Two kinds of muscle contractions: concentric and eccentric. Concentric muscle contraction is just that - contraction of the muscle so that it shortens (like a bicep curl). Eccentric work occurs when the muscle generates tension, but allows the muscle to lengthen under control (like lowering a heavy object). Eccentric muscle contractions generally create more strength changes, as well as soreness, so it only makes sense that we'd look to employ eccentric work in training. Who doesn't want to be sore, anyway?
Here's Carl demonstrating the eccentric phase of KB swings - as the weight moves from the highest position to the lowest point, his body and the KB drop under control, are stopped, and then a reverse movement occurs.
There are interesting things to explore about the rapid concentric contraction (hip thrust to raise the KB) occurring after a fast eccentric contraction (controlled descent of the weight) - plyometric training utilizes this method - but we'll save that for another discussion. For this post, the focus is the eccentric phase of work - controlled lowering, or controlled muscle lengthening.
Either concentric or eccentric contractions can occur fast or slow. For instance, bodybuilders like slow contractions to develop hypertrophy (literally lots of growth), or greater cross sectional area of the muscle. In other words, big muscles and accompanying gains in strength. For power production, faster contractions, particularly faster eccentric contractions, provide a very high return on the effort. This is the reason we utilize KB swings - good hip power development with cardiorespiratory work thrown in as an added bonus. Olympic lifts develop power in the fast catch phase of the clean, jerk, and snatch. As you work to explode up and then under the rack in a clean or jerk, or the overhead lock in a snatch, fast eccentric loading occurs, and voila! - you change neurally, in the architecture of the muscles, density of bones, tensile strength of ligaments and tendons, etc, etc. The end result of that exercise is strength and conditioning, which makes us all harder to injure, maim or kill.
