The eccentric muscle contractions supported by X-trainer current is a unique feature which not only relaxes the muscles but also promotes active regeneration of injured or healthy muscle tissue. The eccentric contractions allow the muscles to elongate actively and relax. This is an important distinction between X-trainer and all other electrical stimulation devices which only cause muscle contraction. As the muscle reaches the fully shortened position, it becomes fully fatigued and the muscles become incapable of further activity. At this point, application of X-trainer instantaneously lengthens the muscle again and allows it to continue the strength work. Relaxing muscles also reduces and can sometimes eliminate delayed onset muscle soreness (DOMS), which can usually be felt in the two to three days following an exercise session. Thus, eccentric contractions help in preventing and treating muscle stiffness. The more relaxed the muscle is, the more strength work it can do and more muscle mass it can build.