Sarcoplasmic reticulum function and muscle contractile character following fatiguing exercise in humans

Short-term aerobic exercise prevents development of glucocorticoid myopathic features in aged skeletal muscle in a sex-dependent manner

Older adults are vulnerable to glucocorticoid-induced muscle atrophy and weakness, with sex potentially influencing their susceptibility to those effects. Aerobic exercise can reduce glucocorticoid-induced muscle atrophy in young rodents. However, it is unknown whether aerobic exercise can prevent glucocorticoid myopathy in aged muscle. The objectives of this study were to define the extent to which sex influences the development of glucocorticoid myopathy in aged muscle, and to determine the extent to which aerobic exercise training protects against myopathy development. Twenty-four-month-old female (n = 30) and male (n = 33) mice were randomized to either sedentary or aerobic exercise groups. Within their respective groups, mice were randomized to either daily treatment with dexamethasone (DEX) or saline. Upon completing treatments, the contractile properties of the triceps surae complex were assessed in situ. DEX marginally lowered muscle mass and soluble protein content in both sexes, which was attenuated by aerobic exercise only in females. DEX increased sub-tetanic force and rate of force development only in females, which was not influenced by aerobic exercise. Muscle fatigue was higher in both sexes following DEX, but aerobic exercise prevented fatigue induction only in females. The sex-specific differences to muscle function in response to DEX treatment coincided with sex-specific changes to the content of proteins related to calcium handling, mitochondrial quality control, reactive oxygen species production, and glucocorticoid receptor in muscle. These findings define several important sexually dimorphic changes to aged skeletal muscle physiology in response to glucocorticoid treatment and define the capacity of short-term aerobic exercise to protect against those changes. KEY POINTS: There are sexually dimorphic effects of glucocorticoids on aged skeletal muscle physiology. Glucocorticoid-induced changes to aged muscle contractile properties coincide with sex-specific differences in the content of calcium handling proteins. Aerobic exercise prevents glucocorticoid-induced fatigue only in aged females and coincides with differences in the content of mitochondrial quality control proteins and glucocorticoid receptors.

Phosphate toxicity and SERCA2a dysfunction in sudden cardiac arrest

Almost half of the people who die from sudden cardiac arrest have no detectable heart disease. Among children and young adults, the cause of approximately one-third of deaths from sudden cardiac arrest remains unexplained after thorough examination. Sudden cardiac arrest and related sudden cardiac death are attributed to dysfunctional cardiac ion-channels. The present perspective paper proposes a pathophysiological mechanism by which phosphate toxicity from cellular accumulation of dysregulated inorganic phosphate interferes with normal calcium handling in the heart, leading to sudden cardiac arrest. During cardiac muscle relaxation following contraction, SERCA2a pumps actively transport calcium ions into the sarcoplasmic reticulum, powered by ATP hydrolysis that produces ADP and inorganic phosphate end products. Reviewed evidence supports the proposal that end-product inhibition of SERCA2a occurs as increasing levels of inorganic phosphate drive up phosphate toxicity and bring cardiac function to a sudden and unexpected halt. The paper concludes that end-product inhibition from ATP hydrolysis is the mediating factor in the association of sudden cardiac arrest with phosphate toxicity. However, current technology lacks the ability to directly measure this pathophysiological mechanism in active myocardium, and further research is needed to confirm phosphate toxicity as a risk factor in individuals with sudden cardiac arrest. Moreover, phosphate toxicity may be reduced through modification of dietary phosphate intake, with potential for employing low-phosphate dietary interventions to reduce the risk of sudden cardiac arrest.

Effects of Exercise on Circulating Extracellular Vesicles in Cardiovascular Disease

The evidence that physical exercise has multiple beneficial effects and is essential to a healthy lifestyle is widely accepted for a long-time. The functional and psychological changes promoted by exercise improve clinical outcomes and prognosis in several diseases, by decreasing mortality, disease severity, and hospital admissions. Nonetheless, the mechanisms that regulate the release, uptake, and communication of several factors in response to exercise are still not well defined. In the last years, extracellular vesicles have attracted significant interest in the scientific community due to their ability to carry and deliver proteins, lipids, and miRNA to distant organs in the body, promoting a very exciting crosstalk machinery. Moreover, increasing evidence suggests that exercise can modulate the release of those factors within EVs into the circulation, mediating its systemic adaptations.In this chapter, we summarize the effects of acute and chronic exercise on the extracellular vesicle dynamics in healthy subjects and patients with cardiovascular disease. The understanding of the changes in the cargo and kinetics of extracellular vesicles in response to exercise may open new possibilities of research and encourage the development of novel therapies that mimic the effects of exercise.

