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Optimization of post-activation potentiation in girls and womenBackground Maximal conditioning contractions (CCs) can lead to the enhancement of evoked-twitch characteristics in human skeletal muscle. This phenomenon is termed post-activation potentiation (PAP). In the knee extensors, PAP is greater in men compared with boys. In adults, the optimal CC duration for PAP is ~ 10 s. We examined child–adult differences in PAP among females and aimed to determine the optimal CC duration in girls and women. Methods Eleven girls (9.3 ± 1.4 years) and 13 women (23.4 ± 2.7 years) participated in this study. Maximal isometric evoked twitches were recorded in the knee extensors before and after 4 maximal CCs of different durations (5, 10, 20, and 30 s), in a random order. PAP was calculated as the percent-change in peak torque (Tpeak) and peak rate of torque development (RTDpeak) after each CC. Results There was a group-by-duration interaction (p < 0.001), reflecting greater Tpeak PAP in women compared with girls following 5 and 10 s CCs, and lower RTDpeak PAP in women following the 30 s CC. The 5 and 10 s CCs lead to the greatest Tpeak and RTDpeak PAP amongst the women while there were no differences between CC durations in girls. Conclusion After both a 5 and 10 s CC, women have greater PAP compared with girls. The optimal CC duration for the knee extensors in women appears to be ~ 5-10 s, while CC durations between 5 and 30 s do not appear to affect levels of PAP in girls.
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Isometric and dynamic strength and neuromuscular attributes as predictors of vertical jump performance in 11- to 13-year-old male athletesIn explosive contractions, neural activation is a major factor in determining the rate of torque development, while the latter is an important determinant of jump performance. However, the contribution of neuromuscular activation and rate of torque development to jump performance in children and youth is unclear. The purpose of this study was to examine the relationships between the rate of neuromuscular activation, peak torque, rate of torque development, and jump performance in young male athletes. Forty-one 12.5 ± 0.5-year-old male soccer players completed explosive, unilateral isometric and dynamic (240°/s) knee extensions (Biodex System III), as well as countermovement-, squat-, and drop-jumps. Peak torque (pT), peak rate of torque development (pRTD), and rate of vastus lateralis activation (Q30) during the isometric and dynamic contractions were examined in relation to attained jump heights. Isometric pT and pRTD were strongly correlated (r = 0.71) but not related to jump performance. Dynamic pT and pRTD, normalized to body mass, were significantly related to jump height in all 3 jumps (r = 0.38-0.66, p < 0.05). Dynamic normalized, but not absolute pRTD, was significantly related to Q30 (r = 0.35, p < 0.05). In young soccer players, neuromuscular activation and rate of torque development in dynamic contractions are related to jump performance, while isometric contractions are not. These findings have implications in the choice of training and assessment methods for young athletes.
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Effect of passive heat exposure on cardiac autonomic function in healthy childrenPurpose: The aim of this study was to examine the effect of passive heat stress on heart rate variability parameters in healthy children. Method: Fifteen children (9.3±1.6 years) of both sexes (8 male) participated in two randomized experimental conditions separated by 5 to 12 days. Children were seated for 2 h in an environmental chamber for 2 sessions: Neutral (22.4±0.1°C, 40.4±6.5%RH) and Hot (34.9±0.3°C, 36.6±6.2%RH) conditions. Electrocardiogram, mean skin temperature, tympanic temperature, and blood pressure were recorded. Five min epochs were averaged for analysis of cardiac autonomic function over the 2 h protocol. Result: Mean skin and tympanic temperatures and heart rate increased during the Hot condition (all p<0.01) while mean arterial pressure decreased (p<0.01). During the Hot condition, root-mean-square difference of successive normal RR intervals (45±9 to 38±7 ms), low- (LF, 1536±464 vs. 935±154 ms2) and high-frequency power (HF, 1544±693 vs. 866±355 ms2) decreased, whereas LF/HF ratio increased (1.64±0.24 vs. 2.40±0.23 au); all indices were different from Neutral (all p<0.05). These were all unchanged throughout the Neutral condition (all p>0.05), except for LF/HF ratio which decreased during the Neutral condition (p<0.05). Conclusion: Mild hyperthermia elicited marked changes in cardiac autonomic control in young children. These data suggest that, in healthy children, vagal withdrawal is responsible for the cardiac autonomic response to hyperthermia.
