Albertas Skurvydas

Children are more susceptible to central fatigue than adults

Performance in high‐intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19–27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12–14 years). The volunteers (n = 7 in each group) performed three 5‐s maximal voluntary contractions (MVCs) and a continuous 2‐min MVC. The VA and fatigue of the muscles was assessed by applying 250‐ms 100‐HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2‐min MVC, VA in boys and girls was more depressed than in adults. The end‐exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are more susceptible to central fatigue than adults. This should be taken into account when evaluating results of fitness tests that require high levels of motor unit activation. Muscle Nerve, 2007

The effect of two kinds of T-shirts on physiological and psychological thermal responses during exercise and recovery

The aim of this study was to investigate the physiological and psychological responses during and after high-intensity exercise in a warm and humid environment in subjects wearing shirts of different fabrics. Eight healthy men exercised on two separate occasions, in random order, wearing two types of long-sleeve T-shirt: one made of polyester (PES) and the other of cotton fabric (CT). They performed three 20 min exercise bouts, with 5 min rest between each, and then rested in a chair for 60 min to recover. The ambient temperature was 25 °C and relative humidity was 60%. The exercise comprised of treadmill running at 8 km/h at 1° grade. Rectal temperature, skin temperatures at eight sites, heart rate, T-shirt mass and ratings of thermal, clothing wettedness, and shivering/sweating sensation were measured before the experiment, during the 5 min rest period after each exercise bout, and during recovery. Nude body mass was measured before the experiment and during recovery. The physiological stress index showed that the exercise produced a state of very high heat stress. Compared with exercise wearing the CT shirt, exercise wearing the PES fabric produced a greater sweating efficiency and less clothing regain (i.e., less sweat retention), but thermophysiological and subjective sensations during the intermittent high-intensity exercise were similar for both fabrics. However, skin temperature returned to the pre-exercise level faster, and the thermal and rating of shivering/sweating sensation were lower after exercise in the warm and humid environment in subjects wearing PES than when wearing the more traditional CT fabric.

Ryanodine receptor fragmentation and sarcoplasmic reticulum Ca2+ leak after one session of high-intensity interval exercise

Significance High-intensity interval training (HIIT) has become popular because it is a time-efficient way to increase endurance. An intriguing and so-far-unanswered question is how a few minutes of HIIT can be that effective. We exposed recreationally active men to one session of three to six sets of 30-s high-intensity cycling exercise. Muscle biopsies taken 24 h later showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca2+ channels, the ryanodine receptor 1 (RyR1). In isolated mouse muscle fibers, this fragmentation was accompanied by increased SR Ca2+ leak, which can trigger mitochondrial biogenesis. The HIIT-induced RyR1 fragmentation did not occur in muscles exposed to antioxidant, which offers an explanation for why antioxidants blunt effects of endurance training. High-intensity interval training (HIIT) is a time-efficient way of improving physical performance in healthy subjects and in patients with common chronic diseases, but less so in elite endurance athletes. The mechanisms underlying the effectiveness of HIIT are uncertain. Here, recreationally active human subjects performed highly demanding HIIT consisting of 30-s bouts of all-out cycling with 4-min rest in between bouts (≤3 min total exercise time). Skeletal muscle biopsies taken 24 h after the HIIT exercise showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca2+ release channel, the ryanodine receptor type 1 (RyR1). The HIIT exercise also caused a prolonged force depression and triggered major changes in the expression of genes related to endurance exercise. Subsequent experiments on elite endurance athletes performing the same HIIT exercise showed no RyR1 fragmentation or prolonged changes in the expression of endurance-related genes. Finally, mechanistic experiments performed on isolated mouse muscles exposed to HIIT-mimicking stimulation showed reactive oxygen/nitrogen species (ROS)-dependent RyR1 fragmentation, calpain activation, increased SR Ca2+ leak at rest, and depressed force production due to impaired SR Ca2+ release upon stimulation. In conclusion, HIIT exercise induces a ROS-dependent RyR1 fragmentation in muscles of recreationally active subjects, and the resulting changes in muscle fiber Ca2+-handling trigger muscular adaptations. However, the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes, which may explain why HIIT is less effective in this group.