On-field low-frequency fatigue measurement after repeated drop jumps

Purpose: Monitoring fatigue is now commonly performed in athletes as it can directly impact performance and may further increase the risk of injury or overtraining syndrome. Among the exercise-induced peripheral alterations, low-frequency fatigue (LFF) assessment is commonly restricted to in-lab studies. Measuring LFF on-field would allow athletes and coaches to assess muscle fatigability on a regular basis. The aim of the present study was therefore to validate a new portable device allowing quadriceps LFF assessment in the field.

Methods: LFF was assessed in 15 active and healthy participants before (PRE) and after (POST) a series of drop jumps. LFF was assessed, thanks to a dedicated device recording evoked force to muscle submaximal electrical low- and high-frequency stimulation. Changes in the low- to high-frequency force ratio (further referred to as Powerdex® value) were compared to the changes in the ratio of evoked force induced by paired-pulse femoral nerve electrical stimulation at 10 and 100 Hz (i.e., DB10/DB100 ratio). Maximal voluntary contraction (MVC) and voluntary activation (VA) were also measured.

Results: MVC decreased (p < 0.001), whereas VA was not affected by the fatiguing task (p = 0.14). There was a decrease in the DB10/DB100 ratio (from 96.4% to 67.3%, p < 0.001) as well as in the Powerdex value (from 74.0% to 55.7%, p < 0.001). There was no significant difference between POST values (expressed in percentage of PRE values) of the DB10/DB100 ratio and Powerdex (p = 0.44), and there was a significant correlation between the changes in Powerdex® and DB10/DB100 (r = 0.82, p < 0.001).

Conclusion: The on-field device we tested is a valid tool to assess LFF after a strenuous exercise consisting of repeated drop jumps as it evidences the presence of LFF similarly to a lab technique. Such a device can be used to monitor muscle fatigability related to excitation–contraction in athletes.

Effect of Mental Task on Sex Differences in Muscle Fatigability: A Review

Previous research demonstrated that there are observable sex differences in developing muscle fatigue when mental task during fatiguing activity is present; however, there is no available review on this matter. Therefore, this review aimed to summarize the findings of previous studies investigating the effect of mental task on muscle fatigue in men and women. To conduct the review, we utilized searches using the electronic databases Web of Science, PubMed, Scopus, and EBSCO Cinahl Ultimate. The studies included had no limited publication date and examined the effects of mental task on muscle fatigue in a healthy adult population of any age. The evaluation was performed using the following criteria: time to failure, or subjective scale in various modifications (visual analog scale-VAS, rate of perceived effort-RPE, rate of perceived fatigue-RPF, rate of perceived discomfort-RPD). A total of seven studies met the set criteria, which were subsequently analyzed. Heavy mental task (more demanding math tasks) can reduce the time to failure for both men and women, with the reduction being more pronounced for women than for men. For light mental task (simple math tasks), no reduction in time to failure was observed to a great extent. The mental task in any of the included studies did not affect the subjective perception of fatigue, effort, discomfort, or pain. Although the studies investigating the effect of mental task on sex differences in muscle fatigability are limited, based on our findings we can assume that in jobs requiring heavier mental task, women may be more prone to the faster development of muscle fatigue; thus, employers might consider paying attention to the possibility of adequate rest.

Capsaicin and Its Effect on Exercise Performance, Fatigue and Inflammation after Exercise

Capsaicin (CAP) activates the transient receptor potential vanilloid 1 (TRPV1) channel on sensory neurons, improving ATP production, vascular function, fatigue resistance, and thus exercise performance. However, the underlying mechanisms of CAP-induced ergogenic effects and fatigue-resistance, remain elusive. To evaluate the potential anti-fatigue effects of CAP, 10 young healthy males performed constant-load cycling exercise time to exhaustion (TTE) trials (85% maximal work rate) after ingestion of placebo (PL; fiber) or CAP capsules in a blinded, counterbalanced, crossover design, while cardiorespiratory responses were monitored. Fatigue was assessed with the interpolated twitch technique, pre-post exercise, during isometric maximal voluntary contractions (MVC). No significant differences (p > 0.05) were detected in cardiorespiratory responses and self-reported fatigue (RPE scale) during the time trial or in TTE (375 ± 26 and 327 ± 36 s, respectively). CAP attenuated the reduction in potentiated twitch (PL: -52 ± 6 vs. CAP: -42 ± 11%, p = 0.037), and tended to attenuate the decline in maximal relaxation rate (PL: -47 ± 33 vs. CAP: -29 ± 68%, p = 0.057), but not maximal rate of force development, MVC, or voluntary muscle activation. Thus, CAP might attenuate neuromuscular fatigue through alterations in afferent signaling or neuromuscular relaxation kinetics, perhaps mediated via the sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) pumps, thereby increasing the rate of Ca2+ reuptake and relaxation.

