# Effects of Resisted Methods upon Sprint Performance in Rugby Players: A Systematic Review
## 4 Discussion
### Acute Effect on the Sprint's Kinematics and Kinetics  
The acute effect on running kinematics and kinetics is another consequence of weighted training, so the loads used represent other variables to be taken into account when determining their application on athletes. Although research claims that this type of weighted training benefits the strength of the lower extremities by increasing the sprint performance, there is concern that overload work has negative influences on the acceleration kinematics, reducing the athlete's performance [22], or on the kinematics of movement, modifying the optimal angles of the joints during the application of force, application of horizontal force [20], and muscle activation [49].  
As for the percentage of additional load, studies confirm that the heavier the additional load, the greater the interruptions in muscle activity [19], the greater the increase in the alteration of the muscle activation patterns of the lower extremities [48], and, consequently, the greater the changes in running technique [33]. In this regard, [PERSON] et al. [33] state that lighter sled loads (<40% BM) appear to be more suitable for improving speed capacity without causing drastic changes in the unloaded sprint technique, while heavier loads may be more suitable for optimizing horizontal force production and therefore acceleration performance, especially in the first steps. In addition, it should be noted that loads less than 30% BM are optimal loads for sprint training without causing major interruptions in running technique (contact and flight time, stride length and frequency, and trunk angle) [19].  
Studies with football, rugby union, and Australian football athletes concluded that RS training with loads close to 10% of body mass ensures that the acceleration kinematics are not negatively affected while improving the application of power in a horizontal direction [40]. On the other hand, in terms of velocity reduction, it is established that a reduction of 30% can cause an alteration in running mechanics, although it can produce beneficial effects in the first steps of the acceleration phase, as found in their study by [19].  
Regarding the training weighted with different implements or different anchor points of the harness, the studies show that depending on the use of one or the other system, an alteration of the factors of the kinematic parameters is observed. On the one hand, the anchor point that causes the least alteration of the kinematics parameters of the sprint is the waist [20]. On the other hand, [PERSON] et al. [48] found that sprint work with resisted parachutes did not change the kinematic or the muscle activation, despite producing a loss of speed, while the increase in the load of the sled thrust caused interruptions in the sprint technique and altered the muscle activation patterns of the lower extremities. Consequently, all these aspects can help determine the choice of the most suitable load in the resisted sprint job as well as the use of the right implement.  
Finally, at the beginning of the study, it was hypothesized that the resisted method would produce improvements in sprint performance as well as benefits in PAP, which has been corroborated, provided that a 6-week training period is applied and appropriate overloads (% of body mass) are used.