One of the most valuable techniques for athletes trying to quickly put on muscle is to vary their rep tempos during a workout.
There are three basic types of muscle contraction:

• Concentric: where the muscle shortens under tension
• Isometric: where the muscle remains the same length regardless of tension
• Eccentric: where the muscle lengthens under tension

Eccentric muscle contraction results in muscle growth, so when it comes to putting on some serious size, that’s your moneymaker. (Click here for a full explanation.) By slowing down your sets, you can put more emphasis on the eccentric phase of your lifts. More eccentric stress leads to greater gains in muscle size.
In addition to increasing muscle mass, slow eccentrics are great for developing strong, durable tendons and connective tissue, crucial for injury prevention.
An easy example of varying the speed of an exercise is performing a Push-Up with a 2-0-2-0 tempo. That simply means lowering yourself (eccentric) for two seconds with no pause at the bottom, pushing up (concentric) for two seconds and again not pausing at the top. This keeps constant tension on the muscles throughout the set and increases the difficulty tenfold.
If you are trying slow eccentrics for the first time, don’t overdo it. Start with controlled two-second eccentrics and see how your body responds. Eccentric stress is more damaging to muscle tissue, so expect to be more sore than usual. Gradually build yourself up to six-second eccentrics.
Want more challenge? Try double tension training.