Inactivity – fuelled by cars and a sedentary work life – has been dubbed the biggest public health problem of the 21st century, a global pandemic with dramatic impact on peoples wellbeing. The latest reports suggest that around the world it was responsible for 5.3 million deaths in 2008 – around one in 10 – more deaths than smoking.

Not only does exercise make you fitter, it can also ward off numerous and often unexpected diseases, from heart attacks, to diabetes, some forms of cancer and dementia. There are tentative signs it might even make you cleverer, by boosting cognitive performance and brain function.

"It's irrefutable that physical activity and exercise are beneficial for health," says sports scientist Chris Easton, at the University of West Scotland.

Latest government guidelines recommend adults under 65 should do 150 minutes of moderate physical activity every week, in bouts of at least 10 minutes. Worryingly, only a fraction of the population manages even that. In the UK just 14% of adults exercise regularly. However, a growing number of sports scientists advocate brief intensive training sessions to achieve the same health gains. The latest class to hit London's Fitness First gyms is called Tabata, which claims that just 4 minutes of exercise, in 20 second bursts interspersed by 10 seconds of rest, is enough to significantly boost your fitness and get you in shape. The workout is named after the Japanese scientist Izumi Tabata who tested it on the Japanese speed skating team in the 1990s. When Tabata put a group of students through this regime five times a week for six weeks, they ended up significantly fitter than another group of students who ran at a medium intensity for an hour on a treadmill.

But can just four minutes really beat an hour of running? When it comes to fitness, the idea is that during the short bursts your body exceeds its VO2 max and switches into anaerobic respiration, giving you a much bigger impact on fitness. It also seems to help in people with some medical problems, such as metabolic syndromes – which includes diabetes.

Not everyone is convinced though. David Stensel, at the University of Loughborough, cautions that although the benefits for physical fitness are well established, when it comes to preventing disease more research is needed. Besides, he says, "you're not going to persuade large numbers of middle aged people to do that form of exercise on a regular basis. I don't think it's going be the answer from a public health standpoint."

WEIGHT TRAINING

The latest government guidelines prescribe weekly weight training for all. Is is 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."

Any other benefits?

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 ageing 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, 6-10 reps," Timmons says.

What is he looking at?

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.

Doesn't it also boost your metabolism?

One of the most widely touted benefits is the idea that weight training, including increasingly popular choreographed weights classes such as Body Pump, 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 weights 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 push ups 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%.

Why?

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.

CYCLING

Cycling is fun and easy – but is it good exercise?

cycling.jpg Photograph: Observer

There is no doubt that cycling is a great workout – both building muscles in the legs and improving cardiovascular fitness. What's more, cycling is low- impact, which means it's easier on the joints than running.

If you cycle to work, is that a decent workout?

Studies in countries where commuting by bike is common show just how good regular cycling can be. One Danish study for instance found a 28% reduction in all-cause mortality in adults who cycled to work and back, compared to those who did not, even when other forms of physical activity were accounted for. Another study found that Chinese women who cycled to work were less likely to die from all forms of cancer than those who didn't.

So far, it's not clear whether things are different in the UK, which might be because the Danish and Chinese cycle so much. The Danes average three hours a week and the Chinese women almost six.

A large study in Norfolk found that commuters spent only an hour a week on their bikes in total. That said, even in the UK, people who commute by bike do seem to do more overall moderate to intense physical activity than those who don't, which means cyclists do not reduce the amount of exercise they do the rest of the time to compensate.

Can you get the same benefit from a spinning class?

Spinning classes are advertised as burning 400-600 calories in a class lasting 45 minutes to an hour. Of course, because each participant is in control of their own resistance dial, it's clear some people might be feigning the intensity to which they follow the class. But the opportunity to go hard, which might not be possible on the roads especially in cities, could be beneficial. One study of cyclists in Denmark found that to get significant reduction in all-cause mortality and coronary heart disease, you need to cycle vigorously, rather than slowly.

Could spinning be the answer?

Measuring people as they do spinning classes suggests it could. This is the case even though overall activity during the class is moderate, including the downhills or the cooldown periods, because there are extensive periods during a spinning class where when the intensity is so high that it could even be considered a useful way to increase the fitness off athletes off-season.

One study of 20 healthy students for instance measured their VO2max – a common measure of physical fitness which involves measuring the maximum volume of oxygen that your body consumes while you exercise at maximum capacity. After measuring this on a treadmill, the students had their VO2 and heart rate measured during two spinning classes. The researchers were astonished at the intensities reached during significant portions of the class. They concluded that despite the overall moderate intensity of the class, spinning had to be categorised as a high intensity activity.

On yer bike, then?

Yes, but not if you are totally unfit – as well as the potential for spinning to really boost physical fitness through these high intensity bursts, the authors also warn that there could be associated risks: for instance heart attacks, especially for those who are not used to exercising at such high intensities, or who are already at risk of cardiovascular disease.

RUNNING

Isn't running best left to kids in the playground?

Photograph: Observer

They are certainly on to something – running is cheap and easy and probably one of the easiest ways to burn calories and lose weight. But it's not for everyone – running is high impact and puts a massive strain on joints

But only if you run in your fat-burning zone, right?

This is one of the biggest misconceptions with physical exercise, says Easton – the idea that to get the best results you need to run at lower intensities, or at a heart rate that is often termed your "fat burning zone".

"It stems from good science but is totally misinterpreted," he says. "This idea of a fat-burning zone is exercising at low to moderate intensity, and for most people that would be a walk or a very light jog – then using heart-rate monitoring to establish 50-70% of your maximum heart rate."

While it's true that exercising at light intensities uses fat as the dominant source of energy, the overall energy requirement is substantially smaller, so you are burning far fewer calories than you would be if you were exercising at higher intensity, Easton says. "So in terms of achieving weight loss, that's totally counter intuitive."

Rather than thinking about fat burning zones, "people should think more about calories in and calories out," says Timmons.

So more intense running is better?

Weight loss aside, there are other benefits to ramping up your runs, because working at higher intensities should also make more of an impact on overall fitness – something that is starting to be reflected in government guidelines which give the time saving option of 75 minutes of more vigorous exercise each week, rather than 150 minutes of moderate activity.

Is it possible to get too much of a good thing?

Yes. One study this year followed amateur marathon runners taking part in the Quebec City marathon as they trained and took part in the race, and followed up afterwards. In more than half of the runners – especially those who were less fit and who had done less training – there were substantial decreases in heart function after the race. The good news is that the damage was short lived, and heart function bounced back to normal, but the authors did caution that it shows the importance of proper training.

Is that why some people drop dead during a marathon?

It's very unlikely you'll drop dead during a marathon – around 0.5 or 1 out of 100,000 participants. But this short-term heart damage could be a red flag to more sustained damage if this cycle is repeated over a number of years.

And even if you aren't keeling over during the race, some argue that running excessively – for instance taking part in marathons – can undo some of the cardiovascular benefits of regular running, for instance through increased calcium plaque build up in the heart, which increases the risk of coronary heart disease.

Is there an optimal amount?

O'Keefe likens regular vigorous exercise to a potent drug – if you don't take enough, you won't be getting the optimum effects. And like a drug, he warns that an overdose could do serious harm to the body.

Perhaps, but the biggest benefits are seen when people go from being unfit to being moderately fit. At higher fitness levels, you don't see the same benefits in longevity and disease risk reduction. And although vigorous is good, you don't need to go overboard. A study of 20,000 people in Denmark over years found that those who ran lived on average for six years longer than those who did not. And the biggest benefits came from running between one hour and two and a half hours a week, over two or three sessions, at a comfortable pace where you felt out of breath, but not too breathless.

O'Keefe thinks this could be because long periods of vigorous physical activity, over an hour or two, puts too much strain on heart muscle, causing it to stretch and tear, which – over years and decades – can lead to stiffening and scarring.

Indeed, one 2011 study found higher levels of fibrosis, or scarring, in the heart muscle of competitive endurance athletes aged 50 or over, compared to others of the same age.

What if you really want to do a marathon?

For the best health benefits, O'Keefe suggests moderate intensity running, in small doses – of about 30 to 50 minutes at a time. If you must run a marathon, just do one or two, tick the box, and return to more moderate levels of training.

SWIMMING

Swimming is for softies, isn't it?

Not so. "People see swimming as something really easy but if you're swimming non-stop for half an hour, that is a really good and intense workout, no question," says Easton. How much you get out of it will obviously depend on the intensity of the swim. Some strokes like front crawl are more taxing.

Any other benefits?

Yes, if you are feeling the strain of your workout you could do worse than take a load off in the water. One of the biggest health benefits of swimming is that it's non-weight bearing, which means you won't be putting any strain on muscles and joints. Sadly, that same effect might be your downfall if you want to swim to lose weight, because you aren't lifting your own body weight as you are with running, which makes for a comparatively easier workout. In fact, the more fat a person carries, the more buoyant they are which means that unlike exercising under the full force of gravity, lean people actually work harder when they swim than those carrying a spare tyre.

