The intervention will be a physical exercise program for colorectal cancer patients during the adjuvant chemotherapy. The exercise program aims on reducing the side effects of the treatment and improving patients' quality of life. In addition, the investigators try to improve endurance and resistance training level, in order to achieve greater physical functionality, survival and general well-being. For this, the investigators will carry out an exercise program based mainly on muscular strength and cardiorespiratory condition. It will last 6 months, with a frequency of 3 days per week, including sessions of 60 minutes. Sessions will consist of three parts: warm-up, main part (endurance and resistance training), and cool down. An individualized and supervised progression of training will take place. The intensity levels will always be adjusted to the initial levels of the participants, always considering their preferences and comfort.

Participants' preferences and exercise history will be considered through an initial interview. Motivational strategies based on self-determination theory will be applied, since it is one of the most used theories in the field of physical exercise. This theory proposes that all people need to feel competent, autonomous and socially related. If these three needs are satisfied, participants will develop more positive (autonomous) forms of motivation, which are related to better consequences such as vitality, enjoyment, quality of life and adherence to physical activity. In addition, motivational strategies will be applied for families and healthcare professionals.

Before starting the program and at the end of it, each eligible patient will be evaluated through:

• Physical activity: strength test of lower and upper limbs, agility test, stress test for cardiorespiratory fitness, physical activity levels, physical condition and body composition.
• Psychological factors: autonomy support, basic psychological need satisfaction, motivation, quality of life, perceived barriers, depression, anxiety, hope, quality of life.
• Clinical parameters: survival rate, side effects, biological factors, treatment delays and planned treatment completion.

Objectives and justification of the project In Spain, cancer is the second cause of death after cardiovascular diseases. However, if it is divided by sex, cancer is the leading cause of death for men and the second cause for women. In recent years there has been an increase in new cases of cancer due to population growth and aging, since 2/3 of cancer patients are older than 65 years. In addition to the impact of this disease on people's quality of life, a high economic cost has been estimated for the health system. For example, the total cost of cancer in 2015 in Spain was 7,168 million euros. This situation represents an important public health problem and therefore implies that research on the factors that can prevent or alleviate this disease should be considered as a priority.

In this regard, there is increasing scientific evidence of the physical and psychosocial benefits of exercise both during and after the treatment of this disease. Nevertheless, only 30-40% of cancer survivors perform the necessary physical activity (PA) recommended by the main medical institutions. In fact, a significant number of patients are obese (33%) and have a low cardiorespiratory condition (45%). Despite this, PA continues to decrease after diagnosis and only 5-10% of patients are physically active during treatment and 20-30% after treatment. Therefore, the development of intervention programs aimed at promoting PA should be a fundamental objective in the fight against cancer and its physical and psychosocial consequences. In this line, our general objective will be to develop a motivation PA program for colorectal cancer patients. Specifically, and considering the gaps existing in the current literature, our general objective is subdivided into the following specific objectives:

• To evaluate the physical, psychosocial and behavioural effects, in the short and medium term, of a physical exercise program in which motivational strategies based on self-determination theory are applied.
• To explore qualitatively the benefits of the physical exercise program, as well as the troubles encountered in its implementation.
• To develop an educational guide and an intervention protocol to encourage motivation and adherence to PA of colorectal cancer patients.

Methodology This study will require a sample size of 34 patients. This calculation was made through the G*power software, by means of an analysis of variance (ANOVA), adjusting the values to p <.05, with a statistical power of 80% and an effect size of 0.5. The patients will be recently diagnosed with colorectal cancer in phases II-III by the oncology service of the Hospital Universitario Puerta de Hierro de Majadahonda. Inclusion criteria include to be between 18 and 75 years old, to be undergoing chemotherapy treatment, and to not have any type of associated pathology that prevents them from performing PA. A intervention with an experimental group and a control group will be conducted, where 17 patients will be assigned to a control group, and 17 patients will perform a supervised motivational exercise program.

Measures Cardiorespiratory fitness. Stress tests adapted to the characteristics of the patients will be performed, both in the type of ergometer and in the protocol. We will use electrocardiographic recording and gas exchange analysis. Aerobic and anaerobic exercise thresholds will be established.

Quality of life. The validated Spanish version of the EORTC-QLQ-CR30 will be used for patients with colorectal cancer.

Body composition. Anthropometries will be carried out to estimate body composition parameters.

Biological parameters. The usual analytics carried out in the hospital during the treatment will be used to check the patient's health status and the side effects of the chemotherapy treatment.

Anxiety. The validated version in Spanish of the Hamilton Anxiety Scale will be used.

Depression. The Spanish version of the Hamilton Depression Scale will be used. Autonomy support. The Spanish version of the Perceived Autonomy Support for Exercise Settings will be used.

Perceived barriers to do PA. This scale will be translated and validated to the Spanish context.

Basic psychological needs in exercise. We will use the Basic Psychological Needs in Exercise Scale.

Basic psychological needs in life. The Spanish version of the Basic Psychological Need Satisfaction and Frustration Scale will be used.

Types of motivation. To measure motivation towards PA, the Behavioural Regulation in Exercise Questionnaire will be for PA levels. The Spanish version of the IPAQ questionnaire will be used. On the other hand, PA will be measured in a direct and objective way through portable GT3X accelerometers.

Design, procedure and data analysis The intervention program will be carried out at the Hospital Universitario Puerta de Hierro of Majadahonda. The program will last 6 months, with three weekly one-hour sessions, and will comply with the recommendations established by the American Cancer Society and the American College of Sports Medicine. The safety precautions and contraindications identified by these institutions for colorectal cancer patients will be especially considered. The program will include aerobic sessions at moderate and vigorous intensities, as well as strength work. Patients will be monitored to model exercise loads individually.

The measurements will be made just before starting the PA program (T1) in the case of the experimental group, at an initial time of the equivalent treatment in the case of the control group. The same measures will be carried out 6 months later (T2). For PA levels, instead of measuring them just after the end of the program, we will wait another month so that patients have been able to recover their usual PA levels after the shock that can mean for them to face the end of our program. In the control group, PA levels will also be evaluated one month after T2. To test the effects of the program, all measurements will be performed 6 months after T2 in both groups (T3).

Additionally, qualitative methodology will be used, so that the 17 patients of the experimental group will be interviewed to know the factors related to their adherence to the PA program and the benefits perceived by them with their participation. Qualitative data collection will be reinforced with field notes compiled in research diaries by observers who will be present during the development of the program. Relatives and health professionals will also be interviewed to learn their perspective regarding the benefits of the program and adherence to it in these patients.

In order to promote adherence to the program and to educate in autonomous PA habits when it ends, motivational strategies based on the self-determination theory will be used. Specific strategies will be developed for this group, based on previous works and educational guides for colorectal cancer patients. The instructor in charge of directing the program will be trained in several sessions to use motivational strategies based on this theory. Once the program is finished, an educational guide of motivational strategies for the promotion of PA in patients with colorectal cancer will be developed and published from the perspective of self-determination theory.

Inclusion criteria:

• Being more than 18 years
• Colorectal cancer stage II and III
• Be receiving adjuvant chemotherapy treatment after surgery
• Ecog > 1
• No medical contraindications

Exclusion criteria:

• Have a disability
• Colorectal cancer stage IV
• Presence of metastasis
• Ecog < 1