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.

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.

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.

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 &lt; 0.001) only after the first exercise. RF µ was elevated up to 2 weeks after the end of the first exercise (p &lt; 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 &lt; 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.

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.

A basal heat stress test to detect military operational readiness after a 14-day operational heat acclimatization period

A basal heat stress test (HST) to predict the magnitude of adaptive responses during heat acclimatization (HA) would be highly useful for the armed forces. The aim was to identify physiological markers assessed during a HST (three 8-min running sets at 50% of the speed at VO_2max) performed just before a 14-day HA period that would identify participants still at "risk" at the end of HA. Individuals that responded poorly (large increases in rectal temperature [T_rec] and heart rate [HR]) during the initial HST were more likely to respond favorably to HA (large reductions in T_rec and HR). However, they were also more likely to exhibit lower tolerance to HST at D15. Basal T_rec was found to efficiently discriminate participants showing a T_rec > 38.5°C after HA, who are considered to be "at risk". Finally, participants were classified by quartiles based on basal T_rec and HR at the end of the HST and physiological strain index (PSI). Most of the participants "at risk" were among the upper quartile (i.e. the least tolerant) of T_rec and PSI (p = 0.011 for both). Overall, these results show that the individuals who are less tolerant to a basal HST are very likely to benefit the most from HA but they also remain less tolerant to heat at the end of HA than those who better tolerated the basal HST. A basal HST could therefore theoretically help the command to select the most-ready personnel in hot conditions while retaining those who are less tolerant 6.

Contralateral effects of eccentric resistance training on immobilized arm

This study compared the effects of contralateral eccentric-only (ECC) and concentric-/eccentric-coupled resistance training (CON-ECC) of the elbow flexors on immobilized arm. Thirty healthy participants (18-34 y) were randomly allocated to immobilization only (CTRL; n = 10), immobilization and ECC (n = 10), or immobilization and CON-ECC group (n = 10). The non-dominant arms of all participants were immobilized (8 h·day^-1 ) for 4 weeks, during which ECC and CON-ECC were performed by the dominant (non-immobilized) arm 3 times a week (3-6 sets of 10 repetitions per session) with an 80%-120% and 60%-90% of one concentric repetition maximum (1-RM) load, respectively, matching the total training volume. Arm circumference, 1-RM and maximal voluntary isometric contraction (MVIC) strength, biceps brachii surface electromyogram amplitude (sEMG_RMS ), rate of force development (RFD), and joint position sense (JPS) were measured for both arms before and after immobilization. CTRL showed decreases (P < .05) in MVIC (-21.7%), sEMG_RMS (-35.2%), RFD (-26.0%), 1-RM (-14.4%), JPS (-87.4%), and arm circumference (-5.1%) of the immobilized arm. These deficits were attenuated or eliminated by ECC and CON-ECC, with greater effect sizes for ECC than CON-ECC in MVIC (0.29: +12.1%, vs -0.18: -0.1%) and sEMG_RMS (0.31:17.5% vs -0.15: -5.9%). For the trained arm, ECC showed greater effect size for MVIC than CON-ECC (0.47 vs 0.29), and increased arm circumference (+2.9%), sEMG_RMS (+77.9%), and RDF (+31.8%) greater (P < .05) than CON-ECC (+0.6%, +15.1%, and + 15.8%, respectively). The eccentric-only resistance training of the contralateral arm was more effective to counteract the negative immobilization effects than the concentric-eccentric training.

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.

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.

Resting Muscle Shear Modulus Measured With Ultrasound Shear-Wave Elastography as an Alternative Tool to Assess Muscle Fatigue in Humans

The aim of this study was to investigate the time course of the resting vastus lateralis (VL) muscle shear elastic modulus (μ) measured with ultrasound shear-wave elastography during repetition of isometric maximal voluntary contractions (MVCs) of the knee extensors (KE). Fifteen well-trained young males repeated 60 5-s isometric MVCs. Evoked electrical stimulations and the VLμ were measured every ten MVCs at rest. The resting VLμ significantly decreased (−34.7 ± 6.7%; P < 0.001) by the end of the fatigue protocol. There was also a 38.4 ± 12.6 % decrease in MVC after exercise (P < 0.001). The potentiated doublet and single twitch torque amplitudes and properties were significantly modified by the end of exercise (P < 0.001). This study shows the time course of the resting VLμ during the repetition of maximal voluntary fatiguing exercise of the KE muscles. The decrease of the resting VLμ could directly affect the force transmission capabilities accounting for peripheral fatigue.

A Moderate Supplementation of Native Whey Protein Promotes Better Muscle Training and Recovery Adaptations Than Standard Whey Protein - A 12-Week Electrical Stimulation and Plyometrics Training Study

The purpose of this study was to assess if native whey protein (NW) supplementation could promote recovery and training adaptations after an electrostimulation (ES) training program combined to plyometrics training. Participants were allocated into three groups, supplemented 5 days/week, either with 15 g of carbohydrates + 15 g of NW (n = 17), 15 g of carbohydrates + 15 g of standard whey protein (SW; n = 15), or placebo (PLA; 30 g of carbohydrates; n = 10), while undergoing a 12-week ES training program of the knee extensors. Concentric power (Pmax) was evaluated before, immediately after, as well as 30 min, 60 min, 24 h, and 48 h after the 1st, 4th and last ES training session. The maximal voluntary contraction torque (MVC), twitch amplitude, anatomical cross-sectional area (CSA) and maximal voluntary activation level (VA) were measured before (T0), and after 6 (T1) and 12 weeks of training (T2). P_max recovery kinetics differed between groups (p < 0.01). P_max started to recover at 30 min in NW, 24 h in SW and 48 h in PLA. Training adaptations also differed between groups: MVC increased between T0 and T2 in NW (+11.8%, p < 0.001) and SW (+7.1%, p < 0.05), but not PLA. Nevertheless, the adaptation kinetics differed: MVC increased in NW and SW between T0 and T1, but an additional gain was only observed between T1 and T2 in NW. VA declined at T1 and T2 in PLA (−3.9%, p < 0.05), at T2 in SW (−3.5%, p < 0.05), and was unchanged in NW. CSA increased, but did not differ between groups. These results suggest that NW could promote a faster recovery and neuromuscular adaptations after training than SW. However, the mechanisms underlying this effect remain to be identified.