Kynurenine aminotransferase isoforms display fiber-type specific expression in young and old human skeletal muscle

Conversion of kynurenine (KYN) to kynurenic acid (KYNA) is the main pathway for free tryptophan degradation in skeletal muscle and has emerged as an important mechanism of how exercise is linked to promotion of mental health. Metabolism of KYN to KYNA mainly depends on the expression of kynurenine aminotransferases (KATs) that is under control of the mitochondria biogenesis regulator PGC-1α. We therefore hypothesized that expression of KATs would vary between muscle fibers that differ in mitochondrial content, i.e. oxidative type I vs more glycolytic type II muscle fibers. Moreover, we tested the hypothesis that KAT expression differs with age. Single muscle fibers were isolated from biopsies taken from the vastus lateralis muscle in young and old healthy subjects. In young and old subjects the abundance of KAT I, KAT III and KAT IV was greater in Type I than Type II fibers without age-dependent difference in the KAT isoform expressions. The link to mitochondrial content was further seen as the expression of KAT IV correlated to mitochondrial cytochrome c oxidase IV (COX IV) abundance in both fiber types. In conclusion, we describe for the first time the expression pattern of KAT isoforms with respect to specific fiber types and age in human skeletal muscle.

Acute effects of very low-volume high-intensity interval training on muscular fatigue and serum testosterone level vary according to age and training status

PURPOSE

To compare the acute physiological responses of three different very low-volume cycling sessions (6 × 5 s, 3 × 30 s, and 3 × 60 s) and their dependence on age and training status.

METHODS

Subjects were untrained young men (mean ± SD; age 22.3 ± 4.6 years, VO₂peak 42.4 ± 5.5 ml/kg/min, n = 10), older untrained men (69.9 ± 6.3 years, 26.5 ± 7.6 ml/kg/min, n = 11), and endurance-trained cyclists (26.4 ± 9.4 years, 55.4 ± 6.6 ml/kg/min, n = 10). Maximal voluntary contraction (MVC) and electrically stimulated knee extension torque, and low-frequency fatigue, as ratio of stimulation torques at 20-100 Hz (P20/100), were measured only 24 h after exercise. Serum testosterone (Te) and blood lactate concentrations were measured only 1 h after exercise.

RESULTS

All protocols increased the blood lactate concentration and decreased MVC and P20/100 in young men, but especially young untrained men. In old untrained men, 6 × 5 s decreased P20/100 but not MVC. Te increased after 3 × 30 s and 3 × 60 s in young untrained men and after 3 × 60 s in older untrained men. The increase in Te correlated with responses of blood lactate concentration, MVC, and P20/100 only in old untrained men.

CONCLUSIONS

As little as 6 × 5 s all-out cycling induced fatigue in young and old untrained and endurance-trained cyclists. Slightly higher-volume sessions with longer intervals, however, suppressed contractile function more markedly and also transiently increased serum testosterone concentration in untrained men.

Effect of sprint cycling and stretch-shortening cycle exercises on the neuromuscular, immune and stress indicators in young men

Selection of optimal physical load is essential for desired adaptation including health benefits. We hypothesized that neuromuscular, immune and stress indicators will be higher after energy demanding sprint interval exercise (SIE) than to mechanically demanding stretch-shortening cycle exercise (SSE). The main aim of this study was to assess and compare the kinetics of blood brain-derived neurotrophic factor (BDNF), norepinephrine (NE) and cortisol (as stress indicators) and proinflammatory (IL-6) and anti-inflammatory (IL-10) cytokines within 24 hours after metabolically demanding SIE and after muscle damage inducing SSE. Twenty healthy physically active young men randomly assigned to two equal groups to complete 12 bouts of 5 s stationary cycling sprints every 3 min (SIE) or 200 drop-jumps with 30 s interval between each jump (SSE), respectively. Quadriceps muscle maximal voluntary contraction torque and voluntary activation and soreness were measured and blood samples collected before and 2 min, 1 hour, 12 hours and 24 hours after the SIE and SSE. The BDNF, cortisol, IL-6 and NE levels increased more at 2 min after SIE than SSE (P < 0.05); however, the IL-10 level did not differ between SIE and SSE. BDNF and cortisol levels were decreased at 24 h after both SIE and especially after SSE. The higher was the initial BDNF level, the greater was its decrease at 24 h after both type of exercise. Before exercise BDNF level correlated closely with the change in central fatigue (decrease in voluntary activation) after both SIE and SSE. We thus conclude that both metabolically demanding SIE and muscle damage inflicting SSE induced long-lasting decrease in circulating BDNF which may not promote brain health. The level of circulating BDNF, but not cortisol, IL-6, IL-10 or NE, was associated with changes in central motor fatigue.

