Muscle energetics changes throughout maturation: a quantitative 31P-MRS analysis

Simultaneous creatine and phosphocreatine mapping of skeletal muscle by CEST MRI at 3T

PURPOSE

To confirm that CrCEST in muscle exhibits a slow-exchanging process, and to obtain high-resolution amide, creatine (Cr), and phosphocreatine (PCr) maps of skeletal muscle using a POlynomial and Lorentzian Line-shape Fitting (PLOF) CEST at 3T.

METHODS

We used dynamic changes in PCr/CrCEST of mouse hindlimb before and after euthanasia to assign the Cr and PCr CEST peaks in the Z-spectrum at 3T and to obtain the optimum saturation parameters. Segmented 3D EPI was employed to obtain multi-slice amide, PCr, and Cr CEST maps of human skeletal muscle. Subsequently, the PCrCEST maps were calibrated using the PCr concentrations determined by 31 P MRS.

RESULTS

A comparison of the Z-spectra in mouse hindlimb before and after euthanasia indicated that CrCEST is a slow-exchanging process in muscle (<150.7 s-1 ). This allowed us to simultaneously extract PCr/CrCEST signals at 3T using the PLOF method. We determined optimal B1 values ranging from 0.3 to 0.6 μT for CrCEST in muscle and 0.3-1.2 μT for PCrCEST. For the study on human calf muscle, we determined an optimum saturation time of 2 s for both PCr/CrCEST (B1  = 0.6 μT). The PCr/CrCEST using 3D EPI were found to be comparable to those obtained using turbo spin echo (TSE). (3D EPI/TSE PCr: (2.6 ± 0.3) %/(2.3 ± 0.1) %; Cr: (1.3 ± 0.1) %/(1.4 ± 0.07) %).

CONCLUSIONS

Our study showed that in vivo CrCEST is a slow-exchanging process. Hence, amide, Cr, and PCr CEST in the skeletal muscle can be mapped simultaneously at 3T by PLOF CEST.

Alteration of skeletal muscle energy metabolism assessed by 31 P MRS in clinical routine: Part 2. Clinical application

In this second part of a two-part paper, we intend to demonstrate the impact of the previously proposed advanced quality control pipeline. To understand its benefit and challenge the proposed methodology in a real scenario, we chose to compare the outcome when applying it to the analysis of two patient populations with significant but highly different types of fatigue: COVID-19 and multiple sclerosis (MS). 31 P-MRS was performed on a 3 T clinical MRI, in 19 COVID-19 patients, 38 MS patients, and 40 matched healthy controls. Dynamic acquisitions using an MR-compatible ergometer ran over a rest (40 s), exercise (2 min), and a recovery phase (6 min). Long and short TR acquisitions were also made at rest for T1 correction. The advanced data quality control pipeline presented in Part 1 is applied to the selected patient cohorts to investigate its impact on clinical outcomes. We first used power and sample size analysis to estimate objectively the impact of adding the quality control score (QCS). Then, comparisons between patients and healthy control groups using the validated QCS were performed using unpaired t tests or Mann-Whitney tests (p < 0.05). The application of the QCS resulted in increased statistical power, changed the values of several outcome measures, and reduced variability (standard deviation). A significant difference was found between the T1PCr and T1Pi values of MS patients and healthy controls. Furthermore, the use of a fixed correction factor led to systematically higher estimated concentrations of PCr and Pi than when using individually corrected factors. We observed significant differences between the two patient populations and healthy controls for resting [PCr]-MS only, [Pi ], [ADP], [H2 PO4 - ], and pH-COVID-19 only, and post-exercise [PCr], [Pi ], and [H2 PO4 - ]-MS only. The dynamic indicators τPCr , τPi , ViPCr , and Vmax were reduced for COVID-19 and MS patients compared with controls. Our results show that QCS in dynamic 31 P-MRS studies results in smaller data variability and therefore impacts study sample size and power. Although QCS resulted in discarded data and therefore reduced the acceptable data and subject numbers, this rigorous and unbiased approach allowed for proper assessment of muscle metabolites and metabolism in patient populations. The outcomes include an increased metabolite T1 , which directly affects the T1 correction factor applied to the amplitudes of the metabolite, and a prolonged τPCr , indicating reduced muscle oxidative capacity for patients with MS and COVID-19.

Physiological mechanisms of muscle strength and power are dependent on the years post obtaining peak height velocity in elite juniors rowers: A cross-sectional study

BACKGROUND

It is not yet known whether the years after peak height velocity (PHV) are associated with the physiological mechanisms of muscle strength and power in Juniors rowers.

OBJECTIVE

To identify the association between years post PHV (YPPHV) with muscle power and strength in Juniors rowers.

METHODS

We tested 235 Brazilian rowing athletes (male: 171, female: 64, Juniors category). We measured: power (indoor rowing over 100-m, 500-m, 2,000-m and 6,000-m) and muscle strength (one repetition maximum (1RM) test in squat, deadlift, bench press and bent row on the bench). Biological maturation was index by age of PHV. The sample was divided into groups considering YPPHV recent (2.5 to 3.9), median (2.51 to 4.9) and veteran (>4.9). We use a Baysian approach to data handling.

RESULTS

When compared to their peers in the recent and median post PHV groups, the male veteran group were superior in muscle power (Absolute: 100-m (BF10: 2893.85), 500-m (BF10: 553.77) and 6,000-m (BF10: 22.31). Relative: (100-m (BF10: 49.9)) and strength (BF10≥10.0 in squat, bench press and deadlift), and in the female the veteran group were superior in test time (500-m, BF10: 88.4).

CONCLUSION

In elite Juniors rowers the increasing YPPHV are associated with muscle power performance in both sexes and muscle strength performance in males.

Chemical exchange saturation transfer imaging of creatine, phosphocreatine, and protein arginine residue in tissues

Chemical exchange saturation transfer (CEST) MRI has become a promising technique to assay target proteins and metabolites through their exchangeable protons, noninvasively. The ubiquity of creatine (Cr) and phosphocreatine (PCr) due to their pivotal roles in energy homeostasis through the creatine phosphate pathway has made them prime targets for CEST in the diagnosis and monitoring of disease pathologies, particularly in tissues heavily dependent on the maintenance of rich energy reserves. Guanidinium CEST from protein arginine residues (i.e. arginine CEST) can also provide information about the protein profile in tissue. However, numerous obfuscating factors stand as obstacles to the specificity of arginine, Cr, and PCr imaging through CEST, such as semisolid magnetization transfer, fast chemical exchanges such as primary amines, and the effects of nuclear Overhauser enhancement from aromatic and amide protons. In this review, the specific exchange properties of protein arginine residues, Cr, and PCr, along with their validation, are discussed, including the considerations necessary to target and tune their signal effects through CEST imaging. Additionally, strategies that have been employed to enhance the specificity of these exchanges in CEST imaging are described, along with how they have opened up possible applications of protein arginine residues, Cr and PCr CEST imaging in the study and diagnosis of pathology. A clear understanding of the capabilities and caveats of using CEST to image these vital metabolites and mitigation strategies is crucial to expanding the possibilities of this promising technology.