Neuromuscular responses to isometric, concentric and eccentric contractions of the knee extensors at the same torque-time integral

PURPOSE

The present study compared isometric, concentric and eccentric contractions at the same torque-time integral for changes in neuromuscular fatigue and muscle damage parameters.

METHOD

Healthy men (18-24 years) were placed to either isometric (ISO), concentric (CONC), or eccentric (ECC) group (n = 11/group) that performed corresponding contractions of the knee extensors to exert the same amount of torque-time integral (24,427 ± 291 Nm·s). Changes in maximal voluntary contraction (MVC) torque, voluntary activation, evoked torque at 10 Hz and 100 Hz and its ratio, M-wave amplitude, and muscle soreness were assessed immediately before and after, 1 h, 1 day and 2 days after each exercise, and were compared among the groups.

RESULTS

MVC torque decreased immediately after ISO (- 17.0 ± 8.3%), CONC (- 21.7 ± 11.5%) and ECC (- 26.2 ± 15.6%) similarly (p = 0.35), but the decrease sustained longer (p < 0.05) for ECC (2 days post-exercise: - 12.9 ± 14.8%) and ISO (- 5.5 ± 7.9%) than CONC (+ 5.0 ± 11.0%). Muscle soreness developed after ECC (25.1 ± 19.8 mm) and ISO (17.5 ± 21.0 mm) similarly (p = 0.15). Voluntary activation decreased immediately (- 3.7 ± 6.6%) and 1 h post-exercise (- 4.7 ± 7.6%) for all groups similarly. Electrically evoked forces decreased greater immediately (- 30.1 ± 15.6%) and 1 h post-exercise (- 35.0 ± 12.8%) for ECC than others, and the decrease in 10/100 Hz ratio was also greater immediately (- 30.5 ± 12.6%) and 1 h after ECC (- 23.8 ± 10.3%) than others.

CONCLUSION

ISO, CONC and ECC with the same torque-time integral produced similar neuromuscular fatigue at immediately post-exercise, but the force loss was longer-lasting after ISO and ECC than CONC, and the changes in peripheral fatigue parameters were the greatest after ECC, suggesting greater muscle damage.

Muscle Contractile Characteristics During Exhaustive Dynamic Exercise and Recovery

Our aim was to provide an in vivo assessment of human muscle twitch characteristics during and following an exhaustive dynamic exercise to explore temporal alterations of the rate of force development (RFD) and relaxation (RFR). Eleven healthy participants (mean age ± SD: 24 ± 3 years) completed a dynamic knee-extensor exercise in randomized order at three different intensities, eliciting exhaustion after ∼9 min (56 ± 10 W), ∼6 min (60 ± 10 W), and ∼4 min (63 ± 10 W), in addition to a low-intensity (28 ± 5 W) bout. In a novel setup, an electrical doublet stimulation of m. vastus lateralis was applied during exercise (every 30 s) and recovery for frequent evaluation of key contractile properties (maximal force, RFD, RFR, and electromechanical delay) in addition to M-wave characteristics. RFD and RFR remained stable throughout the low-intensity trial but declined in all exhaustive trials to reach a similar level of ∼40% of pre-exercise values at task failure but with the exponential decay augmented by intensity. Following exhaustion, there was a fast initial recovery of RFD and RFR to ∼80% of pre-exercise values within 1 min, followed by a longer suppression at this level. The M-wave characteristics remained unchanged during all trials. In conclusion, this is the first study to quantify the intensity-dependent alterations of RFD and RFR during and after exhaustive dynamic exercise in humans. A hypothesized reduction and fast reversion of RFD was confirmed, and a surprising compromised RFR is reported. The present unique experimental approach allows for novel insight to exercise-induced alterations in human muscle contractile properties which is relevant in health and disease.

Resistance Training Affects Neuromuscular Fatigue But Not Efficiency in Elite Rowers

PURPOSE

To investigate how resistance training (RT) in a regular training program affects neuromuscular fatigue (NMF) and gross efficiency (EGROSS) in elite rowers.

METHODS

Twenty-six elite male rowers performed 4 RT sessions within 10 days. At baseline and after the first and fourth RT, EGROSS and NMF were established. From breathing gas, EGROSS was determined during submaximal rowing tests. Using a countermovement jump test, NMF was assessed by jump height, flight time, flight-to-contraction-time ratio, peak power, and time to peak power. Muscle soreness was assessed using a 10-cm-long visual analog scale.