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Measurement and Interpretation of Maximal Aerobic Power in ChildrenThe assessment of maximal aerobic power (VO2max) in both children and adults is an invaluable tool for the evaluation of exercise performance capacity and general physical fitness in clinical, athletic, public health, and research applications. The complexity of means and considerations, as well as varying specific aims of VO2 max testing, has prevented the formulation of a universally applicable, standard testing protocol, in general, and for children in particular. Numerous tester-controllable factors, such as exercise modality, metabolic measurement system, testing protocol, or data reduction strategies, can affect both the measurement and interpretation of VO2max data. Although the general guiding principles are similar, children differ from adults in several aspects. One notable difference is the frequent absence of a discernible VO2max plateau in children. Thus, the proper choice of equipment and procedures may be different for children than for adults. It is therefore the aim of this article to highlight the general and pediatric-specific considerations that may affect VO2max measurement and interpretation of results.
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Comparison of laser speckle contrast imaging and laser-Doppler fluxmetry in boys and menObjective: We compare microvascular reactivity assessed by laser-Doppler fluxmetry (LDF) and laser speckle contrast imaging (LSCI) of boys and men during rest, post-occlusive reactive hyperaemia (PORH), and cycling exercise. Methods: 19 boys (9±1 y) and 18 men (22±2 y) participated. LDF and LSCI measures were taken of the forearm during rest, PORH, and exercise. Results: For all 3 assessments, the LSCI presented with higher flux values than the LDF for both boys and men (p<0.001). Bland-Altman analyses indicated that there was a positive linear bias between LSCI and LDF measurements in both boys and men. Regression analyses showed that the responses for the two methods were variable, depending on the particular assessment. For instance, at rest in boys there was no relationship between LDF and LSCI (r2=0.002), whilst in men there was a strong relationship (r2=0.86). Conclusions: LSCI presented with higher values than LDF during rest, PORH, and exercise; the disparity between the two measures was larger as blood flow increased. The assessments were generally consistent, both methods appear to provide usable data for the assessment of microvascular reactivity in both boys and men. There are biases to each method and the data are not interchangeable between LDF and LSCI.
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Isometric-based EMG threshold in girls and womenBackground: The electromyographic threshold (EMGTh) has been suggested to indicate the onset of accelerated higher-threshold (type-II) MU recruitment. Previous research has demonstrated that boys’ EMGTh occurs at higher relative exercise intensities than men’s in both cycling- and isometric-based testing. Girls‒women EMGTh differences were demonstrated only in cycling, but findings were clouded by low EMGTh-detection rates in women (68%) and particularly in girls (45%). Purpose: To examine the EMGTh, in girls and women, using the same males-employed isometric-based test protocol, and compare the females’ findings with those previously obtained in the males. Methods: Seventeen girls and 17 women had their EMGTh determined as well as their one repetition-maximum isometric knee-extension strength (1RM). Vastus-lateralis sEMG root mean square was recorded and the EMGTh was defined as the exercise intensity (%1RM) at the bi-segmental point of the least sum of squares. Results: EMGTh was detected in 88.2% of girls and 94.1% of women, and occurred at higher relative intensities in the girls than in women (56.0±11.1 vs. 47.7±8.0 %1RM). The girls’ 1RM (normalized to lean body mass) was only 69.1% that of the women. Conclusions: Girls’ EMGTh values are higher compared with women’s, possibly reflecting lower ability to activate higher-threshold (type-II) motor units. The females’ EMGTh and detection rate values were similar to the corresponding values previously observed in males. The females’ age-related difference in the recruitment of higher-threshold motor units, as reflected by the EMGTh, appears to be on par with the males.