High-Intensity Interval Training for Rowing: Acute Responses in National-Level Adolescent Males

Background: This study investigated the acute effects of two high-intensity interval training (HIIT) programs on physiological responses and internal workload. Methods: Ten national-level adolescent male rowers (age: 15.7 ± 0.2 years; maximal oxygen uptake (VO2max): 60.11 ± 1.91 mL∙kg−1∙min−1) performed two HIIT testing sessions: short (S-HIIT) and long (L-HIIT). In S-HIIT, the rowers performed 25 reps of 30 s at 100% power at VO2max (Pmax) interspersed with 30 s at P@20% Pmax; whereas in L-HIIT, the rowers executed 4 × 4 min at P@90% Pmax interspersed with 3 min of active recovery (P@30% Pmax). Results: The acute physiological responses and internal workload were evaluated. The significance level was set at p < 0.05. Oxygen uptake (VO2) (p < 0.05), time spent per session at ~90% VO2max (p < 0.01), total VO2 consumed (p < 0.01), total distance (p < 0.001), the rating of perceived exertion, blood lactate concentration and heart rate (always p < 0.0001) were significantly higher in L-HIIT than in S-HIIT. However, peak power output was significantly lower in L-HIIT compared to S-HIIT (p < 0.0001). Conclusion: In adolescent rowers, both HIIT tests stimulated aerobic and anaerobic systems. The L-HIIT test was associated with acute cardiorespiratory and metabolic responses, as well as higher perceptions of effort than the S-HIIT test. In adolescent rowers, HIIT emerges as an asset and could be introduced into a traditional in-season, moderate-intensity and endurance-based rowing program once a week.

Influence of Advancing Biological Maturation on Aerobic and Anaerobic Power and on Sport Performance of Junior Rowers: A Longitudinal Study

Background: Researches are linking Biological Maturation (BM) with the performance of adolescent rowers from both genders. Despite this, there is still not enough information about the influence of BM advancement correlating to sports, aerobic and anaerobic performance in adolescent athletes at the sport modality rowing. Objective: Investigate the influence of Biological Maturation on sports performance and aerobic and anaerobic power in adolescent rowing athletes. Methods: A longitudinal observational study, developed over 3 years, with a sample of 52 adolescents, rowing athletes, of both genders (61% male and 39% female) mean age of 16.0 ± 0.5 years old at the start and 18.4 ± 0.5 years old at the end of the study. Analysis was performed once a year. BM was evaluated through maturational groups generated from Age Peak Height Velocity; maximum aerobic power [VO₂Max (ml/kg/min)] and mean anaerobic power (Watts) through the ergometer test (indoor rowing); peak anaerobic power (Watts) through a mathematical model derived from competition time, to determine sports performance analyzed the race time during world championship tryouts. Results: The advancement of BM influenced the reduction of the test time and increase of the mean anaerobic power (Watts) in indoor rowing (η2p > 0.36, p < 0.05), as well as an improvement in performance in sports competition (η2p > 0.35, p < 0.05). However, the advancement of BM did not affect VO2Max (ml/kg/min) in young elite rowing athletes of both sexes (p > 0.05). Conclusion: Advances in biological maturation have been shown to influence the anaerobic and sports performance (reduction of the execution time in 2,000-m) of adolescent rowing athletes of both genders.

Sex-specific maturation of muscle metabolites carnosine, creatine, and carnitine over puberty: a longitudinal follow-up study

Due to the invasiveness of a muscle biopsy, there is fragmentary information on the existence and possible origin of a sexual dimorphism in the skeletal muscle concentrations of the energy delivery-related metabolites carnosine, creatine, and carnitine. As these metabolites can be noninvasively monitored by proton magnetic resonance spectroscopy, this technique offers the possibility to investigate if sexual dimorphisms are present in an adult reference population and if these dimorphisms originated during puberty using a longitudinal design. Concentrations of carnosine, creatine, and carnitine were examined using proton magnetic resonance spectroscopy in the soleus and gastrocnemius muscles of an adult reference population of female (n = 50) and male adults (n = 50). For the longitudinal follow-up over puberty, 29 boys and 28 girls were scanned prepuberty. Six years later, 24 boys and 24 girls were rescanned postpuberty. A sexual dimorphism was present in carnosine and creatine, but not carnitine, in the adult reference population. Carnosine was 28.5% higher in the gastrocnemius (P < 0.001) and carnosine and creatine were respectively 19.9% (P < 0.001) and 18.2% (P < 0.001) higher in the soleus of male when compared with female adults. Through puberty, carnosine increased more in male subjects compared with female subjects, both in the gastrocnemius (+10.43% and -10.83%, respectively; interaction effect: P = 0.002) and in the soleus (+24.30% and +5.49%, respectively; interaction effect: P = 0.012). No significant effect of puberty was found in either creatine (interaction effect: P = 0.307) or carnitine (interaction effect: P = 0.066). A sexual dimorphism in the adult human muscle is present in carnosine and creatine, but not in carnitine.NEW & NOTEWORTHY This is the first study to investigate sexual dimorphisms in skeletal muscle carnosine, creatine, and carnitine concentrations in a substantial adult reference population (n = 100). A sexual dimorphism is present in both carnosine and creatine at adult age. The origin of the sexual dimorphisms is investigated using a longitudinal design over puberty in 24 males and 24 females. The sexual dimorphism in carnosine originated partly during puberty for carnosine, but not for creatine.

Influence of Biological Maturity on the Muscular Strength of Young Male and Female Swimmers

The present study compared the evolution of strength and muscle mass in swimmers of both sexes across different chronological and biological ages. Seventy-six swimmers (55 males and 21 females) from 10 to 20 years of age underwent evaluations of sexual maturation using the developmental stage of the genitals (G1, G2, G3 G4 and G5) and pubic hair (P1, P2, P3, P4 and P5) (Tanner criteria), lean mass using the dual-energy X-ray absorptiometry method, and isokinetic strength of the knee extensor muscles. Boys did not present a significant difference in strength among G1, G2, and G3 (p > 0.05) nor between 10 and 13 years of age (p > 0.05), although significant differences in strength of the extensor (p = 0.038) muscles of the knee were found between G3 and G4 and between athletes of 10 and 14 years of age (extensor muscles, p = 0.033). Muscle mass was different between G3 and G4 (p = 0.003) and between 10 and 14 years of age (p = 0.020). The mean age of boys in the G4 pubertal stage was 13.7 ± 2.0 years, although from 12 to 16 years of age, boys are typically in the G4 stage. Girls showed no significant differences in strength nor muscle mass among the pubertal stages (p > 0.05). We conclude that by equating boys by chronological age, individuals with different pubertal development levels, muscle mass, and strength levels can be grouped. Therefore, the relationship between the level of pubertal deve-lopment and chronological age seems to be a useful strategy for grouping boys, while grouping girls by chronological age seems adequate for training and competition.

