How does training on the hardest difficulty with a stationary bike affect your body?

I often cycle on a stationary bike using a difficulty (around level 8-10) which I can ride at 40-50kph comfortably and can go over 65kph in short bursts. I can ride this way for at least an hour if i really push myself so you can see it isn't extremely difficult.

The other day, I decided to try going on max difficulty (which is level 16 on my bike) for a little while. My aim was to give everything I had for 30 seconds. For the first 20 seconds, I rode between 40 and 45kph. For the next 10 seconds, I cycled at over 45kph. I pushed myself to the absolute limit here and had to get off the bike immediately. Straight away, my legs tightened up and I could barely stand my legs felt that weak. I had a shower and once i got out (which was something like 20 minutes after I finished riding), my legs still felt really weak and I could barely climb the stairs.

This got me thinking..
If I can cycle at level 8 for 30-60 minutes, I can increase my fitness and aerobic endurance.
How does the body respond to cycling as fast as you can on the hardest difficulty for 30 seconds? Does this improve leg strength, or muscular endurance..??

Could someone please help me figure this out?
Thanks heaps in advance.

Technorati Tags: fitness programs, stationary bike