Effect of androgenic-anabolic steroids and heavy strength training on patellar tendon morphological and mechanical properties

Combined androgenic-anabolic steroids (AAS) and overloading affects tendon collagen metabolism and ultrastructure and is often associated with a higher risk of injury. The aim of this prospective study was to investigate whether such effects would be reflected in the patellar tendon properties of individuals with a history of long-term resistance training and AAS abuse (RTS group), compared with trained (RT) and untrained (CTRL) nonsteroids users. Tendon cross-sectional area (CSA), stiffness, Youngs modulus, and toe limit strain were measured in vivo, from synchronized ultrasonography and dynamometry data. The patellar tendon of RT and RTS subjects was much stiffer and larger than in the CTRL group. However, stiffness and modulus were higher in the RTS group (26%, P < 0.05 and 30%, P < 0.01, respectively) than in the RT group. Conversely, tendon CSA was 15% (P < 0.05) larger in the RT group than in RTS, although differences disappeared when this variable was normalized to quadriceps maximal isometric torque. Yet maximal tendon stress was higher in RTS than in RT (15%, P < 0.05), without any statistical difference in maximal strain and toe limit strain between groups. The present lack of difference in toe limit strain does not substantiate the hypothesis of changes in collagen crimp pattern associated with AAS abuse. However, these findings indicate that tendon adaptations from years of heavy resistance training are different in AAS users, suggesting differences in collagen remodeling. Some of these adaptations (e.g., higher stress) could be linked to a higher risk of tendon injury.

Repeated Bout Effect Was More Expressed in Young Adult Males Than in Elderly Males and Boys

This study investigated possible differences using the same stretch-shortening exercise (SSE) protocol on generally accepted monitoring markers (dependent variables: changes in creatine kinase, muscle soreness, and voluntary and electrically evoked torque) in males across three lifespan stages (childhood versus adulthood versus old age). The protocol consisted of 100 intermittent (30 s interval between jumps) drop jumps to determine the repeated bout effect (RBE) (first and second bouts performed at a 2-week interval). The results showed that indirect symptoms of exercise-induced muscle damage after SSE were more expressed in adult males than in boys and elderly males, suggesting that the muscles of boys and elderly males are more resistant to exercise-induced damage than those of adult males. RBE was more pronounced in adult males than in boys and elderly males, suggesting that the muscles of boys and elderly males are less adaptive to exercise-induced muscle damage than those of adult males.

The effect of multiple sclerosis and gender on central and peripheral fatigue during 2-min MVC

OBJECTIVE The aim of this study was to evaluate the effect of secondary progressive MS disease on central and peripheral fatigue in women and men during continuous 2-min maximal voluntary contractions (MVC) of the quadriceps muscle. METHODS We studied age-matched (40-50-years of age) healthy individuals (men, n=9; women, n=10) and MS patients (men, n=9; women, n=9). The inclusion criteria for MS patients were a Kurtzke Expanded Disability Status Score <4 and a Fatigue Severity Scale Score >5. RESULTS The electrically-induced torque, MVC, and voluntary activation (VA) values were significantly greater in healthy persons than in MS patients. At the end of exercise, the fatigue index of MVC and VA was greater in MS patients than in the healthy cohort, whereas the electrically-induced torque was significantly decreased in healthy individuals. Peripheral fatigue was directly correlated with VA in MS patients; in contrast, it was inversely correlated with VA in healthy men. The variability of torque increased significantly during exercise and did not depend on MS and gender. CONCLUSIONS MS increased central fatigue and diminished peripheral fatigue significantly during 2-min MVC; however, only peripheral fatigue was significantly greater in healthy men than in healthy women. SIGNIFICANCE These results suggest the specified pattern of central and peripheral fatigue in MS patients and could be further exploited for potential therapeutic uses, for instance, to model exercise-induced less central fatigue, and this may not be differentiated between men and women.

Human alpha-actinin-3 genotype association with exercise-induced muscle damage and the repeated-bout effect.