Sex-related differences in accumulated O2 deficit incurred by high-intensity rowing exercise during childhood and adolescence

PURPOSE

The aims of the present study were to determine during childhood and adolescence (i) the effect of sex on non-oxidative energy production, quantified by the accumulated oxygen deficit (AOD), and (ii) the influence of AOD on high-intensity performance.

METHODS

Thirty-nine boys and 35 girls aged 10-17 years performed a 60 s all-out test on a rowing ergometer to determine AOD and mean power output (MPO). Multiplicative allometric modelling was used to assess the concurrent effects of lean body mass (LBM) and age on AOD.

RESULTS

AOD significantly increased with age in both sexes (p < 0.001) with boys exhibiting significantly higher AOD than girls from the age of 14 years (10-11.9 yr: 1.9 vs 1.9 L, 12-13.9 yr: 2.4 vs 2.7 L, 14-15.9 yr: 2.8 vs 4.6 L and 16-17.9 yr: 2.9 vs 5.2 L, in girls and boys respectively, p < 0.001). However, a sex difference was no longer significant when AOD was analysed using an allometric model including age and LBM (p = 0.885). Finally, significant correlations were found between AOD and MPO in boys and girls but with lower evidence in girls (r² = 0.41 vs. 0.89).

CONCLUSION

Non-oxidative energy production increased more extensively in boys than girls from the age of 14 years. Age and LBM accounted for the sexual differentiation of AOD during childhood and adolescence. In addition, AOD was found to be a determinant factor of high-intensity performance, more particularly in boys.

Eccentric Resistance Training in Youth: A Survey of Perceptions and Current Practices by Strength and Conditioning Coaches

BACKGROUND

Eccentric resistance training (ERT) in youth is advocated for aiding performance and injury risk. However, research investigating the applied practices of ERT in youth is in its infancy. In this study, we surveyed the perceptions and practices of practitioners utilizing ERT in youth to provide an understanding of its current application in practice.

METHODS

Sixty-four strength and conditioning coaches completed an online survey reporting their current use of ERT in youth using both open and closed questions.

RESULTS

Coaches deemed the inclusion of ERT important in youth with its inclusion based upon factors such as maturation status, training age and strength levels. Coaches also displayed an awareness of the physiological responses to eccentric exercise in youth compared to adults. ERT was primarily used for injury prevention, with the majority of coaches using body-weight and tempo exercises. Furthermore, utilizing eccentric hamstrings exercises was reported as highly important. The frequency of ERT tended to increase in older age groups and coaches mainly prescribed self-selected rest intervals. Finally, the need for further research into the training guidelines of ERT in youth was highlighted, in which coaches require more information on how maturation influences training adaptations and the fatigue-recovery responses.

CONCLUSION

Coaches emphasized the importance of including ERT for both performance and injury prevention factors in youth although further research is required to generate practical guidelines for coaches in order to support its inclusion within practice.

Heart Rate Responses during Sport-Specific High-Intensity Circuit Exercise in Child Female Gymnasts

This study examined heart rate (HR) responses during a sport-specific high-intensity circuit training session to indirectly assess cardiorespiratory stress in child athletes. Seventeen, female gymnasts, aged 9-11 years performed two 5-min 15 s sets of circuit exercise, interspersed by a 3 min rest interval. Each set included five rounds of five gymnastic exercises (7 s work, 7 s rest) executed with maximal effort. During the first circuit training set, peak heart rate (HR) was 192 ± 7 bpm and average HR was 83 ± 4% of maximum HR (HRmax), which was determined in a separate session. In the second set, peak HR and average HR were increased to 196 ± 8 bpm (p < 0.001, d = 0.55) and to 89 ± 4% HRmax (p < 0.001, d = 2.19), respectively, compared with the first set. HR was above 80% HRmax for 4.1 ± 1.2 min during set 1 and this was increased to 5.1 ± 0.4 min in set 2 (p < 0.001, d = 1.15). Likewise, HR was above 90% of HRmax for 2.0 ± 1.2 min in set 1 and was increased to 3.4 ± 1.7 min in set 2 (p < 0.001, d = 0.98). In summary, two 5-min 15 s sets of high-intensity circuit training using sport-specific exercises, increased HR to levels above 80% and 90% HRmax for extended time periods, and thus may be considered as an appropriate stimulus, in terms of intensity, for improving aerobic fitness in child female gymnasts.

Effect of Long-Duration Adventure Races on Cardiac Damage Biomarker Release and Muscular Function in Young Athletes

The aim of the present study was to examine the effect of 1- and 2-day adventure races on cardiac muscle damage and skeletal muscle soreness and function in young athletes. Twelve male trained adolescents (14-15 years) completed both 1-day (48.2 km) and 2-day (66.0 km) races that included trail running, mountain biking, kayaking, and in-line skating separated by 10 weeks. Myocardial damage biomarker concentrations (cTnI and CK-MB), maximal voluntary isometric contraction (MVIC) torque, perceived knee extensor (KE) muscle soreness (PMS), and drop and squat jump heights were measured before and after each race. Heart rate was also monitored throughout. Mean heart rate (% cardiac reserve) was higher during the 1-day (66.6 ± 6.4%) than 2-day (62.6 ± 7.8%, p = 0.038) race. The amplitude of cardiac damage biomarker release was also higher following the 1-day than the 2-day race (peak cTnI: 0.14 vs. 0.03 ng/mL, p = 0.045; peak CK-MB: 20.30 vs. 11.98 ng/mL, p = 0.020). However, cardiac biomarker concentrations returned to baseline at 24-48 h post-exercise, except for CK-MB after the 2-day race (p = 0.017). Eight and three participants exceeded the cTnI cut-off for myocardial injury in 1- and 2-day races, respectively, but none exceeded the cut-off for acute myocardial infarction. While there was a significant decrease in drop jump height (-5.9%, p = 0.003), MVIC torque and squat jump height remained unchanged after both races. PMS was increased at 24 h after both races (p < 0.001) but returned to baseline levels by 72 h post-race. In conclusion, the shorter, more intense race produced more cardiac damage, although this probably represents a standard exercise intensity-dependent response rather than pathological response. Skeletal muscle functional and soreness responses were moderate and similar between races.

Maturation-related changes in the development and etiology of neuromuscular fatigue

PURPOSE

The aim of the present study was to investigate the role of maturation on the etiology of neuromuscular fatigue induced by repeated maximal voluntary isometric contractions (MVIC).

METHODS

Nine prepubertal boys (9.9 ± 1.3 years), eight male adolescents (13.6 ± 1.3 years) and eleven men (23.4 ± 3.0 years) performed a series of repeated isometric MVICs of the knee extensors until the MVIC torque reached 60% of its initial value. Magnetic stimulations were delivered to the femoral nerve every five MVICs to follow the course of voluntary activation level (VA) and the potentiated twitch torque (Qtw_pot).

RESULTS

Task failure was reached after 52.9 ± 12.7, 42.6 ± 12.5, and 26.6 ± 6.3 repetitions in boys, adolescents and men, respectively. VA remained unchanged in men whereas it decreased significantly and similarly in boys and adolescents (p < 0.001). In contrast, Qtw_pot remained unchanged in boys and decreased significantly less in adolescents than adults (p < 0.05).

