Modification Does Not Mean Failure
At some point in your training, you are likely going to experience discomfort with certain movements. This can be the result of a variety of different reasons such as a strength imbalance, muscle strain, or overuse to name a few. But, just because you're experiencing discomfort with a particular movement, doesn't mean that your training session is now wasted because you can't do what was programmed.
It's leg day, and the meat-and-potatoes of the program is focused around the squat- the king of the lifts! We know the squat is the single most important movement that really defines how strong your body is as a whole system, not just the legs. Therefore, missing a day to push the squat really sucks because it feels like you're falling behind. Many look at it this way, and it's not just the squat, it could be ANY core lift, which for whatever reason, you're not feeling great performing on that particular day. So, what do you do? How do you make that session worthwhile, and feel like you're improving, not failing? How do you feel like you lifted something?
"Just go lighter." --This is often what you'll hear, and many times, that can work. But, that recommendation depends on the context. If you can squat 300lbs. but your light warm up sets at 135lbs, and 165lbs both feel like sh*t, then going lighter in this particular case is NOT the appropriate option. However, if that 300lb squatter had to do the majority of their work at say 250lbs that day, and it only started to feel "off" when they got closer in warming up to 250lbs (their working sets) then, going lighter would be both appropriate, and useful. In the first case though, what about them? The best answer here would be to modify, and/ or simplify both the load, AND the movement.
First, objectively look at what the goal of the day is. Is it leg strength? The fact that you can't do what was written in the program (squat 80+%...) without discomfort does NOT define success, or failure. Do front squats, or goblet squats feel the same? How about if you slow down your tempo, or add pauses? Many times, this will work because it's a way to still lift "heavy" assuming you can adhere to the tempo/ pauses, but the loading is comparatively restricted to what you would have been doing that was causing the discomfort. Now, if all of the above doesn't work, what next? What about single leg squats such as the Bulgarian split squat? In most cases these DO feel okay, or rather, they don't cause the same (if any) discomfort like the squat, other than that incredible burning sensation you get in the legs from doing them! So what is this doing? Simplifying the movement pattern, and further unloading the movement that's causing the discomfort. However, simple DOES NOT mean easy. You are still getting adequate, and progressive work to the same muscles that you would otherwise be using to squat, just without pain, or discomfort. Modification does not mean regression! Fighting through it will most often eventually CAUSE regression.
Though the above focused on the squat, that was just an example. There are appropriate ways to do this with just about any core lift you can think of. Not making these appropriate modifications when necessary will leave you consistently experiencing pain, or discomfort. What's the point? If you're always in "pain" because of something you're doing, then you need to change what you're doing if you truly want to improve your fitness.