If you are riding a bike and want to maximize your effort then getting a bike fit from your local bike shop is a must do!
When you are in your saddle for a 100 km bike ride and your seat height being out by an inch can make a huge difference in comfort and power output.
Make sure that when you get these measurements from the bike store you write them down and also mark the seat post in case you have to remove it or it slides down over time.
If you are riding a stationary bike or an Airdyne (see fixing the uncomfortable airdyne write up here) Then you should be able to get it close by having the leg at almost full extension while you are at the bottom of the pedal stroke. If you have the seat too low you will know by the burning sensation in your quads.
This stuff matters, there are really quite a few websites where you can nerd out about bike fit and angles, but if you are comfortable on your bike then you are more apt to ride it. No two people are going to be the same, and we are all built a little differently, so the sizing charts may put you in the ballpark in terms of positioning.
Experiment with the positioning and seek out a good bike fitter to put you in a good position to maximize your effort!