These seem simple yet are more brutal than you think for as each rep is extended out by the constant up and down motion (hence elevator)
It's best to test these out with bodyweight only and then build up using some light weights - I usually do goblet squats with my clients.
There is also a temptation to perform these at ludicrous speed (Light Speed > Ridiculous Speed > Ludicrous Speed) which would remove a lot of the benefits of the extended reps.
So slow it down and perform at normal speeds.
You will just have to suck up the ...slight... burning feel in your quads.
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!