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Salivary and Serum Concentrations of Cortisol and Testosterone at Rest and in Response to Intense Exercise in Boys Versus MenThis study compared salivary and serum concentrations of testosterone and cortisol at rest and in response to intense multitask exercise, in boys and men. Early morning saliva and venous blood samples were obtained pre- and 15 min post-exercise from 30 competitive swimmers (15 boys, 14.3±1.9y; 15 men, 21.7±3.1y). Exercise included a swim-bench maximal strength task, an all-out 200m swim, followed by a high-intensity interval swimming protocol (5 x 100m, 5 x 50m, and 5 x 25m). At baseline, fasting testosterone (but not cortisol) concentration was higher in men than boys in serum and saliva (p<0.05). Salivary and serum cortisol increased post-exercise, with a greater increase in men compared with boys (men: 226% and 242%, respectively, boys: 78% and 64%, respectively; group-by-time interaction, p<0.05). Testosterone was reduced post-exercise in serum but not in saliva (men: -14.7% and 0.1%, respectively, boys: -33.9% and - 4.5%, respectively, fluid-by-time interaction, p<0.01). Serum and salivary cortisol (but not testosterone), pre- and post-exercise values were strongly correlated in both men and boys (r=0.79 and 0.82, respectively; p<0.01). In summary, early morning high-intensity exercise results in a decrease in testosterone in serum, but not saliva, and an increase in cortisol irrespective of the fluid used, in both boys and men. When examining immediate post-exercise changes, the lack of correlation in testosterone between saliva and serum suggests that saliva may not be an appropriate fluid to examine changes in testosterone. The high correlation observed between serum and saliva for cortisol indicates that, in both boys and men, saliva may be used to monitor the immediate cortisol response to exercise.
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Isometric-based test improves EMG-threshold determination in boys vs. menBackground: Children have been hypothesized to utilize higher-threshold (type-II) motor units (MUs) to a lesser extent than adults. Two recent studies, using a cycling-based EMG-threshold (EMGTh) protocol, supported the hypothesis, showing children’s EMGTh intensities to be higher than adults’. Conclusions, however, were hampered by children’s low EMGTh detection rates. Insufficiently high contractile forces at exhaustion were postulated as the reason for non-detection, predominantly in children. An intermittent isometric contraction test (IICT) protocol facilitates higher contractile forces prior to exhaustion and was shown effective in EMGTh testing of adults. Purpose: Determine whether an IICT protocol would enhance EMGTh detection in children, and consequently increase the magnitude of the previously observed child–adult EMGTh differences. Methods: 18 boys and 21 men completed one-repetition-maximum (1RM) isometric knee-extension test. The IICT protocol followed, commencing at 25%1RM and comprising five isometric contractions per load, incremented by ~ 3%1RM to exhaustion. Vastus lateralis surface EMG was recorded and EMGTh, expressed as %1RM, was defined as the onset of the EMG-response’s steeper segment. Results: EMGTh was detected in 88.9% of boys and 95.2% of men, and occurred at higher relative intensities in boys (56.4 ± 9.2%1RM) than in men (46.0 ± 6.8%1RM). This 10.4% difference was 57% greater than the corresponding, previously reported cycling-based age-related difference. Conclusions: With the boys’ detection rate nearly on par with the men’s, the IICT protocol appears to overcome much of the intensity limitation of cycling-based protocols and provide a more sensitive EMGTh detection tool, thus extending the previously observed boys‒men difference. This difference adds supports to the notion of children’s more limited type-II MU recruitment capacity.
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The effect of acute low-load resistance exercise with the addition of blood flow occlusion on muscle function in boys and menPurpose: In adults, low-load resistance training with blood flow occlusion (BFO) mimics strength increases that occur from high-load training, without the need to experience high mechanical stress. In view of child–adult differences in exercise responses, this study examined whether BFO during exercise elicits differential changes in maximal voluntary contraction (MVC) and electromyographical (EMG) activity in children and adults. Methods: Sixteen men (24.4 ± 2.5 years) and 14 boys (10.7 ± 2.0 years) performed low-load resistance exercise (25 repetitions at 35% MVC) of the wrist flexors with and without BFO. MVC wrist flexor force and EMG activity of the flexor carpi radialis (FCR) were obtained at the beginning and end of the exercise. Results: Both groups demonstrated a larger decrease in MVC force following BFO (− 18.6 ± 12.5%) than the control (without BFO) condition (− 6.2 ± 15.0%; p < 0.001). Whereas the men’s EMG amplitude increased 16.3 ± 20.5% (p = 0.005) during BFO, the boys’ EMG amplitude did not change over time or between conditions. In both groups, the mean power frequency (MPF) of the EMG signal decreased more during BFO (− 20.1 ± 9.6%; p < 0.001) than the control condition (− 5.6 ± 9.7%; p = 0.002). Conclusions: Low-load exercise with BFO resulted in similar neuromuscular responses between boys and men, except for an observed increase in the EMG amplitude in men but not boys. While this result might suggest that men relied on a greater activation of higher-threshold motor units during BFO, it does not explain why there were similar decreases in MPF between groups. Therefore, it remains unclear whether the effectiveness of BFO training is similar for children and adults.
