In world 5 there is a leg gym that has you doing all the leg exercises one after another with no rest.

First came knee lifts which are fine I can do those all day, then came the squats (and it makes you hold every single one of them) which I started to get tired near the end. After the squats the game introduces the mountain climber for the first time and boy did I really feel these after the squats, last came the thigh squeeze which is rightfully a rest after the last two exercises.

Here is a video of the mountain climbers for those who may not know.The game wants you to do them rapidly similar to how the knee lifts work. https://www.youtube.com/watch?v=hPE3S55dgFY

This level was by far harder than any other in the game so far, and its short. I managed to get a score of 99 for the stage due to the joycon not picking up one or two of the mountain climbers as a great.

Edit (10/22/19): I redid the leg gym today since it was a request from a townsperson and it wasnt nearly as bad today. I guess its because I did a bunch of planks last time since it my attack that required the most effort in my moveset so my abs were already tired. Now I have unlocked hell again though with the wide squat attack.