Doesn't swimming just make you feel famished?

That is another potential pitfall: swimming can stoke the appetite more than other forms of exercise. Long distance running, for instance, seems to regulate appetite hormones and decrease hunger, at least for a while after exercise, despite all the energy that has been used. But this is not the case with swimming.

"We've done many studies now and when you run at a reasonable intensity – 70% of someone's VO2max – we find that the hormone ghrelin is suppressed," Easton says. This is the hormone responsible for stimulating hunger. Another hormone, peptide YY, which suppresses hunger, is also increased after fairly vigorous exercise.

Walking on the other hand did not have the same effect: "It didn't make any difference to ghrelin or to people's hunger."

But swimming is different. Easton found that people felt hungrier after swimming than after other forms of vigorous exercise.

So you could lose what you have gained by overeating afterwards?



Actually, despite feeling hungrier, people still seem to resist. "On average we found that people did say they felt a little bit hungrier after swimming, but that didn't translate into them increasing their food intake," says Stensel.

And anyway, says Easton, perhaps the hunger stems from swimming being an unexpectedly tough workout. "One of the great things about swimming is it that you use both the upper body and lower body." This uses more muscle mass, "so you increase your metabolic rate to quite a substantial level," says Easton.

BIKRAM YOGA

What's it all about?

Photograph: Observer

Devotees of Bikram – a series of 26 intense postures practiced repeatedly for 90 minutes in a sauna-like humid room heated to about 40C – claim far reaching benefits, from toning and weight loss, to increased flexibility, endurance and stamina, as well as emotional and psychological effects. The classes tend to be expensive, and are usually performed in a little more than ones underwear. But there can be adverse effects, such as dehydration and heat stroke.

So is there any evidence to back up the claims?

Given its recent rise in popularity, there has been relatively little research into the physiological effects until recently. The first peer-reviewed study on Bikram led by Brian Tracy at Colorado State University in 2008, found that it led to substantial improvements in balance and a moderate increase in leg muscle strength. But despite the gruelling challenge of Bikram, there is little evidence that it burns more calories than other exercise. In a study yet to be published, Tracy found that men burn around 460 calories during a Bikram class and women 330. Not a bad workout, but instead of 90 minutes in a nauseatingly hot room some might prefer a gentle jog for half an hour.

A small study last year assessed the effects of Bikram on 10 healthy young adults, and found that practising Bikram three times a week for 90 minutes over two months led to just moderate weight loss, and no improvement in cardiovascular health or aerobic fitness.

So you won't sweat off the pounds then?

Not likely, but of course the incentive to do yoga tends not to be weight loss. And the same study did see improved flexibility and increased back strength, compared to a control group who underwent no training.

Could they have got those benefits from yoga without the sweat?

Easton suspects so. "Bikram is one of these fads at the moment but if you step back and focus on yoga itself, there is good support for the benefits." These include core-strengthening exercises, strength training and weight training using your body weight, he says. And of course there's the increased flexibility, which you should get from any form of yoga – heated or not. One study by the American Council for Exercise compared hot yoga sessions – in rooms heated to around 30C – to normal yoga, and found no difference in the workout, except how hard the participants found it. Until more studies are published on Bikram it seems more of a psychological workout, with the same physical benefits you could get from a regular yoga class.

So it's a bit of a washout?

Not for everyone. You may be surprised to hear that those who have the most to gain from Bikram might actually be those who are obese and at risk of developing metabolic diseases such as diabetes. It seems to increase their glucose tolerance, whereas no such effects have been proved in lean people. Stacy Hunter a cardiovascular ageing specialist at the University of Texas, who conducted the study, says that previous studies have shown that increasing core temperature several times a week by hot tub therapy improves glucose metabolism in the absence of exercise. "With that being said, I believe that the yoga postures themselves are enough to bring about certain improvements but the heat heightens the beneficial effect," she says.

Any other benefits?

One of the big promises of yoga is psychological. As a form of exercise that promotes mindfulness, it has been shown in clinical studies to have antidepressive effects. But what about Bikram? Scientists at the Massachusetts General Hospital are looking at whether the heated environment might bring added benefits to those who suffer from persistent depression, and have launched a pilot trial. But until the results are in, for now the benefits of bikram don't seem worth the sweat when cooler yoga offers many of the same benefits. "I haven't seen anything that convinces me that practising yoga in a very hot room achieves anything above and beyond what yoga can," Easton says.