Maybe it's a collective midlife crisis, a late realisation of our mortality or an old-fashioned desire to lose the spare tyre. But one thing is certain, the middle aged are getting fit.
Research from 4.5 million Strava users has found that the most active age group is the over 40s. And it's never too late to start. A flurry of studies last year showed that exercising in middle age is the elixir of youth. One, published in the European Heart Journal, found that sedentary, middle-aged people who took up aerobic exercise for six months developed longer telomeres - the tiny caps on the end of cells that shorten as we get older. Scientists refer to the length of people's telomeres as a sign of how quickly they are ageing.
Likewise, another study published in the journal Circulation found that middle-aged people who began a two-year-long programme of brisk walking or jogging for 30 minutes, four to five times a week, were able to reverse age-related stiffening of their arteries.
Far from giving in to middle-aged limitation, many of us are the fittest we've ever been. Cindy Crawford and Drew Barrymore have posted gym selfies and yoga videos, while Davina McCall has reached new fitness heights - swimming the channel in her 40s and qualifying as a personal trainer at 50.
I have tried to keep fit since my mid-20s, but it was always with the same goal: to be thin. I did back-to-back aerobics classes and ran five times a week in my 30s. By the time I reached my 40s, my knees were ruined and I had Achilles' tendonitis. I was forced to slow down, so took up yoga and qualified as an instructor at 42. But while the poses calmed me down, they still didn't give me the lean, toned body I wanted. So, in my mid-40s I got myself a personal trainer. She had me stop running, start walking everywhere and do strength training in our twice weekly sessions together.
After six months, I was lean and had finally dropped that last five kilos. So at 49, I decided that I would qualify as a personal trainer.
I want to look nice in jeans. But you know what I want more? To be able to get out of a chair without that geriatric 'oof'. Hell, I want to be able to stand up on long train journeys when I am 70, and not complain about it. Do you want that, too? Here's what I've learnt about second-life fitness...
Each decade after 30, your muscles decline by 3-8pc - a process known as sarcopenia. Muscle has a higher metabolic rate than fat, so the more muscle you have, the more calories you burn during exercise and rest.
"Muscle requires more blood and oxygen to be supplied to it than fat, and that increases the energy expenditure the body has to do to maintain it," explains Professor John Brewer, Head of Sports and Exercise Science at St Mary's University, Twickenham. One study from the American College of Sports Medicine asserts that muscle loss is the single greatest contributor to age-related decline in metabolism, and claim that by adding just 2-4lbs of muscle to your body, you could burn 100 extra calories a day at rest (that's 3,000 calories in a month, enough to lose a pound). And it takes a surprisingly short time to build muscle. One study from the Harvard School of Public Health followed 10,500 men aged over 40 for 12 years. It found that, of all the activities they did, weight training for 20 minutes, three times a week, had the greatest effect on preventing age-related abdominal fat.
Weight training helps control blood sugar levels in patients with Type 2 diabetes and one analysis concluded that resistance training should be recommended in the prevention and management of the condition. It might help age-related bone loss, too. With age comes a decrease in oestrogen for women, a hormone that helps with calcium production. Studies from Glasgow Caledonian University have found that strength training improved bone density in the post-menopausal.
But middle-aged fitness is not created with weight training alone. This is the decade to find out what works for you and do it: leaving fads to the young 'uns. That means understanding that fitness has three main pillars. Firstly strength, achieved by resistance exercise - anything from your own body weight (say, a push up) to free weights, such as dumb bells or resistance bands. Secondly, we need flexibility, because becoming more flexible means our muscles can contract and expand during everyday life, when sitting, standing or reaching up to a shelf. Thirdly, we need endurance, which is achieved by doing cardiovascular exercise, such as brisk walking, running, cycling or swimming. The Strava research also found the two things that worked for the over 40s were setting goals and training with friends.
We know that over-training for sustained periods may lead to weakened bones, burnout and even abnormal heart rhythms. But it can also tax your immune system and hinder fat loss. That's thanks to cortisol, a hormone emitted by the adrenal glands in response to prolonged mental or physical stress. Exercising too hard has been shown to affect immunity because of cortisol overload, which has also been associated with an increased risk of storing fat around the middle. Not to mention, you'll probably just get sore and give up.
There was a point when everyone was talking about High Intensity Interval Training (HIIT) to burn fat. But HIIT - short bursts of exercise at high intensity - is tough on the body, so it needs to be built up to and, in my opinion, limited to around once a week in middle age. Low Intensity Steady State training (LISS) on the other hand, done at 70pc of your maximum heart rate, is ideal for burning fat. Think of anything that leaves you quite puffed and not quite able to hold a conversation - brisk walking, cycling, power walking or dancing. A few LISS sessions, for 35 minutes to one hour, three times a week, are essential. Slowly begin to increase your pace or the length of time you do LISS training for because your body becomes more efficient the more you do. Or add some HIIT training into your routine. The ideal weekly line-up is 3-4 sessions of LISS that last 35 minutes or more; resistance training about 2-3 times a week, stretching before and after; and perhaps a yoga or Pilates session.
I used to wonder at some people in the gym who would be swinging their legs back and forth, and side to side, or making fast circles with their arms. Now I know they were doing dynamic stretching, which is the best type to do at the start of your workout because it helps release synovial fluid into your joints. This is the prime lubricator of bones, making you more mobile and decreasing injury risk.
Do 8-15 reps of four dynamic stretches before exercise and focus on the body parts you'll be working. For example, before resistance training do 20 fast squats with your arms swinging down and up overhead. You can also try leg swings, arm rotations and shoulder shrugs. After your workout, do static stretches. That's where the stretch is held for 10-15 seconds to release the muscle. Stretching afterwards is essential because your muscles contract during exercise and need to be stretched adequately in order to prevent soreness and injury. Again, focus on the parts you worked most.
Anna Magee is the editor of Healthista.com