There are a couple of conditions that could be causing this unusual hind leg action you have noticed. If he is big and young or not very well muscled, he could be having “delayed release” in his stifle. This occurs when the patellar ligaments, that normally allow a horse to lock the stifle and sleep standing up, become engaged at the wrong moment and catch during the forward swing of his stride. Smart horses quickly figure out how to release this by exaggerating the sideways movement of the leg. The most extreme version of this problem is a “locked” stifle, when the ligaments cannot release and the horse stands with the leg stretched straight out backwards. This problem is generally encountered in big growthy young horses who do not have much strength in the quadriceps muscles, and in debilitated horses who have been on stall rest so they lack muscle tone. Usually a steady exercise program focused on increasing hind end strength, incorporating lots of hill work, lateral work, and raised trot poles, will resolve the issue. More stubborn cases can be treated with internal blistering, joint injection, and surgery to release the medial patellar ligament.
The other most likely possibility is that this horse has shivers. Nobody is truly certain why this occurs but it is thought to be a muscular/nerve condition that results in a “pins and needles” sensation in the hind limb when the horse starts to move or is asked to pick up the hind leg. They will jerk the hind leg up and it will quiver for a few moments before relaxing, but once the horse is warmed up and moving it tends to resolve. While this is not a painful condition it does tend to cause anxiety especially if we get impatient and frustrated with them, so a patient and sympathetic farrier is a huge asset- this can be an incredibly tiring type of horse to shoe behind! The good news is that this condition does not interfere with a horses athletic ability. I have had some success with putting these horses on a high fat diet as if they were diagnosed with PSSM, because I predominantly see the condition in large bodied warmblood types.
You mentioned that he also likes to stand crouched in the stall. This can be a sign of stifle pain, so a veterinary examination might be a good idea. If he has a fundamental problem within the stifle joint, such as OCD, the discomfort cold have prevented him from using the stifles, resulting in weak muscles and the patellar ligament problem.
Melissa McKee DVM