Combined Endurance and Strength or Only Endurance Training? Effects of Training Mode on Neuromuscular Characteristics and Functional Abilities in Obese Adolescent Girls Enrolled in a Weight-Reduction Program.

The aim of the present study was to examine whether combining strength and endurance training would promote better improvements in neuromuscular characteristics and functional abilities than endurance training alone in obese adolescent girls enrolled in a weight-reduction program. Twenty-four obese adolescent girls (12-15 years) volunteered to participate in a 9-month training program. Participants were allocated into two groups following either (i) combined training (endurance + strength; E+ST) or (ii) endurance training (ET) program. Absolute and specific maximal torque, muscle size, and maximal voluntary activation level (VA) of the knee extensor (KE) and plantar flexor (PF) muscles were assessed. Moreover, functional abilities such as balance and fatigability during a maximal isometric intermittent contraction test of the KE muscles were measured before and after the intervention. The force of the adductor pollicis (AP) muscles was used as a control to account for any effect of growth or mechanical unloading on neuromuscular properties and muscle size. While absolute and specific torque of the KE (+14.7 ± 12.1% and +14.4 ± 15.5%; p < 0.05) and PF (+19.2 ± 16.7% and +18.3 ± 17.5%; p < 0.001) muscles increased in the E+ST group, PF torque decreased, and KE torque did not change in the ET group (-22.6 ± 10.5% and -15.0 ± 17.2%; p < 0.001). Moreover, the VA of the KE muscles increased for the E+ST (+6.1 ± 5.6%; p < 0.01) group and decreased for the ET group (-5.4 ± 5.4%; p < 0.05). In contrast, VA remained similar in the PF muscles for both groups. The number of repetitions during the fatigability test increased in the ET group (38.4 ± 22.3 vs. 84.1 ± 33.3; p = 0.032) and was unchanged in the E+ST group (50.8 ± 14.1 vs. 54.2 ± 37.8), but it was associated with a higher force level. Moreover, balance improved in the E+ST group, but not in the ET group. To conclude, physical training combining strength and endurance training promoted larger improvement in neuromuscular characteristics and functional abilities than endurance training alone in obese adolescent girls. Greater neuromuscular adaptations resulting from the E+ST training may be beneficial for preserving or even increasing functional abilities and possibly induce greater engagement in the active lifestyle of obese adolescents. However, the endurance component seems necessary in training programs to reduce fatigability during daily living activities.

Sex-Related Neuromuscular Adaptations to Youth Obesity: Force, Muscle Mass, and Neural Issues.

Young obese are generally stronger than their typically developing counterparts. Strength differences could be partly ascribed to nervous adaptations, due to the loading effect of carrying overweight. We hypothesized that central adaptations of the muscles highly involved in weight bearing, i.e., plantar flexors (PF) and knee extensors (KE) could be greater in girls than boys due to their reduced potential for muscle hypertrophy. Furthermore, it is possible that neuromuscular adaptations in weight-bearing muscles will be greater compared to the unloaded muscles such as the adductor pollicis (AP).Twenty-four non-obese and 21 obese (body mass index: 33 ± 4 kg·m-2) adolescent girls and boys (12-15 years) performed maximal voluntary isometric contractions (MVC) of the PF and KE muscles. Voluntary activation (VA), assessed with the twitch interpolation technique, the antagonist co-activation (Co-Act) level, and the normalized root-mean-square value (RMS) of the agonist muscles were measured to account for central adaptations.The results revealed a weight status effect (p < 0.001) on the absolute MVC torque and VA of both KE and PF muscles. Moreover, these differences were also related to the sex of the participants (p < 0.05) for the PF muscles. While the VA, absolute, and specific MVC torque were greater in obese compared with non-obese girls, no difference was found between boys. A similar Co-Act level was observed between groups, whatever the sex and muscle group considered. Finally, no significant differences were found for the AP regarding peripheral and neural factors.This study highlighted a favorable effect of obesity on the central mechanisms (i.e., VA) responsible for force production within the lower limb muscles. However, obesity-related central adaptation was only observed in girls for the PF muscles. Thus, the excess of body mass supported by the muscles involved in weight-bearing could act as a chronic training stimulus responsible for these adaptations in obese adolescents but mostly in girls.

