The purpose of this randomized controlled trial is to evaluate and compare the effectiveness of adding chitosan phonophoresis as an adjunct to a rehabilitation-based exercise program compared to a rehabilitation-based exercise program with sham chitosan phonophoresis in athletes with recurrent hamstring strain injuries on the following outcomes: Hamstring-specific pain and function, Muscle strength and endurance, Lower extremity functions, Hamstring Flexibility symmetry and Hamstring injury healing score.

Hamstring injuries represent one of the most prevalent and recurrent soft tissue injuries in athletes, accounting for a significant percentage of time lost from training and competition across a variety of sports disciplines. These injuries encompass a spectrum of diagnoses, including muscle strains, proximal tendon avulsions, tendinopathies, and referred posterior thigh pain. The prevalence of recurrent hamstring strain injuries varies widely in studies, ranging from about 12% to as high as 63%. On average, recurrent hamstring strains account for around 26% of all hamstring injuries in sports such as Australian football, soccer, and rugby.

Inclusion Criteria:

Participants will be included in the study if they fulfill the following criteria:

1. Male recreational football players who participate in non-professional football training or matches for at least two sessions per week (each session ≥ 45 minutes) for at least the previous 12 months
2. Aged 18-35 years
3. Clinically diagnosed with a new episode of acute unilateral recurrent hamstring strain. Recurrent injury is defined as a new acute hamstring injury in the same limb and muscle group (biceps femoris, semitendinosus, or semimembranosus) as at least one previous hamstring strain that was professionally diagnosed, treated with rehabilitation, and after which the athlete returned to full participation. The previous injury must have occurred within the last 24 months
4. Current injury onset within 3-10 days of enrollment.
5. Current injury classified as Peetrons grade I or II on diagnostic musculoskeletal ultrasound performed within 7 days of enrollment by a single sonographer with at least 5 years of experience, using a 12-18 MHz linear probe in prone position (Peetrons, 2002; Heiss et al., 2025). The specific muscle involved, location within the muscle, and lesion volume (π/6 × L × W × D) will be documented at baseline
6. VISA-H score ≤ 75 at screening
7. Body mass index between 18.5 and 30.0 kg/m².
8. Able to walk independently without an assistive device.

Exclusion Criteria:

Participants will be excluded if they have any of the following:

1. History of hamstring tendon avulsion (complete or partial) or surgical repair in either limb.
2. Hamstring strain injury in the contralateral limb within the previous 12 months.
3. Concomitant lower extremity or lumbopelvic pathology that prevents safe completion of testing at maximal effort, including but not limited to knee ligament injury, meniscal injury, hip labral pathology, sacroiliac joint dysfunction, or lumbar disc herniation with radiculopathy.
4. Neurological disorder affecting lower extremity motor or sensory function such as peripheral neuropathy, lumbar radiculopathy, or multiple sclerosis.
5. Known allergy or hypersensitivity to chitosan, chitin, or shellfish, identified by self-report on a standardized allergy screening questionnaire at enrollment
6. Active skin disease, open wounds, or dermatitis over the posterior thigh of the affected limb.
7. Corticosteroid injection to the ipsilateral hamstring within the previous 6 months
8. Systemic conditions impairing healing such as diabetes mellitus, immunosuppressive therapy, chronic kidney disease, autoimmune connective tissue disorders, or coagulation disorders.
9. Use of pharmacological agents or supplements with known effects on tissue healing (such as growth hormone, anabolic steroids, or collagen supplements) within the previous 3 months.