RESULTS

No significant differences were found for EGROSS (P = .565, ω2 = .032). Muscle soreness (P = .00, ω2 = .500) and time to peak power (P = .08, ω2 = 0.238) were higher compared with baseline at all test moments. Flight-to-contraction-time ratio, jump height, and peak power after the fourth RT differed from baseline (P < .05, ω2 = .36, ω2 = .38, and ω2 = .31) and from results obtained after the first RT (P < .05, ω2 = .36, ω2 = .47, and ω2 = .22).

CONCLUSIONS

RT in general does not influence EGROSS, but large individual differences (4.1%-14.8%) were observed. NMF is affected by RT, particularly after multiple sessions. During periods of intensified RT, imposed external load for low-intensity endurance training need not be altered, but rowers are recommended to abstain from intensive endurance training. Individual monitoring is strongly recommended.

Fatigue development and perceived response during self-paced endurance exercise: state-of-the-art review

Performance in self-paced endurance exercises results from continuous fatigue symptom management. While it is suggested that perceived responses and neuromuscular fatigue development may determine variations in exercise intensity, it is uncertain how these fatigue components interact throughout the task. To address the fatigue development in self-paced endurance exercises, the following topics were addressed in the present review: (1) fatigue development during constant-load vs. self-paced endurance exercises; (2) central and peripheral fatigue and perceived exertion interconnections throughout the self-paced endurance exercises; and (3) future directions and recommendations. Based on the available literature, it is suggested (1) the work rate variations during a self-paced endurance exercise result in transitions between exercise intensity domains, directly impacting the end-exercise central and peripheral fatigue level when compared to constant-load exercise mode; (2) central and peripheral fatigue, as well as perceived exertion response contribute to exercise intensity regulation at the different stages of the trial. It seems that while neuromuscular fatigue development might be relevant at beginning of the trial, the perceived exertion might interfere in the remaining parts to achieve maximal values only at the finish line; (3) future studies should focus on the mechanisms underpinning fatigue components interactions throughout the task and its influence on exercise intensity variations.

Greater decrements in neuromuscular function following interval compared to continuous eccentric cycling

Our aim was to determine the demands and consequences of a single session of continuous (CONT) or interval (INT) eccentric cycling. Fourteen healthy males performed 'work-matched' CONT and INT eccentric cycling in a cross over design. Measures of maximal voluntary contraction (MVC), resting twitch force, voluntary activation (VA), muscle soreness and creatine kinase (CK) were taken at baseline, immediately post, and 24, 48 and 72 h post the first exercise bout. The second bout was used to characterise within session demands. Decreases in MVC (INT 19%, CONT 13%), twitch force (INT 31%, CONT 18%), and VA (INT 10%, CONT 6%) were observed immediately post session (p < 0.05). Reductions in twitch force were greater after INT (p < 0.05) and lasted 48 h. Muscle soreness was greater following INT, versus CONT (p < 0.05), although no differences in CK were observed. Metabolic demands (% of V̇O_2peak and [BLa]) were greater during INT vs. CONT (32 ± 6% 28 ± 6%; p < 0.001), [BLa] (1.0 ± 0.4 vs. 0.8 ± 0.2 mmol·L^-1; p < 0.001) and RPE (12 ± 1 vs. 11 ± 1; p < 0.001), respectively. Total time under tension was 48% greater in CONT compared to INT (p < 0.001), whereas average torque (during exercise) was 40% greater during INT compared to CONT (p < 0.001). Interval eccentric cycling exacerbates muscle soreness, decrements in muscle function and lengthens recovery compared to a work matched continuous bout, which is attributable to increased force rather than time under tension.

Fatigue development and perceived response during self-paced endurance exercise: state-of-the-art review

Performance in self-paced endurance exercises results from continuous fatigue symptom management. While it is suggested that perceived responses and neuromuscular fatigue development may determine variations in exercise intensity, it is uncertain how these fatigue components interact throughout the task. To address the fatigue development in self-paced endurance exercises, the following topics were addressed in the present review: (1) fatigue development during constant-load vs. self-paced endurance exercises; (2) central and peripheral fatigue and perceived exertion interconnections throughout the self-paced endurance exercises; and (3) future directions and recommendations. Based on the available literature, it is suggested (1) the work rate variations during a self-paced endurance exercise result in transitions between exercise intensity domains, directly impacting the end-exercise central and peripheral fatigue level when compared to constant-load exercise mode; (2) central and peripheral fatigue, as well as perceived exertion response contribute to exercise intensity regulation at the different stages of the trial. It seems that while neuromuscular fatigue development might be relevant at beginning of the trial, the perceived exertion might interfere in the remaining parts to achieve maximal values only at the finish line; (3) future studies should focus on the mechanisms underpinning fatigue components interactions throughout the task and its influence on exercise intensity variations.