Neuromuscular and Metabolic Responses during Repeated Bouts of Loaded Downhill Walking.

INTRODUCTION: The aim of this study was to compare vastus lateralis (VL) and rectus femoris (RF) muscles for their nervous and mechanical adaptations during two bouts of downhill walking (DW) with load carriage performed 2 wk apart. Moreover, we investigated cardiometabolic and perceived exertion responses during both DW bouts. METHODS: Seventeen participants performed two 45-min sessions of loaded DW (30% of body mass; slope, -25%; speed, 4.5 km·h -1 ) separated by 2 wk. Rating of perceived exertion, cost of walking, heart rate, and EMG activity of thigh muscles were assessed during the DW. Muscle shear elastic modulus ( µ ) of RF and VL were assessed before each exercise bout. Maximal voluntary contraction torque was assessed before (PRE), immediately after (POST), and 24 and 48 h after the two exercise bouts. RESULTS: Maximal voluntary contraction torque decreased from POST (-23.7% ± 9.2%) to 48 h (-19.2% ± 11.9%) after the first exercise (Ex1), whereas it was significantly reduced only at POST (-14.6% ± 11.0%) after the second exercise (Ex2; P < 0.001). Rating of perceived exertion (Ex1: 12.3 ± 1.9; Ex2: 10.8 ± 2.0), heart rate (Ex1: 156 ± 23 bpm; Ex2: 145 ± 25 bpm), cost of walking (Ex1: 4.5 ± 0.9 J·m -1 ·kg -1 ; Ex2: 4.1 ± 0.7 J·m -1 ·kg -1 ), and RF EMG activity (Ex1: 0.071 ± 0.028 mV; Ex2: 0.041 ± 0.014 mV) were significantly decreased during Ex2 compared with Ex1 ( P < 0.01). RF µ was significantly greater in Ex2 (0.44 ± 0.18) compared with Ex1 (0.56 ± 0.27; P < 0.001). CONCLUSIONS: The RF muscle displayed specific mechanical and nervous adaptations to repeated DW bouts as compared with VL. Moreover, the muscle adaptations conferred by the first bout of DW could have induced greater exercise efficiency, inducing lesser perceived exertion and cardiometabolic demand when the same exercise was repeated 2 wk later.

Contractile properties are less affected at long than short muscle length after eccentric exercise.

PURPOSE: The aim of the present study was to investigate whether the electrically evoked muscle responses are differently affected over time by the knee joint angle after an exercise-induced muscle damage (EIMD). We hypothesized that low-frequency-evoked responses would be less affected at long than short muscle length, and that mechanisms located within the muscle and tendinous tissues would be involved. METHODS: Fifteen males performed 45 min loaded downhill walking (DW) exercise. Maximal voluntary contraction torque (MVC), optimal angle for torque production, voluntary activation level (VAL), twitch, doublet at 10 and 100 Hz (Db10 and Db100, respectively), rate of torque development (RTD), post-activation potentiation (PAP), muscle shear elastic modulus (µ) and aponeurosis stiffness were assessed before, after, and 4, 24, 48, 72 and 168 h after the exercise at a knee angle of 40°, 90° and 120° (0°: full extension). RESULTS: MVC, VAL and Db100 were similarly decreased across joint angles after the DW and optimal angle was not affected. Twitch, Db10, Db10/Db100, PAP and RTD were less affected and muscle µ more increased at long than short muscle lengths (p < 0.05), especially during the first 24 h after the DW exercise. CONCLUSION: Low-frequency-evoked responses were more preserved at long than short muscle length the first 24 h after the DW exercise, suggesting that joint angle should be taken into account to assess muscular alterations after EIMD. This length-dependence could be associated to the higher sensitivity to Ca2+ and the higher increase in muscle stiffness at long than short muscle length.

Circulating microRNA levels after exercise-induced muscle damage and the repeated bout effect.