CONCLUSIONS

Children and adolescents experience less peripheral and more central fatigue than adults. However, adolescents experience more peripheral fatigue than children for a comparable amount of central fatigue. This finding supports the idea that the tolerance of the central nervous system to peripheral fatigue could increase during maturation.

Relationship between (non)linear phase II pulmonary oxygen uptake kinetics with skeletal muscle oxygenation and age in 11-15 year olds

NEW FINDINGS

What is the central question of this study? Do the phase II parameters of pulmonary oxygen uptake ( V ̇ O 2 ) kinetics display linear, first-order behaviour in association with alterations in skeletal muscle oxygenation during step cycling of different intensities or when exercise is initiated from an elevated work rate in youths. What is the main finding and its importance? Both linear and non-linear features of phase II V ̇ O 2 kinetics may be determined by alterations in the dynamic balance between microvascular O₂ delivery and utilization in 11-15 year olds. The recruitment of higher-order (i.e. type II) muscle fibres during 'work-to-work' cycling might be responsible for modulating V ̇ O 2 kinetics with chronological age.

ABSTRACT

This study investigated in 19 male youths (mean age: 13.6 ± 1.1 years, range: 11.7-15.7 years) the relationship between pulmonary oxygen uptake ( V ̇ O 2 ) and muscle deoxygenation kinetics during moderate- and very heavy-intensity 'step' cycling initiated from unloaded pedalling (i.e. U → M and U → VH) and moderate to very heavy-intensity step cycling (i.e. M → VH). Pulmonary V ̇ O 2 was measured breath-by-breath along with the tissue oxygenation index (TOI) of the vastus lateralis using near-infrared spectroscopy. There were no significant differences in the phase II time constant ( τ V ̇ O 2 p ) between U → M and U → VH (23 ± 6 vs. 25 ± 7 s; P = 0.36); however, the τ V ̇ O 2 p was slower during M → VH (42 ± 16 s) compared to other conditions (P < 0.001). Quadriceps TOI decreased with a faster (P < 0.01) mean response time (MRT; i.e. time delay + τ) during U → VH (14 ± 2 s) compared to U → M (22 ± 4 s) and M → VH (20 ± 6 s). The difference (Δ) between the τ V ̇ O 2 p and MRT-TOI was greater during U → VH compared to U → M (12 ± 7 vs. 2 ± 7 s, P < 0.001) and during M → VH (23 ± 15 s) compared to other conditions (P < 0.02), suggesting an increased proportional speeding of fractional O₂ extraction. The slowing of the τ V ̇ O 2 p during M → VH relative to U → M and U → VH correlated positively with chronological age (r = 0.68 and 0.57, respectively, P < 0.01). In youths, 'work-to-work' transitions slowed microvascular O₂ delivery-to-O₂ utilization with alterations in phase II V ̇ O 2 dynamics accentuated between the ages of 11 and 15 years.

Children Exhibit a More Comparable Neuromuscular Fatigue Profile to Endurance Athletes Than Untrained Adults

The present study compared neuromuscular fatigue profiles between children, untrained adults and adult endurance athletes during repeated maximal muscle contractions. Eighteen prepubertal boys, 19 untrained men and 13 endurance male athletes performed 5-s maximal voluntary isometric knee extensor contractions (MVICs) interspersed with 5-s recovery until MVIC reached 60% of its initial value. Single and doublet magnetic stimulations were delivered to the femoral nerve to quantify the time course of potentiated twitch amplitude (T_tw,pot), high-frequency torque (T_{100 Hz}) and the low-to-high frequency torque ratio (T_{10 Hz}/T_{100 Hz}), i.e., indicators of peripheral fatigue. M-wave-normalized EMG amplitudes (EMG/M) and the maximal voluntary activation level (VA) were calculated to quantify central fatigue. Adults (15.9 ± 3.9 repetitions) performed fewer MVICs than children (40.4 ± 19.7) and endurance athletes (51.7 ± 19.6), however, no difference was observed between children and athletes (P = 0.13). T_tw,pot (∼52%, P < 0.001), T_{100 Hz} (∼39%, P < 0.001) and T_{10 Hz}/T_{100 Hz} (∼23%, P < 0.001) decreased only in adults. Similar decrements in vastus medialis and vastus lateralis EMG/M were observed in children and endurance athletes (range: 40-50%), and these were greater than in adults (∼15%). Whilst VA decreased more in children (-38.4 ± 22.5%, P < 0.001) than endurance athletes (-20.3 ± 10.1%, P < 0.001), it did not change in adults. Thus, children fatigued more slowly than adults and as much as endurance athletes. They developed less peripheral and more central fatigue than adults and, although central fatigue appeared somewhat higher in children than endurance athletes, both children and endurance athletes experienced greater decrements than adults. Therefore, children exhibit a more comparable neuromuscular fatigue profile to endurance athletes than adults.

Child-adult differences in neuromuscular fatigue are muscle-dependent

The aim of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor (KE) and plantar flexor (PF) muscles during repeated maximal voluntary isometric contractions (MVIC) between children and adults. Twenty-one prepubertal boys (9-11 years) and 24 men (18-30 years) performed two fatigue protocols consisting in a repetition of 5-s isometric MVIC of the KE or PF muscles interspersed with 5-s passive recovery periods until MVIC reached 60% of its initial value. The etiology of neuromuscular fatigue of the KE and PF muscles was investigated by means of non-invasive methods such as the surface electromyography, single and doublet magnetic stimulation, twitch interpolation technique and NIRS. The number of repetitions performed was significantly lower in men (15.4 ± 3.8) than boys (38.7 ± 18.8) for the KE fatigue test. In contrast, no significant difference was found for the PF muscles between boys and men (12.1 ± 4.9 and 13.8 ± 4.9 repetitions, respectively). Boys displayed a lower reduction in potentiated twitch torque, low-frequency fatigue and muscle oxygenation than men whatever the muscle group considered. In contrast, voluntary activation level and normalized EMG data decreased to a greater extent in boys than men for both muscle groups. To conclude, boys experienced less peripheral and more central fatigue during repeated MVICs than men whatever the muscle group considered. However, child-adult differences in neuromuscular fatigue were muscle-dependent since boys fatigued similarly to men with the PF muscles and to a lower extent with the KE muscles than men.

