On April 21, over 30,000 runners will take to the streets to take part in the London Marathon. But what does pounding the pavement for 26 miles do to your body? Samantha Mayers reports.

By Samantha Mayers

Very soon the streets of London will be swarming with Lycra-clad bodies.

Last year, an astonishing 36,748 people managed to complete the course, making the London Marathon the biggest in the world.

This year's runners includes the usual mix of pro athletes, who whizz from start to finish making the 26.2 miles look like a breeze, and celebrities - with singer Katherine Jenkins, Shadow Chancellor Ed Balls and McFly drummer Harry Judd among the varied line-up for 2013 - as well as the hoards of regular folk taking part.

Some take up the challenge for charity, others for the thrill, or simply to test their fitness and stamina.

Whatever the motivation, there's no denying that completing a marathon is a huge achievement, a test of both physical and mental endurance.

Though the hardest part of a marathon is running it, of course, there's a lot more to it than the 26 miles on the day - training (hopefully for a good few months at least) is crucial, and plays a vital part in preventing and minimising injuries on the day.

"The training that you need to complete a marathon is a lot of work, I would advise at least six months training," says Dr David Stensel, reader in Exercise Metabolism at Loughborough University. "You need to gradually build up your mileage for it to be safe."

Marathons are growing increasingly popular. And while the likely assumption is that anybody able to complete a 26-mile run in one piece must be in pretty good shape, are marathons actually entirely good for our bodies and health?

What of the joint damage people talk of, the strained muscles that play up forever more?

Though there have been cases of people dying during the marathon, thankfully this is extremely rare, and there have only been 11 fatalities linked to the London Marathon in its 31-year history (this year's is the 32nd).

Cases of fatal heart failure usually occur due to an undiagnosed existing problem or defect, and for most marathon runners the health consequences will be minor.

Stensel is adamant that the benefits are greater than the risks. "The pros do outweigh the cons if you are sensible," he says.

"There are psychological benefits as exercise makes you feel good," Stensel adds. Indeed, lots of runners cite the 'high' they experience from accomplishing a run as a key motivator for doing it, and there's nothing like embarking on an impossible challenge and smashing it.

As well as the obvious physical benefits of being fit and active, Stensel says running can boost our bodies in many more ways.

"There is evidence to show that marathon runners have higher HDL cholesterol, which is good cholesterol, which could lower your risk of heart disease," he says. Research suggests it can increase bone marrow in the upper legs too, says Stensel, and increase how much blood the heart can pump around the body.

The crucial thing is to be prepared, which includes eating well, training sensibly before taking part in a marathon - bearing in mind that over-training can be potentially hazardous too - and seeking professional advice and being cautious if you have existing injuries or health conditions which may be aggravating by running, such as back and joint injuries.

Here's a run through some of the key factors for marathon health...

:: Muscles Most marathon runners will at some point experience a form of cramp, usually in the calves, thighs or feet, which can range from feeling unpleasant to downright unbearable. They often occur as a result of muscles becoming fatigued and tight and can be your body's way of asking you to stop. You can reduce the risk, and prevent cramps becoming unbearable, by preparing well and staying well-hydrated. Pain from severe cramps can force some people to drop out, but if you've trained well they should wear off pretty quickly, as building up your endurance slowly will have helped strengthen your muscles. Warming up with appropriate stretches before a race is also vital to help loosen muscles, and stretching slowly afterwards will help reduce longer-lasting damage and soreness.

:: Joints There's been a lot of discussion about whether running is damaging to joints. However, Dr David Stensel says: "There is no evidence to show that marathon running does any harm to your joints unless you already have an existing injury, then it could make it worse." When it comes to joints and running, that's the crucial point - being aware of existing weaknesses and adapting your activities accordingly. If you do have niggling pains or existing problems that can flare up, speak to a suitable medic or physiotherapist for advice.

Of course, running can cause aches and strains in and around the joints. Again, building up gradually, rather than attempting a 26-mile slog from scratch, is crucial, as is having a good running technique and suitable well-fitting running shoes, to help minimise impact, especially when running on hard roads.

:: Heart Every minute of every day, our hearts are pumping away ensuring blood carries enough oxygen around our bodies. When you run a marathon, your body needs way more oxygen than usual, so the heart has to work harder. The heart is a muscle so, in basic terms, training and building up endurance slowly is just as important for the heart as it is for your thighs and calves. Training for a marathon (providing you do it sensibly) can be great for long-term heart health, reducing the risk of heart diseases. Staying well hydrated and ensuing your sodium levels are just right is also crucial for helping your heart out during exercise, but over-hydrating can have adverse affects, so don't drink too much too soon. It's quite normal to feel lightheaded after a long run, and some people faint. Heart attacks do happen during marathons but almost always because of an existing problem, such as a congenital defect or undiagnosed arrhythmia. Anybody with concerns should speak to their doctor.

:: Lungs Like hearts, the lungs step up their game during exercise, having to work harder and faster to take in oxygen and get rid of carbon dioxide more quickly. It's normal to feel out of breath during or after exercise, and though this may be unpleasant it's unusually no risk. However, how rapidly you become out of breath can depend on your overall fitness levels. Plus, being a smoker means you're far more prone to breathing difficulties. There's also a big difference between being out of breath, and being short of breath. Being short of breath could be a sign of a more serious problem, including heart problems, or respiratory conditions like asthma. If you do have existing problems of this nature, it's crucial to speak to your doctor before embarking on a marathon and training regime. Providing you take sensible steps, you still may be fine to run.