Muscle cramps are a painful condition arising from hyperactivity of motor units caused by repetitive activation of the nervous system or CNS. The origin of the cramp, in most cases, is believed to be the intramuscular portion of the motor nerve endings. Most muscle cramps are also accompanied by fasciculations in the same muscle and normal serum creatine kinase (CK) activity. Muscle fiber contractions can be caused by vigorous contraction of a shortened muscle, by alterations in the electrolyte composition of the extracellular fluid, and by ear mite infestations. In contrast, muscle contractures, such as those of exertional rhabdomyolysis, are painful muscle spasms that represent a state of muscle contraction not accompanied by depolarization of the muscle membrane. Muscle contractures are invariably accompanied by a marked increase in serum CK activity.
Hypocalcemia is a relatively rare disease in horses, also known as lactation tetany, transport tetany, idiopathic hypocalcemia, and eclampsia.
Synchronous diaphragmatic flutter occurs due to activation of the phrenic nerve in synchrony with atrial depolarization, causing the diaphragm to contract with each heartbeat. This occasionally produces an audible thumping sound.
Infestation by the spiny ear tick (Otobius megnini) in the ear canal can produce extremely painful intermittent muscle cramps, not associated with exercise, lasting from minutes to a few hours and often resembling colic.
Shivers syndrome is a spastic condition of the hind limbs and occasionally the forelimbs of horses, which usually only manifests when the animals are backing up or when their hooves are lifted.
Another very important and sometimes overlooked factor is lactic acid, which is directly linked to cramps because it is a byproduct of glucose metabolism when there is a lack of oxygen in the muscles. It accumulates and causes fatigue and pain, especially after intense exercise. It can be relieved by stretching, hydration, and rest, although the exact cause of cramps may involve imbalances of minerals such as calcium and potassium, as well as factors such as dehydration.
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