Reactive oxygen and nitrogen species in skeletal muscle: acute and long-term effects

Low-level laser therapy reduces the fatigue index in the ankle plantar flexors of healthy subjects

Low-level laser therapy (LLLT) has been suggested as a resource capable of increasing resistance to fatigue and enhancing muscle performance through its metabolic and photochemical effects. This study evaluated the immediate effects of the application of LLLT on neuromuscular performance of the plantar ankle flexors in healthy subjects through a fatigue-induced protocol. This is a randomized controlled clinical trial, attended by 60 young and physically active volunteers of both genders. The subjects were randomly assigned into three groups, control, placebo, and laser, and underwent a preliminary evaluation of the isokinetic performance of plantar flexors and electromyographic activity of the soleus muscle to ensure homogeneity between groups. After the application of the respective intervention protocols, participants were induced to fatigue by performing 100 isokinetic concentric contractions of ankle plantar flexors at a speed of 90°/s. The dynamometric fatigue index (DFI) and median frequency were recorded during the fatigue protocol for comparison between groups. The group receiving the laser application showed significantly lower dynamometric fatigue index (p = 0.036) when compared to control and placebo groups. In relation to the median frequency during the fatigue test, there was a decrease in all groups, however with no differences between them. We suggest that LLLT being applied prior to exercise can reduce the fatigue index in the ankle plantar flexors of healthy subjects.

Increased hypertrophic response with increased mechanical load in skeletal muscles receiving identical activity patterns

It is often assumed that mechanical factors are important for effects of exercise on muscle, but during voluntary training and most experimental conditions the effects could solely be attributed to differences in electrical activity, and direct evidence for a mechanosensory pathway has been scarce. We here show that, in rat muscles stimulated in vivo under deep anesthesia with identical electrical activity patterns, isometric contractions induced twofold more hypertrophy than contractions with 50-60% of the isometric force. The number of myonuclei and the RNA levels of myogenin and myogenic regulatory factor 4 were increased with high load, suggesting that activation of satellite cells is mechano dependent. On the other hand, training induced a major shift in fiber type distribution from type 2b to 2x that was load independent, indicating that the electrical signaling rather than mechanosignaling controls fiber type. RAC-α serine/threonine-protein kinase (Akt) and ribosomal protein S6 kinase β-1 (S6K1) were not significantly differentially activated by load, suggesting that the differences in mechanical factors were not important for activating the Akt/mammalian target of rapamycin/S6K1 pathway. The transmembrane molecule syndecan-4 implied in overload hypertrophy in cardiac muscle was not load dependent, suggesting that mechanosignaling in skeletal muscle is different.

Concentrically trained cyclists are not more susceptible to eccentric exercise-induced muscle damage than are stretch-shortening exercise-trained runners

Here, we test the hypothesis that continuous concentric exercise training renders skeletal muscles more susceptible to damage in response to eccentric exercise. Elite road cyclists (CYC; n = 10, training experience 8.1 ± 2.0 years, age 22.9 ± 3.7 years), long-distance runners (LDR; n = 10, 9.9 ± 2.3 years, 24.4 ± 2.5 years), and healthy untrained (UT) men (n = 10; 22.4 ± 1.7 years) performed 100 submaximal eccentric contractions at constant angular velocity of 60° s(-1). Concentric isokinetic peak torque, isometric maximal voluntary contraction (MVC), and electrically induced knee extension torque were measured at baseline and immediately and 48 h after an eccentric exercise bout. Muscle soreness was assessed and plasma creatine kinase (CK) activity was measured at baseline and 48 h after exercise. Voluntary and electrically stimulated knee extension torque reduction were significantly greater (p < 0.05) in UT than in LDR and CYC. Immediately and 48 h after exercise, MVC decreased by 32 % and 20 % in UT, 20 % and 5 % in LDR, and 25 % and 6 % in CYC. Electrically induced 20 Hz torque decreased at the same times by 61 and 29 % in UT, 40 and 17 % in LDR, and 26 and 14 % in CYC. Muscle soreness and plasma CK activity 48 h after exercise did not differ significantly between athletes and UT subjects. In conclusion, even though elite endurance athletes are more resistant to eccentric exercise-induced muscle damage than are UT people, stretch-shortening exercise-trained LDR have no advantage over concentrically trained CYC.