this during my PhD.