Addressing YoungBlood's post:
As leonp1981 was talking about...
...muscles grow when they're forced to adapt to something. If you curl 45lbs per arm for 8 reps, and then suddenly try 55's for eight, you're biceps will have to adapt to that stress.
Not if there was a dramatic transfer of stress to other muscle groups i.e. swinging etc... in fact there might be less biceps stimulation (unlikely but possible)
Also your concept of progression here is flawed and perhaps is a mark of your own particular experience. Progression in weightlifting tends to be more like week 1 you got 4 reps out of a particular weight then the next week you may get 6... then you decide to increase the load and you get 4... the cycle continues... until a plateau occurs which requires intervention... that is the point at which separates the good from the mediocre lifters
If you then decide to jump on a treadmill for an hour, you may not get sore, you my be burning more calories that are needed to visually grow, but the signal of adaptation has been given to your biceps. If you continue to curl (or attempt) the 55's, your biceps will get the continued stimulus to grow (when you read grow, grow=adapt) and they'll still be able to grow if you take in enough calories.
As Dorian Yates once said about how he maintains similar training during his Mr. O diet prep:
"The program that built the muscle will keep the muscle."
Dorian also induced career ending muscle tears during his contest preparations. He actually now regrets not changing his pre contest training.
As the "natural" guys Willet & LaCour said..."Let the training keep your muscle. And let your diet and cardio shed the body fat."
That goes without saying but there are "supplements" that help retain muscle too.