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Bone Turnover Markers and Osteokines in Adolescent Female Athletes of High- and Low-Impact Sports Compared With Nonathletic ControlsThis study examined differences in resting concentrations of markers of bone formation and resorption, and osteokines between female adolescent (12–16 y) swimmers, soccer players, and nonathletic controls. Resting, morning blood samples were obtained after an overnight fast from 20 swimmers, 20 soccer players, and 20 nonathletic controls, matched for age. carboxyl-terminal cross-linking telopeptide of type I collagen (CTX), amino-terminal propeptide of type I collagen (P1NP), total osteocalcin (OC), sclerostin, osteoprotegerin (OPG), and receptor activator of nuclear factor kappa B ligand (RANKL) were analyzed in serum. After controlling for percent body fat, there were no significant differences between swimmers and nonathletic controls in any of the measured markers. In contrast, soccer players had significantly higher P1NP (89.5 [25.6] ng·mL−1), OC (57.6 [22.9] ng·mL−1), and OPG (1052.5 [612.6] pg·mL−1) compared with both swimmers (P1NP: 66.5 [20.9] ng·mL−1; OC: 24.9 [12.5] ng·mL−1; OPG: 275.2 [83.8] pg·mL−1) and controls (P1NP: 58.5 [16.2] ng·mL−1; OC: 23.2 [11.9] ng·mL−1; OPG: 265.4 [97.6] pg·mL−1), with no differences in CTX, sclerostin, and RANKL. These results suggest that bone formation is higher in adolescent females engaged in high-impact sports like soccer compared with swimmers and controls.
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Boys–men mean-power-frequency differences in progressive exercise to exhaustion, confounded by variability and adiposityBackground: Only scant research has compared children’s mean power frequency (MPF) to adults’, with a clear overview still lacking. A significant obstacle has been MPF’s high variability, which this study aimed to overcome by elucidating the MPF characteristics distinguishing boys from men in progressive exhaustive exercise. Methods: Electromyographic (EMG) data of 20 men (23.5 ± 2.5yrs) and 17 boys (10.2 ± 1.0 yrs), who performed progressively exhausting, intermittent isometric knee extensions, were subjected to secondary MPF analysis. Participants’ vastus lateralis MPF data series were transformed to third-order polynomial regressions and expressed as percentages of the peak polynomial MPF values (%MPFpeak). The resulting curves were compared at 5-% time-to-exhaustion (TTE) intervals, using repeated-measures ANOVA. Raw MPFpeak values were adiposity corrected to 0% fat and used to convert the %MPFpeak data back to absolute MPF values (Hz) for estimating muscle-level MPF. Results: No overall interaction or group effects could be shown between the %MPFpeak plots, but pairwise comparisons revealed significantly higher men’s values at 50–70%TTE and lower at 100%TTE, i.e. boys’ shallower MPF rise and decline. The adiposity-corrected boys’ and men’s composite MPF values peaked at 125.7 ± 2.5 and 166.0 ± 2.4 Hz, respectively (110.7 ± 1.7 and 122.5 ± 2.1 Hz, uncorrected), with a significant group effect (p < 0.05) and pairwise differences at all %TTE points. Conclusions: The boys were lower than the men in both the observed and, more so, in the adiposity-corrected MPF values that presumably estimate muscle-level MPF. The boys’ shallower MPF rise and decline conform to children’s claimed type-II motor-unit activation and/or compositional deficits and their related known advantage in muscular endurance.
