Try and get a copy of 24th Nov Cycling Weekly, large article on the damage they do to knees
I've not read the aritcle but I'm going to say it's bobins anyway, I had a bad injury for years, riding with gears agrivates it more than riding the SS!
If you want to increase leg strength then just try to keep the bike in one "training gear" for the whole ride. Pick one gear and only change down if your cadence drops below 65-70 and only change up if you start to spin out going down a hill.
Isn't that, you know, just normal riding?
I like SS for 3 reasons.
1) Good for quickly getting fit after time off and my riding tends to be sporadic due to work.
2) People are in awe of me, despite me being at the back of the group constantly. "Is it hard?" "does it hurt?" "whats it like?" "do you really need that facial hair?" etc, it's not bum sex but it appears to have a similar reputation round here.
3) Lack of maintenance, I washed my bike last night, first time I've been near an SS bike for more than five minutes and not riding it in almost 2 years.
4) It's actualy fun. Seems an ailien concept to some people that riding slightly quirky bikes can actualy be fun in the same way that riding the latest 20 speed electronicaly shifted fully-boing carbon uber-spaff-bike is fun for them.
5) It removes the need to learn to count.