Most people when it comes to fluidity use a lot of relaxed joints.
Alright, I'll give a quick little tutorial:
When you're going for fluidity, it is crucial that you use relaxed joints. Not, continuous relaxed joints, but when you get a section of your body moving (for example: right rotating chest, extending right pec, contracting left pec - turning your upper body right), you build up enough momentum, then relax some joints, because when you relax joints, they automatically go into whatever position is most efficient for keeping that momentum.
Instead of using 50 frames or something that high, set them to 10. It's a lot more useful for pinpointing areas. But, just using 10 frames per space wont help. When you are trying to perfect something, hold shift, then press space. It uses 1 frame instead of 10.
Finally, it's really, really hard and not known to make a replay in one try. Every time you do something (example: a kick) press R. It will play the replay and you can see your progress. Then, when you find where you left off in the replay, press P and it will pause the replay at the exact frame you pressed P at. Then, if you paused too early, hold shift and press P. It will hop to the next frame. Do this untill you get to the frame you want, Then press
E to jump into editing mode.
Replaymaking is a very tedious and time-consuming process, but it feels great when you finish!
Hope this helped! ^-^
Here are some replays for example: