Frailty syndrome is a complex aging expression determined by ontogenetic and phylogenetic factors. Chronic stress has been shown to have immunosuppressive effects, to accelerate immunosenescence and to cause cumulative disorders in many physiological systems, resulting in frail state. In a recent approach, Linda Fried and colleagues have developed a construct whose bases are muscle loss, negative energy balance and physical inactivity, called 'Frailty Cycle'. They identified five dimensions in the construct: weakness, low resistance to an effort, slowness, low physical activity, and weight loss, which were operationalized on five criteria to identify the Physical Frailty (PF), and divide the population in frail, pre-frail and non-frail. Recently, epidemiological studies reported that cognitive impairments, low immune expression, and others global health dimensions have a powerful association with physical frailty. However, there is a need for the search for new correlated markers for the frail condition, for a better understanding of the phenomenon. On the other hand, exercise has been shown as a co-adjuvant treatment to have positive effects on several factors linked to physical frailty (e.g. improve immunity and prevent chronic diseases), because of it's potential effect on hormonal mediation. Looking at Fried PF Phenotype construct, their dimensions share biological 'commonalities' that can be explained by studying the biopsychological mechanisms with exercise being a key factor in the study of these relationships.

The current research was designed to investigate and characterize the prevalence of the PF in a cross-sectional Portuguese samples (institutionalized participants), to examine the relationship between PF and each one of the general health status domain such as physical fitness and functioning status; neuroendocrine and immune parameters; psychological and cognitive ability of these populations; and to verified the impact of different types of exercise in each domain of general health status. However, this doctoral thesis is presented in the form of articles, divided into five sections and their respective chapters. In total, 3 preliminary studies (2 systematic reviews of studies 1 and 2 and one exercise-intervention pilot study 3), 5 cross-sectional studies (4,5,6,7, and 8) and 3 intervention studies (9,10 and 11) were completed. The cross-sectional design consisted of the assessment of 140 older women (≥75 years old), living in different centres of heath care and social support, located in the city of Coimbra, Portugal. The participants were selected using a non-probabilistic convenience sampling based on the geographical area of the center region of Coimbra city.

This research protocol was planned for approximately 13 months, involving a trained research team and it is built in different phases, according to the type of studies: cross- sectional and experimental. The cross-sectional study (4 weeks' duration) consisted in the evaluation of older people (65 years old), living in different centres of heath care and social support (HCS), located in the city of Coimbra, Portugal. The participants were selected using a non-probalistic convenience sampling based on the geographical area of center region of Coimbra city. The experimental phase characterized as an intervention study with two different chair-based exercise programs, designed to assess the effect of strength/elastic band and multimodal interventions on biopsychological outcomes in frail e pre- frail individuals. Participants in these groups was attended a 45 minutes' exercise session, two-three times per week during 28 weeks and were extracted using the Physical Frailty status from the cross-sectional study design.

Inclusion Criteria:

• inclusion criteria: a) ability to participate in a study with total duration of 12 months and encompassing three phases: cross-sectional study, intervention and detraining periods b) existence of an appropriate physical space to carry out the exercise sessions; c) required support of caregivers to assist with the elders' displacement to the exercise classes;

Exclusion Criteria:

• The following selection criteria for participants included was adopted: a) take a part in this study spontaneously; b) drug therapy and clinical condition controlled and updated (it must be stable and enable participation in the exercise classes as decided by local medical staff); c) not completing the '8-foot-up and go test' in the maximum time of 50 seconds.