If you don’t exercise now—and keep exercising—by the time you're 70 your muscles will have shrunk beyond recognition. There’s room for hope, though.

The muscle atrophying that has been assumed to be a normal part of aging is far from inevitable. This stunning revelation comes from two studies that looked at the effects of aging on muscle fitness.

In the first study, researchers provide remarkable visual evidence of the differences in muscles between long-time exercisers and sedentary people. Using MRI snapshots, they compared the muscle mass in the mid-thighs of masters athletes aged 40 to 81 years old. These are people who train four to five times a week in running, swimming, and cycling. The images in the paper show that the thigh of a 40-year-old masters athlete looks virtually the same as that of a 70-year-old athlete. The area of the mid-thigh muscles did not decrease with age.

An image of a healthy, but sedentary, 70-year-old, however, showed a drastic decrease in the amount of muscle. The second study by other researchers examined the “motor units” of legs muscles. These are the basic units of muscles, with each unit connected to a single neuron. The atrophying of motor units is believed to contribute to the general weakening that occurs as we age.

Sixty-year-old masters runners had similar numbers of motor units in their legs muscles as 20-year-old runners. Older non-runners, however, experienced a 35% decline in the number of motor units. This effect, though, didn’t extend to the arms muscles. Older runners and non-runners had similar declines in motor units in their arms.

Clearly, long-term exercise plays an important role in maintaining our muscle mass as we age. While your hair may turn gray, or your waist grow a little wider, if you keep hitting the road, you’ll still hold on to some of your youth.