Normally lagging body parts are lagging behind for a reason, that reason being that they lack stimulus to grow. Why is this? It’s quite simple really but not something that’s obvious until someone explains it. If we take the example of posterior (rear) deltoid (shoulder) muscles which are a problem for most bodybuilders/fitness athletes. When targeting this muscle it’s more of
a case of mental control/connection than adding more weight, if you are performing a face pull exercise to target this muscle group specifically it can be excellent but at the same time if you don’t focus your attention onto that muscle you will end up using your lats, rhomboids or traps to compete the movement, this situation is compounded by adding more weight. Remember it’s a lagging muscle, it’s weaker than its neighbours so if weight is increased this muscle will become unable to complete the movement no matter how great your mental control (motor-neurone connection). In this case you immediately compensate by incorporating the other “pull” muscles and usually remove all intensity from that area you actually want to target!
How many times do the front of your shoulders (anterior deltoid) hurt after doing bench press? If the answer is anything other than never, you’re going too heavy and actually reducing the intensity of the workout for your pectorals. It’s the same scenario, your pectorals are too weak to lift the weight alone and end up asking any other nearby muscles for help, which not only reduces the load on the target area but also can lead to overtraining of the assisting muscle group (now you can’t train shoulders the next day without overtraining and risk of additional injury).
Woah, that was information overload! Let me summarise:
Don’t fall into the trap of increasing weight to increase intensity, it’s just the easy approach. Make sure you’re lifting nice and slowly, not jerking, feeling the target muscle group work. Use tricks like rest/pause, super-setting, triangle sets, drop sets etc. There are many more ingredients to increase intensity without incurring the risks mentioned above.
Best of luck