The neuromuscular system can quickly adapt to exercise-induced muscle damage (EIMD), such that it is less affected by subsequent damaging exercise, a phenomenon known as the repeated bout effect (RBE). Circulating muscle-specific microRNAs (myomiRs) may be able to potentially predict the long-lasting maximal voluntary contraction (MVC) torque deficit (>24 h), an indicator of EIMD. We aimed to investigate: 1) how plasma myomiR levels are modified by the RBE and 2) whether plasma myomiRs can predict the long-lasting MVC torque deficit. Nineteen participants performed two identical bouts of loaded downhill walking separated by 2 wk. MVC torque, creatine kinase (CK) activity, myoglobin (Mb) concentration, and myomiR levels were measured before and up to 48 h after exercise. Correlation and multiple regression analyses were performed to assess the ability of these markers to predict the largest MVC torque loss beyond 24 h postexercise. Similar to MVC torque, CK activity, and the Mb concentration, the relative abundance of certain myomiRs (hsa-miR-1-3p, and hsa-miR-133a-3p) was less affected after the second bout of exercise relative to the first bout. The CK activity, Mb concentration, and level of several myomiRs (hsa-miR-1-3p, hsa-miR-133a-3p, and hsa-miR-206) correlated with long-lasting MVC torque loss. Multiple regression showed that the best combination of markers to predict the long-lasting deficit of MVC torque included several myomiRs, Mb, and CK. Certain myomiR levels increased less after exercise bout 2 than after exercise bout 1, indicating the presence of the RBE. The measurement of myomiR levels in combination with Mb concentrations and CK activity could improve the prediction of the long-lasting MVC torque deficit.NEW & NOTEWORTHY The present study is the first to show that plasma muscle-specific microRNA (myomiR) levels can be modified by the repeated bout effect, as their levels increased less after the second exercise bout relative to the first. This study is also the first to suggest that myomiR levels could be used to partially predict maximal voluntary contraction torque loss at 24 h postexercise (i.e., the magnitude of exercise-induced muscle damage). Interestingly, the combined measurement of certain myomiR levels with those of myoglobin and creatine kinase improved the predictive value.

Appropriateness of indirect markers of muscle damage following lower limbs eccentric-biased exercises: A systematic review with meta-analysis.

PURPOSE: The aim of this review was to (1) characterize the time-course of markers of exercise-induced muscle damage (EIMD) based on the level of maximal voluntary contraction torque loss at 24-48h post-exercise (MVCloss24-48h), (2) identify factors (e.g., exercise and population characteristics) affecting the level of MVCloss24-48h, and (3) evaluate the appropriateness of EIMD markers as indicators of MVCloss24-48h. METHODS: Magnitude of change of each EIMD markers was normalized using the standardized mean differences method to compare the results from different studies. Time-course of EIMD markers were characterized according to three levels of MVCloss24-48h based on a clustering analysis of the 141 studies included. Association between MVCloss24-48h levels and participant´s characteristics or exercise type/modalities were assessed. Meta-regressions were performed to investigate the associations between MVCloss24-48h and EIMD markers changes at <6h, 24h, 48h, 72h and >96h after exercise. RESULTS: Time-course of EIMD markers recovery differs between levels of MVCloss24-48h. Training status and exercise type/modality were associated with MVCloss24-48h level (p<0.05). MVCloss24-48h was correlated to changes in myoglobin concentration (<6h), jump height (24h) and range of motion (48h) (p<0.001). CONCLUSION: As the exercise could differently affect markers as function of the EIMD severity (i.e., MVCloss24-48h levels), different markers should be used as function of the timing of measurement. Mb concentration should be used during the first hours after the exercise (<6h), whereas jump height (24h) and range of motion (48h) could be used as surrogate for maximal voluntary contraction later. Moreover, training status and exercise type/modality could influence the magnitude of MVCloss24-48h.

Muscle Shear Elastic Modulus Provides an Indication of the Protection Conferred by the Repeated Bout Effect.