Impaired cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with Barth syndrome

Barth syndrome (BTHS) is an X‐linked condition characterized by altered cardiolipin metabolism and cardioskeletal myopathy. We sought to compare cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with BTHS and unaffected controls and examine their relationships with cardiac function and exercise capacity. Children/adolescents and young adults with BTHS (n = 20) and children/adolescent and young adult control participants (n = 23, total n = 43) underwent ³¹P magnetic resonance spectroscopy (³¹P‐MRS) of the lower extremity (calf) and heart for estimation of skeletal muscle and cardiac bioenergetics. Peak exercise testing (VO_2peak) and resting echocardiography were also performed on all participants. Cardiac PCr/ATP ratio was significantly lower in children/adolescents (BTHS: 1.5 ± 0.2 vs. Control: 2.0 ± 0.3, P < 0.01) and adults (BTHS: 1.9 ± 0.2 vs. Control: 2.3 ± 0.2, P < 0.01) with BTHS compared to Control groups. Adults (BTHS: 76.4 ± 31.6 vs. Control: 35.0 ± 7.4 sec, P < 0.01) and children/adolescents (BTHS: 71.5 ± 21.3 vs. Control: 31.4 ± 7.4 sec, P < 0.01) with BTHS had significantly longer calf PCr recovery (τPCr) postexercise compared to controls. Maximal calf ATP production through oxidative phosphorylation (Qmax‐lin) was significantly lower in children/adolescents (BTHS: 0.5 ± 0.1 vs. Control: 1.1 ± 0.3 mmol/L per sec, P < 0.01) and adults (BTHS: 0.5 ± 0.2 vs. Control: 1.0 ± 0.2 mmol/L sec, P < 0.01) with BTHS compared to controls. Blunted cardiac and skeletal muscle bioenergetics were associated with lower VO_2peak but not resting cardiac function. Cardiac and skeletal muscle bioenergetics are impaired and appear to contribute to exercise intolerance in BTHS.

Are Prepubertal Children Metabolically Comparable to Well-Trained Adult Endurance Athletes?

It is well acknowledged that prepubertal children have smaller body dimensions and a poorer mechanical (movement) efficiency, and thus a lower work capacity than adults. However, the scientific evidence indicates that prepubertal children have a greater net contribution of energy derived from aerobic metabolism in exercising muscle and reduced susceptibility to muscular fatigue, which makes them metabolically comparable to well-trained adult endurance athletes. For example, the relative energy contribution from oxidative and non-oxidative (i.e. anaerobic) sources during moderate-to-intense exercise, the work output for a given anaerobic energy contribution and the rate of acceleration of aerobic metabolic machinery in response to submaximal exercise are similar between prepubertal children and well-trained adult endurance athletes. Similar conclusions can be drawn on the basis of experimental data derived from intra-muscular measurements such as type I fibre percentage, succinate dehydrogenase enzyme activity, mitochondrial volume density, post-exercise phosphocreatine re-synthesis rate and muscle by-product clearance rates (i.e. H⁺ ions). On a more practical level, prepubertal children also experience similar decrements in peak power output as well-trained adult endurance athletes during repeated maximal exercise bouts. Therefore, prepubertal children have a comparable relative oxidative contribution to well-trained adult endurance athletes, but a decrease in this relative contribution occurs from childhood through to early adulthood. In a clinical context, this understanding may prove central to the development of exercise-based strategies for the prevention and treatment of many metabolic diseases related to mitochondrial oxidative dysfunction (e.g. in obese, insulin-resistant and diabetic patients), which are often accompanied by muscular deconditioning during adolescence and adulthood.

Recovery of upper-body muscle power after short intensive exercise: comparing boys and men

PURPOSE

Boys' lower-body muscle power generation (PO) recovers faster than men's following intensive exercise. The purpose of this study was to examine whether boys differ from adult men in recovering from upper-body muscle power generation following intensive exercise.

METHODS

Fifteen prepubertal boys (M ± SD age 10.6 ± 1.0 years) and 13 men (31.1 ± 5.0 years) performed two upper-body Wingate Anaerobic Tests (WAnT), separated by either 2-min or 10-min recovery intervals. WAnT parameters, pre-and post-WAnT heart rates (HR), and blood lactate ([La]) were measured during recovery from the WAnTs.

RESULTS

Boys' mean power (MP) of the repeated WAnT (WAnT₂) following 2- and 10-min recoveries was 97.3 ± 7.2% and 99.4 ± 3.9%, respectively, compared to MP of the first test (WAnT₁) (p > 0.05 for both tests). In contrast, in men's MP of the WAnT₂ following the 2-min recovery, was significantly lower than that of the WAnT₁ (84.4 ± 6.7%, p = 0.0001). While boys' and men's HR recovery after 2 min differed significantly (p = 0.046), no between-group differences were found following the 10-min recovery. Peak [La] in boys was 37-44% lower than that in men (p = 0.002).

CONCLUSIONS

The faster recovery of PO in boys after supra-maximal upper-body exercise is partially explained by the lower power generated by boys, attributed in part to a lower anaerobic capacity and to the greater relative contribution of aerobic processes to performance and recovery from anaerobic-type tasks. Further research is needed to determine the physiologic, neurologic and biochemical basis of the rapid muscle power recovery in children.

Metabolic and Fatigue Profiles Are Comparable Between Prepubertal Children and Well-Trained Adult Endurance Athletes

The aim of this study was to determine whether prepubertal children are metabolically comparable to well-trained adult endurance athletes and if this translates into similar fatigue rates during high-intensity exercise in both populations. On two different occasions, 12 prepubertal boys (10.5 ± 1.1 y), 12 untrained men (21.2 ± 1.5 y), and 13 endurance male athletes (21.5 ± 2.7 y) completed an incremental test to determine the power output at VO_2max (PVO_2max) and a Wingate test to evaluate the maximal anaerobic power (P_max) and relative decrement in power output (i.e., the fatigue index, FI). Furthermore, oxygen uptake (VO₂), heart rate (HR), and capillary blood lactate concentration ([La]) were measured to determine (i) the net aerobic contribution at 5-s intervals during the Wingate test, and (ii) the post-exercise recovery kinetics of VO₂, HR, and [La]. The P_max-to-PVO_2max ratio was not significantly different between children (1.9 ± 0.5) and endurance athletes (2.1 ± 0.2) but lower than untrained men (3.2 ± 0.3, p < 0.001 for both). The relative energy contribution derived from oxidative metabolism was also similar in children and endurance athletes but greater than untrained men over the second half of the Wingate test (p < 0.001 for both). Furthermore, the post-exercise recovery kinetics of VO₂, HR, and [La] in children and endurance athletes were faster than those of untrained men. Finally, FI was comparable between children and endurance athletes (-35.2 ± 9.6 vs. -41.8 ± 9.4%, respectively) but lower than untrained men (-51.8 ± 4.1%, p < 0.01). To conclude, prepubertal children were observed to be metabolically comparable to well-trained adult endurance athletes, and were thus less fatigable during high-intensity exercise than untrained adults.

Exercise-induced fatigue in young people: advances and future perspectives

PURPOSE

In recent decades, the interest for exercise-induced fatigue in youth has substantially increased, and the effects of growth on the peripheral (muscular) and central (neural) mechanisms underpinning differences in neuromuscular fatigue between healthy children and adults have been described more extensively. The purpose of this review is to retrieve, report, and analyse the findings of studies comparing neuromuscular fatigue between children and adults. Objective measures of the evaluation of the physiological mechanisms are discussed.

METHOD

Major databases (PubMed, Ovid, Scopus and Web of Science) were systematically searched and limited to English language from inception to September 2017.

