According to aficionados, it will boost your immune system, burn calories and leave you with an all-natural high.  There’s no doubt swimming boosts aerobic fitness, reduces fat and increases muscle. Add icy water though, and the risks may start to outweigh the benefits. “Immersion in cold water puts a significant strain on the heart, increases your blood pressure, and leads to uncontrollable hyperventilation that can be a precursor to drowning,” says Mike Tipton, professor of human and applied physiology at the University of Portsmouth, and a keen triathlete. This “cold water shock response” begins below 15C, and is responsible for most drownings in the UK (where the average water temperature is 10C) – most often, the shock of the cold causes people to inhale water, but it can also prompt heart attacks.  That’s not to say you can’t get used to the cold. “Provided you go about it sensibly, as few as five, five-minute immersions in cold water will reduce that shock response and the problems associated with it,” says Tipton. Anyone considering outdoor swimming should have health checks, be accompanied, and build up to longer swims at colder temperatures slowly.  Going into cold water will give you a buzz and leave you feeling wide-awake, because it activates the fight or flight response and releases hormones such as adrenaline and cortisol. But is it better than regular swimming? The jury’s still out.  Several studies have found cold water activates the immune system, but that isn’t necessarily the same thing as improving the immune system. Neither is it clear whether you’d get the same immune system boost by simply swimming in warmer water – this is something that Tipton’s team is currently investigating.  Does it burn more calories? It depends on how fast you’re moving. If you’re sitting still in cold water and shivering, you’ll burn more than if you were on dry land, but who wants to put themselves through that? Once you get moving and stop shivering, any difference in the number of calories burned will be negligible, Tipton says.  No gym is complete without its squadron of spinning bikes, often accompanied by pumping techno music and an instructor hollering at the class. Spinning, or group indoor cycling, promises to burn fat and boost strength and endurance as you power through virtual sprints, hill climbs and headwinds - all without having to actually go outdoors.  Generally, riding on a stationary bike isn’t as good as the real thing. One recent study found cyclists produced more power – and thus burnt more calories – for the same perceived effort, when they were outdoors.  On the other hand, in a spinning class you get the added benefit of a coach pushing you harder than you might otherwise go cycling alone. “Outdoors can be quite strenuous, but it is more self-paced,” says Carl Foster, director of the human sports laboratory at the University of Wisconsin, La Crosse. “Face it, the instructors make their money motivating people, so spinning classes tend to be at the upper end of the intensity continuum.” In another study, Foster’s team found spinning class participants regularly exceeded their maximum aerobic capacity for short periods – so were working very hard. Spinning should make you fitter and burn lots of calories – just beware your appetite will also increase as a result.  Pilates combines elements of yoga and martial arts training into a predominantly mat-based routine that it’s claimed boosts strength, flexibility and posture. Celebrities, including Madonna and Jennifer Aniston, swear by its body-sculpting powers, but pilates is also recommended for those with back pain to strengthening muscles that support the spine.  A key claim is that it activates and strengthens “core” abdominal muscles, which is probably true. In a recent study, Duncan Critchley at Kings College, London and his colleagues divided a group of regular gym users into two groups – one group did two 45-minute sessions of pilates a week for eight weeks; the other did conventional weight training – and then compared their abdominal muscles. “Those who had done pilates training were using their deepest tummy muscles more than those who had done the strength training,” says Critchley. In another study, he looked at the effect of pilates on those with lower back pain, and found that while most reported feeling better after a pilates course, this didn’t necessarily correlate with how well their core abdominal muscles were working.  Even so, pilates probably will make you stronger. Another recent study showed that a twice-weekly class led to significant increases in the endurance of muscles in the abdomen and upper body, although it detected no improvements in posture or balance.  If you want to get fit, but never have time, then HIIT (high intensity interval training) might seem the perfect solution. The idea is to alternate very short (30 second) bursts of extremely intense aerobic exercise with slower recovery periods. This is claimed to produce the same benefits as more conventional aerobic exercise (such as running or cycling) for far longer.  