I just did in the server room some self-concocted single leg half squats with mini resistance bands (40lb + 15lb). I think can achieve nearly 20kg at the top. Gonna be getting more mini bands from TB.
Band over the base of 1 leg and my left or right hand would hold onto the other end.
Half squat down to kneecap 90 deg, eccentric down slowly, explosive up.
Not too risky for lower back discs, think low risk for herniation, coz isolated to single leg so the load there is halved. Better to isolate left and right from each other, coz quite common for people to develop 1 side more strength/power than the other side. Only -ve is that it takes more time.
Will do this in future in addition to the on bike ultra low cadence on a slope training. Yesterday I pulled up on my handlebars until now it developed a slight creaking sound, maybe bent something the clamp or joint or what.
Bonus