Muscles can be subjected to 4 different types of Contraction in order to achieve hypertrophy, increased physical strength and agility.

The first type is a concentric contraction where the muscle gets shorter as it contracts. This type of contraction is used in the up movement of almost every exercise. Bicep curls, leg extensions, chest press, shoulder press, bent over row, etc... all have an upwards movement that is concentric.

The second type of contraction is eccentric where the muscle gets longer as it contracts. The down movement of most exercises is eccentric. Performing sets of eccentric contractions is typically known as performing "negatives". For example if performing a bicep curl, you might bring up the weight at a normal speed but then lower it slowly to achieve an intense eccentric contraction. It should be noted that all muscles are stronger in the eccentric movement than the concentric movements. So if you plan to do negatives, you should always use a heavier weight than when doing concentric exercises.

The third type of contraction is isometric where the muscle remains contracted but there is no change in length. For example, pick up any weight in your hand and lift your arm up straight in front of you (front deltoid raise) and stop there. Hold the weight there are long as you can. Congratulations, you just performed an isometric contraction of your front deltoid. Because isometric contractions usually consist of one continued contraction, best results can be achieved by choosing a slightly lighter weight.

Lastly, there are plyometric contractions. Plyometric contractions are also known as explosive contractions. A plyometric contraction is achieved by performing a concentric contraction with the maximum amount of force, intensity and strength as possible for each repetition. Jumping is the best known plyometric exercise. When broken down to a muscular level, jumping is essentially a squat performed with enough force to lift the body off the ground. Some examples of exercises that can be performed as plyometric include squat (jumping), push-ups, pull ups, chest press. All power lifting exercises (clean pulls, high pulls, clean and press, power snatch, etc...) are made up of plyometric contractions.

I always recommend that a good resistance training program include all 4 types of contractions at least once per muscle group. Not only is it a good practice because it promotes higher density muscles and greater strength it's also a very good way to tire up your muscle very quickly. So for those of you who are pressed for time, this can be a great way to get the max out of your workout in the least amount of time.