Patellar tendinopathy (PT) is a common overuse injury in sports that involve jumping and directional changes. The biomechanical differences between individuals with PT and healthy controls during side-cutting, as well as whether both legs are similarly affected, remain unclear. This study aimed to compare the kinematics and kinetics of the affected leg in patients with PT, their unaffected leg, and healthy controls during side-cutting. Methods: Thirty patients with PT and thirty healthy participants performed a side-cutting task. Motion capture and force plates were used to collect data. Five peak ground reaction forces (GRFs) and the corresponding hip, knee, and ankle joint angles and moments were analyzed. Statistical comparisons were performed using t-tests with Bonferroni correction.

Patellar tendinopathy (PT) is a musculoskeletal disorder characterized by localized pain at the junction of the inferior pole of the patella and the patellar tendon insertion. It is considered an overuse injury, predominantly affecting highly active populations participating in sports that involve repetitive jumping, running, and side-cutting, such as volleyball, soccer, and basketball, where the patellar tendon repeatedly stores and releases energy. The long-term accumulation of mechanical load in these activities can lead to structural changes in the tendon. Current biomechanical research on patellar tendinopathy has primarily focused on jumping tasks. Previous studies suggest that individuals with PT tend to adopt a pain-avoidance strategy by reducing knee flexion during landing to decrease patellar tendon loading, and they also demonstrate significantly reduced knee extension moments and rates of moment development at initial contact. However, some studies have found no significant differences in knee flexion angle or extension moment compared to healthy controls, yet these individuals still exhibit increased local patellar tendon stress, indicating that patellar tendon loading may be triplanar and should not be analyzed solely in the sagittal plane, highlighting the importance of also considering the frontal and transverse planes. Notably, research using individualized finite element models has systematically quantified the effects of femoral and tibial rotations in the frontal and horizontal planes on peak principal stress in the patellar tendon, showing that rotation in the horizontal plane has a significant impact on tendon stress. Since the hip joint serves as the proximal driver of femoral horizontal rotation, hip motion in the horizontal plane warrants particular attention. However, biomechanical studies examining hip, knee, and ankle joint mechanics in the horizontal plane in individuals with PT remain scarce. The side-cutting maneuver is a complex task involving single-leg landing, change of direction, and push-off, during which the lower limb experiences multi-directional impact forces from the sagittal, frontal, and horizontal planes. Research on side-cutting has primarily focused on acute injuries, such as anterior cruciate ligament rupture, with limited attention to chronic conditions like PT. Existing evidence not only confirms that side-cutting increases patellar tendon loading but also suggests that individuals with anterior knee pain may be at greater risk of injury during side-cutting, emphasizing the need for biomechanical studies of side-cutting in PT populations. A proposed framework divides the side-cutting maneuver into three phases: the initial contact-deceleration phase, the stance pivot phase, and the push-off phase, which are defined by five peak ground reaction force events, including four horizontal and one vertical peak. Previous research indicates that multi-planar loading tasks, particularly horizontal landing phases, are more sensitive than vertical landing tasks in detecting aberrant biomechanical patterns in individuals with PT. Although ground reaction forces do not directly represent patellar tendon loading, they reflect the external horizontal and vertical loads experienced by the lower limb. Therefore, using a ground reaction force-based phase analysis framework for side-cutting is particularly valuable in examining vertical and horizontal movement strategies in individuals with PT. Based on this framework, the present study aims to assess, during side-cutting, the magnitudes of each peak ground reaction force and the corresponding three-dimensional joint angles and moments of the knee, hip, and ankle in the affected limb of individuals with PT, their unaffected limb, and healthy controls.

Inclusion criteria for the PT group were as follows:

• Age 18-40 years and regular participation in sports, defined as engaging in physical activities for at least 90 minutes per week over the past three months, such as basketball, soccer, tennis, volleyball, long-distance running, and other sports activities,
• Structural tendon changes on imaging, including tendon thickening, hypoechogenicity, and/or increased neovascularization on Doppler ultrasound, or increased signal intensity on T2-weighted Magnetic Resonance Imaging (MRI) sequences relative to normal tendon tissue, with observable localized edema, inflammation, or degenerative changes,
• positive tenderness response upon palpation of the patellar tendon,
• reproduction of patellar tendon pain during resisted knee extension and/or during a standardized single-leg squat test,
• a score of ≤ 80 on the VISA-P (Victorian Institute of Sport Assessment-Patella) questionnaire.

Inclusion criteria for the control group were as follows:

• Participants were reported regular participation in sports, defined as engaging in physical activities for at least 90 minutes per week over the past three months, such as basketball, soccer, tennis, volleyball, long-distance running, and other sports activities,
• no history of knee injury,
• no history of lower-limb surgery,
• normal range of motion of the lower-limb joints (hip, knee, and ankle joints with no limitations in active or passive movements, within the normal physiological range),
• no history of chronic joint disorders, including current or pre-existing knee pain, PT, or functional impairment
• no evidence of patellar tendon pathology on ultrasound or MRI.

Exclusion criteria for both groups were as follows:

• a history of trauma to other lower limb joints,
• a history of surgery on any lower limb joint,
• use of analgesic medication in the past two weeks,
• the presence of other lower limb pathologies.

Patients with PT were recruited from the outpatient clinic of Peking University Third Hospital and via hospital posters, whereas healthy controls were recruited through posters targeting healthy individuals. All participants read and signed the approved informed consent form before data collection.