As well as physical fitness during a time when you can't being doing laps, its important to keep your mind active and focused ready for when you get back in the seat. There are a number of things you can do to keep mentally sharp.
Practicing visualisation is one excellent way to rehires laps in your mind. Think of it as free testing, you can chose any kart, any track and any condition and do as many laps as your mind will let you. This might seem pointless, I mean it's not real is it? Well research has shown that practicing visualisation does in fact improve performance in real life.
Here's and example of two well known F1 drivers racing each other...in their minds. So what are you waiting for, if its good enough for them..
