The participants in the athlete group were volleyball players competing in the 1st or 2nd division of the Portuguese National Championship, training and/or competing for more than 20 hours per week.

The participants in the non-athlete group had no prior experience in any jumping sport.

Ultrasound Tissue Characterization (UTC) captures tendon structure and quantifies the number of altered fibers. This technology classifies tendon structure into four echo types, reflecting tendon integrity and fibrillar organization. Data collection for the patellar tendon will be conducted with participants seated, hips and knees at 90° flexion, and feet supported on a stable surface. For the Achilles tendon, the participants will be lying down with ankle in neutral position.

This positioning ensures the probe is aligned transversely to the tendons, enabling the acquisition of optimal ultrasound images without anisotropy.

Ultrasound parameters will be standardized for all scans: a 40-second B-mode ultrasound using a 7-10 Hz linear transducer, with the probe attached to a tracking device that moves automatically along the tendon's perpendicular axis, capturing consecutive images at 0.2 mm intervals. A single scan of the tendons will be performed for each knee.

The data collection protocol includes the following tasks: countermovement jump, single-leg countermovement jump with the dominant leg, single-leg countermovement jump with the jumping leg, single-leg horizontal jump with the jumping leg and single-leg horizontal jump with the dominant leg. Kinematic variables will be analyzed by recording the spatial positions of reflective markers placed on specific anatomical regions. Data will be captured using the Qualisys Motion Capture System (version 2021.2).

Ground reaction forces will be measured using a force platform connected to a signal amplifier. The platform records the three components of ground reaction forces-anteroposterior, mediolateral, and vertical-via four sensors.

The image capture hardware is connected to the Qualisys USB Analog Acquisition interface, synchronizing temporal, kinetic, and kinematic data with the Qualisys Track Manager (QTM) software.