# Acceleration Capacity and Vertical Jump Performance Relationship in Prepubertal Children
## 4 Discussion
The ability to rapidly produce force is also influenced by the rate of muscle activation [47]. [PERSON] et al. [48] showed a greater rate of torque development associated with a greater rate of muscle activation in children trained in gymnastics than children trained in endurance. This relationship between sprint and jump abilities in children highlights the importance of lower body strength and power output for these two actions. A recent article [49] has already suggested that inter-individual differences in children's sprint performance could be linked to the ability to produce force, and not to a technical difference. This idea is supported by some studies that have demonstrated the positive effect of strength development on sprint and jump performance. A 6-week strength training program composed of full squats performed twice a week [50] induced sprint and jump performance improvement with under 13-year-old soccer players. It also has been demonstrated that an electromyostimulation protocol training developing strength resulted in an improvement in specific jumps among gymnast prepubertal girls [51]. In contrast, no correlation was observed between horizontal jump and short-sprint performance. Different hypotheses could explain this result. Firstly, different joint contributions between horizontal and vertical jumps could induce different associations with sprinting. Indeed, vertical jumping implies a similar contribution of the hip, knee, and ankle articulation, while horizontal jump performance is mostly determined by hip and ankle action [36]. Considering that the sprint acceleration phase is characterized by greater knee flexion at foot strike and a greater knee extensor contribution [52], this suggests a greater similitude between the acceleration phase's motor pattern and that of vertical jumping, compared with horizontal jumping. Secondly, a low-balance capacity causing a submaximal performance during a horizontal jump is plausible. It could affect the stabilization phase and reduce performance. Indeed, an association between horizontal jump performance and balance capacity has already been shown in children [53; 54]. This problem could be accentuated by the technical aspects associated with this jump modality. However, this study noticed a correlation between 20 m sprint time and the hop test performance, as shown by a previous study with 15-year-old adolescents [55]. This could suggest that unilateral jump performance could be more associated with long-sprint performance in children. This hypothesis needs to be explored. However, even if the horizontal jump is commonly used to assess general fitness conditions in children [56] and presents a method that is easily applied in practice without a specific instrument, it is probably not appropriate to assess acceleration capacity in children. A significant difference between girls and boys was identified in the APHV, which is coherent with previous studies demonstrating earlier development for girls than for boys [32]. A significant sex effect was observed for the vertical jump performances, while no difference between girls and boys was observed for horizontal jump performances. These results support the idea of a lower sensitivity for horizontal jumps than vertical jumps to reveal performance differences.