Also see Myopathies in Horses.
Damage to the muscles is probably the most common cause of back soreness in horses. This most commonly involves the longissimus dorsi muscle, which acts to extend and laterally flex the vertebral column. All or part of the longissimus muscles usually are strained during ridden exercise, and clinical signs are associated with altered performance and acute or chronic back pain. The principal sites of damage are the caudal withers and cranial lumbar regions (just in front of and behind the saddle area). Most of these injuries respond to rest and physiotherapy, although several weeks may be needed for full recovery. Abnormalities of the thoracolumbar musculature are also frequently seen in exertional rhabdomyolysis (tying-up syndrome, see Exertional Myopathies in Horses).