When foaling goes wrong, it can have serious consequences for both the mare and the foal. Japanese researchers have determined that a difficult birth—and sometimes even apparently normal births—can rupture a hind leg muscle called the gastrocnemius in foals. And unfortunately, that muscle’s rupture can be fatal.

“Gastrocnemius destruction can occur during parturition (birth) such as in a hip-locked dystocia (difficult birth),” said Fumio Sato, PhD, of the Hidaka Training and Research Center at the Japan Racing Association in Hokkaido. “But, in some cases, the gastrocnemius muscle might be overextended when the foals fall backwards during their attempts to stand up after parturition.”

The gastrocnemius muscle is the strong, rounded muscle that runs along the back of the hind leg, between the stifle joint and the hock. The human equivalent is the calf muscle.

When the gastrocnemius muscle is injured during or just after birth, foals tend to not bear weight on that leg or are severely lame, Sato said. The rupture can lead to significant internal bleeding of the muscle, which can cause anemia, and the bruising (hematomas) within the muscle can cause calcification (bone mass formation inside the muscle tissue)