WHAT IT IS
High-intensity training (often called HIT) is based on the theory that brief, highly intense workouts are the best way to build muscle and strength. Though there are many variations, most HIT programs usually tell you to perform only one set of each lift to failure and to train just three times each week.
WHERE IT CAME FROM
High-intensity training was popularized in the '70s by Arthur Jones, the inventor of Nautilus machines, and pro bodybuilder Mike Mentzer.
HOW TO USE IT
Varying the amount of weight and the intensity with which you lift is superior to any program that recommends using the same weight over and over. Four weeks of short, heavy, all-out workouts could lead to swift gains.
Since most guys do too many sets and don't lift heavy enough, high-intensity training can spark new gains. It ensures your workouts stay focused and challenging.
It's controversial, partly because most research suggests that multiple sets are more effective for muscle growth than a single set. Also, training to failure isn't necessary to make gains and can lead to overtraining and injury.