Surely someone who trains using mostly big lifts (thus handling bigger weights) will be stronger than someone whose workouts are mostly single-joint exercises?
Well, you must first understand the difference between having strong muscles and being good at demonstrating strength in a few specific exercises. Demonstrating strength in an exercise requires both a physical capacity (strength) and a movement skill.
In a sense, it’s no different than being able to hit a baseball a mile. You can have very strong and powerful muscles individually without being able to apply that to a powerful home run swing.
Most powerlifters are stronger and more powerful than the best homerun hitters, but they can’t usually apply their strength to the bat swing because they lack movement skill.
It’s the same with strength: just because all your individual muscles are strong doesn’t mean you have the skill to coordinate their strength together, much less demonstrate maximum strength on heavy lifts.
If a bodybuilder never squats or deadlifts, even if he has very strong muscles individually, his performance on those big lifts is perceived as poor. That’s not because his muscles are just inflated goo with no strength. It’s because he doesn’t have the motor skill to apply the individual potential of all of his muscles into one maximal effort on those complex lifts. You can have a subpar performance on the big lifts even when all the involved muscles are strong and developed.
If a big bodybuilder who built his physique mostly with single-joint exercises were to do a training phase focusing only on the big lifts, his performance would quickly be brought up to par. He’d get there simply by practicing applying maximal force on a set of movements. It wouldn’t take long; the strength is already there.
Researchers studied biceps strength and size gains from the curl versus a supinated lat pulldown, which activates the biceps just as much as a curl. The results? There was no difference in biceps strength or size gains between both groups (2).
Any exercise that can properly load a specific muscle – and with which you can gradually add more weight over time – will lead to similar gains in that individual muscle. In fact, some single-joint exercises might be superior for making a specific muscle stronger because you can better focus on maximizing tension.
That’s why Westside Barbell’s conjugate system uses up to 80% of its training volume in the form of assistance exercises (often single-joint) rather than the competition lifts. They use assistance exercises to build muscle and strength and practice their competition lifts to develop the skill to apply their strength.