Background: The neuromuscular system is able to quickly adapt to exercise-induced muscle damage (EIMD), such that it is less affected by subsequent damaging exercise, a phenomenon known as the repeated bout effect (RBE). The objective was to determine whether the mechanical properties of the quadriceps, as evaluated by shear wave elastography (SWE), were less affected when a second bout of eccentric-biased exercise was performed 2 weeks later. It was hypothesized that the first bout would confer protection against extensive muscle damage through an adaptation of the muscle stiffness before the second bout (i.e., higher muscle stiffness). Methods: Sixteen males performed two identical bouts of downhill walking separated by 2 weeks (45 min at 4.5 km.h-1; gradient: 25%; load: 30% of the body mass). Rectus femoris (RF) and vastus lateralis (VL) resting shear elastic modulus (µ) and EIMD symptoms were measured before and up to 7 days following the exercise bouts. Changes in neuromuscular function was evaluated by maximal voluntary contraction torque, voluntary activation level, evoked mechanical response to single and double (10 and 100 Hz doublets) electrical stimulation. An index of protection (IP) was calculated for EIMD symptoms to assess magnitude the RBE. Results: EIMD symptoms were less affected after the second than the first exercise bout. RF and VL-µ increased (p < 0.001) only after the first exercise. RF µ was elevated up to 2 weeks after the end of the first exercise (p < 0.001) whereas VL µ was only increased up to 24 h. The increase in µ observed 2 weeks after the end of the first exercise was correlated with the IP; i.e., attenuation of alterations in muscle µ, 10 Hz-doublet amplitude and rate of torque development after the second exercise bout (p < 0.05). Conclusion: We showed that muscle µ assessed by SWE was sensitive to the RBE, with a differential effect between VL and RF. The persistent increase in µ was associated with the attenuation of neuromuscular impairments observed after the second bout, suggesting that the increased muscle stiffness could be a "protective" adaptation making muscles more resistant to the mechanical strain associated to eccentric contractions.

Fluctuations in food and fluid intake during a 24-h World Championship: analysis of the deviation from nutritional programs.

Background: A food and fluid intake program is essential for ultraendurance athletes to maximize performance and avoid possible gastrointestinal symptoms (GIS). However, the ability to follow such a program during a race has been under-assessed. We thus investigated the fluctuations of food and fluid intake during the 24-h run World Championship of 12 elite athletes (6 men and 6 women; age: 46 ± 7 years, height: 170 ± 9 cm, weight: 61.1 ± 9.6 kg, total distance run: 193-272 km) and assessed their ability to follow their nutritional program. Methods: Real-time overall intake (fluids, energy, and macronutrients) was recorded and compared to that of their program. The temporal difference in absolute values and the degree of divergence from their program were assessed, divided into four 6-h periods. GIS were recorded during the race. A questionnaire identifying the details of their nutritional program and the self-assessed causes of their inability to follow it was completed by the participants the day after the race. Results: Water, total fluid, carbohydrates (CHO), and energy intake decreased during the last quarter of the 24-h ultramarathon relative to the first half (p = 0.024, 0.022, 0.009, and 0.042). However, the differences were no longer significant after these values were normalized by the number of passages in front of the supply tent. The participants progressively failed to follow their nutritional program, with the intake of their planned items dropping to approximately 50% during the last quarter. However, this was adequately compensated by increases in unplanned foods allowing them to match their expected targets. GIS, lack of appeal of the planned items, and attractivity of unplanned items were the main explanations given for their deviation from the program (64, 27, and 27%, respectively). Conclusion: Despite evident difficulty in following their nutritional programs (mostly attributed to GIS), elite ultraendurance runners managed to maintain high rates of fluid and food intake during a 24-h ultramarathon and therefore still met their planned elevated nutritional objectives.Abbreviations: CHO: carbohydrates, GIS: gastrointestinal symptoms.

Circulating microRNAs after a 24-h ultramarathon run in relation to muscle damage markers in elite athletes.

