# Patellar Tendon Force Differs Depending on Jump-Landing Tasks and Estimation Methods
## 1 Introduction  
Patellar tendinopathy, also known as \"jumper's knee\", is a chronic overuse injury of the patellar tendon which is characterized by activity-related anterior knee pain [1]. Most injuries develop gradually over time, with no identifiable event responsible for their occurrence. The development of patellar tendinopathy is believed to result from the combination of mechanical overload applied to the tendon and the repetition of this load [2]. This explains its prevalence in sports involving repeated jumping and landing [3]. The prevalence of patellar tendinopathy is 44.6% in elite volleyball players and 31.9% in elite basketball players [3]. Prevention of patellar tendinopathy is important as this injury causes long-lasting symptoms that can lead to the end of a sports career [4]. Many risk factors for patellar tendinopathy have been suggested in the scientific literature. The prevalence of this injury is higher among male compared to female athletes and higher among highly skilled compared to less skilled athletes [3, 5]. Lack of quadriceps and hamstring muscles flexibility and quadriceps strength have been suggested to be associated with patellar tendinopathy [6, 7]. Lower flexibility and greater muscle strength may increase tendon strain, transmit greater forces through the patellar tendon, and thus lead to tendon overload. Therefore, high patellar tendon force (PTF) associated with high loading rate of this force are thought to be the primary causative factors of patellar tendinopathy in sports that involve repeated jumping and landing [8, 9, 10]. The landing phase of a jump-landing maneuver induced a large eccentric load on the knee extensor muscles between the time of the touchdown and the time the centerof mass reached its lowest position [11]. During this phase, eccentric quadriceps muscle contraction can load the patellar tendon beyond its inherent capacity and may cause microtrauma of tendon fibers [12]. Therefore, the eccentric phase of landing is thought to be responsible for patellar tendinopathy [13]. As this knee extensor mechanism is mainly involved in the development of patellar tendinopathy, focusing on the landing phase of a jump-landing task seems relevant in investigating this injury [1]. A recent systematic review reported that a greater knee flexion angle at initial contact followed by hip extension rather than hip flexion during a horizontal landing were the only two variables associated with asymptomatic patellar tendinopathy that could be replicated between studies [14]. In this study, asymptomatic patellar tendinopathy was characterized by the presence of an ultrasound abnormality. Moreover, this review identified conflicting results due to the different jump-landing tasks used in the scientific literature. Each jump-landing task induced different mechanical demands and elicited different biomechanical responses [15]. During sports activities such as volleyball and basketball, athletes perform jumps not only with a vertical component but also in various horizontal directions. A previous study compared two phases of a single stop-jump task: the horizontal landing during the approach phase (before jumping) and the vertical landing of the stop-jump [8]. The landing of the approach phase was called a horizontal landing because this movement referred to the braking phase where the participants decelerated their center of mass-horizontal velocity after the first acceleration steps. After the braking phase, participants performed a vertical propulsion followed by a landing which was called the vertical landing. Their results revealed higher PTF during the horizontal landing phase compared to the vertical landing phase of the stop-jump. This first evidence seemed to show that the additional forward deceleration of the center of mass during landing induced greater patellar tendon loading compared to landing with a vertical velocity only.