## Impact of Flexibility on Vertical Jump, Balance and Speed in Amateur Football Players
While some authors argue that the changes observed in the range of motion after a stretching program are due to anatomical or structural tissue changes, when large volumes and intensities are used for considerable periods [30; 31] others mention that they are due to the participants' increased tolerance to the discomfort caused by stretching (neurophysiological effect) [25; 32]. The literature is not consensual concerning stretching, and this practice seems to be falling into disuse, with its replacement by other tools, namely strength training [18; 33]. In addition to the numerous systemic benefits, recently documented by [PERSON] et al. [37], strength training has also been shown to be an effective strategy for gaining ROM in athletic populations, which may favor the choice of this type of intervention over stretching programs [38]. An increase in the length of the fascicle, a better agonist-antagonist coactivation (reciprocal inhibition) and an enhancement of the lengthening-shortening cyclesare the main mechanisms proposed for these changes in flexibility as a result of strength training [33]. The contribution of flexibility to performance is still unclear [7; 23; 24; 25; 26; 27; 28]. In static stretching, for example, it is hypothesized that the increase in ROM around a joint occurs because of the maintenance of maximum amplitudes for longer periods, compared to alternative modalities (dynamic, ballistic or PNF), causing the increase in tendon elasticity and decrease in muscle viscosity, which produces a reduction in the stiffness of the musculotendinous unit and passive torque, generating greater stretching of the tissues [24]. However, studies have shown that this stretching modality can induce acute harmful effects, resulting from these neural and peripheral mechanisms, particularly from the reduction of musculotendinous stiffness, which effects will increase with stretching duration because of a decrease in muscle contractile capacities and the production of maximum muscle strength and power [24; 25]. Hence, the literature shows that static stretching should be used with caution, or even avoided during warm-up, to prevent subsequent potentially deleterious effects on performance, except for specific cases such as athletes who will resort to extreme ranges (handball goalkeepers, gymnasts or ballerinas, for example) [18; 24]. The inclusion of dynamic stretching exercises, on the other hand, can help in the warm-up and preparation process for the activity, given the increase in muscle temperature, when combined with aerobic exercise of submaximal intensity and activities specific to the sport in question [25; 32]. Concerning recovery, the assumptions for the use of stretching are related to the reduction of DOMS (delayed onset muscle soreness). Evidence indicates that stretching does not reduce muscle pain or the presence of inflammatory muscle markers in the days following (up to 72 h) after exercise, whether applied before or after the activity [25; 27; 28]. [PERSON] et al. [25] also concluded that static stretching exercises as a form of recovery do not present benefits compared to other passive modalities proposed in the literature, such as cryotherapy, contrast baths and electrical stimulation. Flexibility is an important component of an athlete's physical capabilities, with important implications for their overall performance [7; 8; 12; 17; 39]. Although the acute effects of stretching on sports performance are well-established [23; 24; 25; 26; 27; 28; 34], few studies [7] analyze the impact of lower limb (LL) muscle flexibility concerning skills such as vertical jump, balance and speed in adult football players. Given the above, this work aims to better understand these relations and find the associations between lower limb flexibility and vertical jump, balance and speed in amateur football players, to help develop new guidelines and recommendations alluding to flexibility, its training and its practical applicability and usefulness in the sports context and football in particular.