iOS devices are stuffed with all sorts of sensors too. Some of them are used in ways you'd expect; the GPS chip in all iPhones but the original model is great for mapping your runs - indeed, the RunKeeper app can map your run live so that friends and family can watch your route as you run it.

Some of the sensors are used in innovative ways; apps such as CrunchFu can count how many sit-ups you do if you hold an iPhone on your chest under your crossed arms, just by using the accelerometer.

And that particular sensor is so sensitive that it can even detect your heart rate in apps such as Cardiograph. And sometimes, developers use sensors in ways we bet Apple never imagined.

Instant Heart Rate, for example, can read your heart rate when you press your finger against the camera on the back of your iPhone 4/4S - the flash lights up and shines through your skin, and the camera picks up the change in the light levels as your heart pumps!

Using your iPhone itself, with all its built-in sensors and connectivity, or augmenting it with accessories that you plug in or connect wirelessly, is usually a more cost-effective strategy than buying a drawerful of stand-alone gadgets that only do one thing. It's not just to do with the cost of buying these widgets; it's how good the experience is once you start using them compared to how it could be on iOS.

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Sure, you could buy a cheap pedometer with a basic numeric LCD, but while that would tell you how many steps you've taken today, say, if you were to use a Fitbit Ultra instead, your iPhone would display the results on rich, full-colour graphs, be able to store weeks, months or even years of data that its powerful processor can manipulate and present in different ways - to help you spot trends, set goals and more.

It's easy to forget the benefits of having everything linked together, with your iOS device as the nexus; Medisana's range of health-focused accessories for iOS - for measuring blood sugar, blood pressure, weight and more - all use the same app, so if you choose, you can have all your data in one place. This makes it easy to spot trends, or share results with your doctor who could then see correlation between things that might have been tricky to identify otherwise.

Your iPhone, iPad or iPod touch itself, then, can be an useful bit of hardware at the centre of a terrific ecosystem of accessories, apps and services, but it's not just this that makes it an invaluable companion when you're trying to achieve or maintain a healthy, active lifestyle.

There's a subtle but effective and affecting psychological benefit as well; the mere act of writing things down can make a big difference. Partly, this is because you can actually keep track of changes and trends in, say, your weight rather than relying on your trousers feeling tight or loose. We're often surprised to find that the raw data we note down is at odds with what we presumed was happening, both in a positive and negative way at times, a testament to the fact that our minds are a mess of preconceptions and downright lies about ourselves.

Partly, too, it helps because you can see, live, how close you are to the goals you've set, and we've been amazed at the difference this makes to us in the months we've been using this kind of kit.

With the Nike FuelBand, for example, you can, at the press of a button, get a little row of LEDs that show you how close to your activity goal you are, and it's fascinating to note how this can help us make different choices.