A Paradigm Shift in Musculoskeletal Rehabilitation Assessment After ACL Reconstruction Using Near-Infrared Spectroscopy

Assessment of muscle haemodynamics after musculoskeletal injury has traditionally relied on a limited number of near-infrared spectroscopy (NIRS) sensors placed on isolated muscles, without a simultaneous, multi-site approach. This study addresses that methodological gap in the context of anterior cruciate ligament (ACL) reconstruction.

This study aims to establish and compare muscle haemodynamic response patterns, measured using NIRS, in physically active individuals with ACL reconstruction at different time points after surgery, compared with healthy controls.

It is a multicentre, observational, case-control, cross-sectional study, conducted in three groups of males aged 18 to 65 years: ACL1 (less than 6 months since ACL reconstruction), ACL2 (more than 2 years since ACL reconstruction, with normalised physical activity and no longer undergoing rehabilitation), and a healthy control group with no history of ACL injury.

Each participant completes a single incremental interval cycling test on a cycle ergometer. During the test, 24 portable NIRS devices (Moxy) simultaneously and bilaterally record haemodynamic data at 12 muscle locations: rectus femoris (proximal and distal thirds), vastus medialis, vastus lateralis, biceps femoris (proximal and distal thirds), semitendinosus, gluteus maximus, tibialis anterior, gastrocnemius, medial deltoid, and biceps brachii; combining propulsive, stabilising, and postural musculature and reflecting a multi-joint, whole-body approach to the assessment of exercise-induced muscle haemodynamics.. Data are analysed using a proprietary multiparametric interpretation algorithm designed to evaluate the haemodynamic performance of multiple muscle groups simultaneously and comparatively. Assessors and statisticians remain blinded to group assignment throughout data collection and analysis.

Primary outcomes are muscle oxygen saturation, capillary haemoglobin concentration, and muscle blood flow, recorded continuously via NIRS at each of the 12 bilateral locations during exercise and through 2 to 3 minutes of post-exercise recovery (an individually variable recovery period). Secondary outcomes include heart rate (continuous Bluetooth monitoring), rating of perceived exertion (Borg CR-10 scale, 0-10), anthropometric and adiposity indicators (height, weight and body mass index), sociodemographic and clinical variables (age, toxic habits, injury and sports history, and physical activity in the preceding 48 to 72 hours), and study dropouts with their reason.

Sample size was calculated using G-Power, based on a moderate effect size (Cohen's d = 0.5) estimated from a study in the process of publication, with a 95% confidence level (Z = 1.96), 80% statistical power (β = 0.20), and a 10% non-response rate, yielding a required sample of 43 participants per group (total n = 129).

The study is conducted at four collaborating centers: Espai Físic de Salut Integral, Fisioplanet, Hospital Universitari Son Espases (HUSE), and Sant Joan de Déu Palma-Inca (SJD). It is approved by the Clinical Research Ethics Committee of the Arnau de Vilanova University Hospital (CEIC-3203)

General inclusion criteria

• Men aged 18 to 65 years.
• Adipose tissue thickness of less than 2 cm at the NIRS device placement sites, assessed with a skinfold calliper.
• Functional capacity to complete an incremental interval test on a cycle ergometer.
• Able to understand and communicate, both orally and in writing, in Spanish, Catalan or English.

General exclusion criteria

• Cardiometabolic, endocrine, neurological, or other severe comorbidity that could compromise participant safety, alter physical performance, or interfere with the interpretation of the results.
• Disabling pain preventing completion of the physical assessment as established in the protocol.

Eligibility criteria (LCA1 group)

• Physically active before the injury, with an exercise frequency greater than 3 days per week.
• Surgically treated with ACL reconstruction.
• Time since surgery of 6 months or less.
• Explicit authorisation from the physiotherapist or rehabilitation physician responsible for their recovery, confirming that the participant is able to safely tolerate the physical demands of the test.

Eligibility criteria (LCA2 group)

• History of ACL reconstruction surgery, with a minimum of 2 years since the intervention.
• Regular physical exercise practice (more than 3 days per week) during at least the 2 to 3 months prior to assessment.

Eligibility criteria (CON group)

• Regular physical exercise practice (more than 3 days per week) during at least the 2 to 3 months prior to assessment.
• No history of ACL injury or meniscal injury in either knee.