Effect of Foot Position During Rehabilitation of Achilles Tendinopathy on Tendon Mechanics and Symptoms.

Ultrasound images of the Achilles tendon will be recorded using a probe (Hitachi L64, Tokyo, Japan) put in an external probeholder during multiple rehabilitation exercises: Birise Bidrop Standing, Unirise Unidrop Standing Knee Extended, Unirise Unidrop Standing Knee Bent and squat. The device we will use is the Arietta 650, FUJIFILM, which allows to record B-mode ultrasound images. The probe will be placed distally on the Achilles tendon, just above the calcaneal insertion (used as reference point). Using a speckle tracking algorithm in Matlab, the displacement of 6 rows within the Achilles tendon is calculated. Intra-tendinous sliding is calculated as the relative difference between the displacement of the superficial and deep border.

At pretest, 3, 6, 9, posttest, 15, 18, 21, and 24, the VISA-A questionnaire will be completed online. During the pre- and post-test assessments, participants will complete the questionnaire on a laptop provided in the lab, ensuring it is administered online and in the same manner as throughout the intervention, where they fill it in at home.

The VISA-A questionnaire is an 8-item survey used to assess the severity of Achilles tendinopathy. It evaluates pain, daily functional activities, and sporting ability, with scores ranging from 0 to 100, where a score of 100 indicates no symptoms.

Heel-rise test: participants perform as many heel-raises as possible at a frequency of 25 per minute (controlled by a metronome). The test will be discontinued when the participant does not reach maximum height, cannot keep the frequency, cannot keep the knee straight or stops due to pain or fatigue. By using an application, the foot will be filmed and heel work will be calculated.

• Hopping till pain: the participant hops on a single leg at a pace of 2 jumps per second (controlled by a metronome) where the goal is to achieve 25 pain-free hops, but the test will be stopped when the participant starts to experience pain.

SF-12 questionnaire: used to assess health-related quality of life.

Intra-tendinous vascularity will be assessed in the sagittal plane with Doppler Ultrasonography (Arietta 650, FUJIFILM). The ultrasound transducer will be aligned parallel to the tendon and will be applied with no pressure and it will be positioned so that it visualizes the proximal portion of the calcaneal bone and the Achilles tendon. For this, participants will be in prone position with the knee extended and the ankle joint passively stabilized at 90°. Three scans with a duration of 4 seconds will be recorded at both the insertion and midportion of the tendon after all rehabilitation exercises in the lab are performed.

During rest in the prone position, multiple scans of the lower leg will be acquired using 3D freehand ultrasound. This technique combines conventional B-mode ultrasonography (FUJIFILM Healthcare, Arietta 650DI) with a motion analysis system to generate in vivo three-dimensional reconstructions of tendon structures. The 2D ultrasound images will be transformed into a global coordinate system using the open-source medical image computing and visualization software 3D Slicer, enabling reconstruction of a 3D tendon volume. Based on this technique, the length and volume of the free Achilles tendon (AT) will be estimated.

As a supplementary check, free AT length will also be measured manually by determining the distance between the calcaneal insertion and the soleus myotendinous junction, identified using ultrasound imaging. In addition, two ultrasound images (transverse and sagittal planes) will be obtained at the swollen region of the tendon to provide indications of tendon width.

In the lab, patients will perform four rehabilitation exercises: Bilateral heel raise-drop, unilateral heel raise-drop (knee extended and knee bent version) and squat. Besides that, walking and optionally (if the patient can) running on the treadmill. 3D-motion capture data, EMG (Delsys electrodes) and force plate data will be collected. Through musculoskeletal modeling in OpenSim, Kinematics, kinetics, muscle activations and forces (of Gastrocnemius medialis (GM) and lateralis (GL), soleus (SOL), tibialis posterior and anterior and peroneus) will be analyzed. Besides that, EMG will be recorded through Delsys electrodes on Gastrocnemius medialis and lateralis, soleus, tibialis anterior and peroneus.

Achilles Tendon Load will be calculated as the sum of the tendon force from GM, GL and SOL from the musculoskeletal model - muscle analysis after dynamic optimization.

Throughout the training program (and also the 12 weeks after as follow-up), all patients wear a Fitbit Inspire 3. This activity tracker will measure the physical activity of the patient. Most important features here are amount of daily steps and automatically tracked activities (cycling, swimming, running, walking) with according time. Besides that, all patients have a logbook where they write down every week as well which activities they did as an extra check-up.

(The logbook is only used during the training program).

During the post-test assessment in the lab, following the 12-week training program, a live interview will be conducted with each participant. The principal investigator will ask questions regarding the participant's experiences with the training program. The interview will include both open-ended questions and closed-ended questions using a Likert scale. It will take approximately 10 minutes and will provide important information regarding the feasibility of the program for patients performing 30 minutes of exercises daily.

TENDINS-A questionnaire: a patient-reported outcome measure (PROM) designed to assess the severity of disability in individuals with Achilles tendinopathy. It evaluates pain, symptoms, and physical function to determine the impact of the condition on daily life. As this questionnaire was only recently developed and published in 2024, limited research is available on how its outcomes relate to the VISA-A. Therefore, collecting both measures at pre- and post-test will allow comparison between the two instruments.

13-item FRESS questionnaire: used to assess return-to-sport outcomes.

Tampa Scale for Kinesiophobia (TSK): used to assess fear of movement and reinjury.