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Skin Blood Flow Responses to Acetylcholine, Local Heating, and to 60% VO2max exercise with and without Nitric Oxide inhibition, in Boys vs. GirlsPurpose: To determine sex-related differences in the skin blood flow (SkBF) response to exercise, local heating, and acetylcholine (ACh) in children, and to assess nitric oxide contribution to the SkBF response. Methods: Forearm SkBF during local heating (44°C), ACh iontophoresis, and exercise (30-min cycling and 60% of maximum oxygen consumption) was assessed, using laser Doppler fluxmetry, in 12 boys and 12 girls (7–13 y old), with and without nitric oxide synthase inhibition, using Nω-nitro-L-arginine methyl ester iontophoresis. Results: Local-heating-induced and ACh-induced SkBF increase were not different between boys and girls (local heating: 1445% [900%] and 1432% [582%] of baseline, P = .57; ACh: 673% [434%] and 558% [405%] of baseline, respectively, P = .18). Exercise-induced increase in SkBF was greater in boys than girls (528% [290%] and 374% [192%] of baseline, respectively, P = .03). Nω-nitro-L-arginine methyl ester blunted the SkBF response to ACh and during exercise (P < .001), with no difference between sexes. Conclusion: SkBF responses to ACh and local heat stimuli were similar in boys and girls, while the increase in SkBF during exercise was greater in boys. The apparent role of nitric oxide was not different between boys and girls. It is suggested that the greater SkBF response in boys during exercise was related to greater relative heat production and dissipation needs at this exercise intensity. The response to body size-related workload should be further examined.
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The skin blood flow response to exercise in boys and men and the role of nitric oxidePurpose: Children thermoregulate effectively during exercise despite sweating rate being consistently lower when compared with adults. The skin blood flow (SkBF) response of children to exercise is inconsistent, when compared with adults. We examined the SkBF response to exercise in children and adults, along with the potential contribution of nitric oxide to the SkBF response. Methods: Forearm SkBF during cycling (30 min at 60% O2max) was investigated in 12 boys (10 ± 1 years) and 12 men (22 ± 2 years) using laser-Doppler flowmetry and Nω-nitro-l-arginine methyl ester (L-NAME) iontophoresis to inhibit nitric oxide synthase. Results: The exercise-induced SkBF increase was similar in boys and men (mean ± SD, 540 ± 127 vs. 536 ± 103% baseline, respectively, p = 0.43, d = 0.01 [− 0.8 to 0.8]). However, the total hyperaemic response to exercise (area-under-the-curve, AUC) indicated that boys had a greater vasodilatory response (cutaneous vascular resistance, CVC) (p < 0.01, d = 0.6 [− 1.2 to 2.8] than the men (134,215 ± 29,207 vs. 107,257 ± 20,320 CVC·s−1). L-NAME blunted the SkBF response more in boys than in men (group-by-treatment interaction, p < 0.001) and resulted in smaller AUC in boys (56,411 ± 23,033 CVC·s−1; p < 0.001, d = 1.4 [− 0.4 to 3.2] compared with men (80,556 ± 28,443 CVC·s−1; p = 0.08, d = 0.8 [0.0–1.6]). Boys had a shorter delay from the onset of exercise to onset of SkBF response compared with men (205 ± 48 and 309 ± 71 s, respectively; p < 0.01, d = 1.7 [0.9–2.8]). L-NAME increased the delay in boys and men (to 268 ± 90 and 376 ± 116 s, respectively; p = 0.01, d = 1.0 [0.4–2.1]) but this delay was not significantly different between the groups (p = 0.85). Conclusions: These findings suggest that boys experience greater vasodilation and faster increases in SkBF during exercise compared with men. The contribution of nitric oxide to the SkBF response to exercise appears to be greater in boys than in men.
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Reporting of Adverse Events in Muscle Strengthening Interventions in Youth: A Systematic ReviewClear definition, identification, and reporting of adverse event (AE) monitoring during training interventions are essential for decision making regarding the safety of training and testing in youths. Purpose: To document the extent to which AEs, resulting from intervention studies targeting muscle strengthening training (MST) in youth, are reported by researchers. Methods: Electronic databases (CINAHL, PubMed, SPORTDiscus, and Web of Science) were searched for English peer-reviewed articles published before April 2018. Inclusion criteria were: (1) average age <16 years, (2) use of MST, (3) statement(s) linked to the presence/absence of AEs, and (4) randomized controlled trials or quasi-experimental designs. Risk of reporting bias for AEs followed recommendations by the Cochrane Collaboration group. Results: One hundred and ninety-one full-text articles were screened. One hundred and thirty met all MST criteria, out of which only 44 (33.8%; n = 1278, age = 12.1 [1.1] y) included a statement as to the presence/absence of adverse events. The 86 other studies (66.2%) included no such statement. Of the reporting 44 studies, 18 (40.1%) indicated one or more adverse events. Of the 93 reported adverse events, 55 (59.1%) were linked to training or testing. Conclusions: Most MST studies in youth do not report presence/absence of adverse events, and when reported, adverse events are not well defined.
