To analyze the effects of a high intensity exercises program by intervals on sarcopenia, active aging and oral microbiota in older adults.

All participants will be advised to lead healthy lifestyle habits according to the recommendations of the "Plan for the Promotion of Physical Activity and Balanced Eating of the Junta de Andalucía" http://www.juntadeandalucia.es/educacion/webporta/ishareservlet/content/86cf9950-abaa-403b-9fc3 9bc5ecae4614. A submaximal stress test will be performed at the beginning and end of the intervention. Before the test, the participants will become familiar with the measuring instrument with a warm-up that allowed them a gain in understanding the gear system. The test will start with a pedaling cadence of 50 revolutions, with an output power of 0 W, increasing by 25 W every 2 minute period. During the test there will be a continuous recording of blood pressure, heart rate and maximum oxygen consumption (VO2max). With this stress test, the aerobic and anaerobic threshold of each participant will be determined to mark the intensity of work at which each subject must perform physical activity in the intervention period.

The interventions of the 2 experimental groups are carried out with a frequency of 3 weekly trainings, for a total period of 16 weeks. The intensity of the training will be assigned to each subject individually based on the stress test performed in the previous phase. Experimental group 1 (HIIT): The HIIT protocol will be carried out after a 10-minute warm-up period with joint mobility exercises and dynamic stretching. The intervention will consist of training in which intense work intervals and active rest periods are combined using a cycle ergometer. The session will be divided into 4 intervals of 4 minutes duration at an intensity of 85-95% of the maximum heart rate [HRM], individually controlled during all sessions (Polar Team pro®), followed by active rest intervals 3 minute duration at 50-70% FCM. After the training, there will be a 10-minute cool down of gentle pedaling and joint range exercises. Experimental group 2 (MICT): Heating phase identical to the previous one. Subsequently, the participants will perform a training on the cycle ergometer with an intensity close to 70% FCM maintained for 40 minutes and will be controlled individually (Polar Team pro®). The session will conclude with a cool down phase identical to the previous one. Control group (CTRL): The participants will receive general advice on the positive effects of the regular practice of physical activity, and they will be given the guide of recommendations for the promotion of physical activity published by the Junta de Andalucía: http: // www.juntadeandalucia.es/salud/servicios/contenidos/andaluciaessalud/docs/130/Guia\_Recomendaciones\_AF.pdf

Inclusion Criteria:

• Age ≥ 65 years
• SARC-F ≥4 or handgrip strength <16 kg for women and < 27 kg for men.
• Not being afflicted by any condition preventing physical activity.
• Not being engaged in regular physical activity >20 minutes and >3 days/week.
• Being able to communicate without difficulty.
• Being able to read and understand the informed consent form as well as the goals of the study.

Exclusion Criteria:

• Acute or terminal illness.
• Unstable cardiovascular conditions.
• Conditions or medical treatments affecting the normal production of saliva (i.e. autoimmune disorders such as Sjögren's syndrome or medications like anxiolytics or antidepressants).
• Being or having been medicated with antibiotics up to one month prior to the taking of samples.
• Alcohol abuse or regular drug abuse.
• Severe visual or auditory impairment.
• Being involved in a different research group with the potential to interfere in our results.

The assignment to the groups will be of simple random type and hidden. Those responsible for admitting patients to the intervention phase will not know what group has been assigned each. This assignment will be made in advance by a researcher who will not intervene in the later stages of evaluation, intervention, data recording and elaboration of database. The assignment will be communicated through sealed and totally opaque envelopes.