Drs Chris and Xand Van Tulleken investigate the science of exercise and the dangers of inactivity.

Is modern exercise a wellness cult? Or is it a vital cure for a world that’s struggling with ill health and stuck on the sofa? Most of us might like to get a bit fitter, but how easy is it to actually start exercising and give up sedentary habits?In this series Drs Chris and Xand Van Tulleken examine exercise and how best to do it. How much should we be doing? How does it help our bodies? And how does our surrounding environment stand in the way of us getting fitter? Chris is challenged to examine his scepticism towards exercise. Similarly, Xand is asked to look at his new-found exercise evangelism and see what he is really running from.Recently Xand has discovered the joys of physical activity. He’s running, cycling, heading to the gym and playing ping pong like never before. It’s been a real transformation and a way to keep on top of things after years of unhealthy living.His twin brother Chris, on the other hand, is really feeling the aches and pains of middle age. With a busy job and a young family, he has precious little spare time for exercise. After a very active period in his 20s and 30s, Chris is now embracing his ‘Dad bod’ and sliding into a creaky middle age. Xand wants to help him turn things around. Can he do it?

In Episode 2 - The Scale of the Crisis - the twins meet Dr Fiona Bull, head of the World Health Organization's Physical Activity Unit. She explains how the WHO is tackling the global inactivity crisis and how schools and cities need to change to meet this challenge. Meanwhile, Xand is still trying to inspire Chris to get more active, so has enlisted his help for a run around their local park. Things don’t quite go according to plan…

Presented by Drs Chris and Xand van TullekenProducer: Tom WoolfendenSound Design: Dan King Series Editor: Jo Rowntree A Van Tulleken Brothers and Loftus Media production for BBC Radio 4