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Different discrete motor-unit activation patterns in the flexor carpi radialis in boys and menBackground Lower activation of higher threshold (type-II) motor units (MUs) has been suggested in children compared with adults. We examined child–adult differences in discrete MU activation of the flexor carpi radialis (FCR). Methods Fifteen boys (10.2 ± 1.4 years), and 17 men (25.0 ± 2.7 years) completed 2 laboratory sessions. Following a habitu-ation session, maximal voluntary isometric wrist flexion torque (MVIC) was determined before completing trapezoidal isometric contractions at 70%MVIC. Surface electromyography was captured by Delsys Trigno Galileo sensors and decom-posed into individual MU action potential trains. Recruitment threshold (RT), and MU firing rates (MUFR) were calculated. Results MVIC was significantly greater in men (10.19 ± 1.92 Nm) than in boys (4.33 ± 1.47 Nm) (p < 0.05), but not statisti-cally different after accounting for differences in body size. Mean MUFR was not different between boys (17.41 ± 7.83 pps) and men (17.47 ± 7.64 pps). However, the MUFR–RT slope was significantly (p < 0.05) steeper (more negative) in boys, reflecting a progressively greater decrease in MUFR with increasing RT. Additionally, boys recruited more of their MUs early in the ramped contraction. Conclusion Compared with men, boys tended to recruit their MUs earlier and at a lower percentage of MVIC. This difference in MU recruitment may explain the greater decrease in MUFR with increasing RT in boys compared with men. Overall, these findings suggest an age-related difference in the neural strategy used to develop moderate–high torque in wrist flexors, where boys recruit more of their MUs earlier in the force gradation process, possibly resulting in a narrower recruitment range.
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Developmental changes in motor unit activity patterns: child-adult comparison using discrete motor unit analysisUsing global surface electromyography (sEMG) and the sEMG threshold it has been suggested that children activate their type-II motor unit (MU) to a lesser extent compared with adults. However, when age-related differences in discrete MU activation are examined using sEMG decomposition this phenomenon is not observed. Furthermore, findings from these studies are inconsistent and conflicting. Therefore, the purpose of this study was to examine differences in discrete MU activation of the vastus lateralis (VL) between boys and men during moderate-intensity knee extensions. Seventeen boys and 20 men completed two laboratory sessions. Following a habituation session, maximal voluntary isometric knee extension (MVIC) torque was determined before completing trapezoidal contractions at 70% MVIC. sEMG of the VL was captured and mathematically decomposed into individual MU action potential trains. Motor unit action potential amplitude (MUAPamp), recruitment threshold (RT), and MU firing rates (MUFR) were calculated. We observed that MUAPamp–RT slope was steeper in men compared with boys (p < 0.05) even after accounting for fat thickness and quadriceps muscle depth. The mean MUFR and y-intercept of the MUFR–RT relationship were significantly (p < 0.001) lower in boys than in men. The slope of the MUFR–RT relationship tended to be steeper in men, but the differences did not reach statistical significance (p = 0.056). Overall, our results suggest that neural strategies used to produce torque are different among boys and men. Such differences may be related, in part, to boys’ lower MUFR and lesser ability to activate their higher-threshold MUs. Although, other factors (e.g., muscle composition) likely also play a role.
