STIFF SHOULDERS? Aching back? Headaches? It's easy to blame it on work stress, but fitness experts believe the root cause could be physical. Put simply, spending too much time at our desks or on the couch has caused us to go soft inside - and I don't mean emotionally. Without the support of a strong core, even simple activities such as sitting put strain on other parts of the body. Addressing the issue, experts have started a new trend - core training. By focusing on a deep abdominal muscle called the tranversus abdominis (TVA), core training works from the inside out to improve everything from backache to your back hand. 'The TVA is the most important muscle in the body. It's like a corset that runs from your lower back to your belly button,' says Adam Menhennett, a personal trainer who also lectures at the National Academy of Sports Medicine in the United States. 'Your central nervous system engages the TVA 30 to 110 milliseconds prior to any other muscle.' The star of the postural show, the TVA's role is to stabilise your lumber spine. By creating the right balance of tension, flexibility and length in this and all the other small muscles (the stabilisers) that connect your core, you provide a solid centre from which every other activity is performed. It's like learning to ride a bike: without those stabilisers on you're going to wobble and fall over. Although she probably didn't know it at the time, switching on these core muscles is what your mother was talking about when she told you to stand up straight. So if our body knows those core muscles are so important, why do we have to work so hard to make sure they do their job? The answer, like so many of today's health issues, is in our increasingly sedentary lifestyles. 'The 'sit-down posture' of most lifestyles today means the important postural muscles have become decommissioned,' says Menhennett, 'so the external mobiliser muscles try to act like stabilisers instead.' The result is that the hip, back, chest, neck and shoulder muscles become overloaded, creating the tension, backache and headache we often associate with stress. Before you can switch on your core muscles, you have to learn to switch off the mobilisers that have become used to doing all the work. 'If you do a headstand in yoga and don't know how to switch off your shoulder muscles and switch on your core, for example, you could make the stress in your shoulder and back muscles worse,' says Menhennett. Unlike Pilates and yoga, core training is based on the science of physiology and is recommended by sports scientists, physiotherapists and osteopaths. Young, old, sedentary or active, it's never too late (or too early) to get with the programme. Gym fitness schedules are filling up with core-training classes designed to activate and strengthen those inner muscles. Some, such as stability ball and fitball, use large, inflated balls. Others, such as Body Balance, use a series of slow, controlled movements. Like all new exercise programmes, it is recommended you consult an expert before embarking on a core training session. In this case, a one-on-one assessment with a physiotherapist or qualified trainer can determine your key areas of weakness. As Menhennett says: 'You can't find a solution if you don't know what problems to address.' And, even if you are fit, it doesn't mean your core is. When you see a rippling body builder or a lean marathon runner, it's easy to assume they must be strong in every way. But the reality is, unless they specifically work to strengthen those core muscles, they will encounter the same postural problems as the rest of us. Whether it's running, hitting a ball or just sitting at your desk, your core muscles underwrite every movement. If they become inhibited through lack of use, you can't carry out even simple tasks in an optimum fashion. So, at best, playing a sport when you have a weak core will hinder your performance and, at worse, could put you at higher risk of injury. Remember, your core muscles are so deep, ordinary sit-ups won't touch them. So forget that elusive six-pack - for the ultimate life-changing work out, start from the inside.