Effects of 120 vs. 60 and 90 g/h Carbohydrate Intake during a Trail Marathon on Neuromuscular Function and High Intensity Run Capacity Recovery

BACKGROUND

Current carbohydrate (CHO) intake recommendations for ultra-trail activities lasting more than 2.5 h is 90 g/h. However, the benefits of ingesting 120 g/h during a mountain marathon in terms of post-exercise muscle damage have been recently demonstrated. Therefore, the aim of this study was to analyze and compare the effects of 120 g/h CHO intake with the recommendations (90 g/h) and the usual intake for ultra-endurance athletes (60 g/h) during a mountain marathon on internal exercise load, and post-exercise neuromuscular function and recovery of high intensity run capacity.

METHODS

Twenty-six elite trail-runners were randomly distributed into three groups: LOW (60 g/h), MED (90 g/h) and HIGH (120 g/h), according to CHO intake during a 4000-m cumulative slope mountain marathon. Runners were measured using the Abalakov Jump test, a maximum a half-squat test and an aerobic power-capacity test at baseline (T1) and 24 h after completing the race (T2).

RESULTS

Changes in Abalakov jump time (ABKJT), Abalakov jump height (ABKH), half-squat test 1 repetition maximum (HST1RM) between T1 and T2 showed significant differences by Wilcoxon signed rank test only in LOW and MED (p < 0.05), but not in the HIGH group (p > 0.05). Internal load was significantly lower in the HIGH group (p = 0.017) regarding LOW and MED by Mann Whitney u test. A significantly lower change during the study in ABKJT (p = 0.038), ABKH (p = 0.038) HST1RM (p = 0.041) and in terms of fatigue (p = 0.018) and lactate (p = 0.012) within the aerobic power-capacity test was presented in HIGH relative to LOW and MED.

CONCLUSIONS

120 g/h CHO intake during a mountain marathon might limit neuromuscular fatigue and improve recovery of high intensity run capacity 24 h after a physiologically challenging event when compared to 90 g/h and 60 g/h.

Mechanism of Fatigue Induced by Different Cycling Paradigms With Equivalent Dosage

Leg cycling is one of the most common modes of exercise used in athletics and rehabilitation. This study used a novel cycling setting to elucidate the mechanisms, central vs. peripheral fatigue induced by different resistance with equivalent works (watt∗min). Twelve male adults received low and relatively high resistance cycling fatigue tests until exhausted (RPE > 18) in 2 weeks. The maximal voluntary contraction, voluntary activation level, and twitch forces were measured immediately before and after cycling to calculate General (GFI), central (CFI), and peripheral (PFI) fatigue indices of knee extensors, respectively. The results showed that the CFI (high: 92.26 ± 8.67%, low: 78.32 ± 11.77%, p = 0.004) and PFI (high: 73.76 ± 17.32%, low: 89.63 ± 11.01%, p < 0.017) were specific to the resistance of fatigue protocol. The GFI is influenced by the resistance of cycling to support the equivalent dosage. This study concluded that the mechanism of fatigue would be influenced by the resistance of fatigue protocol although the total works had been controlled.

Greater Short-Time Recovery of Peripheral Fatigue After Short- Compared With Long-Duration Time Trial

The kinetics of recovery from neuromuscular fatigue resulting from exercise time trials (TTs) of different durations are not well-known. The aim of this study was to determine if TTs of three different durations would result in different short-term recovery in maximal voluntary contraction (MVC) and evoked peak forces. Twelve trained subjects performed repetitive concentric right knee extensions on an isokinetic dynamometer self-paced to last 3, 10, and 40 min (TTs). Neuromuscular function was assessed immediately (<2 s) and 1, 2, 4, and 8 min after completion of each TT using MVCs and electrical stimulation. Electrical stimulations consisted of single stimulus (SS), paired stimuli at 10 Hz (PS10), and paired stimuli at 100 Hz (PS100). Electrically evoked forces including the ratio of low- to high-frequency doublets were similar between trials at exercise cessation but subsequently increased more (P < 0.05) after the 3 min TT compared with either the 10 or 40 min TT when measured at 1 or 2 min of recovery. MVC force was not different between trials. The results demonstrate that recovery of peripheral fatigue including low-frequency fatigue depends on the duration and intensity of the preceding self-paced exercise. These differences in recovery probably indicate differences in the mechanisms of fatigue for these different TTs. Because recovery is faster after a 3 min TT than a 40 min TT, delayed assessment of fatigue will detect a difference in peripheral fatigue between trials that was not present at exercise cessation.

