There are 3 basic types of muscle contractions, concentric, isometric, and eccentric. Concentric contractions are when the muscle is contracting while shortening. Think of the up portion of a one arm dumbbell curl. An isometric contraction is when the muscle contracts without shortening. In this case if we curl the dumbbell up half way and hold it, we would be doing an isometric contraction during the static hold portion. The muscle is contracting, but not shortening. If we lower the dumbbell slowly, under control, we are executing an eccentric contraction. In this case, the muscle is engaged and contracting, but it is lengthening, not shortening.
A muscle can produce more force during an eccentric contraction. However, when executing a lift in a normal fashion, the eccentric portion of the lift results in less force than the concentric portion. If you think about this, it makes sense. During a normal lift, you are having to produce more force to lift a weight than to lower it.
Still, research has shown that if you eliminate the eccentric portion of a lift (i.e., drop deadlifts, where you drop the weight from the top position, rather than lower it), strength and hypertrophy (muscle growth)
gains are reduced. Tension is a powerful stimulus for increases in muscle strength and hypertrophy. Even a muscle that is bathed in anabolic steroids will not grow at all unless there is a tension stimulus. Eccentric contractions can produce a powerful tension stimulus if done correctly. The way to do this is to perform forced negatives, or partner assisted negatives. For example, to perform partner assisted negatives on a biceps curl, the weight would be curled up into the top position, then the partner would push down on the bar, while the lifter resists and tries to keep the bar from lowering. In this case the eccentric contraction is producing MORE force than the concentric contraction. This provides a very powerful tension stimulus. It is so powerful in fact, that it can easily be overdone. Only a few repetitions should be performed in this manner. It can produce incredible soreness if overdone. Eccentric contractions can produce lots of micro damage in the muscles. Also, the eccentric contraction should be done slowly and under control. Explosive eccentric contractions can result in tearing of the muscle.
A good strategy is to perform a few forced eccentric contractions each week in 2-3 muscles that are lagging and are being targeted specifically for extra work. Trying to train the whole body this way would likely lead to over-training quickly. For example, if we are trying to bring up lagging arms, we might add 5-6 forced negative preacher curls, and 5-6 forced negative dumbbell overhead triceps extensions. Forced negatives are technically difficult to perform. Not all exercises can accomodate forced negatives. For example, forced negatives for deadlifts or barbell squats are very much a varsity move and should be attempted by the most experienced lifters only, and even then only after careful planning and with experienced spotters.
Be cautious when performing eccentric contractions. Make sure that you are warmed up thoroughly first. Use a partner for spotting and assistance. Do the eccentric portion slowly and under control. Do fewer reps than you think you need. You will be very sore the next day. Trust me on this one!
With forced negatives, just a little is enough.
