It is an experimental, longitudinal prospective, controlled trial. The project will be developed in the General University Hospital of Valencia (CHGUV), and the University of Valencia.

Patients will be recruited from a previous cohort. The patients of this cohort will be contacted and proposed to participate. Those patients who accept, will be divided into the following experimental groups:

• Group 1: Patients in this group will perform the water exercise protocol, 3 times a week, for 12 weeks.
• Group 2: Patients in this group will not perform any type of exercise during the 12 weeks of the treatment.

Variables related to muscular quality by ultrasound, frailty, physical function, independence in the activities of daily life, balance, cognitive function, quality of life and sleep, the social sphere will be evaluated, and the intensity of pain and fatigue, before and after12 weeks of intervention.

OBJECTIVE To evaluate changes in muscle architecture by ultrasound, before and after an intervention with physical exercise directed in water in frail individuals; and determine the correlation of the image findings with the changes detected in their morbidity, functionality and quality of life.

SAMPLE

The sample will be composed of at least 40 frail men and women, according to Frail Phenotype criteria. Patients will be recruited from a previous cohort. For the assessment of frail, Linda Fried's criteria will be used: unintentional weight loss, feeling of general exhaustion, slow gait, low level of physical activity or muscular weakness. A point will be assigned to each positive criterion. A final score formed by the sum of the five criteria. According to this score, subjects will be classified as follows:

• Non-frail: 0 points
• Pre-frail: 1 or 2 points
• Frail: 3 or more points

STUDY DESIGN It is an experimental, prospective, controlled study. Patients will be recruited from a previous cohort performed at the General University Hospital of Valencia (CHGUV). This study was entitled "Muscular architecture study by ultrasound for the diagnosis of frail with correlation of clinical, physical and biochemical markers". The patients of this cohort will be contacted by telephone, where they will be proposed to participate in the present study. Patients that accept will be ask to undergo a muscle ultrasound. At that time those who meet the inclusion criteria and none of the exclusion criteria, will be presented with the option of participating in the targeted exercise intervention. The subjects will be divided into Group 1 (Exercise) or Group 2 (Control). Those who agree to participate will be given the informed consent, offering them the appropriate time to read it and resolve doubts about it.

All eligible subjects will be informed of the objectives of the study and will be asked to collaborate in it. Before the ultrasound examination, the patients will be explained that the procedure is experimental and what is the purpose of the study. Being a study with intervention, the need to maintain maximum adherence to the physical exercise regime during the 12 weeks, as well as the need to perform all assessments both before and after the intervention.

Variables related to the diagnosis of muscular quality by ultrasound, frailty condition, morbidity, physical function, independence in the activities of daily life, balance, cognitive function, quality of life and sleep, the social sphere will be evaluated, and the intensity of pain and fatigue, before and after the 12 weeks of intervention.

The muscular ultrasound will be performed in the Radiology Department of the Hospital. At the conclusion of the ultrasound, a muscle strength measurement will be performed with a dynamometer and patients will fill out a questionnaire of epidemiological data, medical history, comorbidities and, criteria of frailty. A second appointment will be scheduled to make a blood extraction for biochemical analysis. After concluding the intervention, patients will be called again to perform a follow-up ultrasound, questionnaires, physical function assessment and biochemical analysis.

INTERVENTION For the exercise in the water (aqua aerobics), there will be an expert physiotherapist in the field of hydrotherapy that will guide the exercises in controlled sessions of approximately 45 minutes. Exercises of joint mobility, balance, strength and coordination with / without aquatic material (tables, cork anklets, long corks, cork dumbbells, pull-boy, and inflatable collars, among others) will be included. At all times, qualified health personnel will monitor any symptom or sign that advises the removal of the subject of the study (maintained dizziness, hypertension or uncontrolled hypotension, chest pain, muscular or tendon injuries, etc).

MUSCULAR ULTRASOUND Studies of muscle architecture through ultrasound show a non-invasive method to detect isometric contraction of individual muscles. In addition to being non-invasive, accessible and economical, it allows the dynamic exploration of the muscle under study.

The ultrasound allows, on the one hand, assessment of the quantitative parameters at rest and contraction, measuring muscle thickness, anatomical and physiological area puncture angle and fascicular length. On the other hand, muscle intensity echo will be assessed as a qualitative parameter. The muscle architecture of the anterior compartment of the leg will be evaluated by ultrasound in B mode, obtaining transverse and longitudinal images with a linear transducer (6-15 MHz). The measurements of the muscle ultrasound will be made three times and the average of said measurements will be used in the data analysis. The images made during the scan will be stored in the hospital picture archiving and communication system (PACS).

