Previous studies have shown that the neuroplasticity of the residual corticospinal fibers, the motor cortex and the spinal neurons plays an important role in the spontaneous functional recovery of people with neurological or musculoskeletal pathology. However, it is also possible to stimulate the neuroplasticity mechanisms of these structures through techniques aimed at rehabilitating different deficits (for example, motor function or sensitivity). In general, intervention programs are usually carried out, in most cases, using low-cost strategies such as therapeutic physical exercise programs.

The objective of this study is to analyze the effectiveness of visual illusion therapies in combination with conventional exercises on the symptoms and signs related to incomplete spinal cord injury that affects the upper limb.

The study will include the realization of three measurements that will be carried out one day before starting the program, one day after finishing it, and one month later (follow-up).

The clinical assessment will be composed of the study of the following variables: Motor function and motor skills, Upper limb isometric force, Muscle activation, Muscle tone, Quality of life, Functionality. All interventions will last eight weeks and will be planned according to the availability of volunteers. In each session, it will be recorded if any type of adverse effect occurs. There will be four types of interventions: i. Visual Illusion (IV) and therapeutic exercise program (PE), ii.placebo and PE, iii. IV, iv. IV placebo.

Approximately 60% of people with a spinal cord injury have an incomplete injury. This causes alterations in strength, sensitivity, and muscle tone, which leads to a marked loss of functionality. This supposes a profound psychological and socioeconomic impact on the life of the affected person, in addition to the implicit physical impact.

After an injury of this type, there is a spontaneous functional recovery that may have a different path depending on the characteristics of the injury and the person. Previous studies have shown that the neuroplasticity of the residual corticospinal fibers, the motor cortex, and the spinal neurons play an important role in the spontaneous functional recovery of people with neurological or musculoskeletal pathology. However, it is also possible to stimulate the neuroplasticity mechanisms of these structures through techniques aimed at rehabilitating different deficits (for example, motor function or sensitivity). In general, intervention programs are usually carried out, in most cases, using low-cost strategies such as therapeutic physical exercise programs.

This nervous reorganization and recovery can also be influenced by the activity of mirror neurons, which are found in motor and premotor areas and also in other cortical and subcortical areas; and they report activation when they observe an action with a specific purpose. The activity of these neurons has been studied in different populations with involvement of the nervous and musculoskeletal system, such as cerebrovascular accidents, head injuries, Parkinson's, Alzheimer's, or carpal tunnel syndrome. For its study, experimental investigations have been carried out applying different interventions that modify the activity of mirror neurons, such as mirror therapy, virtual reality therapies, or Action-Observation therapies. Of these, in general, promising results have been obtained in all these populations, except in the case of Alzheimer's disease in an advanced stage.

However, these treatments have not been investigated in the recovery of the upper limb in people with incomplete spinal cord injury.

For all the above, the objective of this study is to analyze the effectiveness of visual illusion therapies in combination with conventional exercises on the symptoms and signs related to different pathologies of the nervous and musculoskeletal systems that affect the upper limb. Specifically, it is intended to know the impact of visual illusion therapy on:

• The motor function of the arm and hand.
• The functionality.
• The isometric strength of the upper extremities.
• Muscle tone, rigidity, and elasticity of the muscles of the arms.
• Neuropathic pain.
• The fine motor skills of the hand.
• The upper limb functionality.

Therefore, this study aims to provide knowledge to the scientific community on how neuroplasticity mechanisms and specifically mirror neurons can influence the rehabilitation of people with incomplete spinal cord injury and, in addition, propose a specific treatment methodology for the rehabilitation of the upper extremities in this type of population.

All procedures will be carried out under the principles of the Declaration of Helsinki (World Medical Association) and will be approved by the ethics committee of the University of Valencia. In addition, all participants will be asked to sign an informed consent that will include all the sections required by this committee.

A minimum of 20 participants per group is established, establishing a power of 80%, an ability to detect a small effect size (Coehn's d = 0.3) for the main variable mobility measured with Nedcodo Agile and a probability of type I error. of 0.05, resulting in a total of 80 participants. In order to minimize possible losses, 25% more patients will be added per group, so 100 people will participate in the study.

The study will include the realization of three measurements that will be carried out one day before starting the program, one day after finishing it, and one month later (follow-up). These will be carried out by team members who are blind to the assignment of the interventions. In the first evaluation, anthropometric and demographic data (i.e. height, weight, age, educational level, possible comorbidities, etc.) will be collected in order to control, in case of heterogeneity, these confounding variables.

The data analysis will be carried out using the SPSS statistical program (version 26). The normality of the sample will be analyzed using the Shapiro Wilk test and homoscedasticity using the Levene test. For the comparison between groups, a mixed factorial ANOVA will be used, and for the comparison between pairs of groups the Bonferroni correction will be used. In the event that there may be a confounding factor that meets the requirements to be analyzed as a covariate, an ANCOVA will be used. Statistically significant differences will be assumed when the p-value is less than 0.05.

All interventions will last eight weeks and will be planned according to the availability of volunteers. In each session, it will be recorded if any type of adverse effect occurs.

There will be four types of interventions resulting of a combination of:

• Visual Illusion (IV): the patient will be seated in a chair with a table in front of it. The front part of the trunk will be covered with a black blanket that will be attached to the table. On the table, you will see arms and hands projected performing different types of functional manual activities that will include mobility and strength tasks. The projected arms will be adapted to the dimensions of each subject so that the patient can recognize the projected arms as theirs. This program will last 10 minutes.
• Therapeutic exercise program (PE): Physical exercise program for the upper extremities will be carried out.

Inclusion Criteria:

• Incomplete cervical spinal cord injury (AIS C, D or E).
• Ability to understand instructions (Mini-Mental State Examination> 23 points)

Exclusion Criteria:

• Traumatic pathology in upper limbs.
• Other alterations of the central nervous system or peripheral
• Alterations in the vestibular system.
• Concomitant diseases.