Trying to hack fitness is a multi-million-dollar industry; we’ve all seen at least one ad featuring a purported miracle product that claims it can make people lose weight and look great–without even trying. From low-effort exercise machines to strange-ingredient diets to fat-burning belts and bands, there’s no shortage of attempts to make it easy to be fit.

A gene therapy trial performed on mice may foreshadow yet another way to hack fitness. In a study done by a team at Washington University in St. Louis’ medical school, mice quickly built muscle mass and reduced obesity after receiving the therapy, even while eating a diet high in fat and not exercising. The results were published last week in a paper in Science Advances.

Sound appealing? Here’s how it worked.

The gene targeted was FST, which is responsible for making a protein called follistatin. In humans and most other mammals, follistatin helps grow muscle and control metabolism by blocking a protein called myostatin, which acts to restrain muscle growth and ensure muscles don’t get too big.

The researchers injected eight-week-old mice with a virus carrying a healthy FST gene (gene therapy involves adding healthy copies of a gene to cells, usually using a virus as a deliveryman).

Over a period of 18 weeks, or about 4 months, the team observed that the muscle mass of the treated mice more than doubled, as did their strength level. They also experienced reduced damage related to osteoarthritis, less inflammation in their joints, and had healthier hearts and blood vessels than mice that didn’t receive the gene therapy–even though all the mice ate the same high-fat diet and did the same amount of exercise.

Going into the study, the researchers worried the muscle growth catalyzed by the gene therapy could harm the heart, mainly through thickening of the heart’s walls. Surprisingly, though, heart function and cardiovascular health of the treated mice actually improved. In subsequent studies, the team will continue to monitor the treatment’s effect on the heart, as complications could emerge over time.

Talk about a fitness hack; imagine being able to build muscle and maintain a healthy metabolism while lounging on the couch eating burgers and fries. There have been similar studies to replicate the effects of exercise by commandeering the genetic instructions that control the way cells interact with proteins; though various “exercise pills” have successfully mimicked the effects of regular cardiovascular activity in mice, scientists still don’t fully understand how, at a molecular level, exercise has the effects it does on the human body.

This may change in the next couple years, though; a National Institutes of Health consortium called the Molecular Transducers of Physical Activity is in the midst of an in-depth study on the molecular effects of exercise on tissues and organs in 3,000 people.

If the muscle-building gene therapy eventually reaches a point where it can be used in humans, though, the research team isn’t viewing it as a quick-fix health hack. Rather, it would be used to help get people with conditions like muscular dystrophy or severe obesity to a baseline from which they could adopt tried-and-true muscle-building practices like weight lifting or physical therapy.

“In cases of severe obesity or muscle loss, it is extremely difficult–if not impossible–to lose weight or improve muscle strength through normal exercise and diet,” said Farshid Guilak, orthopedic surgery professor and director of research at Shriners Hospitals for Children in St. Louis. “The goal of this study was to show the importance of muscle strength in overriding many of the harmful effects of obesity on the joint.”

If every condition, process, and trait in our bodies is tightly linked to our genes, it’s conceivable that almost any aspect of our health could be manipulated using gene therapy and related tools. Maybe one day there will indeed be a pill we can take or a shot we can get to give us svelte, muscular bodies without any of the effort.

The fact that this would ruin the pleasure and satisfaction of a good workout is another conversation–and one not everybody would be interested in having. But even if genetic or chemical exercise-replacement tools become safe to use in humans in the foreseeable future, they’ll likely be limited, at least at first, to those who need them due to debilitating health conditions.

That said–for the time being, keep hitting the treadmill, the weight room, or your other off-the-couch, effort-intensive workout of choice.