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Post-activation potentiation and potentiated motor unit firing patterns in boys and menBackground Post-activation potentiation (PAP) describes the enhancement of twitch torque following a conditioning contraction (CC) in skeletal muscle. In adults, PAP may be related to muscle fibre composition and is accompanied by a decrease in motor unit (MU) firing rates (MUFRs). Muscle fibre composition and/or activation is different between children and adults. This study examined PAP and MU firing patterns of the potentiated knee extensors in boys and men. Methods Twenty-three boys (10.5 ± 1.3 years) and 20 men (23.1 ± 3.3 years) completed familiarization and experimental sessions. Maximal isometric evoked-twitch torque and MU firing patterns during submaximal contractions (20% and 70% maximal voluntary isometric contraction, MVIC) were recorded before and after a CC (5 s MVIC). PAP was calculated as the percent-increase in evoked-twitch torque after the CC. MU firing patterns were examined during submaximal contractions before and after the CC using Trigno Galileo surface electrodes (Delsys Inc) and decomposition algorithms (NeuroMap, Delsys Inc). MU action potential amplitudes (MUAPamp) and MUFRs were calculated for each MU and exponential MUFR-MUAPamp relationships were calculated for each participant and trial. Results PAP was higher in men than in boys (98.3 ± 37.1% vs. 68.8 ± 18.3%, respectively; p = 0.002). Following potentiation, the rate of decay of the MUFR-MUAPamps relationship decreased in both contractions, with a greater decrease among boys during the high-intensity contractions. Conclusion Lower PAP in the boys did not coincide with smaller changes in potentiated MU firing patterns, as boys had greater reductions in MUFRs with potentiation compared with men in high-intensity contractions.
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Child-Adult differences in antagonist muscle coactivation: A systematic reviewAntagonist coactivation is the simultaneous activation of agonist and antagonist muscles during a motor task. Age-related changes in coactivation may contribute to observed differences in muscle performance between children and adults. Our aim was to systematically summarize age-related differences in antagonist muscle coactivation during multi-joint dynamic and single-joint isometric and isokinetic contractions. Electronic databases were searched for peer-reviewed studies comparing coactivation in upper or lower extremity muscles between healthy children and adolescents/young adults. Of the 1083 studies initially identified, 25 met eligibility criteria. Thirteen studies examined multi-joint dynamic movements, 10 single-joint isometric contractions, and 2 single-joint isokinetic contractions. Of the studies investigating multi-joint dynamic contractions, 83% (11/13 studies) reported at least one significant age-related difference: In 84% (9/11 studies) coactivation was higher in children, whereas 16% (2/11 studies) reported higher coactivation in adults. Among single-joint contractions, only 25% (3/12 studies) reported significantly higher coactivation in children. Fifty six percent of studies examined females, with no clear sex-related differences. Child-adult differences in coactivation appear to be more prevalent during multi-joint dynamic contractions, where generally, coactivation is higher in children. When examining child–adult differences in muscle function, it is important to consider potential age-related differences in coactivation, specifically during multi-joint dynamic contractions.
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Cytokine concentrations in saliva vs. plasma at rest and in response to intense exercise in adolescent athletesBackground: Salivary measures are advantageous in conducting large paediatric studies involving repeated measures. However, research measuring salivary cytokines in youth is limited. Aim: Compare salivary with plasma concentrations of inflammatory cytokines at rest and following exercise in adolescent swimmers (21 male, 22 female). Methods: Following collection of resting saliva and blood samples, participants performed a bout of high-intensity interval swimming, with samples taken again 15 min post-swimming and analysed for interleukin-6 (IL-6), interleukin 10 (IL-10), and tumour necrosis factor-alpha (TNF-a). Results: Resting IL-10 was significantly lower, while IL-6 and TNF-a were significantly higher in saliva compared with plasma. IL-10 increased from pre- to post-swimming in plasma, but less so in saliva (51% vs. 29%; p ¼ 0.02). TNF-a decreased post-swimming in saliva, but not in plasma (–27% vs 1%; p ¼ 0.01). IL-6 decreased post-swimming in saliva compared with plasma (–21% vs. 3%; p ¼ 0.06). Intraclass correlation coefficients (ICC) revealed no association between salivary and plasma IL-6 and TNF-a, while IL-10 showed a weak correlation only at rest (ICC ¼ 0.39; p ¼ 0.05). Conclusions: Differences in concentrations and exercise responses, along with weak correlations, suggest that salivary cytokine levels are not an accurate representation of blood cytokine levels, and should not be used as a surrogate measure in paediatric studies.
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Motor unit fring rates during slow and fast contractions in boys and menBackground Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation difers, depending on the rate of contraction. We investigated the efect of contraction rate on discrete MU activation in boys and men. Methods Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist fexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and fexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains. Results In both groups and muscles, the initial MU fring rate (MUFR) was greater (p<0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p<0.05). Mean MUFR was signifcantly lower during fast contractions only in the FCR (p<0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p<0.05) during fast compared with slow contractions. Conclusion In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.