Similar Cardioventilatory but Greater Neuromuscular Stimuli With Interval Drop Jump Than With Interval Running

CONTEXT

Drop jumps and high-intensity interval running are relevant training methods to improve explosiveness and endurance performance, respectively. Combined training effects might, however, be achieved by performing interval drop jumping.

PURPOSE

To determine the acute effects of interval drop jumping on oxygen uptake (V˙O2)-index of cardioventilatory/oxidative stimulation level and peripheral fatigue-a limiting factor of explosiveness.

METHODS

Thirteen participants performed three 11-minute interval training sessions during which they ran 15 seconds at 120% of the velocity that elicited maximal V˙O2 (V˙O2max) (ITrun), or drop jumped at 7 (ITDJ7) or 9 (ITDJ9) jumps per 15 seconds, interspersed with 15 seconds of passive recovery. V˙O2 and the time spent above 90% of V˙O2max (V˙TO2max) were collected. Peripheral fatigue was quantified via preexercise to postexercise changes in evoked potentiated quadriceps twitch (ΔQT). Power output was estimated during ITDJs using optical sensors.

RESULTS

All participants reached 90% of V˙O2max or higher during ITrun and ITDJ9, but only 11 did during ITDJ7. V˙TO2max was not different between ITrun and ITDJ9 (145 [76] vs 141 [151] s; P = .92) but was reduced during ITDJ7 (28 [26] s; P = .002). Mean ΔQT in ITDJ9 and ITDJ7 was not different (-17% [9%] vs -14% [8%]; P = .73) and greater than in ITrun (-8% [7%]; P = .001). No alteration in power output was found during ITDJs (37 [10] W·kg-1).

CONCLUSION

Interval drop jumping at a high work rate stimulated the cardioventilatory and oxidative systems to the same extent as interval running, while the exercise-induced increase in fatigue did not compromise drop jump performance. Interval drop jumping might be a relevant strategy to get concomitant improvements in endurance and explosive performance.

Characterization of performance fatigability during a self-paced exercise

Pacing during a high-intensity cycling time trial (TT) appears to prevent premature task failure, but the performance fatigability during a self-paced exercise is currently unknown. Therefore, the current study characterized the time course of performance fatigability during a 4-km TT. Eleven male cyclists performed three separated TTs in a crossover, counterbalanced design. The TTs lasted until the end of the fast-start (FS; 600 ± 205 m), even-pace (EP; 3,600 ± 190 m), and end-spurt (ES; 4,000 m) phases. Performance fatigability was characterized by using isometric maximal voluntary contractions (IMVCs), whereas the muscle activation [i.e., voluntary activation (VA)] and contractile function of knee extensors [e.g., peak torque of potentiated twitches (TwPt)] were evaluated using electrically evoked contractions performed before and 1 min after each specific part of the trial. Gas exchange, power output (PO), and electromyographic activity (EMG) were also recorded. EMG/PO showed an abrupt increase followed by a continuous decrease toward the end of FS, resulting in a drop in IMVC (−12%), VA (−8%), and TwPt (−23%). EMG/PO was stable during EP, with no additional drop on IMVC, VA, or TwPt (−12%, −6%, and −22%, respectively). EMG/PO increased abruptly during the ES, but there was no change in IMVCs, VA, or TwPt (−13%, −8%, and −26%, respectively). These findings demonstrate that the performance fatigability during a self-paced exercise is characterized by a large drop in contractile function and muscle activation at the beginning of the trial (i.e., FS), without additional change during the middle and end phases (i.e., EP and ES).

NEW & NOTEWORTHY The time course of performance fatigability throughout a self-paced exercise is currently unknown. The results showed that a large amount of muscle activation and contractile function impairments are attained early on a self-paced exercise (first ∼15% of the total time trial distance) and maintained throughout the test. This novel finding characterizes the performance fatigability from a contractile function and muscle activation perspective, which brings new insights for future studies focused on real-world exercise training and competition.

