# Kinetic Comparison between Drop Jumps and Horizontal Drop Jumps in Elite Jumps and Sprinters
## 4 Discussion  
Here, we compared the kinetic variables of the DJ versus the HDJ in elite jumpers and sprinters. Furthermore, we tested the hypothesis that there are differences between these exercises. This is the first exhaustive study that seeks to quantify the existing differences between the DJ and the HDJ in six kinetic variables. We found differences centered mainly on the preponderance of vertical axis forces over the horizontal axis in both exercises. This phenomenon does not seem to be reversed during athletic exercises. As in other studies [13], we also noticed that P and GRF tend to always be lower on the anteroposterior axis than on the vertical axis. The GRFE represents the landing forces, and it corresponds to the phase where the movement is still in its negative (eccentric) segment (see Figure 1) and can be heavily influenced by the fall strategy [2; 15]. In contrast, GRFC represents the positive (concentric) phase of movement, starting when the velocity reverses direction and increases in value. It is unlikely that GRFC, under normal conditions, is influenced by external factors other than the purely concentric forces of the athlete. The most critical variables for analyzing DJ performance are derived from its measurement. This consideration is also based on the fact that within the SSC, the energy required to perform eccentric actions is significantly lower than that needed for concentric actions [31]. For this reason, although we recognize the benefits of the eccentric phase in increasing muscular stiffness, as well as its effects on the concentric phase function [32], we consider GRFC the most essential variable of GRF in the context of athletic performance assessment. Overall, GRFE was significantly greater in the DJ vs. the HDJ, providing evidence that eccentric force production is higher in vertical jumps than in horizontal jumps. The GRFE found here is higher than the mean reported in previous research [2; 15; 16; 33]. As the HDJ is affected by its countermovement activity, it is logical that it shows similar results to those of previous studies [15], where a decrease in force is evident with this technique. Another result we found is that, for both exercises, GRFE increased considerably with increasing FH, corroborating previous reports [34] and showing that eccentric forces are affected by FH. In another study [35] in which DJ landings from different heights were analyzed, it was found that with increasing FH, the amount of heel strikes during contact also increased. This  
Figure 1: Comparison of GRFE and GRFC between the drop jump (DJ) and the horizontal drop jump (HDJ) with eccentric and concentric phase delineation starting at velocity = 0. (**A**) Force–time trace during DJ30, (**B**) force–time trace during DJ40, (**C**) force–time trace during DJ50, (**D**) force–time trace during HDJ30, (**E**) force–time trace during HDJ40, and (**F**) force–time trace during HDJ50. behavior was equally observable in both DJ and HDJ exercises. Also, the representation of the initial peak force resulting from heel strikes is higher in the DJ than in the HDJ, but is less abrupt and with a longer time in the HDJ. Another study [36; 37] indicates both a horizontal and a vertical component in the HDJ, while the DJ has only a vertical component. While their justification is based on video analysis, this criterion may be unfounded when analyzing the force traces of the DJ, where anteroposterior forces exist but with a visibly smaller component than in the HDJ. During DJ50, we found an increase in PE, PC, and GRFE, but observed a decrease in GRFC, which reinforces the importance of separating the forces (eccentric and concentric) during ground contact. This behavior has been explained previously [38] and should represent a warning point for coaches seeking to work the intensity of pylemetric exercise based on the concentric force production. In DJ50, a slight non-significant increase in ground contact time, flight time, and, therefore, jump height has been found [17].