The below drawings are from a very good German website (harzenspferd.de) and are an excellent visual demonstration of a correctly muscled horse. These are of course the opposite extremes of what we find, so your horse may be somewhere in between with some of the imbalances but not all. Please also be aware that breeding and conformation need to be taken into account.
Lets break this down a little:
Without a strong engine the rest of the horses musculature will have to compensate. Here we can see the difference between an under-muscled rump, with weak gluteals/ quadriceps (and thus little stifle support) and a nice well rounded rear – be very careful not to mix this up with fatty tissue as a rump can look very rounded and not be muscled!! Long slow hill work is great for this as well as raised poles and backwards steps.
So the power generated from that strong rump now has to travel through your horses back. Unfortunately I see signs of this core muscle sagging all to often, from not enough training and also incorrect riding. The horse on the right will struggle to support the riders weight as well as its own and injury/ bad behaviour is often the result. The spine should not protrude but be supported on either side by strong and supple (not tight and reactive) long back muscles and also by the smaller para-spinal muscles which support the vertebrae. Again do not confuse fatty tissue which can build up either side of the spine for muscling.
Finally we look at the shoulders and neck – an area I have to spend so much time on due to the tension and imbalance I often find here. Note how the scapula (the shoulder blade) on the muscled horse is harder to see – it is fully surrounded and supported by well developed muscle. We can also see that we are trying to develop muscling along the top portion of the horses neck, if your horse is showing more development in the lower portion then you should consult with your riding instructor to try and improve how you move together. Again please note that the crest on the horse in the left image is not composed of fatty soft tissue but of muscled supple tissue.
Please do not hesitate to contact Claire for more information on how to improve your horses muscling.