Fitness comes with one stubborn rule: your muscles have to do the work. No one can do your push-ups for you.
Scientists may, however, be getting closer to outsourcing at least some of what working muscle does.
Researchers in China have created living muscle grafts that contract continuously inside mice. Implanted beneath the skin, the engineered tissue developed its own blood supply and kept contracting for months without any conscious movement from the animals.
And the benefits didn’t stay inside the graft.
Muscle That Keeps Contracting Without a Workout
The researchers began with skeletal muscle stem cells.
They took cells from the quadriceps muscles of individual mice, multiplied them in the laboratory and encouraged them to mature toward muscle cells. Around six million cells were then implanted beneath the skin on each animal’s back.
Because the cells came from the recipient animal, the grafts were essentially autologous — made from the mouse’s own tissue. There was no risk of rejection.
Then, once implanted, the cells did something remarkable.
They organized themselves into recognizable muscle tissue, developed a blood supply and began contracting spontaneously. The researchers could see the grafts physically tense and relax beneath the skin. They detected transplanted cells for at least 81 days, and tissue collected later showed organized muscle fibers and blood vessels.
Basically, the muscles were “self-exercising.” It’s not a literal description, as the implants didn’t exactly reproduce the complex cardiovascular and mechanical demands of a workout, but they did contract repeatedly.
When muscle contracts, it releases proteins and other chemical messengers that can influence metabolism, inflammation and distant tissues. Muscle-derived signals can communicate with organs including the liver, fat and brain.
But the most striking results came from aging animals.
The Effects Reached Far Beyond the Graft
The researchers implanted grafts into roughly 18-month-old mice and compared them with animals that underwent a sham procedure. Eight mice were included in each aging group. After eight weeks, grafted animals had retained a greater proportion of lean body mass. Fifteen weeks after transplantation, they also ran farther and produced greater grip strength than control mice. Their tibial bone density was higher as well.
Researchers found other benefits as well. Blood tests showed lower levels of several measures associated with inflammation. The researchers also found less accumulation of inflammatory immune cells in the liver and signs of reduced liver damage. Fat mass was lower, triglycerides fell more and the animals expended more energy over a 24-hour period. The mice with the grafts were also more curious and keen to explore new things, like a new branch of a maze.
So what does this mean?
For starters, don’t give up on exercise just yet. Exercise remains one of the best-supported ways to preserve strength and physical function during aging. Recent reviews continue to find benefits from resistance and other forms of training in older adults with age-related muscle loss.
The attraction of the new approach lies elsewhere: what happens when exercise itself becomes difficult or impossible. For people who are unable to exercise, this could be an option.
But there are still safety issues.
The researchers saw no obvious tumor formation or major toxicity under the conditions studied, but they have not yet performed the extensive toxicology, immune-response and long-term tracking studies required before contemplating human treatment. They also don’t know the ideal quantity of implanted muscle tissue. Too little might do nothing; too much could struggle to receive enough oxygen and develop dead tissue at its center.
Scaling presents another problem.
James White, a physiologist at Duke University who wrote a Nature Aging commentary on the research, called using a muscle graft to mimic exercise response an “intriguing premise.” But he has also cautioned that humans might require multiple grafts to produce effects comparable with those seen in mice, which may create a more dangerous trade-off, particularly for the frail people unable to exercise.
For now, the old rule still stands: no one can do your push-ups for you. But this study suggests scientists may eventually find ways to reproduce some of what working muscle does for the rest of the body.
If that holds up in humans, the biggest beneficiaries may not be people looking for an easier workout.
The study was published in Nature Aging.
