Pull-ups and pull-downs are both exercises that recruit the main muscles of your upper back, primarily your latissimus dorsi. In a pull-up, you pull yourself up against gravity, hanging from a fixed bar. In a pull-down, you are fixed on a bench, and you pull the bar down against resistance provided by a pulley and weight stack. Your body responds to the different sources of resistance in different ways, affecting your goals of mass gain.
Pull-Ups
Pull-ups are simply pulling yourself up from a dead hang until your upper chest touches the bar. It involves the finger and forearm flexors, the elbow flexors such as the brachialis and brachioradialis, and the large muscles of your upper back, primarily the latissimus dorsi and rhomboids. Pull-ups also recruit core muscles in your abdominals and lower back by requiring you to stabilize your lower body. The stability requirement results in a great recruitment of your central nervous system.
Lat Pull-Downs
Lat pull-downs recruit largely the same muscles as pull-ups, but being seated on a bench or otherwise fixed to the floor removes the stability requirement. Most lat pull-down machines are on pulleys attached to selectorized weight stacks. This way, you can more conveniently scale how much resistance you are lifting. The seated position also makes it easier to isolate the upper back muscles, focusing on working against the resistance with your target muscles.
Stimulating Muscle Fibers with Pull-ups
You must stimulate as many muscle cells, or fibers, as possible to gain mass in a muscle. Most muscles, including your lats, are a mixture of different muscle fiber types. Some of these fibers are high-endurance and easy to stimulate, but are relatively small. The other type of fibers are more difficult to stimulate and less enduring, but larger and stronger. Teaching your body to recruit these harder-to-stimulate fibers will lay the foundation for greater mass. Exercises that force you to develop force quickly are ideal for this. Pull-ups, particularly kipping pull-ups, can introduce a ballistic component into your training that stimulates the more power-oriented muscle fibers.
High Volume Training
The sarcoplasm is the fluid component of your muscle cells. This is where the carbohydrate fuel for activities lasting less than 2 minutes is stored. This fluid accounts for 25 to 30 percent of the mass of each muscle cell. Increasing your muscle mass by targeting these fuel stores is best achieved by high-volume training, with multiple sets of 10 to 15 repetitions. Pull-downs, which can focus specifically on your lats and do not fatigue your central nervous system as pull-ups do, fit well into a high-volume program.
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