One idea is that HIIT is better at depleting the glycogen – or sugar stores – in your muscles, than regular exercise. “If you start breaking down this glycogen on a regular basis, the theory is your muscles will get better at taking glucose up from the bloodstream, and this could reduce your risk of diabetes,” says Niels Vollaard, an exercise physiologist at the University of Bath.  Vollaard’s team found that interspersing 10 minutes of easy peddling on a stationary bike with two “all-out” 20-second sprints can achieve the same health benefits as a 45-minute run or cycle ride. Even better, this seems to suppress appetite, unlike more conventional exercise which makes you hungrier.  The drawback? For now at least, most gym bikes and treadmills are geared towards general aerobic exercise, rather than sprint training. And although many gyms claim to offer HIIT classes, what they often mean is interval training which, although hard work, is unlikely to recreate the extremely high intensities of the HIIT research taking place in sports science labs.  To do this, you need to find a static bike that lets you switch the resistance from really low, to really high, extremely quickly. Alternatively, look for a steep hill – ideally with a downward slope immediately before it. The key is to get your legs moving as quickly as possible before you hit the sudden increase in intensity – and then you need to keep on peddling as hard as you can for another 20 seconds. Runners could also try the same thing on foot.  Because of the strain HIIT puts on your heart, Vollaard recommends consulting your doctor before starting any kind of high intensity exercise.  “There is the pain that hurts you, and then there’s the pain that changes you”. It’s a mantra beloved of crossfitters who believe that to get results you need to push yourself hard; very hard. Crossfit uses constantly varied, high-intensity, functional movements – the sort we use in everyday life to sit down at a chair or lift a bag off the floor – to get results. In crossfit, the loads you bear are far heavier, and they’re performed in a competitive environment where you’re encouraged to push yourself harder each week – often with little rest in between.  Recent research suggests crossfit delivers impressive results. When Steven Devor and his colleagues at Ohio State University assigned 54 men and women to a five-day-a-week crossfit programme for ten weeks, they saw big improvements in participants’ aerobic capacity (a marker of general fitness), a 3-4% fall in fat and increases in muscle, regardless of initial level of fitness. “Crossfit generally produces improved results when in body composition and measurements of aerobic fitness compared to traditional weight lifting or aerobic exercise,” says Devor – mainly because you are constantly working at a high intensity, with few rests in between sets, he says. However, this may come at a cost. In Devor’s study, nine of the participants dropped out because of injury.  Running is now second in popularity to swimming in the UK, with booming numbers signing up for half-marathons and marathons. This is prompting some to seek even bigger challenges, like ultramarathons or iron man triathlons. Those who compete in such events tend to be extremely lean and fit, but is running – particularly for long distances – really the best form of exercise?  It depends on what you want to achieve. “If you just want to improve your health and to feel a bit better, then regularly running five to 10km is great,” says Niels Vollaard, an exercise physiologist at the University of Bath. “If you want to build slightly bigger muscles and look a bit more trimmed then you probably want to do a bit of weight training as well.” This is because although running will make your muscles fitter, it won’t usually make them grow any bigger. Your goals will also influence how hard you need to push yourself on each run. If you’re just running for general health and wellbeing, then continuing to run the same distance at the same intensity will continue to burn off excess calories and reduce your risk of disease. But if you want to get even fitter, or improve your time or distance, you will need push yourself harder – either running longer distances, or incorporating some interval training.  However, regularly running longer distances than 10K can have a negative impact on your health. “Training for a marathon is a real challenge for your joints, tendons and cartilage, and so we tend to see regular distance runners developing problems with their knees, hips and shins,” says Vollaard. “Your muscles are happy to keep on adapting and getting better, but the rest of your body will struggle.” And although runners generally have healthier hearts than people who don’t exercise, emerging evidence suggests that constantly training for, and competing in, extreme endurance events such as marathons, ultramarathons, ironman triathlons, and very long-distance bicycle races, might damage the heart and blood vessels.  Before considering any exercise regime, always consult your GP. Never undertake any form of exercise that may be strenuous, unusual or intense without taking medical advice.
From HIIT to pilates, swimming, running, ultramarathons and crossfit, there are scores of ways to get fit. But which will really make a difference?