Have you ever heard people talking about doing an exercise eccentrically? Do you know what they mean when they say it. Gym jargon can be confusing and when folks talk about contraction types it can be hard to figure out exactly what they mean. Here I will explain the different types of contractions as well as provide examples of their use.
Note: I don't like the term muscle contractions because "contraction" means to shorten. Only one type of contraction described below actually involves the muscle shortening. I prefer to say muscle activations, but that's me being a geek. For colloquialism's sake I'll use contractions for this post.
Concentric: This, to me, is the purest form of muscle contraction because it's the only one in which the length of the muscles shortens as the muscle producing force. This is your basic contraction, such as when your trying to show off for the ladies so you start flexing your biceps . That's probably the best example of a concentric contraction that I can think of. As you flex your bicep, the muscle shortens while producing force.
Eccentric: An eccentric contraction is when force is produced while the muscle is lengthening. This is what happens when you do negatives on a flat bench. As you lower the bar to your chest your pecs (and triceps and anterior deltoid ) lengthen, but they still have to produce force or else you'll be crushed by the bar. Another example is walking down hill or down a set of stairs. Your quadriceps work eccentrically to keep you from crumpling to the ground.
Isometric: An isometric contraction is when force is produced, but there is no change in muscle length. Think trying to pull ouf the sword of excalibur. Too geeky? How about trying to deadlift with a weight that you can't lift. Try as you might, that weight isn't coming off the floor, yet you're producing a hell of a lot of force trying to do so.
So there you have it, the short and skinny of muscle contractions.