This article is from the Hardgainer Bodybuilding & Weightlifting FAQ, by Frank J. Kelly and Craig R. Sadler with numerous contributions by others.
Many of us have a weak bodypart or set of muscles we would like
to improve upon. However, until one is 80% of the way to
becoming advanced (the 300-400-500 level), you should
concentrate on the Squat, Deadlift, and Bench/Dip poundages.
If, after that stage, you still want to specialize, the
key is to decrease work on other exercises -- still going
to failure but using fewer sets and reps. Make the weak
area the first muscle you hit in a workout and try
2-3 exercises instead of just 1 but keep the volume low.
Concentrate on increasing weight/reps, but don't
neglect the squat/deadlift.
 
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