All participants will be subject to a standardised rehabilitation exercise region consisting of stretching and eccentric exercises following the Alfredson protocol. The 12-week program will be taught by a physiotherapist or athletic trainer and supplemented by video tutorials. The regime begins with three 30-second static stretches for the gastrocnemius (knee extension) and soleus (knee flexion) muscles with 1-minute rest between each stretch. The 2nd part is eccentric exercises to the calves following the Alfredson protocol. This will be three sets of eccentric exercises lasting 12 weeks, with 15 sets twice a day. If participants are unable to complete three sets of 15 repetitions, they can start with fewer repetitions and sets (at least 2 sets of 10 repetitions each) and gradually increase to the total number of sets. Participants will perform the static stretching exercises from Step 1 at the end of each session

The ATRA will measure participants' tendon length/tension. Participants should be positioned in a prone position with the knee flexed to 90 degrees. The assessor will use a dynamic joint goniometer to measure the angle at which the foot passively falls.

Using B-mode ultrasound scanning, the probe is moved from the medial to the lateral aspect of the Achilles tendon until a planar image of the Achilles tendon is clearly shown. The probe was adjusted straight perpendicular to the tendon fibres. A greyscale sonogram is acquired, and the tendon thickness is measured using the maximum thickness. Less than 5.3 mm was considered a healthy value for the Achilles tendon.

The cross-sectional area will be measured using B mode. After identifying the region of interest (the thickest portion or pathological site) in the longitudinal view, the transducer is rotated 90° to switch to the short-axis view (axial ultrasound view, as shown in the figure) for cross-sectional measurement. The cross-sectional area (CSA) is then recorded.

Vascularity will be documented using the Ohberg score. Using the Doppler mode on the ultrasound machine, the transducer is placed vertically to obtain a sagittal view of the softest area of the Achilles tendon. Pressure on the skin from the transducer should be kept to a minimum to prevent occlusion of blood vessels. The colour box is focused on the dorsal aspect of the tendon. The assessor will spend one minute exploring the blood flow over the tendon to find the area of maximum Doppler flow. After locating the area, sonograms are taken to determine the Öhberg score. Higher scores indicate more Doppler blood flow in the peritendinous and intratendinous tissues. Healthy tendons (Ohberg score of 0) have no blood vessels in the Achilles tendon.

Shear wave elastography (SWE) can be used to quantify soft tissue stiffness. Once the optimal scan plane of the tendon is determined, the SWE function is activated. It enters penetration mode and the measured stiffness range is normalized to 0-600 kPa. The SWE color card (H × W: 1.4 cm × 1.3 cm) is placed immediately above the upper edge of the calcaneus. After the color signal stabilized for five seconds, the electrogram was recorded and stored. Tendon stiffness was measured using a Q-box stiffness meter. Depending on the size of the tendon, the diameter of the circular measuring area (Q-box) is set to 2 mm, which covers the Achilles tendon and excludes other adjacent soft tissues. The Q-box is located in the center of the tendon on the SWE color map. For each Q-box, the average tendon stiffness (in kPa) is measured.

The hand-held dynamometer will be used to quantify isometric muscle strength (in kilograms) during ankle dorsiflexion and plantarflexion. Participants will maintain a maximal voluntary isometric contraction for 3 seconds. Three trials will be performed for each movement, with 30-second rest intervals between trials to minimize fatigue.