The main theories on why muscle forces accelerate the body upward to a greater extent in countermovement jumps than in squat jumps: a brief review In all literature countermovement jumps (CMJ) are considered higher in contrast to squat jumps (SJ) (Bobbert et al. 1996; Kubo et al. 1999; Bobbert et al. 2005). However, the current literature regarding the potential key mechanisms underlying why greater muscle forces accelerating the body upward are observed in the CMJ compared to the SJ is somewhat unclear. A CMJ can be defined as a positioning that begins in a standing position, initiating the downward movement before the upward movement in contrast to an SJ in which the initial position is squatting without preparatory countermovement (Akl 2013). The higher jump heights observed in the CMJ compared to the SJ are evident even if at the beginning of the propulsion phase the body configuration is identical (Bobbert et al. 1996). In past literature three main mechanisms have attempted to provide an explanation for the greater muscle forces observed in the CMJ compared to the SJ. The first plausible theory is that muscle lengthening in the CMJ increases the force capacity production of the contractile mechanism (Edman et al. 1978; Ettema et al. 1992; Herzog et al. 2003). Second, the assumption that muscle fibers are on the descending branch of their force-length relationship at the onset of propulsion in the CMJ and SJ, however in the CMJ, the elongation of a chain of elastic components, are not that far from optimal length, therefore allowing a greater force in the initial phase of their shortening of the range of action, with the stretching of the sequences of elastic components, this then causes the storage of elastic energy which is then reused in the propulsion phase (Ettema et al . 1992). The final explanation... middle of the paper... acts so that vertical force production is not hindered. However this was not seen by Babbert et al. (1996), as motion disintegration was not present in the SJ and the toe position in the CMJ remained the same. In conclusion, this brief review suggests that the literature is contradictory in the theory that muscle fibers during a CMJ are less over-optimal in length than at SJs, in turn allowing greater force in the initial part of their shortening, causing l 'storage of elastic energy which can then be reused during the propulsion phase, therefore further research is necessary. However, it appears that from a biomechanical point of view the development of the active state during the preparatory countermovement in a CMJ can increase the strength of the hip extensors and greater work during shortening in a CMJ and therefore greater vertical strength and height of jumping..