It's a cold evening, it's raining sheets and you have watching telly for so long that Netflix are considering sending someone over to the house to see if you've been eaten by your couch. You know you need to leave the house and get out for a walk or run but you just do not want to.

The question is, if we know exercise is good for us, why have we evolved to avoid it?

Daniel Liberman is a Professor of Human Evolutionary Biology at Harvard University and he's written a new book which looks at the world that has left a lot of people anxious, worried and harassed by the commercialisation of exercise.

Speaking to Dermot and Dave, Daniel revealed how it's not a sign of laziness if you struggle to find the motivation to work out. It's human nature.

And also how he once raced against horses. Yes. Really.

Daniel's book 'Exercised: The Science of Physical Activity, Rest and the Pursuit of Health' is out now...