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.

This randomized controlled trial is part of the QuantFlow research program, a multidisciplinary initiative designed to advance the understanding of patellar tendon pathology through the integration of quantitative ultrasound imaging, Doppler-based intratendinous vascularity assessment, shear-wave elastography, biomechanical performance testing, and clinical evaluation. Within this framework, the present trial evaluates the therapeutic, mechanobiological, and functional effects of a long-hold yielding isometric exercise protocol in competitive weightlifting athletes with vascularized patellar tendinopathy.

Patellar tendinopathy is a common overload-related condition in Olympic weightlifting due to the repetitive exposure to high mechanical loads during movements such as deep squats, cleans, jerks, and ballistic pulling actions. These demands can result in localized tendon pain, reduced load tolerance, and impaired athletic performance. Ultrasound imaging in symptomatic athletes frequently reveals increased tendon thickness, hypoechoic regions, altered fibrillar organization, and varying degrees of Doppler-detected intratendinous vascularity. Although the clinical relevance of vascularity remains debated, its presence may be associated with pain modulation and tissue remodeling. Understanding how targeted tendon-loading strategies influence vascularity, stiffness, structure, and symptoms is therefore central to this trial.

Long-hold yielding isometric loading is a specific exercise strategy characterized by sustained tendon loading for extended durations, typically lasting 30 to 60 seconds per repetition. Compared with short-duration isometric contractions, prolonged loading is thought to stimulate mechanotransductive pathways more effectively, potentially promoting collagen reorganization, modulation of tendon stiffness, and improvements in neuromuscular efficiency. Prolonged isometric loading may also induce short-term analgesic effects and influence intratendinous vascular responses. However, evidence supporting these mechanisms in high-performance weightlifting athletes is limited, and randomized controlled trials in this population are scarce.

The study follows a two-arm, parallel-group randomized controlled design with four assessment time points: baseline, post-intervention at 8 weeks, and follow-up assessments at 12 and 24 weeks. Participants are competitive weightlifting athletes aged 18 to 45 years who train at least three times per week and present with symptomatic patellar tendinopathy of at least six weeks' duration and a baseline VISA-P score below 80. Ultrasound confirmation of structural tendon changes or Doppler-detected vascularity is required. Key exclusion criteria include previous patellar tendon surgery, tendon rupture, recent injection therapies, systemic conditions affecting tendon health, significant concomitant knee pathology, pregnancy, or inability to comply with the study protocol.

Baseline assessments include pain evaluation during tendon-loading activities using the Numeric Pain Rating Scale, functional disability assessed with the VISA-P questionnaire, high-resolution B-mode ultrasound for tendon morphology, Doppler ultrasound for vascularity assessment, and shear-wave elastography for quantification of tendon stiffness. Neuromechanical performance is evaluated using force plate-based countermovement and reactive jump testing, providing measures such as jump height, force-time characteristics, impulse, and rate of force development. Maximal isometric force is assessed using standardized isometric testing procedures. All assessments follow standardized acquisition protocols to ensure reproducibility and inter-session reliability.

Participants are randomly allocated to either the intervention group or a usual-care control group using a computer-generated randomization schedule with allocation concealment. Outcome assessors for imaging and biomechanical testing are blinded to group assignment. The intervention group completes a long-hold yielding isometric exercise program consisting of three to five sets of three to five repetitions, each held for 30 to 60 seconds at a target discomfort level of 3 to 4 out of 10. Exercises are performed three times per week for eight weeks. The control group continues their usual training under standard tendinopathy management recommendations without exposure to long-duration isometric loading.

In addition to the primary and secondary outcome analyses, pre-specified exploratory subgroup analyses will be conducted stratified by biological sex (men and women) to examine whether sex modifies the response to the intervention. These analyses will be performed separately for key clinical, imaging, and biomechanical outcomes, including pain, functional status, tendon vascularity, tendon stiffness, and selected performance measures. The purpose of these analyses is exploratory and hypothesis-generating, and they are not intended to define additional independent outcome measures.

All primary and secondary outcome measures are reassessed at the post-intervention time point, with follow-up evaluations conducted at 12 and 24 weeks to determine the persistence of clinical, structural, and functional adaptations. Data quality is ensured through standardized operating procedures, routine quality-control checks, and secure data management practices. The study is conducted in accordance with international ethical standards for human research, including the principles of the Declaration of Helsinki and Good Clinical Practice.

Inclusion Criteria:

1. Competitive weightlifting athletes (Olympic weightlifting or CrossFit) training ≥3 times per week.
2. Age 18-45 years.
3. Symptom duration of at least 6 weeks.
4. Presence of vascularized patellar tendinopathy, defined as Doppler-detectable intratendinous neovascularization using standardized settings.
5. VISA-P score < 80 at baseline.
6. Ability to perform loaded lower-limb exercises safely.
7. Willingness to refrain from other structured isometric interventions focused on the patellar tendon during the study period.
8. Ability to provide informed consent.
9. Sex/Gender requirement: both men and women eligible; the study includes sex-stratified analysis to examine potential sex-based differences in tendon response.

Exclusion Criteria:

1. Previous knee surgery within the past 12 months.
2. Partial or complete patellar tendon rupture (confirmed clinically or by ultrasound).
3. Coexisting knee conditions (e.g., meniscal tear, ACL/PCL injury, patellofemoral joint instability) that significantly affect load tolerance.
4. Systemic diseases affecting tendon health (e.g., diabetes, rheumatoid arthritis, connective tissue disorders).
5. Corticosteroid injection in the patellar tendon within the last 6 months.
6. Participation in other clinical trials involving exercise or tendon-loading interventions.
7. Neuromuscular or medical conditions limiting safe performance of isometric or plyometric exercise.
8. Pregnancy.
9. Inability to commit to follow-up assessments at 8, 12, and 24 weeks.

De-identified individual participant data (IPD) derived from this study may be shared after completion of the primary analyses and publication of the main results. Shared data will be limited to curated and filtered analytical datasets containing derived quantitative variables used in statistical analyses, such as ultrasound-derived morphology, vascularity and elastography metrics, biomechanical outcomes, and clinical scale scores. Raw data sources, including original imaging files, force recordings and questionnaire forms, will not be shared.

All shared datasets will be fully anonymized in compliance with GDPR (EU 2016/679), Spanish LOPDGDD regulations and institutional ethical requirements. Access to de-identified IPD will be provided upon reasonable request, subject to review by the study investigators, and shared through secure institutional or trusted research repositories after publication of the primary study findings.

De-identified individual participant data and supporting documents will be made available after publication of the primary study results. Data will remain available for a minimum of five years following initial release.

Access to de-identified individual participant data and supporting information will be granted upon reasonable request for research purposes. Requests will be reviewed by the study investigators to assess scientific validity, ethical compliance and alignment with participant consent. Approved requests will receive access to curated and filtered analytical datasets only. Raw data sources, including imaging files and original questionnaires, will not be shared.

Only the outcomes assessors are masked to group allocation. Ultrasound technicians, elastography operators, and biomechanical evaluators remain blinded throughout all assessment time points (T0, T1, T2, T3). Due to the nature of the exercise-based intervention, neither participants nor care providers can be blinded. Investigators responsible for data analysis will work with anonymized, coded datasets to maintain allocation concealment during statistical procedures.