During compound exercises or any exercise that uses more than 1 muscle group, it's not uncommon for beginners and even the regular lifters to have issues using all the muscle groups as much as they could.
For example, while squatting, your core stabilizes your body, your lower back, also part of your core plays an important role in keeping your upper body upright. Your upper back, which should remain tight, ensures you don't round the back and fall forward. Your quads and glutes perform the major part of the lift and are greatly assisted by the hamstrings.
But many times people tend to not use their glutes and hamstrings as much, causing a weak squat and unbalanced lower body.
These kind of things happen with multiple exercises. Delts overworking during curls or bench press. Forearms and biceps overworking during back exercises (I've personally had issues with that for many years).
All of that can of course be fixed by referring to this thread: http://forums.d2jsp.org/topic.php?t=53112687&f=60But thankfully, it's not the only way to do it. Even with a good brain/muscle connection, focusing on so many body parts at once can be difficult.
Muscle ActivationPrior to your lift, you can activate certain muscle groups, similar to the way someone would activate his CNS before heavy lifts. This is NOT to be confused with muscle pre-exhaustion which is a completely different training technique, totally unrelated.
Say you've been squatting for a few months and hardly ever feel your butt being sore. Or you squatted one day and didn't have time to work your hamstrings and took off early, the next day your hamstrings didnt feel sore.
Before squatting, do slow contractions, both on the concentric and eccentric phases, on leg curls. 15-20 reps, for 3 sets.
Light weight. Focus on flexing that muscle on its own. Remember you do
NOT want to exhaust your muscle. You want blood to rush to that area. Think of it like a pump-up bodybuilders do before hitting the stage. Those 3 sets have to feel like you just finished a mildly heavy set. You're not exhausted, you're just ready to pack on some weights.
Same goes for glutes, use those girly machines to work on your ass. Get some blood in there.
The idea is to select an exercise that isolates the muscle
COMPLETELY. If you want to isolate your biceps, dumbbell curls are no good. Use a machine. Machines come in very handy for muscle activation.
Once ActivatedGo do your compound, focus on the flawlessness of your form for a few weeks. Focus on the muscles you just activated. Make sure you know what it feels like to squeeze them during your lift and
their purpose. When you get a good idea of the feeling, keep activating them and go heavier. At some point, you should see a major difference in the easiness to perform the lift in general. You may not lift heavier, but adding weight will be alot faster.