A team of American researchers measured a muscle strength loss of only 29% in black bears after 110 days of total immobilization during hibernation, compared with 54% in human patients who were bedridden for only 90 days. The researchers found that after 110 days of fasting and confinement in their den, the bears had lost about 29% of their muscle strength, versus 54% for humans bedridden for 90 days. A gap that upends the way medicine views prolonged immobilization, whether in hospital or in space.
Key takeaways
- A muscular gap that defies the laws of physiology: why bears resist where humans falter
- Hidden molecular switches in the blood of hibernating bears capable of protecting our muscles
- From Mars to hospital beds: a universal solution taking shape in European laboratories
A body at rest that refuses to degrade
During this period, the bear eats nothing, drinks nothing and excretes no waste. It enters a state of controlled hypothermia: it lowers its body temperature, slows its heart rate, and sinks into a lethargic sleep, surviving without eating or drinking. This lethargy typically lasts between five and seven months each year. On paper, such fasting coupled with total immobility should trigger a cascade of damage: severe muscle wasting, bone demineralization, and the buildup of nitrogenous wastes in the blood.
None of this actually happens. Hibernating animals like the brown bear tolerate months of immobility, fasting, and very low heart rates without lasting consequences, and emerge from their den with preserved muscle strength, intact bones, and a brain and eyes that function normally. Even more striking: the metabolic wastes produced during this period are recycled into useful nutrients rather than eliminated. The return to normal is equally remarkable. Bears come out of their den in spring healthy, with only a marginal loss of muscle mass, and it takes roughly twenty days to regain normal functioning. By contrast, astronauts in microgravity lose 9 to 11% of their muscle strength during a flight of merely seventeen days. The gap between the two organisms is staggering.
The molecular mechanisms that science is starting to uncover
Researchers at the National Institute for Agricultural Research, Food and Environment have uncovered part of the explanation at the cellular level. In the hibernating brown bear, the TGF-β signaling pathway, typically overactivated during muscle atrophy in rodents or humans, is strongly inhibited, while the BMP pathway, known for its role in maintaining muscle mass during inactivity, is activated. This molecular switch acts like a toggle that protects the muscle from its own degradation process.
A team from Hiroshima University in Japan pushed the experiment further by working directly with blood. By injecting bear serum drawn during hibernation into human muscle cells cultured in the lab, researchers observed a notable increase in the protein content of these cells within just 24 hours, whereas serum from an active bear failed to halt the natural degradation of muscle proteins. A “factor” present in this blood therefore appears capable, on its own, of rebalancing the balance between the synthesis and breakdown of muscle proteins.
A study published in 2026 in Acta Physiologica explored another avenue—the mitochondria of Scandinavian brown bears. During hibernation, these animals maintain muscle mass by reducing the number of their mitochondria while increasing their efficiency, and their muscles reorganize their energy circuits to continue functioning at low temperatures, thereby limiting cellular damage. These adaptations could inspire strategies to prevent muscle loss in humans, whether in cases of immobilization or during spaceflight.
What medicine and space expect from these findings
In France, the Bear2Man project led by chemist and biologist Fabrice Bertile at the Hubert Curien Multidisciplinary Institute in Strasbourg follows a comparable path by using proteomics to map all active proteins in the blood of the hibernating bear. The aim is to preserve muscles in people who remain bedridden for long periods, in certain patients, and in astronauts, while improving the autonomy of the elderly. The question even interests space agencies: Fabrice Bertile has been invited by the European Space Agency to participate in a working group focused on hibernation, with the goal of preventing or limiting muscle loss in astronauts on mission.
The intended outcomes extend far beyond the bear. When a human remains in prolonged microgravity, is bedridden in a confined environment, or immobilized in intensive care, the body begins to degrade in a way that affects almost every organ. The envisaged applications range from protecting astronauts on long-space journeys to aiding the recovery of critically ill patients, as well as preventing age-related diseases such as osteoporosis, muscle wasting, or memory problems. On the space front, a team working with the European Space Agency has even sketched the concept of artificial hibernation capsules for a future voyage to Mars. Engineers envision soft-shell capsules, tuned for a calm environment, low lighting, kept below 10 °C and highly humid. Hibernating astronauts could reduce mission costs, cut ship size by about a third, and preserve crew health on the voyage to Mars.
The next steps now take place in the laboratory. The upcoming phases of this work, conducted in the laboratories of the University of Clermont Auvergne and the Hubert Curien Multidisciplinary Institute in Strasbourg, involve identifying and isolating the active principles of bear serum capable of blocking muscle degradation. Once these molecules are identified, they can be tested on cultured human cells, before considering, one day, clinical trials with bedridden patients or crews heading into space.
Sources: recherche.unistra.fr | inrae.fr
