Muscles soreness stems from small tears in the muscle fibers that are aggravated by post-exercise muscle cell inflammation. Muscles can exert the most force during eccentric contractions. Consequently, muscle soreness is more common following this form of exercise. Muscles contract
concentrically (shorten as they contract), eccentrically (lengthen as they contract) or statically (contract without moving). During eccentric contractions (negatives), muscles create more tension at faster velocities.
Australian researchers found that high-speed eccentric exercise created greater muscle soreness and muscle damage than slow-speed eccentrics in untrained subjects. Muscle micro-injuries may stimulate increases in muscle strength and size, so it's possible that high-speed eccentrics may be a good way to train. However, this kind of exercise might cause serious muscle injury, slow recovery or cause long-term or permanent damage. Fast, eccentric training is a good way to overload muscles but it has a higher risk of injury.