TRANSLATIONAL APPROACH TO NEUROMUSCULAR FUNCTION AND ITS RECOVERY: EFFECT OF LIMBS, AGING, AND DISEASE.

Several factors can determine changes along the pathway from the brain to the muscles, and the interaction of different processes during exercise can ultimately determine neuromuscular function fatigability. Furthermore, the time course of neuromuscular function recovery after exercise is essential for planning repeated daily activities and sports. My PhD work aimed to compare the effects of different fatiguing exercises, populations, and interventions on neuromuscular function fatigability and recovery. Daily-living and sporting activities have different physical requirements, and the fatigability of different muscle groups is of interest. As a result, comparisons of the fatigability of upper- and lowerlimb muscles have been investigated, but there is a lack of information about muscle force loss and recovery following dynamic multi-joint exercise involving elbow flexors and knee extensors. The aim of the first study was to evaluate the magnitude and etiology of neuromuscular function changes in elbow flexors and knee extensors from dynamic multi-joint maximal incremental exercise. For this study 10 young, healthy, and physically active men were enrolled. Each participant completed one familiarization session and two experimental sessions consisted of a maximal incremental exercise to task failure on either an arm-cranking ergometer or a cycle ergometer. Before, exactly 1 min and 20 min after exercise cessation, neuromuscular function evaluation was conducted. The results showed that both maximal voluntary isometric force loss and decreases in evoked forces were not different between elbow flexors and knee extensors 1 min after task failure. However, 20 min after task failure, maximal voluntary isometric force, and low-frequency fatigue were greater in elbow flexors than knee extensors. These results suggest that exercise-induced recovery is muscle-specific. The neuromuscular function is affected by age and the changes within the neuromuscular system are associated with a decline in both muscle strength and mass. Since aging affects functional performance tasks such as stair climbing, ambulation, and rising time from a chair, affecting neuromuscular function, it is of interest to investigate muscle force and power fatigability. Sarcopenia, a highly prevalent disease in older than 65 years, exacerbates age-related changes in the neuromuscular system. The second study aimed to determine the mechanistic bases of the knee extensor muscles fatigability in healthy and sarcopenic old men and women elicited by high-velocity concentric contractions. Fifty-seven healthy old, 14 sarcopenic, and 21 young adults were recruited. Neuromuscular function assessment was performed before and immediately after an intermittent isotonic fatiguing exercise. Sarcopenia was associated with a reduction in knee extensor contractile properties, isometric torque, and power. Exercise-induced loss of isometric torque was not different between healthy and sarcopenic adults whereas power reduction resulted higher in sarcopenic III suggesting that the age-related changes in fatigability are contraction-specific and exacerbated by sarcopenia. Since no differences were observed for voluntary activation, the greater power reduction during exercise in sarcopenic adults may be due to intrinsic muscle impairment of contractile and/or metabolic nature. Many research activities have been addressed in counteracting the muscle weakness observed in older people. Since, aging seems to negatively affect plasma nitrate concentration [NO3-] and the potential benefits of dietary NO3- supplementation on skeletal muscle contractile function and muscle fatigue have been recently suggested, the third study aimed to evaluate the effects of prolonged NO3- supplementation on contractile properties and fatiguability of ankle plantar-flexor muscles in elderly subjects. Eleven elderly and 11 young men were recruited. Two main experimental sessions were performed after a 7-day supplementation period with either nitrate-rich beetroot juice or nitratedepleted beetroot juice. Neuromuscular function evaluation, comprising voluntary and electrically evoked isometric contractions of the plantar flexors, was performed on the dominant and nondominant leg. Our results report that after beetroot juice supplementation, young and elderly increased the plasma concentration precursors of nitric oxide. Improving nitric oxide bioavailability had potentially beneficial effects on fatigue resistance, calcium handling and skeletal muscle force production at submaximal forces in elderly people. Thus, the impairment in muscle contractile properties observed with aging can be positively affected by the increase in plasma concentration of nitric oxide precursors. Finally, independent of age, neuromuscular function can be affected by different diseases. Following acute Covid-19, a substantial proportion of patients showed symptoms and sequelae for several months, namely the post-acute sequelae of Covid-19 (PASC) syndrome. Major phenomena were exercise intolerance, muscle weakness, and fatigue. The aim of the fourth and last study of my thesis was to investigate the physiopathology of exercise intolerance in patients with PASC syndrome by structural and functional analysis several months after recovery. At least 3 months after infection, 11 non-hospitalized PASC patients and 12 patients without long-term symptoms visited the laboratory on four non-consecutive days. Spirometry, lung diffusion capacity and quality of life were assessed at rest. Muscle oxidative capacity recovery rate constant was assessed by repeated arterial occlusion by near-infrared spectroscopy. The cardiopulmonary incremental exercise test was performed. Neuromuscular function as knee extensor rate of torque development, maximal voluntary isometric contractions torque and electrically-evoked single and doublet twitches were assessed before, immediately and 10 minutes after 80 maximal velocity contractions with 20 % of the maximal isometric torque load. Histochemical analysis, quantification of key molecular markers of IV mitochondrial biogenesis, dynamics, and mitochondrial respiration were performed in vastus lateralis biopsies. Finally, specific isometric force and the no-load shortening rate of chemically skinned single isolated muscle fibers were measured. PASC patients showed reduced force-generating capacity and slower recovery after a dynamic fatiguing task compared to control subjects. These results seemed attributable to mild muscle atrophy and impairments in the recovery of excitation-contraction coupling, while actin-myosin bridge formation was not affected. Exercise intolerance of PASC patients was primarily due to impaired mitochondrial function as suggested by both functional invivo indexes of muscle oxidative metabolism and ex-vivo measurements. Finally, impairment in neuromuscular function recovery, along with mitochondrial dysfunction, suggests a delayed restoration of metabolic homeostasis after exercise in PASC. Overall, the work in this thesis shows how the combination of in-vivo measurements and ex-vivo analyses can be utilized to investigate impairments along the pathway from the central nervous system to the muscles, elucidating potential sites of alterations in both upper and lower limbs. Moreover, the results suggest that changes within the neuromuscular system are exacerbated by sarcopenia and disease (i.e. COVID) but can be counteracted by interventions such as dietary nitrate supplementation.