Inside Health listener and keep fit enthusiast, David Heathcote, wanted advice on how far he should safely push himself when he's training in the gym.

In this special programme about the health benefits of keeping active, Dr Mark Porter helps David to find the answer to his question about the exercise "sweet spot".

If you struggle to screw the top off a jar, or use your arms to push yourself out of your chair, that's a sure fire sign, according to Dr Philip Conaghan, consultant rheumatologist and Professor of Musculoskeletal Medicine at the University of Leeds, that your muscles are weak. And the good news is that building muscle strength will protect your joints, not damage them. Dr Conaghan tells Mark that there's a worrying lack of understanding about the impact of muscle weakness on arthritic joints.

Over the last decade there's been a growing interest in the relationship between activity and the risk of developing cancer. Studies have demonstrated that exercise appears to have a protective effect against at least four different cancers (breast cancer, colon cancer, endometrial cancer and some upper gastrointestinal cancers) and that being fit helps recovery from cancer too. Dr Denny Levett, a consultant in peri-operative medicine and critical care at University Hospital, Southampton who has a special interest in the relationship between exercise and health, says the reason for the apparent protective effect of fitness is still being researched but the evidence that the effect exists is now widely accepted.

Professor of Clinical Cardiology, Sanjay Sharma from St George's University of London outlines the benefits to our hearts of keeping active and Park Run fan and regular Inside Health contributor, Dr Margaret McCartney, admits how running has become something of an obsession and promises that the evidence shows that when it comes to getting fitter, it's never too late to start.