gettinhuge--what I mean by "muscle imbalances": A lot of guys treat the "upper back" as one muscle group, but different muscles of the back have different functions. The primary movers in pull-ups and pull-downs (the lats) serve to internally rotate the humerus, and the primary movers in rows (rhomboids, mid-traps, and other upper back muscles) serve to externally rotate the humerous. The take-home message is that overdeveloping the lats and pecs while neglecting the other upper-back muscles is often the cause of the hunched back, forward-slouched shoulder posture you see in the gym. This can lead to shoulder problems.
That said, Simon is probably right. If you do the pull-up program for just a few weeks and then go back to a more balanced routine, I doubt you'll have any negative effects.
-Jake
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