Scientists Use Sound Waves to Study How Cold Hurts Muscles — and How to Help

A team of scientists recently ran an experiment to learn what happens to muscles when a body gets dangerously cold. They used special medical tools called ultrasound machines to look inside the bodies of rabbits without cutting them open. The goal was to find out how cold damages muscles and blood flow — and whether warming treatments could help the body recover. The study also tested a method called peritoneal lavage, which uses warm saltwater to heat the body from the inside.

The researchers used 30 healthy adult male rabbits for the main part of the study. Each rabbit weighed about 4 kilograms, which is roughly the size of a small house cat. The rabbits were kept in a room with a comfortable temperature and given plenty of food and water. An ethics committee approved the experiment to make sure the animals were treated as humanely as possible.

To make the rabbits very cold, scientists placed them in a cage with a shallow ice-water mixture. That cage was then put inside a cold chamber set to -4 degrees Celsius, which is colder than a typical home freezer. Researchers checked each rabbit's core body temperature — measured inside the rectum — every 20 minutes. The cold exposure ended when a rabbit's temperature dropped to around 20.5 degrees Celsius and the animal could no longer stand on its own.

The rabbits were split into three groups for the rewarming phase. The first group received both peritoneal lavage and a warming platform. In peritoneal lavage, warm saltwater was carefully pumped into the rabbit's belly, held there for 15 minutes, and then removed. The second group used only a heating platform and a far-infrared heater. The third group had no outside warming help and had to rely on their own shivering to get warm.

Scientists used several types of ultrasound to study what was happening inside the rabbits' bodies at four different time periods. Regular ultrasound measured the thickness of the calf muscle and nearby tendons. Shear wave elastography checked how stiff the muscle was, while color Doppler imaging looked at blood moving through the leg's main artery. A fourth type used a special liquid injected into the ear vein to light up blood flow in the muscle on the screen.

The ultrasound scans showed clear changes in muscles and blood flow as the rabbits got colder and then warmer. When body temperature dropped, blood flow in the leg decreased and the muscle became stiffer. As the rabbits were rewarmed — especially those that received peritoneal lavage — blood flow began to return closer to normal. Scientists also studied the muscle tissue under a powerful microscope and tested for a protein called BNIP3-1, which can signal cell damage in muscles.