Ultra-endurance sports are growing in popularity but can be associated with adverse health effects, such as exercise-induced muscle damage (EIMD), which can lead to exertional rhabdomyolysis. Circulating microRNAs (miRNAs) may be useful to approach the degree of EIMD. We aimed to (1) investigate the relevance of circulating miRNAs as biomarkers of muscle damage and (2) examine the acute response of skeletal/cardiac muscle and kidney biomarkers to a 24-h run in elite athletes. Eleven elite athletes participated in the 24-h run World Championships. Counter-movement jump (CMJ), creatine kinase (CK), myoglobin (Mb), creatinine (Cr), high-sensitive cardiac troponin T (hs-cTnT), and muscle-specific miRNA (myomiR) levels were measured before, immediately after, and 24 and 48h after the race. CMJ height was reduced immediately after the race (-84.0 ± 25.2%, p < 0.001) and remained low at 24 h (-43.6 ± 20.4%, p = 0.002). We observed high CK activity (53 239 ± 63 608 U/L, p < 0.001) immediately after the race, and it remained elevated 24h after (p < 0.01). Circulating myomiR levels (miR-1-3p, miR-133a-3p, miR-133b, miR-208a-3p, miR-208b-3p, and miR-499a-5p) were elevated immediately after the 24-h run (fold changes: 18-124,723, p<0.001) and significantly (p < 0.05) correlated or tended to significantly (p < 0.07) correlate with the reduction in CMJ height at 24 h. We found no significant correlation between CMJ height loss at 24 h and CK (p = 0.23) or Mb (p = 0.41) values. All elite ultramarathon runners included in our study were diagnosed with exertional rhabdomyolysis after the 24-h ultramarathon race. MyomiR levels may be useful to approach the degree of muscle damage.

M-wave and H-reflex recruitment curves in boys and men.

The aim of the present study was to check whether the M-wave and H-reflex recruitment curves differ between prepubertal boys and men. Eleven boys (9-11 yr) and eleven men (18-35 yr) were magnetically stimulated at the tibial nerve in a prone position. M-wave and H-reflex maximal amplitudes (Hmax; Mmax ; Hmax /Mmax ), thresholds, regression slopes (Hslp ; Mslp ; Hslp /Mslp ) were extracted from M-wave and H-reflex recruitment curves and compared between the two age groups. Overall, no significant difference in M-wave and H-reflex recruitment curve parameters was found between the two populations. Nevertheless, the size of the M-wave associated with maximal H-reflex amplitude was lower in boys as compared to men when expressed relative to maximal M-wave amplitude (MHmax /Mmax : 0.18 ± 0.06 vs. 0.31 ± 0.13; p < .05). This result suggests that the development of peripheral nerve was completed in 9 to 11-year-old boys and did not affect the M-wave and H-reflex recruitment curves parameters. In neuromuscular function studies, it implies that Hmax /Mmax and Hslp /Mslp could be used indifferently to compare spinal motoneuron excitability between 9-11-year-old boys and men. Conversely, evoking H-reflexes at a given percentage of Mmax may bias the comparison between boys and men.

Analysis of food and fluid intake in elite ultra-endurance runners during a 24-h world championship.

BACKGROUND: Properly replacing energy and fluids is a challenge for 24-h ultramarathoners because such unusually high intake may induce adverse effects (gastrointestinal symptoms [GIS] and exercise-associated hyponatremia [EAH]). We analyzed such intake for 12 twelve elite athletes (6 males and 6 females; age: 46 ± 7 years, height: 170 ± 9 cm, weight: 61.1 ± 9.6 kg, total distance run: 193-272 km) during the 2019 24-h World Championships and compared it to the latest nutritional recommendations described by the International Society of Sports Nutrition in 2019. We hypothesized that these elite athletes would easily comply these recommendations without exhibiting detrimental adverse symptoms. METHODS: Ad libitum food and fluid intake was recorded in real-time and energy, macronutrient, sodium, and caffeine intake then calculated using a spreadsheet in which the nutritional composition of each item was previously recorded. GIS, markers of dehydration (body mass modifications, plasma and urine osmolality, and plasma volume; samples obtained 26 h before and just after the race) and EAH (plasma and urine sodium concentrations) were also assessed. RESULTS: Fluid, energy, and carbohydrate intake of the 11 finishers was 16.4 ± 6.9 L, 35.1 ± 15.7 MJ, and 1.49 ± 0.71 kg, respectively. Individual analyses showed that all but one (for fluid intake) or two (for energy and carbohydrate intake) consumed more than the minimum recommendations. The calculated energy balance remained, however, largely negative (- 29.5 ± 16.1 MJ). Such unusually high intake was not accompanied by detrimental GIS (recorded in 75%, but only transiently [3.0 ± 0.9 h]) or EAH (0%). The athletes were not dehydrated, shown by the absence of significant body mass loss (- 0.92 ± 2.13%) and modifications of plasma osmolality and an increase in plasma volume (+ 19.5 ± 15.8%). Performance (distance ran) positively correlated with energy intake (ρ = 0.674, p = 0.023) and negatively (ρ = - 0.776, p = 0.005) with fluid intake. CONCLUSIONS: Overall, almost all of these elite 24-h ultramarathoners surpassed the nutritional recommendations without encountering significant or the usual adverse effects.

