Does anybody do these? I recently started adding them in as assistance work after squatting, but I think they could have quite some carry over into sprinting, especially coming out of the blocks. If you do them properly, they teach you to sit back into your squat putting more emphasis on your hips, glutes and hamstrings instead of letting your knees drift forward, and stopping at the bottom of each rep by sitting on the box and letting the tension go in your legs gets rid of the stretch reflex, so it’s going to teach you to explode up from the bottom with a good hip drive.
I’m currently using a box 1 inch above parallel, but it’s harder for me than when I do regular squats breaking parallel, which I was surprised at. Using a weight I can 8 rep below parallel, I could only manage 3 reps, even though it was a shallower squat.
Yeah I do a slightly modified version of the box squat off a single leg. I follow the form you describe above with a few more technical modifications as one stands up and its the most effective exercise I have ever used. It has significantly benefited all steps from the blocks to about step 8 for me. You’ll find (if you had access to emg) that exercise recruits the muscles you would use in the early acceleration phase of sprinting in almost the exact recruitment pattern and relative intensities required during sprinting. The benefit of doing a resistance training exercise that replicates the movement in sprinting though is that you get to stress the muscles (I see you can only do three reps) and cause adaptation = more speed