RESULT

Collectively, the analyzed studies indicate that children experience less muscular and potentially more neural fatigue than adults. However, there are still many unknown aspects of fatigue regarding neural (supraspinal and spinal) and peripheral mechanisms that should be more thoroughly examined in children.

CONCLUSION

Suitable methods, such as transcranial magnetic stimulation, transcranial electrical stimulation, functional magnetic resonance imaging, near-infrared spectroscopy, tendon vibration, H-reflex, and ultrasound are recommended in the research field of fatigue in youth. By designing studies that test the fatigue effects in movements that replicate daily activities, new knowledge will be acquired. The linkage and interaction between physiological, cognitive, and psychological aspects of human performance remain to be resolved in young people. This can only be successful if research is based on a foundation of basic research focused on the mechanisms of fatigue while measuring all three above aspects.

Maturation-Related Differences in Neuromuscular Fatigue After a Short-Term Maximal Run

Purpose. This study investigated maturation-related differences in neuromuscular fatigue after a short-term maximal run. Methods. Eight male children, eight adolescents, and eight adults performed a maximal ca. 50-s run (300/350/400 m, respectively). Mechanisms of neuromuscular fatigue were assessed through isometric plantar flexor tests, electrical stimulation of the posterior tibial nerve, soleus electromyography, and blood tests. Results. All the groups showed a decrease in the running speed (children: -12.2 ± 6.5%; adolescents: -9.8 ± 5.1%; adults: -12.2 ± 3.1%), but only adults revealed a decline in the maximal isometric plantar flexor torque (-16.1 ± 13.0%). On the other hand, the relative pre- to post-fatigue change in the maximal isometric plantar flexor torque differed only between adults and adolescents. The peak torque in the passive twitch test decreased in adolescents (-19.2 ± 12.2%) and adults (-23.7 ± 13.7%). Moreover, post-fatigue minimum blood pH (children: 7.18 ± 0.03; adolescents: 7.14 ± 0.07; adults: 6.97 ± 0.06) differed between the groups. No changes were observed in the neural drive or mechanisms at the spinal level. Conclusions. Despite the loss of running speed, children showed no post-exercise fatigue, whereas adolescents and adults demonstrated fatigue at peripheral sites. Central fatigue could not be established for the studied groups.

In-vivo31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

In addition to direct assessment of high energy phosphorus containing metabolite content within tissues, phosphorus magnetic resonance spectroscopy (³¹P-MRS) provides options to measure phospholipid metabolites and cellular pH, as well as the kinetics of chemical reactions of energy metabolism in vivo. Even though the great potential of ³¹P-MR was recognized over 30 years ago, modern MR systems, as well as new, dedicated hardware and measurement techniques provide further opportunities for research of human biochemistry. This paper presents a methodological overview of the ³¹P-MR techniques that can be used for basic, physiological, or clinical research of human skeletal muscle and liver in vivo. Practical issues of ³¹P-MRS experiments and examples of potential applications are also provided. As signal localization is essential for liver ³¹P-MRS and is important for dynamic muscle examinations as well, typical localization strategies for ³¹P-MR are also described.

Evidence of nonlocal muscle fatigue in male youth

Evidence for nonlocal muscle fatigue (NLMF) has been inconsistent in adults, with no studies investigating youth. The objective was to examine NLMF in youth. Forty-two young males (age, 10-13 years) were tested for maximal voluntary isometric contraction (MVIC) force of the ipsilateral and contralateral knee extensors at 90° and 120° knee flexion, elbow flexors at 90°, handgrip, knee extensor isokinetic torque (300°·s^-1 analyzed at 90° and 120° knee flexion), as well as a unilateral countermovement jump (CMJ) and Y Balance test (YBT). Isokinetic fatigue group (n = 15) had unilateral fatigue induced with 10 sets of 20 repetitions of maximal isokinetic knee extensor contractions at 300°·s^-1. Isometric fatigue group (n = 15) used 10 repetitions of 6-s knee extensor MVIC whereas the control group (n = 12) were not fatigued. There was no significant difference in the response to the isometric- or isokinetic-fatigue intervention protocols. Main time effects indicated that NLMF was evident with the contralateral knee extensor MVIC at 90° (p = 0.008; 8.9%), knee extensor isokinetic torque at 90° (p < 0.001; 11.4%), and 120° (p = 0.05; 5.4%), CMJ (p = 0.02; 11.5%), handgrip (p = 0.06; 4.5%), elbow flexors (p < 0.001; 7.7%), and YBT (p = 0.001; 5.6%). Ipsilateral NLMF deficits occurred with handgrip (p < 0.001; 7.3%), elbow flexors MVICs (p < 0.001; 10.7%), CMJ (p = 0.02; 12.2%), and YBT (p = 0.002; 3.8%). NLMF with similar relative fatigue-induced deficits of fatigued and nonfatigued limbs suggest that youth fatigue is highly dependent upon the extent of activation or inhibition of the nervous system. Coaches of young athletes might consider developing technical motor skills before fatiguing exercise components, which might hinder the proficiency of their performance.

The reproducibility of different metabolic markers for muscle fiber type distributions investigated by functional 31P-MRS during dynamic exercise

PURPOSE

The objective of the study was to investigate the reproducibility of exercise induced pH-heterogeneity by splitting of the inorganic phosphate (Pi) signal in the corresponding ³¹P-MRS spectra and to compare results of this approach with other fiber-type related markers, like phosphocreatine/adenosine triphosphate (PCr/ATP) ratio, and PCr-recovery parameters.

MATERIAL AND METHODS

Subjects (N=3) with different sportive background were tested in 10 test sessions separated by at least 3 days. A MR-compatible pedal ergometer was used to perform the exercise and to induce a pH-based splitting of the Pi-signal in ³¹P-MR spectra of the medial gastrocnemius muscle. The PCr recovery was analyzed using a non-negative least square algorithm (NNLS) and multi-exponential regression analysis to estimate the number of non-exponential components as well as their amplitude and time constant. The reproducibility of the estimated metabolic marker and the resulting fiber-type distributions between the 10 test sessions were compared.

RESULTS

The reproducibility (standard deviation between measurements) based on (1) Pi components varied from 2% to 4%, (2) PCr recovery time components varied from 10% to 12% and (3) phosphate concentrations at rest varied from 8% to 11% between test sessions. Due to the sportive activity differences between the 3 subjects were expected in view of fiber type distribution. All estimated markers indicate the highest type I percentage for volunteer 3 medium for volunteer 2 and the lowest for volunteer 1.

CONCLUSIONS

The relative high reproducibility of pH dependent Pi components during exercise indicates a high potential of this method to estimate muscle fiber-type distributions in vivo. To make this method usable not only to detect differences in muscle fiber distributions but also to determine individual fiber-type volume contents it is therefore recommended to validate this marker by histological methods and to reveal the effects of muscle fiber recruitments and fiber-type specific Pi concentrations on the intensity ratios between the splitted Pi-components.

