BulkingGaining phase · Muscle-gain phase

Bulking is a training phase in which intake is set above maintenance to support building muscle. The traditional distinction is between a lean bulk, using a small surplus, and a dirty bulk, where intake is unrestricted and fat gain is accepted as part of the deal.

It matters because muscle is built fastest with energy available, particularly once you are past the beginner phase. But the rate at which muscle can be added is biologically capped and slows with training age — so beyond a modest surplus, extra calories add fat, not muscle.

In practice a small surplus with adequate protein and progressive resistance training is the version supported by evidence. Longer, larger bulks mean a longer, harder cut afterwards, and starting a bulk from a high body-fat percentage tilts the ratio of gains further towards fat.