DATA COLLECTION The first section of the questionnaire will include questions about general epidemiological data and details of the patient's medical history. The following parameters will be recorded: age, gender, marital status (single, married or widowed) and social situation (spouse, alone or with children). Among the anthropometric parameters, the height and the weighed of the participants will be measured to perform the calculations of the patient's body mass index. The presence or absence of hypertension, hyperlipidemia, diabetes mellitus, heart failure according to Framingham criteria, ischemic heart disease, presence of previous stroke, chronic obstructive disease, smoker (yes, no or ex-smoker) will be included in the patient's clinical history. Visual problems, hearing problems, Parkinson's disease, depression, previous cancer, kidney failure, arthritis, osteoporosis and number of recent fractures will also be assessed. In addition, an index of comorbidities will be carried out according to the Charlson index using an online calculator and number of falls in the previous 6 months. And finally, the number of visits to the primary care physician, emergencies and hospital admissions in the last 6 months.

ASSESSMENT OF PHYSICAL FUNCTION Various tests will be carried out to assess the physical function. Individuals will be asked the following requirements: comfortable clothing and footwear, light food (no fasting prior to the test), no intense exercise in the previous 2 hours, no present conditions that limit walking, and that may interfere with the interpretation of the test, such as injuries to the lower extremities or respect the schedules of the medication that the patient takes regularly. All tests will be conducted by a qualified physiotherapist. The test are 6-minute walk test, Borg Scale, Short Physical Performance Battery (SPPB), and "timed up and go" test. Likewise, different scales will be carried out to assess the independence, and activities of daily life, assessment of balance or risk of falls. For the assessment of cognitive function (Hospital Anxiety and Depression Scale (HADS), for the assessment of quality of life (Older People Quality of Life (OPQOL), and sleep (Pittsburgh Sleep Quality Index), for the assessment of the social sphere (Duke social support), and for the assessment of pain intensity (McGill Questionnaire) and fatigue (fatigue severity scale).

BIOCHEMICAL BIOMARKERS A blood extraction will be requested by the subjects general practitioners. A blood sample will be taken from the CHGUV clinical analysis department before and after the intervention. The biomarkers will be measured in a blood sample drawn between 8:00 a.m. and 10:30 a.m. after fasting for at least 8 hours and your usual procedure will be done with the protocols already established in the bio clinical analysis department.

STATISTICAL ANALYSIS The descriptive data are presented with the mean ± standard deviation (SD) with the distribution of the data verified by the Shapiro-Wilk and / or Kolmogorov-Smirnov normality test. To evaluate differences in physical characteristics and study variables, t-Student was used for parametric and Mann-Whitney variables for non-parametric variables. The evaluation of the different study variables according to the stages of fragility and control group will be determined by means of analysis of variance (ANOVA) for the parametric variables (followed by the Bonferroni post hoc test) and for the non-parametric Kruskal-Wallis test. To assess the sonographic changes after the intervention of the patients with directed exercise, the T-student test will be used for paired data. The correlations will be made with the Pearson coefficient for parametric data and the Spearman range for non-parametric data. The correlations were considered weak for the correlation coefficient of ± 0.01 to 0.35, moderate for ± 0.36 to 0.67 and strong for ± 0.68 to 1.00. To compare the reliability between sonographer and evaluators of functional tests; we perform the intraclass correlation coefficient (ICC) and the Bland-Altman agreement limits of 95%. The ICC values were classified as poor for values ≤0.20, just for 0.21-0.40, moderate for 0.41-0.60, good for 0.61-0.80 and very good for 0.81-1.00. To assess the accuracy of muscle ultrasound, the area under the receiver operating characteristic (ROC) curve or curves will be determined. The statistical significance will be defined a priori as P <0.05. All statistical analyses will be performed with Statistical Package for the Social Sciences (SPSS) version 24.0 for Windows.

Inclusion Criteria:

• Age between 65 and 95 years, be physically inactive (<150 min physical activity a week), meet 1 or more criteria of frailty according to Linda Fried criteria, signature of informed consent.

Exclusion Criteria:

• Institutionalized patients, life expectancy of less than 6 months for any reason, hospital admission in the last 3 months for any reason; major surgery in the last 6 months before the start of the study, oncological patient with active treatment with chemotherapy or radiotherapy; neurological or musculoskeletal pathology that contraindicates physical activity in water; severe disability (score below 15 points on the Barthel scale); severe cardiovascular disease, psychotic disorder or severe cognitive impairment.