Both legs are working, but the stronger your forward leg the less you'll tend to push with the back leg. So make it your goal to push less and less with the back leg. Your (lack of) flexibility in the back leg will affect what you feel in that quad too.
A few months ago I asked Mike Robertson a few questions about the BSS. In part, I asked: Is the quad of the back leg supposed to contract to return you to standing or should you make an effort to relax the back quad and drive upward with the glutes of the forward leg only? How does lengthening the distance from the foot on the ground to the support bench change the muscles that are recruited? Do you always want to spread out as far as possible?
Here's a portion of Mike's answer:
Quote:
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In my opinion, you shouldn’t be using the back leg at all in a BSS; if you do, that means your glutes/posterior chain/working leg aren’t strong enough with the load that you are using. As with any single-leg exercise, the greater the stride out, the greater the emphasis on the posterior chain. The short-stroke versions all put more of a premium on ankle mobility and quad strength.
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He also suggested that I was overthinking it--paralysis by analysis. So, just do it.
