You may want to go from scrawny to brawny in an instant, but muscle building is a process that can't be rushed. University of New Mexico professor Len Kravitz notes that changes in your muscles begin immediately after a bout of resistance exercise, but you may not be able to see and feel those changes for weeks or months.
Muscle Growth Process
When your muscles encounter resistance, they experience trauma. In response to this trauma, cells rush in to repair the fibers and make them stronger. This response happens quickly, but the changes after any single bout of training are minor. If you consistently do resistance training, the cumulative effects create a visible change in your muscles and a feeling of greater strength. Technically, after a week of strength training, you have built muscle, but only a very small amount.
Adequate work and rest are essential in building muscle. During one week, you might participate in three to four strength-training sessions, leaving at least 48 hours between training specific muscle groups to permit the body to repair and grow stronger. Without this rest, you won't build muscle in a week or over time. When you train to build muscle, you must use weights that are heavy enough to make the last few repetitions in a set of eight to 12 total feel nearly impossible to do with proper form. Doing more than one set of an exercise for each muscle group may help you elicit slightly greater muscle gains over time, but won't make a difference in one week.