Muscles that are made up of more slow-twitch fibers don't grow as much when you lift weights as muscles with more fast-twitch fibers.

Resistance training to muscular failure induces equivalent hypertrophy in both slow-twitch (type I) and fast-twitch (type II) muscle fibers, regardless of load or repetition range.

Lifting heavier weights makes your fast-twitch muscle fibers grow bigger than lifting lighter weights does.

Lifting lighter weights might make your slow-twitch muscles grow more than your fast-twitch muscles, according to some new ideas.

Even the muscles that are built for endurance can get bigger—like, over 10% bigger—if you do the right kind of strength training for a few months. This goes against what people used to think.

The calf muscle that helps you push off when you run or jump (gastrocnemius) grows bigger with weight training better than the deeper calf muscle (soleus), because it has more types of muscle fibers that respond well to exercise.