The goal of this clinical trial is to evaluate whether a long-hold yielding isometric exercise program improves pain, tendon vascularity, mechanical properties, and functional performance in weightlifting athletes with patellar tendinopathy. The study will also explore the safety and feasibility of this exercise strategy.
Researchers will compare the long-hold isometric intervention with routine training recommendations to determine its effects on symptoms and tendon function.
Participants will perform the supervised isometric protocol and complete pain ratings, functional tests, and ultrasound evaluations of the patellar tendon.
Competitive weightlifting athletes (Olympic weightlifting or CrossFit) training ≥3 times per week.
Age 18-45 years.
Symptom duration of at least 6 weeks.
Presence of vascularized patellar tendinopathy, defined as Doppler-detectable intratendinous neovascularization using standardized settings.
Previous knee surgery within the past 12 months.
Partial or complete patellar tendon rupture (confirmed clinically or by ultrasound).
Coexisting knee conditions (e.g., meniscal tear, ACL/PCL injury, patellofemoral joint instability) that significantly affect load tolerance.
Systemic diseases affecting tendon health (e.g., diabetes, rheumatoid arthritis, connective tissue disorders).
Parallel
Long-Hold Yielding Isometric Exercise Program
BehavioralThe intervention consists of a structured long-hold yielding isometric (LH-YI) loading program targeting the patellar tendon. Participants perform 3-5 sets of 3-5 repetitions, each held for 30-60 seconds at an intensity corresponding to 3-4/10 discomfort. Exercises are executed in a controlled knee-flexion position, either as a yielding squat or machine-assisted isometric hold. Training takes place three times per week for 8 weeks, with one supervised session weekly to ensure correct load, technique, adherence, and safety. The protocol aims to induce mechanotransductive adaptation, reduce pain, modulate tendon vascularity, and improve tendon stiffness and neuromechanical performance.
Usual-Care Training Condition
Other interventionParticipants maintain their regular weightlifting training following standard recommendations for patellar tendinopathy management. This includes load monitoring, symptom-guided progression, avoidance of high-irritability movements, and general strengthening. No long-duration isometric exercises or targeted LH-YI loading are permitted. This condition serves as an active comparator reflecting real-world practice to isolate the specific therapeutic and mechanobiological effects of the long-hold yielding isometric protocol.
Functional Disability (Victorian Institute of Sport Assessment Scale for Tendinopathy, VISA-P Score)
Patellar tendon-related functional impairment assessed using the Victorian Institute of Sport Assessment for Patellar Tendinopathy (VISA-P) questionnaire (0-100). Higher scores indicate better function. This measure evaluates global clinical improvement and load tolerance in daily living and sports context.
Pain During Tendon-Loading Activities (Numeric Pain Rating Scale)
Pain intensity during tendon-loading activities, such as squatting or jumping, assessed using the Numeric Pain Rating Scale (NPRS, 0-10), where 0 represents no pain and 10 represents the worst pain imaginable. Higher scores indicate greater pain. This outcome reflects symptom severity and clinical response to the intervention.
Patellar Tendon Vascularity (Surface Area Quantification)
Quantitative assessment of intratendinous vascularity using Surface Area Quantification (SAQ) derived from power Doppler ultrasound images. Doppler images are processed using image segmentation in Fiji/ImageJ software to isolate Doppler-positive pixels within a predefined tendon region of interest. Vascularity is expressed as the percentage of Doppler-positive pixel area relative to the total tendon area analyzed, with higher percentages indicating greater intratendinous vascularization.
Patellar Tendon Vascularity (Öhberg Score)
Intratendinous neovascularization assessed using the Öhberg grading scale based on power Doppler ultrasound imaging. The scale ranges from 0 to 4, where 0 indicates absence of Doppler-detected vessels and higher scores indicate increasing degrees of intratendinous neovascularization. Higher scores represent greater vascularity.
Tendon Stiffness (Shear-Wave Elastography in kPa)
Patellar tendon mechanical stiffness quantified using shear-wave elastography and expressed in kilopascals (kPa). Shear-wave elastography provides a continuous quantitative measure of tissue shear modulus, with higher kPa values indicating greater tendon stiffness. Three repeated elastography measurements will be obtained at each assessment and averaged for analysis. Changes in tendon stiffness will be evaluated by comparing values across assessment time points.
Tendon Stiffness (Shear-Wave Velocity)
Patellar tendon mechanical stiffness quantified using shear-wave elastography and expressed as shear-wave propagation velocity in meters per second (m/s). Shear-wave velocity provides a continuous quantitative measure of tissue stiffness, with higher velocity values indicating greater tendon stiffness. Three repeated elastography measurements will be obtained at each assessment and averaged for analysis. Changes in tendon stiffness will be evaluated by comparing values across assessment time points.
Tendon Mechanical Properties: Stiffness (iPulley and Ultrasound)
Patellar tendon mechanical stiffness in newtons per millimeter (N/mm) will be quantified using synchronized force and ultrasound-based tendon elongation measurements obtained during isometric loading tasks. Tendon force-elongation curves will be generated, and stiffness will be calculated as the slope of the linear regression between tendon force and tendon elongation within the linear region of the loading curve, defined between 50% and 100% of the maximal tendon force. Higher stiffness values indicate greater resistance of the tendon to deformation under tensile load.
Maximal Isometric Force (IMTP or iPulley)
Peak isometric force and force-time characteristics measured using an isometric mid-thigh pull setup (TrueStrenght, Hawkins Dynamics) or the iPulley system.
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