Band around the legs adds bite to every lunge.
Fight the band to keep your front knee from caving in. That's where the work is.
The lunge itself is knee-dominant, quads lowering and driving you back up with the glutes assisting from the hip. The band above your knees adds the twist, it drags the front knee inward the entire rep, so the hip stabilisers have to work continuously to keep the knee tracking straight. That makes it as much a knee-control drill as a leg builder, which is exactly what most people's lunges are missing.
Not inherently, forward knee travel is normal in deep lunges. What matters with the band on is that the knee tracks straight over the foot instead of diving inward.
Long enough that both knees can reach about 90 degrees at the bottom. Too short and your front knee shoots far forward, too long and you cannot drive back up through the front heel.
Stationary first. The band's inward pull is easiest to fight when you are not also managing forward travel, add the walking version once your knee holds its line every rep.
