Considering that the basic psychological needs are 3: competence, autonomy and social relationships, the Psychological Need Satisfaction in Exercise Scale (Wilson et al., 2006), composed of eighteen items (six for each of the dimensions) will be used to assess the degree of satisfaction with these basic psychological needs. For its completion, a scale of 1 to 6 will be used, with 1 being "totally false" and 6 "totally true".

Spinal pathologies, including back pain, are among the most common in recent decades, due to the lack of postural hygiene caused by postures maintained over time. Back pain refers to any of the areas that make up the back (cervical, dorsal, lumbar and shoulder areas). With the Nordic Questionnaire (Martínez and Alvarado, 2017), the aim is to obtain information on the regions in which adolescents present back pain. For this purpose, the questionnaire includes images that specify the place to which each region of the back refers, and questions related to the decrease in autonomy caused by pain in those regions.

The KIDMED questionnaire (Serra-Majem et al., 2004) will be used to establish the level of adherence of adolescents to the Mediterranean diet. It consists of 16 questions that are scored with a value of -1 if they denote a negative connotation with respect to the Mediterranean diet, or with a +1 if they are positive in relation to the diet. The index ranges from 0 to 12 points and the results are classified as: optimal Mediterranean diet (>8 points), requires improvement to adjust intake to Mediterranean patterns (4-7 points), and very low diet quality (≤ 3 points). The questions included in the questionnaire are related to the main foods included in this type of diet (fruit, vegetables, bread, etc.).

The 20-m shuttle run test evaluates maximal aerobic capacity by means of an incremental 20-meter out-and-back test. The initial speed of the test is 8.5 km/h and is increased every minute by an audible signal. The test ends when the subject stops or fails to reach the reference line two consecutive times. The speed recorded is the speed reached in the last stage that the subject is able to complete. Through the equations of Leger, Mercier, Gadoury and Lambert (1988) the reference value of the subject's VO2 max. is obtained.

The test will provide information about the isometric force (kg) that the participant is able to develop with the forearm musculature. For this purpose, a digital dynamometer with manual grip (5030J1, Jamar ®, Sammons Preston, United Kingdom) will be used. Prior to the execution, the instrument will be adjusted according to the age of the player and the length of his/her hand. Two measurements will be taken with each arm, with a one minute rest between them. The participant must hold the dynamometer with the executing hand and the arm extended along the torso, without contacting it, and produce the maximum force possible (Rojas, Vázquez, Sánchez, Banik, & Argáez, 2012; Valdes & Yanci, 2016). Manual grip strength determined by dynamometer is significantly associated with proper cardiovascular health in adolescents, so its analysis is of interest as part of an assessment of global physical fitness (García-Hermoso et al., 2019).

The countermovement jump test (CMJ) will be used to measure this variable. This test provides information on the explosive strength of the lower body. The CMJ is a vertical jump that begins with the subject in a standing position, followed by knee flexion to a position of approximately 90 degrees, and a rapid extension of the knees to perform a jump in which the maximum height is reached. It should be noted that there is no stop in the knee flexion position since the intention is to take advantage of the explosive elastic reflex energy of the movement. The subjects will perform two repetitions of the jump 2 minutes apart. A MuscleLab force platform (Ergotest Innovation S.A., Norway) will be used to record the data, which will measure the time of flight from the time the subject takes off from the platform to the time the subject re-enters contact with the platform.

The sit-and-reach test will be used. A 30-centimeter measuring box (Finder Flex-Tester, Novel Products, USA) with a millimeter ruler is required. In the initial position, the subject should be in a seated position, with knees fully extended, feet hip-width apart and soles of the feet perpendicular to the floor and in contact with the box. The subject will be asked to place one hand on top of the other, with the palms of the hands facing downward and the fingers stretched out. The subject will perform a neck flexion to bring the chin to the chest and a trunk flexion with the knees fully stretched and without ballistic movements, trying to reach the greatest possible distance and maintaining the posture for 3 seconds. The distance will be measured in centimeters and the value 0 will correspond to the tangent of the soles of the feet, the values being positive when this distance is exceeded, and negative when it is not reached.

To measure height, a SECA measuring rod should be used. Adolescents should stand barefoot with their backs against the measuring rod, heels of their feet together and their gaze straight ahead. Measurements will be obtained in centimeters and will be repeated 2 times for each subject. A third measurement will only be taken if the difference between the first and second measurement is greater than 1%.

A Tanita BC 418-MA Segmental scale (Tanita, Tokyo) will be used to measure body weight. The subject will step on the scale with both feet inside the scale and looking straight ahead, without leaning the body. Values will be obtained in kilograms and two measurements will be carried out on each subject. If the difference between both measurements is greater than 1%, a third measurement will be taken.

To obtain the fat mass, it will be necessary to previously measure the triceps, thigh and leg folds, and the waist, hip, relaxed arm, thigh and leg perimeters. For these previous measurements, two measurements will be taken in each subject, and a third measurement will be necessary if the difference between the first two is greater than 5%. Once the values of the previous measurements have been obtained, the fat mass can be calculated using the formula of Slaughter et al. (1988)

To obtain the fat-free mass, it will be necessary to previously measure the triceps, thigh and leg folds, and the perimeters of the waist, hip, relaxed arm, thigh and leg. For these previous measurements, two measurements will be taken in each subject, and a third measurement will be necessary if the difference between the first two is greater than 5%. Once the values of the previous measurements have been obtained, the-fat free mass can be calculated using the formula of Poortmans et al. (2005)

Previously obtained height and weight measurements will be used to establish the BMI. The formula used will be weight/height^2, and the result will be obtained in kg/m^2. This index is commonly used and is related to metabolic and cardiovascular diseases in adolescents and adults.