Muscle haemodynamics has so far been assessed using at most 2-3 near-infrared spectroscopy devices on isolated muscles and without correlating variables. At the same time, research on musculoskeletal pathologies is limited, and there are no studies that analyse the haemodynamic changes that occur when the anterior cruciate ligament (ACL) is injured and whether these changes last over time. The aim of the study is to monitor and evaluate the physiological changes produced by ACL injury on muscle performance using non-invasive near-infrared spectroscopy (NIRS) devices in trained athletes. This descriptive cross-sectional observational multicentre multicentre study, with Balearic Islands ethics committee IB 5585/24PI, will include males aged 18-65 years who have undergone ACL surgery, divided into 3 groups that: have an evolution of less than 6 months (ACL1), more than 2 years since the surgical intervention and are already practising normal physical-sports activities (ACL2) and the control group (CON) that has never suffered an ACL affectation to date. The 3 groups will perform an incremental intervallic test (4:1) on a cycloergometer or treadmill (according to the specificity of their sport), where different muscular haemodynamic variables will be assessed using 12-24 NIRS devices. The main study variables will be the muscular haemodynamic values of each muscle analysed and their correlation between them: muscular oxygen saturation, absolute capillary haemoglobin, loaded and unloaded with oxygen and relative muscular haemoglobin blood flow, loaded and unloaded with oxygen. Secondary variables will be heart rate, socio-demographic variables (age, toxic habits, injury and sports history) and subjective perception of effort (RPE-20).

This study aims to investigate the physiological changes occurring in the musculoskeletal system during the rehabilitation process of ACL injuries using Near-Infrared Spectroscopy (NIRS) technology. This study explores how NIRS devices can monitor muscle hemodynamics during specific rehabilitation exercises, providing real-time insights into the muscle performance and recovery status of patients. The research focuses on athletes who have undergone ACL surgery, categorizing them into different groups based on the time since their surgical intervention (<6 months post-surgery and >2 years post-surgery) compared to a control group that has no history of ACL injury. The study follows a multicenter, observational, cross-sectional, descriptive design and is conducted across multiple healthcare centers, including Físic Espai de Salut Integral, Centre Re3MOVE, MET Salut, Hospital Universitari Son Espases, and San Juan de Déu Palma-Inca. These centers have been selected to ensure a diverse and representative sample of participants, focusing on sports rehabilitation specialists and integrating technology for evaluating functional recovery and muscle performance during rehabilitation.

The participants include male athletes aged 18 to 65 years, who are divided into three distinct groups. Group 1 (LCA1) consists of athletes who have undergone ACL surgery within the last 6 months and are currently undergoing rehabilitation. Group 2 (LCA2) consists of athletes who have had ACL surgery more than 2 years ago and have returned to their normal sporting activities. Group 3 (control group) consists of healthy athletes with no history of ACL injury. The primary objective of the study is to evaluate the physiological and hemodynamic changes in muscle function during rehabilitation and to establish how these changes persist over time. Additionally, the study aims to identify residual physiological limitations in the musculoskeletal system post-rehabilitation and surgery. The specific muscles of interest include the rectus femoris, vastus lateralis, gluteus maximus, hamstrings, and other stabilizing muscles in both the lower and upper body, depending on the mode of exercise used in the evaluation.

NIRS technology, which is non-invasive, will be employed to monitor muscle oxygen saturation (SmO2), total hemoglobin concentration (ThB), oxygenated hemoglobin (O2Hb), and deoxygenated hemoglobin (HHb) in real time. These measures will allow the researchers to assess the muscle's oxidative capacity, blood flow, and overall performance during rehabilitative exercises. The tests will be conducted using a cycle ergometer or treadmill, depending on the participant's sport, and will follow an intervallic incremental protocol (4 minutes of work followed by 1 minute of rest), increasing the workload incrementally until the participant reaches their maximum capability.

The methodology incorporates the use of 12 to 24 portable NIRS devices attached to the participants' muscles, allowing for simultaneous, real-time monitoring of muscle hemodynamics during exercise. The devices will be linked to a central software system (PeriPedal), which will collect and analyze the data, providing feedback to the researchers about muscle performance. Secondary outcomes of the study include heart rate monitoring, body composition (using bioimpedance and skinfold thickness measurements), and subjective perception of exertion using the RPE-20 scale. Sociodemographic data, such as age, injury history, and physical activity levels, will also be collected to contextualize the findings.

This study will adopt a pragmatic approach by incorporating the NIRS technology into routine clinical practice, aiming to create a more holistic understanding of muscle hemodynamics during ACL rehabilitation. The primary hypothesis of the study is that ACL injuries produce measurable changes in muscle hemodynamics that persist over time, even after successful rehabilitation. By identifying the factors that limit muscle performance in real-time, the study seeks to provide clinicians with new tools to tailor rehabilitation programs more effectively and reduce the incidence of recurrent injuries.

The findings from this study have the potential to transform the way musculoskeletal injuries are managed, particularly in terms of rehabilitation strategies and monitoring techniques. The study also aims to contribute to the broader understanding of how muscle performance is influenced by ACL injuries and rehabilitation, ultimately improving patient outcomes in both clinical and athletic settings.

In conclusion, the HemoNIRS project is an innovative approach to understanding the physiological and hemodynamic changes in the musculoskeletal system following ACL injuries, providing real-time data that can be used to optimize rehabilitation processes and enhance overall muscle performance.

General inclusion criteria

• Men aged 18 to 65
• An adipose layer of <2 cm in areas of NIRS placement, assessed with a plicometer
• Understand written, read and spoken Spanish, English or Catalan.

General exclusion criteria

• Having any cardio-metabolic, hormonal, neurological or other comorbidities that could affect participation in the study (e.g. severe mental illness, end-stage disease).

Eligibility criteria (LCA1 group)

• Regular athlete with training and/or competition >3 days/wk prior to injury.
• ACL injury <6 months after surgery.
• Able to perform an incremental intervallic test on a cycloergometer or treadmill.

Eligibility criteria (LCA2 group)

• Regular athlete with training and/or competition >3 days a week.
• ACL injury >2 years after surgery.
• Able to perform an incremental intervallic test on a cycloergometer or treadmill.

Eligibility criteria (CON group)

• Regular athlete with training and/or competition >3 days a week.
• Never suffered an ACL injury in any knee - Able to perform incremental intervallic test on cycleergometer or treadmill.

All the groups exclusion criteria

- Incapacitating pain that prevents assessment

The sample, which will be selected by convenience, will be made up of participants who have a diagnosis of ACL involvement and who have undergone surgery (total or partial), either recently or have had an injury in previous years, may be recruited to participate in the study. Healthy, physically active patients who have never had an ACL injury will also be recruited.

The sample will initially come from the Físic Espai de Salut Integral, Centre Re3MOVE, MET Salut, Hospital Universitari Son Espases (HUSE) and San Juan de Déu Palma-Inca centres. Other private and public physiotherapy, rehabilitation and recovery centres, as well as private entities (clubs, federations, associations and organisations) that may contain participants susceptible to being included in the study, are expected to be included as they are accepted by the respective research commissions.