Runners have always had a view on whether treadmill running is easier than doing it outdoors. Michael Mosley weighs up his options.

For those of us who rather optimistically made a New Year's resolution to do a bit more exercise, running is the obvious and popular option. But is it better to do your running outdoors, in the wind and rain, or to go down to your local gym and work up a sweat on the treadmill, while surreptitiously admiring your reflection in a giant mirror?

It's not something I've really thought much about before, possibly because I'm not that crazy about running, but it's certainly a topic that divides running enthusiasts. So what are the arguments, for and against?

Energy

Firstly, which of these approaches is likely to get you fitter? Those who are enthusiastic about running outdoors claim it uses up more energy. The main reason is wind resistance, which you don't get a lot of when you are a gym rat. But fans of the gym say this is not necessarily true. In a study conducted by Exeter University, Prof Andrew Jones took nine male runners and made them run along a road, measuring their energy expenditure. Then they got these volunteers to run at the same speed on a treadmill, but on different inclines. What they found is that runners could adequately compensate for the extra effort of running outdoors by setting the treadmill to a 1% gradient

Speed

It's worth knowing that when you run on a treadmill you tend to overestimate the pace at which you are going. A study carried out in Singapore which asked people to run outdoors and then match their speed on a treadmill, found that when people ran on a treadmill they went significantly slower, even though they thought they were going just as fast. The scientists behind this particular study suggest this is probably because when you are running indoors you don't get the same visual cues.

Or as they put it: "The unmatched perception of speed is likely due to the distortion of normal visual inputs resulting from the discrepancy between observed and expected optic flow."

Whatever the cause, it seems that when we run outside we unconsciously work harder.

Safety

If running outdoors edges indoors when it comes to work rate, which approach is safer? In the gym, of course, you are unlikely to be hit by falling branches, slip on dog poo or trip over a kerb. But when it comes to injuries there is a hazard which is commoner among treadmill runners - overuse injury due to unvarying repetition.

When you are on a treadmill the tendency is to plug away, like a hamster on a wheel, doing the same thing over and over again. For every mile you run your foot will hit the ground about 1,000 times. Repeating the same movement puts you at greater risk of joint or ligament damage. So if you use a treadmill it's a good idea to mix it up a bit by varying the speed and the incline.

If you run outside, particularly if you go off-road, there is inevitably going to be more variety. Each step will be different from the one that went before, simply because you are running on an uneven and varied surface. Research suggests that this constant challenge not only strengthens the ligaments and activates a greater variety of muscles, but also improves your sense of balance.

That said, the belt of a treadmill can be more forgiving than a hard pavement, which is important if you are carrying an injury.

Well-being

It seems intuitively obvious that running in the great outdoors puts you closer to nature and has to be better than running in a confined space. But what's the proof?

Well, a few years ago scientists from the University of Exeter made a serious effort to properly evaluate the evidence and their conclusions were very clear.

Digging through the literature they found that exercising in natural environments, particularly in green spaces, "was associated with greater feelings of revitalisation and positive engagement, decreases in tension, confusion, anger and depression, and increased energy".

They also found that people who exercised outdoors claimed to enjoy it more, find it more satisfying and said they were more likely to do it again. Though whether they actually did is another matter.

One other advantage of exercising outdoors is that you've got a greater chance of being exposed to sunshine and therefore topping up your vitamin D, (although in UK, at this time of year, that is not a particularly convincing argument).

Conclusion

For me outdoor running has clear advantages. It's far more varied and I have yet to convince my dog that watching me on a treadmill is as much fun as running through the fields. But the best advice, especially if you're starting out, is to pick the kind of running that you enjoy and which you're likely to stick to. If that means heading to the nearest treadmill just remember to vary the speed and the incline.

A recent study reported that joggers who exercise strenuously have the same life expectancy as people who do barely any exercise at all. But the author has now admitted he hasn't actually proved this.