Why does it hurt so?!

The woman (Maya) to the right (well, I say woman - it's actually a computer program's depiction - the software in question is, "My Fitness Coach", on the Wii) has made standing up and sitting down a rather painfull excerise (pun almost intended) today. For last afternoon, I decided to start a fitness regime (long overdue), and turned to this piece of computerised muscle to start the process. In all fairness, it's a pretty good workout - I can tell this by the burning in my upper legs each time I stand up this morning! Even so, this is no bad thing, as it shows I must have been working quite hard - next session probably this evening, if my legs are up to it!