Absence of Bilateral Differences in Child Baseball Players with Throwing-related Pain

The aim of this study was to assess whether side-to-side differences in morphology and function of the upper limbs in 11-12 year-old male baseball players with throwing-related pain (n=14) were more pronounced than that of age-matched healthy untrained subjects (n=16). Baseball players 1) had played baseball≥4.5 h·wk^-1 for ≥ 4 years and (2) suffered from moderate-intensity (3-6 points on 10-point questionnaire scale) throwing-related pain in the shoulder or elbow in at least 2 training sessions within the past month. The range of motion (ROM), function and structure of the elbows and shoulders were assessed using goniometry, isokinetic dynamometry and ultrasonography. While the ROM and eccentric external peak torque of internal shoulder rotation were lower, the thickness of the supraspinatus tendon, the ulnar collateral ligament and articular cartilage of the humeral head were larger in baseball players than controls. There were, however, no significant side-to-side differences in any parameter in either group. In conclusion, it is unlikely that side-to-side differences in shoulder and upper limb structure and function contributed to the throwing-related pain in young baseball players, but low shoulder eccentric external peak torque and range of internal rotation may predispose to throwing-related pain.

Stretching exercise volume for flexibility enhancement in secondary school children

AIM

The study aim was to establish the threshold of stretching volume for flexibility enhancement during physical education lessons in secondary school children.

METHODS

Subjects were 239 tenth grade children randomly assigned to four groups (boys 107, girls 132, mean age 15.1 ± 0.4). Children involved in after-school sports were not included in the study. Physical education lessons were performed twice a week for 45 minutes in duration. The intervention lasted for five weeks comprising 10 physical education lessons. Flexibility was determined from sit and reach test before and after intervention. Subjects in group 1 performed standard "sit and reach" test of four trials in every physical education lesson; in group 2 received one stretching exercise of four repetitions; group 3 received four stretching exercises of four repetitions; in group 4 no stretching was performed.

RESULTS

Flexibility improvement in group 3 were the greatest (21.6%; P<0.05), smaller in group 2 (12.6%, P<0.05) and smallest in 1 group (5.1%, P<0.05), while control group changes were insignificant (1.7%, P>0.05).

CONCLUSION

The main finding was that single flexibility test performed twice a week for five weeks was sufficient stimulus to increase range of motion in secondary school children. Stretching exercises provides exceptional prospects to achieve youths' improvement since schoolchildren are very sensitive to flexibility training.

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.

METHODS

Healthy untrained men (n = 7) performed SSC exercise consisting of 100 maximal drop jumps at 30 s intervals. The same exercise was repeated 4 weeks later. Peak quadriceps muscle force evoked by electrical stimulation at 15 (P15) and 50 (P50) Hz was measured before exercise, after 10, 25, 50 and 100 jumps as well as 1 and 24 h after exercise.

RESULTS

P15 and P50 were higher during the initial phase of the repeated bout compared with the first exercise bout, but there was no difference between the bouts at the end of the exercise periods. P15 and P50 were again larger 24 h after the repeated bout. The P15/P50 ratio during exercise was not different between the two bouts, but it was higher after the repeated bout.

CONCLUSION

A prior bout of SSC exercise temporarily protects against impaired contractile function during a repeated exercise bout. The protection can again be seen after exercise, but the underlying mechanism then seems to be different.

Effect of age on metabolic fatigue and on indirect symptoms of skeletal muscle damage after stretch-shortening exercise

AIM

The purpose of this study was to establish the main differences between men (M) and adolescents (A) (males) in metabolic fatigue and damage induced by exercise performed at maximal intensity.

METHODS

Healthy A (age 13.4 0.6 years, n = 12) and healthy adult M (age 25.4 1.7 years, n = 12) participated in this study. To induce muscle damage and metabolic fatigue stretch-shortening exercise (SSE) (5 bouts of 20 jumps with counter-movement to 90 degrees angle in the knee with 10 s between bouts) has been chosen. The following data were measured: the force of the quadriceps muscle, aroused by electrical stimulation at different frequencies, maximal voluntary contraction force, height of jump (JH), muscle soreness, lactate (La) concentration and creatine kinase (CK) activity in the blood. All the parameters mentioned were measured before exercise and 2-5 min, 20 min, 24 h and 48 h after the SSE, except for La concentration changes in the blood measured before exercise, 2 min and 20 min after the SSE.

RESULTS

The main findings in this study are the following: 1) during SSE JH decreased significantly (P < 0.05) more in M than in A, whereas La concentration in the blood after SSE increased more in M than in A; 2) indirect symptoms of muscle damage were more evident in M than in A; 3) there was secondary decrease in electrically induced muscle force at 10-20 Hz from 3 min until 20 min after SSE but only in the muscles of M; 4) low frequency fatigue after SSE was more evident in M than A.

CONCLUSIONS

The results of the present study indicate that the muscles of adolescents are more resistant to both metabolic fatigue and exercise-induced damage than those of men.