The loin on a horse is equivalent to our lower back. (Anybody suffer from lower back pain?)
The lumbar span is what we are talking about when we refer to the "loin" area on a horse. The lumbar span goes from the lumbar sacral joint to the top of the hip. It is the WEAKEST point of the back because there is no support either from the pelvis or the ribcage under it. It's only slightly bigger than the size of your palm. Due to its structure and the fact it is already the weakest spot on the back, you look for signs of strength in a loin. In my opinion, for a riding horse, the lumbar span is a deal breaker. The old saying "no foot, no horse".... well, no loin, no rider. The power that comes from the hind end of a horse must pass through this relatively small lumbar sacral joint.It joins the massive engine in the hind end, to the massive front end assembly, head and neck. It's like a drinking straw joining 2 lakes.
The lumbar sacral joint is found by palpating the back of the horse as it cannot be "seen" along the topline. It's inside the horse. Starting from the middle of the back, press down with your fingers and then press again and move an inch back towards the tail. Keep doing this and you will feel: bone, bone, bone, bone, bone, mush, mush, bone, croup. (P.S. if your horse is overweight it will be more difficult to find, do this on a lean horse at first). The "mush, mush" is the location of the lumbar sacral joint.
So what do we look for in a strongly coupled loin? Four things.
A short loin is an indication of strength in that area. Interestingly, a short loin makes it easier for the horse to raise its back during collection.
If the lumbar sacral joint is located BEHIND the point of hip this is considered poor placement. If it is located AT the point of hip it is an average placement. If it is located in FRONT of the point of hip it is an excellent placement.
The depth of the horse through his "loin girth". A tiny waist (wasp-wasted) is an indicator of a weak loin. You want that depth to be as near to the heart girth as possible.
No bumps and lumps. You want the loin, croup and gluteal muscles to be smoothly convex. You don't want any dips, bulges or other insults to the outline of the horse in that area.
A wasp waist suggests a long, weak loin, but it is only one indicator. How does a long, weak loin limit my horse, you ask? The ability for a horse to step well under itself and engage the ring of muscles, is centered in the loin. If the loin is able to coil, then the pelvis is able to come forward, and the back and base of the withers can rise. It's the "rounding" that people talk about, although people with an uneducated eye can only "see" rounding in the neck.
Here's an example of a weak loin.
Here is an example where ALL of the 4 requirements for a strong loin are in place.