With chronic bed rest, muscle will atrophy. Every athlete knows this. If you’ve ever had your arm or leg in a cast, you know how weak the limb feels after only a few weeks of immobilization. However with reuse- it will begin to hypertrophy and quickly normalize in function. Jones et al 2004 demonstrated that after 2 weeks of immobilization rest, a host of genes are epigenetically manipulated. These genes control the rate of growth, maintenance and decay of muscle. They found that the genes responsible for wasting muscle were turned on while those responsible for growing and maintaining muscle were not. There is generally a balance between these factors leading to a more or less constant muscle mass (which can be changed with training, nutrition, recovery and other factors). With 1 session of weight training, these changes began to reverse and remained reversed for 4-8 hours. Thus, chronic weight lifting will literally put off disuse atrophy for years. If you are worried about bulking up- you should probably be worrying about wasting away. No one has died from too much muscle or been hospitalized for that reason. On the other hand, hundreds of elderly fall and risk fractures and concussions due to weakness and disuse atrophy. Stay active, move more, lift weights.