As with the break in the cervical vertebrae, the injury of the lumbosacral junction is extremely common and many owners/riders don’t think twice about it.
The equine athlete has 18 thoracic vertebrae which lead into the 6 lumbar vertebrae, which lead into the sacrum. Unlike the thoracic and lumbar vertebrae, the sacrum consists of 5 fused vertebrae and is immobile except where it connects to the last lumbar vertebrae and the coccygeal vertebrae (tail). The junction is between L6 and S1.
Injury to this area can be caused by poor trimming resulting in a broken-backwards hoof angle (long toe, low heel) causing the pelvis to tilt backwards. In addition, improper riding (hollow, disengaged, false/forced frame) and ill fitting tack (too long, too narrow gullet channel, improper tree width/angle) can have the same effect on the back in causing the pelvis to be forced out of its neutral position to accommodate the drop in the back.
The dip before the muscle is typically caused by atrophy of the gluteus medius muscle and over development of the semitendinosus and biceps femoris muscles through working/training in poor/incorrect posture. This can also contribute to the lack of development from the lumbosacral junction to the dock, giving it a flat and sloped appearance. The hind end should be round, with no sudden dips, slopes or divots along the musculature.