Effects of muscle disuse/hospitalization and physical training/retraining on the neuromuscular function in healthy older adults: iso-inertial resistance exercise training as a potential countermeasure form

Population ageing is a global phenomenon and it is causing many concerns regarding the economic, social and health consequences that it might bring to individual countries. Ageing is associated with the decline of the physical performance and the greater risk for the occurrence of physical disabilities. Everyday tasks require good levels of muscle performance, such as muscle force and power abilities. Hence, older people, due to their reduced physical capability levels, might have difficulties in performing these daily actions. Even the walking ability might result impaired in this population which might be associated with a loss in physical independence and, in extreme cases, death. Physical performance deterioration depends on primary ageing processes and lifestyle habits, as well. Within the lifestyle factors, periods of inactivity (i.e. defined as periods where muscles are not stimulated adequately, muscle disuse) are frequent in older people and this might exacerbate performance deterioration in this population. Indeed, muscle disuse causes further impairments in muscle force and power abilities which, added to the intrinsic alterations brought about by ageing processes, might influence everyday tasks to a greater extent. However, influences of muscle disuse on the walking ability in older people are not well studied. Weather muscle disuse can produce negative consequences to this well established and natural motor action in older people is investigated in this thesis. Muscle disuse effects on muscle power and force of the lower limb in older people are studied as well in the current work. Furthermore, to better understand how muscle performance changes with muscle disuse, investigations on the motor control, muscle volume and muscle fibers characteristics accompany the previous analyses. Information about how muscle disuse influences the neuromuscular function of older people might be of clinical importance. Indeed, muscle disuse might be associated to hospitalization periods due to health problems. Interventions that preserve the deterioration of muscle performance before and even after hospitalization might be required to rapidly reintegrate older patients into society. Thus, studying the effects of potential countermeasures, that might be involved before and after hospitalization, on the neuromuscular function of older individuals, is another theme that the current thesis faces. In particular, physical training has been chosen as the promising countermeasure that might help older people to recover from hospitalization or even other muscle disuse periods. As muscle power performance decreases to a greater extent compared to muscle force due to both ageing and muscle disuse conditions, attention is given to a particular form of resistance training: the flywheel iso-inertial resistance training. Although this training has been shown to call for greater muscle power improvements, compared to traditional gravity dependent trainings, its effects on other neuromuscular function features (i.e. muscle force, architecture, etc.) and on the walking ability are not studied in depth. Hence, the current thesis provides new evidence on this topic by further compared it with traditional resistance training adaptations.