Skeletal muscle alkaline Pi pool is decreased in overweight-to-obese sedentary subjects and relates to mitochondrial capacity and phosphodiester content

Defects in skeletal muscle energy metabolism are indicative of systemic disorders such as obesity or type 2 diabetes. Phosphorus magnetic resonance spectroscopy ((31)P-MRS), in particularly dynamic (31)P-MRS, provides a powerful tool for the non-invasive investigation of muscular oxidative metabolism. The increase in spectral and temporal resolution of (31)P-MRS at ultra high fields (i.e., 7T) uncovers new potential for previously implemented techniques, e.g., saturation transfer (ST) or highly resolved static spectra. In this study, we aimed to investigate the differences in muscle metabolism between overweight-to-obese sedentary (Ob/Sed) and lean active (L/Ac) individuals through dynamic, static, and ST (31)P-MRS at 7T. In addition, as the dynamic (31)P-MRS requires a complex setup and patient exercise, our aim was to identify an alternative technique that might provide a biomarker of oxidative metabolism. The Ob/Sed group exhibited lower mitochondrial capacity, and, in addition, static (31)P-MRS also revealed differences in the Pi-to-ATP exchange flux, the alkaline Pi-pool, and glycero-phosphocholine concentrations between the groups. In addition to these differences, we have identified correlations between dynamically measured oxidative flux and static concentrations of the alkaline Pi-pool and glycero-phosphocholine, suggesting the possibility of using high spectral resolution (31)P-MRS data, acquired at rest, as a marker of oxidative metabolism.

Muscle metabolism changes with age and maturation: How do they relate to youth sport performance?

AIM

To provide an evidence-based review of muscle metabolism changes with sex-, age- and maturation with reference to the development of youth sport performance.

METHODS

A narrative review of data from both invasive and non-invasive studies, from 1970 to 2015, founded on personal databases supported with computer searches of PubMed and Google Scholar.

RESULTS

Youth sport performance is underpinned by sex-, age- and maturation-related changes in muscle metabolism. Investigations of muscle size, structure and metabolism; substrate utilisation; pulmonary oxygen uptake kinetics; muscle phosphocreatine kinetics; peak anaerobic and aerobic performance; and fatigue resistance; determined using a range of conventional and emerging techniques present a consistent picture. Age-related changes have been consistently documented but specific and independent maturation-related effects on muscle metabolism during exercise have proved elusive to establish. Children are better equipped for exercise supported primarily by oxidative metabolism than by anaerobic metabolism. Sexual dimorphism is apparent in several physiological variables underpinning youth sport performance. As young people mature there is a progressive but asynchronous transition into an adult metabolic profile.

CONCLUSIONS

The application of recent developments in technology to the laboratory study of the exercising child and adolescent has both supplemented existing knowledge and provided novel insights into developmental exercise physiology. A sound foundation of laboratory-based knowledge has been established but the lack of rigorously designed child-specific and sport-specific testing environments has clouded the interpretation of the data in real life situations. The primary challenge remains the translation of laboratory research into the optimisation of youth sports participation and performance.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Abnormal Weight and Body Mass Index in Children with Juvenile Huntington's Disease

BACKGROUND AND OBJECTIVES

The hallmark clinical manifestation of Huntington's disease (HD), namely lower weight and BMI has been reported in prodromal HD (PreHD) adults and also in PreHD children. Here, we aim to evaluate anthropometric measures of growth and development (height, weight, body mass index (BMI)) in a group of children, adolescents, and young adults diagnosed with Juvenile Onset Huntington's Disease (JHD).

METHODS

Growth measures for 18 JHD patients, documented prior to or shortly after diagnosis, were obtained through medical records. JHD growth measures were compared to a large sample (n = 274) of healthy children, as well as the Center for Disease Control (CDC) growth norms.

RESULTS

After controlling for sex and age, the JHD subjects had no significant differences in height. However, they were an average of 10% lower than controls in weight and BMI. Using CDC norms, the JHD subjects had the same pattern of normal height but decrement in weight. Length of cytosine-adenine-guanine (CAG) repeat in the huntingtin gene was significantly correlated to measures of weight with longer CAG repeats being associated with more severe weight reduction. A subset of 4 subjects had measures that pre-dated onset of any symptom and were therefore prodromal JHD (preJHD). These subjects also had a significant decrement in BMI compared to CDC norms.

CONCLUSIONS

Children with JHD have normal height, but significantly reduced weight and BMI, indicative of a specific deficit in body weight. As the preJHD subjects were also low in BMI, this suggests that these changes are directly due to the effect of the mutated gene on development, rather than symptom manifestation of the disease itself. Potential mechanisms of the weight decrement include energy deficiency due to mitochondrial dysfunction during development.

Effect of fat- and carbohydrate-rich diets on metabolism and running performance in trained adolescent boys

OBJECTIVES

A randomized crossover trial was designed to analyze the impact of a short-term, isoenergetic fat-rich or carbohydrate (CHO)-rich diet on substrate oxidation rates during submaximal exercise and on performance in a 10,000-m running time trial in trained, mid- to late-pubertal boys.

METHODS

An incremental test was performed to determine the peak oxygen uptake (VO2peak). After 2 days on a fat-rich (24.2% ± 0.8% CHO, 60.4% ± 0.3% fat, and 15.5% ± 1.0% protein), CHO-rich (69.3% ± 1.2% CHO, 15.9% ± 2.1% fat, and 15.1% ± 1.1% protein), or habitual (56.1% ± 7.0% CHO, 27.5% ± 4.9% fat, and 16.5% ± 4.0% protein) diet, 19 trained adolescent boys (15.2 ± 1.5 years) performed a 10-minute constant run at 65% VO2peak to determine the respiratory exchange ratio (RER) during exercise and 10,000-m running on an outdoor track.

RESULTS

During the constant run, the RER and CHO contribution to energy expenditure were lower, and fat contribution higher, in the fat-rich diet than in the CHO-rich diet (P < 0.05), but the results were not different from those of the habitual diet. Performance in the 10,000-m run after consuming CHO- and fat-rich diets was similar to performance after a habitual diet (50.0 ± 7.0, 51.9 ± 8.3, and 50.9 ± 7.4 minutes, respectively), but consuming a CHO-rich diet enhanced performance compared with that after a fat-rich diet (P = 0.03).

CONCLUSIONS

These findings indicate that a CHO-rich diet provides additional benefits to 10,000-m running performance in trained adolescent boys compared with a fat-rich diet.

Influence of thigh activation on the VO₂ slow component in boys and men

PURPOSE

During constant work rate exercise above the lactate threshold (LT), the initial rapid phase of pulmonary oxygen uptake (VO₂) kinetics is supplemented by an additional VO₂ slow component (VO₂Sc) which reduces the efficiency of muscular work. The VO₂Sc amplitude has been shown to increase with maturation but the mechanisms are poorly understood. We utilized the transverse relaxation time (T₂) of muscle protons from magnetic resonance imaging (MRI) to test the hypothesis that a lower VO₂ slow component (VO₂Sc) amplitude in children would be associated with a reduced muscle recruitment compared to adults.