Neuromuscular Factors Contributing to Reductions in Muscle Force After Repeated, High-Intensity Muscular Efforts

Multiple neuromuscular processes contribute to the loss of force production following repeated, high-intensity muscular efforts; however, the relative contribution of each process is unclear. In Experiment 1, 16 resistance trained men performed six sets of unilateral isometric plantar flexor contractions of the right leg (3 s contraction/2 s rest; 85% maximal voluntary contraction torque; 90-s inter-set rest) until failure with and without caffeine ingestion (3 mg kg^-1) on two separate days. Corticospinal excitability and cortical silent period (cSP) were assessed before and immediately, 10 and 20 min after the exercise. In Experiment 2, electrically evoked tetanic force and persistent inward current (PIC)-mediated facilitation of the motor neuron pool (estimated using neuromuscular electrical stimulation with tendon vibration) were assessed before and after the same exercise intervention in 17 resistance trained men. Results showed decreases in peak plantar flexion torque (Experiment 1: -12.2%, Experiment 2: -16.9%), electrically evoked torque (20 Hz -15.3%, 80 Hz -15.3%, variable-frequency train -17.9%), and cSP (-3.8%; i.e., reduced inhibition) post-exercise which did not recover by 20 min. Electromyographic activity (EMG; -6%), corticospinal excitability (-9%), and PIC facilitation (-24.8%) were also reduced post-exercise but recovered by 10 min. Caffeine ingestion increased torque and EMG but did not notably affect corticospinal excitability, PIC amplification, or electrically evoked torque. The data indicate that a decrease in muscle function largely underpins the loss of force after repeated, high-intensity muscular efforts, but that the loss is exacerbated immediately after the exercise by simultaneous decreases in corticospinal excitability and PIC amplitudes at the motor neurons.

Acute and Delayed Neuromuscular Alterations Induced by Downhill Running in Trained Trail Runners: Beneficial Effects of High-Pressure Compression Garments

Introduction: The aim of this study was to examine, from a crossover experimental design, whether wearing high-pressure compression garments (CGs) during downhill treadmill running affects soft-tissue vibrations, acute and delayed responses in running economy (RE), neuromuscular function, countermovement jump, and perceived muscle soreness. Methods: Thirteen male trail runners habituated to regular eccentric training performed two separate 40-min downhill running (DHR, -8.5°) sessions while wearing either CGs (15-20 mmHg for quadriceps and calves) or control garments (CON) at a velocity associated with ∼55% of VO2max , with a set of measurements before (Pre-), after (Post-DHR), and 1 day after (Post-1D). No CGs was used within the recovery phase. Perceived muscle soreness, countermovement jump, and neuromuscular function (central and peripheral components) of knee extensors (KE) and plantar flexors (PF) were assessed. Cardiorespiratory responses (e.g., heart rate, ventilation) and RE, as well as soft-tissue vibrations (root mean square of the resultant acceleration, RMS Ar ) for vastus lateralis and gastrocnemius medialis were evaluated during DHR and in Post-1D. Results: During DHR, mean values in RMS Ar significantly increased over time for the vastus lateralis only for the CON condition (+11.6%). RE and cardiorespiratory responses significantly increased (i.e., alteration) over time in both conditions. Post, small to very large central and peripheral alterations were found for KE and PF in both conditions. However, the deficit in voluntary activation (VA) was significantly lower for KE following CGs (-2.4%), compared to CON (-7.9%) conditions. No significant differences in perceived muscle soreness and countermovement jump were observed between conditions whatever the time period. Additionally, in Post-1D, the CGs condition showed reductions in neuromuscular peripheral alterations only for KE (from -4.4 to -7.7%) and perceived muscle soreness scores (-8.3%). No significant differences in cardiorespiratory and RE responses as well as countermovement jump were identified between conditions in Post-1D. Discussion: Wearing high-pressure CGs (notably on KE) during DHR was associated with beneficial effects on soft-tissue vibrations, acute and delayed neuromuscular function, and perceived muscle soreness. The use of CGs during DHR might contribute to the enhanced muscle recovery by exerting an exercise-induced "mechanical protective effect."

Anodal transcranial direct current stimulation does not influence the neural adjustments associated with fatiguing contractions in a hand muscle

PURPOSE

The objective of the current study was to investigate the mechanisms responsible for the briefer time to failure of a submaximal contraction (C2) when performed 60 min after a similar contraction (C1), and the influence of anodal transcranial direct current stimulation (a-tDCS) applied over the motor cortex on these mechanisms.

METHODS

In two sessions, ten adults sustained two isometric contractions (35% of maximum) to failure with the abductor pollicis brevis (APB). Before C2, either a-tDCS or sham stimulation was applied over the motor cortex. Fatigue-related changes in Hoffmann (H) and long-latency (LLR) reflexes, motor-evoked potential (MEP) induced by transcranial magnetic stimulation and associated silent period (SP), maximal motor wave (M_max), voluntary activation (VA), electromyographic (EMG) activity and peak force (PT₃) evoked by a 3 pulse-train (100 Hz) were investigated.

