Hi. I’ve been doing 5:2 for 11 years and haven’t lost any leg muscle. However, I have also suffered leg pain around the knee area when on the stairs or when trying to get out of the bicycle seat to make an extra effort. In my case it turned out that it was down to my IT band being tense, which in turn was caused by my sitting at a slightly skewed angle when watching TV. Over the years that had put tension I was almost unaware of into muscles in the right-middle of my back. Changing my seating position so I’m square to the TV has removed the back muscle tension, relaxed the IT band and removed the pain from my left knee area (and indeed my left ankle).

This was all figured out by a local sports physiotherapist who took a good look at my posture rather than just leaping in to work on the areas I was reporting issues with. Her diagnosis was spot on, but also helped by doing some intense work on the tight muscles and IT band.

It may be worth trying to see a sports physio to see if you have something similar. It’s made a world of difference to me and I would never have figured it out myself.

Best of luck.