This three-year research project aims to test the hypothesis that strain and displacement characteristics of human soft tissues are influenced by (1) habitual physical activity, (2) functional impairment, and (3) training interventions. Over the three years, the ulnar collateral ligament of the elbow, the ankle tendon, and the hamstring muscles will be investigated sequentially. Ultrasound speckle-tracking techniques will be employed to verify these hypotheses and to provide a basis for clinical risk assessment of injury, development of therapeutic strategies, and evaluation of rehabilitation outcomes.

In the first year, strain and displacement of the medial ulnar collateral ligament (MUCL) in both longitudinal and transverse directions during passive isokinetic elbow valgus loading will be analyzed in baseball pitchers at high and low injury risk and in healthy controls. The results will be compared with ultrasonic shear-wave elastography. Participants will then undergo an 8-week low- to moderate-intensity blood-flow restriction resistance training program. This phase is expected to characterize ligament mechanical adaptations to long-term sports training, elucidate the relationship between such adaptations and injury risk, and evaluate the effects of resistance training on enhancing medial elbow joint stability.

In the second year, individuals with posterior tibial tendon dysfunction and healthy controls will be examined. Longitudinal strain and displacement, as well as transverse rotation and displacement of the tibialis posterior tendon during active ankle movements will be quantified and compared with shear-wave elastography measurements. These results are expected to clarify the relationship between tendon strain-displacement behavior and tendon dysfunction, provide mechanistic insights into tendon pathology, and inform optimal therapeutic strategies.

In the third year, patients following anterior cruciate ligament (ACL) reconstruction will be investigated. Longitudinal strain and displacement of the hamstring graft donor site, specifically the semitendinosus muscle-tendon unit, following eccentric training will be assessed and compared with shear-wave elastography and electromyography. This phase is expected to characterize post-harvest semitendinosus muscle activity patterns and their associations with postoperative muscle strength deficits.

By integrating findings across the three phases, the anticipated clinical implications of this project include determining whether:

1. adaptive responses of ligaments to repetitive tensile loading during long-term sports participation (including strain behavior and mechanical properties) can serve as screening indicators for injury risk, and evaluating the effects of blood-flow restriction resistance training in these populations;
2. three-dimensional strain, displacement, and rotation of tendons in response to muscle contraction and joint motion are associated with tendon dysfunction; and
3. strain and displacement characteristics of the semitendinosus muscle (in relation to muscle recruitment) can be improved through postoperative functional training following its use as a graft source.

Inclusion Criteria:

Study 1

• male pitchers aged between 18 and 35 years
• more than five years of pitching experience
• at least 200 counts of pitching training per week

Study 2

• Age between 20 and 65 years
• diagnosed with grade II posterior tibial tendon dysfunction and recommended by a physician for surgical intervention

Study 3

• The participants were between 18 and 45 years old
• had undergone unilateral anterior cruciate ligament semitendinosus tendon autograft reconstruction surgery within the past 6 years
• were able to fully participate in the sports and training they had before their injury

Exclusion Criteria:

Study 1

• diagnosed with partial or full thickness tears of the UCL by arthrographic/ nonarthrographic MR techniques in past medical records
• having a positive result in the "moving valgus stress test" screening24
• unable to participate in regular baseball pitching training or absent from the competition during the March 2023 to March 2025 season
• found to show suspicious signs, such as hypoechoic foci and calcification, which suggest UCL injury under ultrasound screening4
• with a <80% compliance rate in the eight-week BFRT

Study 2

• The lower extremities have undergone surgical intervention
• The lower extremities have a known history of nerve or vascular injury
• The ankles and feet have known ligament tears
• Fixed deformities of the foot joints
• The patient has a known physical condition that would affect the conduct of the experiment

Study 3

• Having any orthopedic condition that prevents them from participating in the competition, such as arthritis or knee pain
• having a history of hamstring muscle strain in the knee within the past 3 months
• having an attendance rate of less than 80% in the subsequent eight weeks of training