METHODS

Eight boys (mean age 11.4 ± 0.4) and eight men (mean age 25.3 ± 3.3 years) completed repeated step transitions of unloaded-to-very heavy-intensity (U → VH) exercise on a cycle ergometer. MRI scans of the thigh region were acquired at rest and after VH exercise up to the VO₂Sc time delay (ScTD) and after 6 min. T₂ for each of eight muscles was adjusted in relation to cross-sectional area and then summed to provide the area-weighted ΣT₂ as an index of thigh recruitment.

RESULTS

There were no child/adult differences in the relative VO₂Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (∆) in area-weighted ΣT₂ between the ScTD and 6 min was not different between groups (Boys 1.6 ± 1.2 vs. Men 2.3 ± 1.1 ms, P = 0.27, ES = 0.6). A positive and strong correlation was found between the relative VO₂Sc amplitude and the magnitude of the area-weighted ∆ΣT₂ in men (r = 0.92, P = 0.001) but not in boys (r = 0.09, P = 0.84).

CONCLUSIONS

This study provides evidence to show that progressive muscle recruitment (as inferred from T₂ changes) contributes to the development of the VO₂Sc during intense submaximal exercise independent of age.

Interrelation of 31P-MRS metabolism measurements in resting and exercised quadriceps muscle of overweight-to-obese sedentary individuals

Phosphorus magnetic resonance spectroscopy ((31)P-MRS) enables the non-invasive evaluation of muscle metabolism. Resting Pi-to-ATP flux can be assessed through magnetization transfer (MT) techniques, and maximal oxidative flux (Q(max)) can be calculated by monitoring of phosphocreatine (PCr) recovery after exercise. In this study, the muscle metabolism parameters of 13 overweight-to-obese sedentary individuals were measured with both MT and dynamic PCr recovery measurements, and the interrelation between these measurements was investigated. In the dynamic experiments, knee extensions were performed at a workload of 30% of maximal voluntary capacity, and the consecutive PCr recovery was measured in a quadriceps muscle with a time resolution of 2 s with non-localized (31)P-MRS at 3 T. Resting skeletal muscle metabolism was assessed through MT measurements of the same muscle group at 7 T. Significant linear correlations between the Q(max) and the MT parameters k(ATP) (r = 0.77, P = 0.002) and F(ATP) (r = 0.62, P = 0.023) were found in the study population. This would imply that the MT technique can possibly be used as an alternative method to assess muscle metabolism when necessary (e.g. in individuals after stroke or in uncooperative patients).

Muscle metabolism during fatiguing isometric quadriceps exercise in adolescents and adults

Children and adolescents are less susceptible to muscle fatigue during repeated bouts of high-intensity exercise than adults, but the physiological basis for these differences is not clear. The purpose of the current investigation was to investigate the muscle metabolic responses, using 31-phosphorus magnetic resonance spectroscopy, during fatiguing isometric quadriceps exercise in 13 adolescents (7 females) and 14 adults (8 females). Participants completed 30 maximal voluntary contractions (6-s duration) separated by 6 s of rest. Fatigue was quantified as the relative decrease in force over the test. Fatigue was not significantly different with age (p = 0.20) or sex (p = 0.63). Metabolic perturbation (change in phosphocreatine, inorganic phosphate, and ADP concentrations) was significantly greater in adults compared with adolescents; no sex effects were present. Muscle pH did not differ with age or sex. Phosphocreatine recovery following exercise was not significantly different with age (p = 0.27) or sex (p = 0.97) but a significant interaction effect was present (p = 0.04). Recovery tended to be faster in boys than men but slower in girls than women, though no significant group differences were identified. The results of this study show that at a comparable level of muscle fatigue, the metabolic profile is profoundly different between adolescents and adults.

Effects of exercise-induced intracellular acidosis on the phosphocreatine recovery kinetics: a 31P MRS study in three muscle groups in humans

Little is known about the metabolic differences that exist among different muscle groups within the same subjects. Therefore, we used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to investigate muscle oxidative capacity and the potential effects of pH on PCr recovery kinetics between muscles of different phenotypes (quadriceps (Q), finger (FF) and plantar flexors (PF)) in the same cohort of 16 untrained adults. The estimated muscle oxidative capacity was lower in Q (29 ± 12 mM min(-1), CV(inter-subject) = 42%) as compared with PF (46 ± 20 mM min(-1), CV(inter-subject) = 44%) and tended to be higher in FF (43 ± 35 mM min(-1), CV(inter-subject) = 80%). The coefficient of variation (CV) of oxidative capacity between muscles within the group was 59 ± 24%. PCr recovery time constant was correlated with end-exercise pH in Q (p < 0.01), FF (p < 0.05) and PF (p < 0.05) as well as proton efflux rate in FF (p < 0.01), PF (p < 0.01) and Q (p = 0.12). We also observed a steeper slope of the relationship between end-exercise acidosis and PCr recovery kinetics in FF compared with either PF or Q muscles. Overall, this study supports the concept of skeletal muscle heterogeneity by revealing a comparable inter- and intra-individual variability in oxidative capacity across three skeletal muscles in untrained individuals. These findings also indicate that the sensitivity of mitochondrial respiration to the inhibition associated with cytosolic acidosis is greater in the finger flexor muscles compared with locomotor muscles, which might be related to differences in permeability in the mitochondrial membrane and, to some extent, to proton efflux rates.

Changes in phosphocreatine concentration of skeletal muscle during high-intensity intermittent exercise in children and adults

PURPOSE

The aim of the present study was to test the hypotheses that a greater oxidative capacity in children results in a lower phosphocreatine (PCr) depletion, a faster PCr resynthesis and a lower muscle acidification during high-intensity intermittent exercise compared to adults.

METHODS

Sixteen children (9.4 ± 0.5 years) and 16 adults (26.1 ± 0.3 years) completed a protocol consisting of a dynamic plantar flexion (10 bouts of 30-s exercise at 25 % of one repetition maximum separated by 20-s recovery), followed by 10 min of passive recovery. Changes of PCr, ATP, inorganic phosphate, and phosphomonoesters were measured by means of (31)Phosphorous-magnetic resonance spectroscopy during and post-exercise.

RESULTS

Average PCr (percentage of [PCr] at initial rest (%[PCr]i)) at the end of the exercise (adults 17 ± 12 %[PCr]i, children 38 ± 17 %[PCr]i, P < 0.01) and recovery periods (adults 37 ± 14 %[PCr]i, children 57 ± 17 %[PCr]i, P < 0.01) was significantly lower in adults compared to children, induced by a stronger PCr decrease during the first exercise interval (adults -73 ± 10 %[PCr]i, children -55 ± 15 %[PCr]i, P < 0.01). End-exercise pH was significantly higher in children compared to adults (children 6.90 + 0.20, -0.14; adults 6.67 + 0.23, -0.15, P < 0.05).

CONCLUSIONS

From our results we suggest relatively higher rates of oxidative ATP formation in children's muscle for covering the ATP demand of high-intensity intermittent exercise compared to adults, enabling children to begin each exercise interval with significantly higher PCr concentrations and leading to an overall lower muscle acidification.