# Patellar Tendon Force Differs Depending on Jump-Landing Tasks and Estimation Methods
## 4 Discussion
In volleyball players, knee flexion angles were also strong predictors of the patellar tendon pain during both spike and block jumps [10]. The latter study stated that the likelihood of patellar tendinopathy was higher with greater knee flexion angles associated with high impact forces and rates of force development in the knee extensor mechanism [10]. Findings of these studies suggested that the knee flexion angle at initial contact may be related to patellar tendon load. The more the knee is flexed, the more the tendon is placed under tension, which may contribute to greater tensile loading on the region of the tendon and a greater ratio of PTF and quadriceps force [28,32]. From a biomechanical perspective, a larger knee flexion angle will increase the distance of the lever arm between the center of the knee joint and the line of action of the resulting GRF vector which will increase the knee extension moment and thus lead to a larger PTF. The greater knee flexion observed during FJL was the first element to explain the greater PTF compared to VJL. For the hip joint, athletes with asymptomatic patellar tendinopathy extended rather than flexed their hip after initial contact of a horizontal landing of a stop-jump compared to asymptomatic athletes with normal patellar tendons [14]. In the present study, although participants displayed greater hip flexion angles during the FJL, they flexed their hip after the initial contact for both tasks (Table 2). This result is not surprising, firstly, because the FJL task was different from the horizontal landing task of the previous studies. The horizontal jump task referred to the braking phase where the participants decelerated their center of mass velocity after the first acceleration steps whereas the FJL was a complete task aiming to perform a higher jump. Secondly, because the population of the current study was composed of healthy volleyball players with no recent history of lower-limb injuries. The results also showed that the maximum eccentric quadriceps force was 22% higher and the knee extension moment was 23% higher during the FJL compared to the VJL. Similar observations have been made in a previous study which reported higher knee joint moments during the horizontal landing of the approach phase of a stop-jump (before jumping) compared to the vertical landing phase in male athletes with healthy patellar tendons [8]. It is likely that the higher knee joint moment observed in the FJL was mainly due to the higher peak of anteroposterior GRF. This additional anteroposterior component may modify the direction of the vector of the resultant GRF and increase the perpendicular distance between the knee joint center and the line of action of this vector [8]. Greater quadriceps force was also required to resist the forward velocity component during landing. Although participants jumped at the same height for both tasks, our results indicated that the peak vertical GRF and its loading rate were higher for the FJL compared to the VJL. The additional forward velocity during FJL influenced not only the anteroposterior component of the GRF but also the vertical component, which could explain the greater quadriceps force production required compared to VJL. As a result, the greater quadriceps force may transmit greater forces through the patellar tendon. However, it was suggested that athletes who were not able to cope with eccentric patellar tendon loads during landing may be more prone to developing patellar tendinopathy [13]. Therefore, our results indicated that the forward velocity component of the aerial phase of a jumping task may play an important role in the development of patellar tendinopathy during landing. The second purpose of this investigation was to compare two estimation methods for PTF available in the scientific literature: the inverse dynamic method and the static optimization method. The inverse dynamic method calculates the PTF by dividing the knee joint moment by the estimated patellar tendon moment arm [26].