The latest government guidelines prescribe weekly weight training for all. Is it all strength and no cardio?
Not so. "Lifting weights doesn't conjure up images of something that would change your heart structure," says Easton, "but in actual fact it does. And there is quite good evidence now that resistance training, if it's done in the right way, can be beneficial to cardiovascular health." Easton says that these benefits of weightlifting pretty much follow the same principles as high intensity exercise such as spinning. "Essentially weight training is very high intensity exercise."
Strength training has also been shown to help ward off diabetes, arthritis, and depression. Resistance training is important for the elderly because it helps to maintain muscle mass.
With the aging population, one of the big concerns is people getting frail, says Timmons, who is studying weight training with elderly people.
"At 60, you don't tend to die of frailty. If you keep going to 80 then you tend to have weak muscles and bones," he says. "If you fall, lose your balance, fracture -- that's very much what drives poor quality of life and mortality."
How much should you do? The government guidelines on exercise recommend two sessions a week, for instance lifting weights at the gym or merely carrying shopping bags. The subjects in his research spend time weight training at the gym like anyone else. "We take them to the gym three times a week and we do progressive weight training -- multiple sets, 6 to 10 reps," Timmons says.
Timmons hopes his research will help shed light on why a quarter of people don't seem to improve their muscle strength at all despite the training. "They are switching on all the processes we expect would lead to muscle growth, but for some reason that's not been integrated into a gain in muscle tissue function." He hopes that by next year, he will have a way of diagnosing these non-responders, so he can get to work on trying to find a solution -- for instance they may need more recovery days or a different diet.
One of the most widely touted benefits is the idea that weight training, including increasingly popular choreographed weights classes, is that lifting weights keeps your metabolism stocked up for hours after you leave the gym.
"I don't think it is correct to put a figure on how long it is increased for after a weight training session because it will be dependent on the person and the intensity of the session, but probably something in the region of between three and six hours is not unreasonable for a very vigorous session," Easton says. "If you do a few pushups in your lunch hour then you are not going to get those benefits."
Lifting weights could also help boost your metabolic rate -- the calories you burn without doing any exercise -- just going about your daily business. According to the American Centers for Disease Control, weight training regularly can boost your metabolic rate by as much as 15 percent.
This is partly to do with replacing fat with muscle, because muscle is a more metabolically active tissue that fat, which means it will burn through more energy just at rest. The more you have the more energy it will consume, plus, you will have to use more energy to carry it around (although the same could also be said for fat, of course).
So by having more muscle you automatically burn more calories? Yes, but how much energy muscle mass burns is often overstated. A common myth is that each kilogram of muscle burns about 100 calories per day, so if you gained, say, 1kg of muscle through weight training you would burn an extra 100 calories a day without needing to lift a finger (plus all the calories you have spent getting the muscles in the first place).
Sadly, those figures are inflated, according to the experts, and the real figure is a fraction of that, although it is pretty hard to measure precisely. But in the long term, even a small effect is not to be sniffed at, says Timmons.© 2014 Guardian Newspapers Limited.