Alpha-actinin-3 (ACTN3) is an integral part of the Z line of the sarcomere. The ACTN3 R577X (rs1815739) polymorphism determines the presence or absence of functional ACTN3, which may influence the extent of exercise-induced muscle damage. This study aimed to compare the impact of, and recovery from, muscle-damaging eccentric exercise on subjects with or without functional ACTN3. Seventeen young men (20-33 years old), homozygous for the R (n = 9) or X (n = 8) alleles, performed two bouts of stretch-shortening exercise (50 drop jumps) two weeks apart. Muscle soreness, plasma creatine kinase (CK) activity, jump height, maximal voluntary isometric torque (MVC), peak concentric isokinetic torque (IT), and electrically stimulated knee extension torques at 20 and 100 Hz were measured at baseline and at a number of time points up to 14 days after each bout. There were no significant baseline differences between the groups. However, significant time point × genotype interactions were observed for MVC (p = 0.021) and IT (p = 0.011) for the immediate effect of eccentric exercise in bout 1. The RR group showed greater voluntary force decrements (RR vs. XX: MVC, -33.3% vs. -24.5%; IT, -35.9% vs. -23.2%) and slower recovery. A repeated-bout effect was clearly observed, but there were no differences by genotype group. The ACTN3 genotype modulates the response of muscle function to plyometric jumping exercise, although the differences are modest. The ACTN3 genotype does not influence the clearly observed repeated-bout effect; however, XX homozygotes recover baseline voluntary torque values faster and thus may be able to undertake more frequent training sessions.

Peripheral and central fatigue after muscle-damaging exercise is muscle length dependent and inversely related.

Healthy untrained men performed 10 series of 12 knee eccentric extension repetitions (EE) at 160 degrees /s. The maximal voluntary isometric contraction force of the quadriceps muscle, the maximal rate of electrically induced torque development (RTD) and relaxation (RTR), isokinetic concentric torque at 30 degrees /s, the electrostimulation-induced torque at 20 and 100Hz frequencies were established before and after EE at shorter and longer muscle lengths. Besides, voluntary activation (VA) index and central activation ratio (CAR) were tested. There was more peripheral fatigue than central after EE. We established more central fatigue as well as low frequency fatigue at a shorter muscle length compared to the longer muscle length. Relative RTD as well as relative RTR, improved after EE and did not depend on the muscle length. Finally, central fatigue is inversely significantly related with the eccentric torque reduction during eccentric exercise and with the changes in muscle torque induced by low frequency stimulation.

Prolonged Muscle Damage Depends on Force Variability

Skeletal muscle damage frequently occurs after eccentric exercise, however, the relationship between intraindividual variability of eccentric exercise and skeletal muscle damage is not clear yet. The aim of the study was to test the hypothesis that prolonged decrease in skeletal muscle force depends on intraindividual variability of eccentric exercise. Eleven healthy men were asked to perform knee eccentric extension of 10 series with 12 repetitions (EE) with maximal intensity at 160 degrees/s. The maximal voluntary knee extension isometric torque (MVC) of the quadriceps muscle and isokinetic concentric torque (IT) at 30 degrees/s of knee angles were established before and 2 min, 60 min, 24 h and 14 days after EE. At 24 h and 48 h after EE muscle soreness and creatine kinase activity before and at 48 h after EE were calculated. The intraindividual variability of EE and autocorrelation coefficient of EE were calculated. We observed a significant decrease in MVC and IT after EE. The maximal isokinetic torque shifted to longer muscle length after EE. It was concluded that longer lasting (within 14 days) indirect symptoms of exercise induced muscle damage (decrease in muscle force) are closely related to variability of EE.

The decrease in electrically evoked force production is delayed by a previous bout of stretch–shortening cycle exercise

Aim:  Unaccustomed physical exercise with a large eccentric component is accompanied by muscle damage and impaired contractile function, especially at low stimulation frequencies. A repeated bout of eccentric exercise results in less damage and improved recovery of contractile function. Here we test the hypotheses that (1) a prior stretch–shortening cycle (SSC) exercise protects against impaired muscle function during a subsequent bout of SSC exercise and (2) the protection during exercise is transient and becomes less effective as the exercise progresses.