Changes in the Viscoelastic Properties of the Vastus Lateralis Muscle With Fatigue.

We investigated the in vivo effects of voluntary fatiguing isometric contractions of the knee extensor muscles on the viscoelastic properties of the vastus lateralis (VL). Twelve young males (29.0 ± 4.5 years) performed an intermittent voluntary fatigue protocol consisting of 6 sets × 10 repetitions of 5-s voluntary maximal isometric contractions with 5-s passive recovery periods between repetitions. Voluntary and evoked torque were assessed before, immediately after, and 20 min after exercise. The shear modulus (µ) of the VL muscle was estimated at rest and during a ramped isometric contraction using a conventional elastography technique. An index of active muscle stiffness was then calculated (slope from the relationship between shear modulus and absolute torque). Resting muscle viscosity (η) was quantified using a shear-wave spectroscopy sequence to measure the shear-wave dispersion. Voluntary and evoked torque decreased by ∼37% (P < 0.01) immediately after exercise. The resting VL µ was lower at the end of the fatigue protocol (-57.9 ± 5.4%, P < 0.001), whereas the resting VL η increased (179.0 ± 123%, P < 0.01). The active muscle stiffness index also decreased with fatigue (P < 0.05). By 20 min post-fatigue, there were no significant differences from the pre-exercise values for VL η and the active muscle stiffness index, contrary to the resting VL µ. We show that the VL µ is greatly reduced and η greatly enhanced by fatigue, reflecting a more compliant and viscous muscle. The quantification of both shear µ and η moduli in vivo may contribute to a better understanding of the mechanical behavior of muscles during fatigue in sports medicine, as well as in clinical situations.

Plantar flexor muscle-tendon unit length and stiffness do not influence neuromuscular fatigue in boys and men.

PURPOSE: The twofold purpose of this study was (1) to compare differences in development and etiology of neuromuscular fatigue at different plantar flexor (PF) muscle-tendon unit (MTU) lengths between boys and men, and (2) to examine the relationship between musculotendinous stiffness and peripheral fatigue. METHODS: Nineteen pre-pubertal boys and 23 men performed three intermittent fatigue protocols at different PF MTU lengths (short: S, neutral: N and long: L), consisting of repeating maximal voluntary isometric contractions (MVIC) until the torque reached 60% of the initial value of MVIC. The etiology of the neuromuscular fatigue and the gastrocnemius medialis aponeuroses and tendon stiffness (KGM) were investigated using non-invasive methods. RESULT: The number of repetitions did not differ between men and boys, regardless of the PF MTU length (S: 16.5 ± 5.4 and 17.6 ± 5.8; N: 15.8 ± 4.5 and 13.3 ± 3.6; L: 13.6 ± 4.8 and 12.6 ± 4.6, respectively). Boys displayed a lower decrement of potentiated twitch torque (Qtwpot; p < 0.001) and greater decrease of voluntary activation level than men (p < 0.001). Although boys showed lower KGM values than men at S, no significant correlation was found between KGM and Qtwpot. CONCLUSION: PF MTU length had no effect on differences in the development and etiology of neuromuscular fatigue between boys and men. Although both groups displayed similar development of fatigue, central mechanisms mainly accounted for fatigue in boys and peripheral mechanisms were mainly involved in men. Additionally, musculotendinous stiffness did not account for difference in peripheral fatigue between children and adults.

Effect of Muscle-Tendon Unit Length on Child-Adult Difference in Neuromuscular Fatigue.

