Why it's harder to build muscle now than when you were young — and the best way to overcome it

Today, it might seem the opposite is true. You have to hit the gym several times a week just to avoid losing the muscle you do have.

In fact, the average 30-year-old loses about a quarter of her muscle strength by her 70th birthday and half of it by age 90, according to a Harvard Medical School health report called "Strength and Power Training for All Ages."

This isn't just because you become less physically active with age.

Age-related muscle loss, or sarcopenia as it's known in the medical community, is the result of a handful of factors that doctors still don't fully understand. For starters, your body becomes less efficient at turning protein into fuel; you also have fewer of the nerve cells responsible for sending signals from the brain to the muscles. But researchers believe a cascade of other issues including hormonal changes, inflammation, and other illnesses that become more common as we get older play a role too.

That doesn't mean muscle loss is completely inevitable, though. Experts at Harvard and the Mayo Clinic suggest strength (or resistance) training is best way to overcome it.

"Strength training is critical to preserving the ability to perform the most ordinary activities of daily living," wrote the authors of the Harvard Healthbeat newsletter, which summarizes the report's takeaways.

At its most basic, strength training involves using weight to create resistance against the pull of gravity. That weight can be your own body, free weights like barbells or dumbbells, elastic bands, or weighted ankle cuffs.

Research suggests you can use either heavy weights and a small number of reps or lighter weights and more reps to build stronger, more sturdy muscles.

Chris Jordan, the exercise physiologist who came up with the viral 7-minute workout (officially called the "Johnson & Johnson Official 7 Minute Workout"), told Business Insider that healthy adults should aim to work out at least three to five days each week, and incorporate resistance training on two to three of those days.

The results you see will vary based on your current fitness level. If you're new to regular workouts and start doing resistance training two or three days each week, you may start building muscle in just a few weeks. But if you've been a regular at a gym or yoga studio for several months, you'll want to step it up a notch and work your body at least four days per week.

Research suggests you can also use high-intensity interval training (HIIT), which combines the cardiovascular benefits of cycling or running with resistance training, to achieve the same or similar results. If you like HIIT, the 7-minute workout is a great place to start.

Whichever workout you try, however, the most important thing is to keep doing it.

"To achieve results," said Jordan, "consistency is key."