So right at about forty to fifty miles, the muscles on the inside of my knees start "cramping" and pedalling becomes painful. Even if I slow down for a bit, it seems to inevitably kick in at the end of my longer rides. I'm use to doing twenty mile rides and over the last month have stepped it up to forty and last weekend a fiffty. Anything over forty and I'm hurtin'. Is this just a combination of hydration/nutrition issues or is there something else I should be looking at? Thanks!
Muscles inside your knees? I think there are no muscles inside it, however if your knee hurts you should go and see a physician. I got similar symptoms about a year ago and I was diagnosed with chondromalatia.
What would also help is to make sure your seat is set up optimally. A lot of people have their seats too low and too far back. Easy test is to place your heel over the pedal-spindle and pedal backwards. At the very bottom where the crank is parallel to the seat-tube, your leg should be completely extended. This is the minimum height and a lot of people ride slightly higher (depending upon shoe+cleat thickness, crankarm-length, road, or track, etc.).
also use easy gears and spin fast. You should hardly ever feel yourself pushing on the pedals at all. But rather feel like you're spinning madly in circles like a hamster... This lowers peak muscle-force required to generate any given amount of power & speed. The lower forces will give you more endurance, lets you ride farther and faster, and reduces aches & pains in your joints.
I agree with Ricardo, but if you as allergic as I am to Md's, you may want to try spinning with low strain and build up over time. According to you, you suddenly went from twenty miles to fifty. Well, perhaps you need to slowly condition for that. I did.
I worry about Md's. They often do not know how to run their own lives much less my.
Perhaps it is helpful that more than one person tells you the same. Danno said "check saddle position".
I was riding with a young, strong and fit guy. He was less than half my age. We had identical bikes but his was adjusted wrong. He could not keep up with me and was exhausted after 25 miles.
His saddle was several inches too low. He sat vertical. Used toe straps. Did not understand spinning. His handle bars were way too high up also.
Have a good LBS look at what you are doing.
Cramps are caused by overexertion over time. Although you should certainly make sure your saddle height is correct, you probably should try to ride a little slower from the beginning of the ride, not just wait until you start cramping.
Hydration and electrolytes can aggrivate cramping, but on a 40 mile ride, these are unlikely to be significant factors.