A well executed hack squat does wonders for knee strength. The above video clip demonstrates this nicely. It also shows the challenges of this exercise, since the main key to the hack squat lies in keeping the body as upright as possible. This drives the knees forwards and keeps the effort in the legs – and this the young man does well. However, I would say the weight is too heavy for the chap since his knees are well wobbly – and therefore his knees will not be getting much stronger for all his effort. The exercise is so difficult, that even doing a hack squat holding a broom handle will challenge most knees at first.
Full range of motion is essential, so to get right down with the back reasonably upright, it may be necessary to widen the stance. The other option is to lift the heels. The old fashioned school of weight lifting ((The old fashioned school include such famous names as Vince Gironda.)) just allowed the heels to raise as the body lowered, and this presents a good challenge for foot strength too. However, someone like Charles Poliquin would say that the extra balance challenge of letting the heels raise means we can shift less weight, so in this case it is better to stand with a wedge under the heels. Ultimately it comes down to what the goals of the workout are and deciding upon what needs strengthening most – knees or knees and ankles/feet. Whichever the answer the hack squat, done well, presents a challenge that the brave (with a good sense of humour) will rise to and their resultant stronger knees will thank them for it.