Lifting heavy weight damages muscle fibers. It is only when the muscles repair themselves that they grow larger and stronger. For this reason it is very important that each muscle group is given plenty of rest following each workout. This is why most body builders alternate muscle groups. For example, you might work on your back on Monday, your legs on Tuesday, your chest on Wednesday, and your arms on Friday. Most successful body builders will recommend that you not work a particular muscle group more than twice per week.
The question may come up of whether it is best to use weight machines or free weights. The general consensus is that free weights are your best option for increasing muscle mass. Machines are typically used for isolating muscles and this is something that only professional body builders need to do before a contest.
