Muscle residual force enhancement: a brief review

Muscle residual force enhancement has been observed in different muscle preparations for more than half a century. Nonetheless, its mechanism remains unclear; to date, there are three generally accepted hypotheses: 1) sarcomere length non-uniformity, 2) engagement of passive elements, and 3) an increased number of cross-bridges. The first hypothesis uses sarcomere non-homogeneity and instability to explain how "weak" sarcomeres would convey the higher tension generated by an enhanced overlap from "stronger" sarcomeres, allowing the whole system to produce higher forces than predicted by the force-length relationship; non-uniformity provides theoretical support for a large amount of the experimental data. The second hypothesis suggests that passive elements within the sarcomeres (i.e., titin) could gain strain upon calcium activation followed by stretch. Finally, the third hypothesis suggests that muscle stretch after activation would alter cross-bridge kinetics to increase the number of attached cross-bridges. Presently, we cannot completely rule out any of the three hypotheses. Different experimental results suggest that the mechanisms on which these three hypotheses are based could all coexist

Force produced after stretch in sarcomeres and half-sarcomeres isolated from skeletal muscles

The goal of this study was to evaluate if isolated sarcomeres and half-sarcomeres produce a long-lasting increase in force after a stretch is imposed during activation. Single and half-sarcomeres were isolated from myofibrils using micro-needles, which were also used for force measurements. After full force development, both preparations were stretched by different magnitudes. The sarcomere length (SL) or half-sarcomere length variations (HSL) were extracted by measuring the initial and final distances from the Z-line to the adjacent Z-line or to a region externally adjacent to the M-line of the sarcomere, respectively. Half-sarcomeres generated approximately the same amount of isometric force (29.0 ± SD 15.5 nN·μm(−2)) as single sarcomeres (32.1 ± SD 15.3 nN·μm(−2)) when activated. In both cases, the steady-state forces after stretch were higher than the forces during isometric contractions at similar conditions. The results suggest that stretch-induced force enhancement is partly caused by proteins within the half-sarcomere

Epilepsia e atividade física: estudos em humanos e animais

Epilepsy is the most common chronic neurological disorder in the world that influences negatively the quality of people's life affected by this disease. Although the favorable effect of physical activity on general health is unquestionable, the appropriate physical exercise for people with epilepsy is still controversial. Studies have shown beneficial effect of physical exercise on frequency of seizures as well as on life quality. However, people with epilepsy frequently are discouraged and excluded from participation in physical exercise due to the fear that the participation in a physical exercise program can precipitate epileptic seizures. Therefore, the aim of this study was to review the effect of physical exercise based on clinical and experimental studies of epilepsy.A epilepsia é considerada o distúrbio neurológico crônico mais prevalente no mundo, influenciando negativamente a qualidade de vida de indivíduos com epilepsia. Apesar do efeito favorável do exercício físico sobre a saúde ser inquestionável, a realização de um programa de exercício físico por pessoas com epilepsia ainda é um assunto controverso. Estudos têm mostrado efeitos benéficos do exercício físico na freqüência de crises assim como na qualidade de vida. Entretanto, indivíduos com epilepsia são freqüentemente desencorajados e excluídos da participação em programas de exercício físico pelo medo que tal participação possa precipitar crises epilépticas. Sendo assim, o objetivo desse estudo foi revisar o efeito do exercício físico baseado em dados de estudos clínicos e experimentais de epilepsia

Post-polio syndrome: renaissance of poliomyelitis?

Poliomyelitis is an acute and infectious viral disease, transmitted primarily through oral-fecal contact or directly, person to person. Approximately 90% of the individuals infected by the polio virus do not present symptoms; however, the affected individuals can show a variety of symptoms if the virus reaches the bloodstream. In up to 2% of cases, the virus reaches the central nervous system  preferably infecting and destroying the motor neurons, resulting in muscular weakness and acute flaccid paralysis. Despite the expressive reduction in the number of cases, many people live with the consequences of the acute illness, thus representing a burden to the public healthcare systems. Many of these people present new manifestations as signs and symptoms that are called post-polio syndrome. It can be defined and characterized by new neuromuscular symptoms, which occur at least 15 years after a period of clinical and functional stability in patients with previous history of symptomatic poliomyelitis. The signs and symptoms characterizing the post-polio syndrome include new muscular weakness, muscular fatigue and atrophy, pain in joints and muscles, sleep disorders, intolerance to cold, respiratory and swallowing difficulties, and recent weight gain. Therefore, the aim of this review is to present the physiological changes caused by the new manifestation of symptoms in individuals with poliomyelitis