PURPOSE: The purpose of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor muscles at different muscle-tendon unit (MTU) lengths during repeated maximal voluntary isometric contractions (MVIC) between boys and men. METHODS: Twenty-two prepubertal boys (9-11 yr) and 22 men (18-30 yr) performed three knee extensor fatigue protocols at short (SHORT), optimal (OPT), and long (LONG) MTU lengths, consisting of repeating 5-s MVIC interspersed with 5-s passive recovery periods until torque reached 60% of the initial MVIC torque. The etiology of neuromuscular fatigue was identified using noninvasive methods such as surface electromyography, near-infrared spectroscopy, magnetic nerve stimulation and twitch interpolation technique. RESULTS: The number of repetitions was significantly lower in men at OPT (14.8 ± 3.2) and LONG (15.8 ± 5.8) than boys (39.7 ± 18.4 and 29.5 ± 10.2, respectively; P < 0.001), whereas no difference was found at SHORT between both age groups (boys, 33.7 ± 15.4; men, 40.9 ± 14.2). At OPT and LONG boys showed a lower reduction in the single potentiated twitch (Qtwpot) and a greater decrease in the voluntary activation level than men. At SHORT, both populations displayed a moderate Qtwpot decrement and a significant voluntary activation reduction (P < 0.001). The differences in maximal torque between boys and men were almost twice greater at OPT (223.9 N·m) than at SHORT (123.3 N·m) and LONG (136.5 N·m). CONCLUSIONS: The differences in neuromuscular fatigue between children and adults are dependent on MTU length. Differences in maximal torque could underpin differences in neuromuscular fatigue between children and adults at OPT and SHORT. However, at LONG these differences do not seem to be explained by differences in maximal torque. The origins of this specific effect of MTU length remain to be determined.

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 (Ttw,pot), high-frequency torque (T100 Hz) and the low-to-high frequency torque ratio (T10 Hz/T100 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). Ttw,pot (∼52%, P < 0.001), T100 Hz (∼39%, P < 0.001) and T10 Hz/T100 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.

Characteristics of motor unit recruitment in boys and men at maximal and submaximal force levels.

The aim of this study was to compare voluntary activation (VA) and motor units (MU) recruitment patterns between boys and men at different contraction levels of the knee extensor muscles. We hypothesized that boys and men would display similar VA and MU recruitment patterns at low submaximal force levels, but that boys would display a lower utilization of their higher-threshold MU and a lower VA at near-maximal and maximal force levels than men. 11 prepubertal boys and 13 men were tested at the optimal knee angle. Next, VA was assessed using the twitch interpolation technique during maximal (MVC) and submaximal isometric voluntary contractions. Mean firing rate (MFR), recruitment threshold (RT) and motor unit action potential size (MUAPSIZE) were extracted to characterize neural strategies. No significant difference between groups was found for VA at every contraction level. Similarly, no significant difference was found for the MFR vs. RT relationship parameters between groups. For the vastus lateralis (VL) muscle, the MUAPSIZE vs. RT relationship differed between boys and men independent of the contraction level (p < 0.05). Boys also displayed a different MFR vs. MUAPSIZE relationship on the VL muscle independent of the contraction level (p < 0.05). To conclude, no difference between boys and men was found for VA regardless of the contraction level investigated. Differences in motor unit recruitment parameters between boys and men seem to be explained by different muscle dimensions between groups.

Child-adult differences in neuromuscular fatigue are muscle dependent.

The aim of the present 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 (MVICs) between children and adults. Prepubertal boys (n = 21; 9-11 yr) and men (n = 24; 18-30 yr) performed two fatigue protocols consisting of 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 noninvasive methods, such as the surface electromyography, single and doublet magnetic stimulation, twitch interpolation technique, and near-infrared spectroscopy. 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 electromyography 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.NEW & NOTEWORTHY Child-adult differences in neuromuscular fatigue during repeated maximal voluntary contractions are specific to the muscle group since children fatigue similarly to adults with the plantar flexor muscles and to a lower extent with the knee extensor muscles. Children experience less peripheral fatigue and more central fatigue than adults, regardless of the muscle group considered.