Science. 1984 Sep; 225(4667): 1172-4.

The twitch in horses: a variant of acupuncture.

Lagerweij E, Nelis PC, Wiegant VM, van Ree JM.

The twitch procedure in horses attenuates the increase in the heart rate evoked by pain-inducing stimuli and the reaction of the animals to such stimuli. Endorphin systems are probably involved in the effectiveness of the twitch, since its action is blocked by naloxone and its application increases plasma concentrations of immunoreactive beta-endorphin. The mode of action of the twitch cannot be explained by the generally accepted theory of divertive pain and may resemble that of classical acupuncture.