RESULTS

The results indicate that regardless of session, the time to failure was briefer (- 13%, p < 0.05) for C2 than C1, with no a-tDCS effect. During C1, MEP amplitude, SP duration and LLR amplitude increased, H-reflex amplitude did not change, and M_max, VA and PT₃ decreased (p < 0.05). Except for EMG activity that was greater during C2 than C1 (p < 0.001), all variables were similar in C1 and C2 (p > 0.05), and recovered their initial values after the 60-min rest, except PT₃.

CONCLUSIONS

The results of the current study indicate that a-tDCS did not influence corticospinal excitability and time to failure of C2 when performed with the APB. These observations may reflect a peripheral origin of the briefer C2 time to failure in the APB.

Child-adult differences in neuromuscular fatigue are muscle-dependent

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

Corticospinal changes induced by fatiguing eccentric versus concentric exercise

The present study assessed neuromuscular and corticospinal changes during and after a fatiguing submaximal exercise of the knee extensors in different modes of muscle contraction. Twelve subjects performed two knee extensors exercises in a concentric or eccentric mode, at the same torque and with a similar total impulse. Exercises consisted of 10 sets of 10 repetitions at an intensity of 80% of the maximal voluntary isometric contraction torque (MVIC). MVIC, maximal voluntary activation level (VAL) and responses of electrically evoked contractions of the knee extensors were assessed before and after exercise. Motor evoked potential amplitude (MEP) and cortical silent period (CSP) of the vastus medialis (VM) and rectus femoris (RF) muscles were assessed before, during and after exercise. Similar reductions of the MVIC (-13%), VAL (-12%) and a decrease in the peak twitch (-12%) were observed after both exercises. For both VM and RF muscles, MEP amplitude remained unchanged during either concentric or eccentric exercises. No change of the MEP amplitude input-output curves was observed post-exercise. For the RF muscle, CSP increased during the concentric exercise and remained lengthened after this exercise. For the VM muscle, CSP was reduced after the eccentric exercise only. For a similar amount of total impulse, concentric and eccentric knee extensor contractions led to similar exercise-induced neuromuscular response changes. For the two muscles investigated, no modulation of corticospinal excitability was observed during or after either concentric or eccentric exercises. However, intracortical inhibition showed significant modulations during and after exercise.

Wide-pulse, high-frequency, low-intensity neuromuscular electrical stimulation has potential for targeted strengthening of an intrinsic foot muscle: a feasibility study

Background

Strengthening the intrinsic foot muscles is a poorly understood and largely overlooked area. In this study, we explore the feasibility of strengthening m. abductor hallucis (AH) with a specific paradigm of neuromuscular electrical stimulation; one which is low-intensity in nature and designed to interleave physiologically-relevant low frequency stimulation with high-frequencies to enhance effective current delivery to spinal motoneurones, and enable a proportion of force produced by the target muscle to be generated from a central origin. We use standard neurophysiological measurements to evaluate the acute (~ 30 min) peripheral and central adaptations in healthy individuals.

Methods

The AH in the dominant foot of nine healthy participants was stimulated with 24 × 15 s trains of square wave (1 ms), constant current (150% of motor threshold), alternating (20 Hz–100 Hz) neuromuscular electrical stimulation interspersed with 45 s rest. Prior to the intervention, peripheral variables were evoked from the AH compound muscle action potential (M_wave) and corresponding twitch force in response to supramaximal (130%) medial plantar nerve stimulation. Central variables were evoked from the motor evoked potential (MEP) in response to suprathreshold (150%) transcranial magnetic stimulation of the motor cortex corresponding to the AH pathway. Follow-up testing occurred immediately, and 30 min after the intervention. In addition, the force-time-integrals (FTI) from the 1st and 24th WPHF trains were analysed as an index of muscle fatigue. All variables except FTI (T-test) were entered for statistical analysis using a single factor repeated measures ANOVA with alpha set at 0.05.

Results

FTI was significantly lower at the end of the electrical intervention compared to that evoked by the first train (p < 0.01). Only significant peripheral nervous system adaptations were observed, consistent with the onset of low-frequency fatigue in the muscle. In most of these variables, the effects persisted for 30 min after the intervention.

Conclusions

An acute session of wide-pulse, high-frequency, low-intensity electrical stimulation delivered directly to abductor hallucis in healthy feet induces muscle fatigue via adaptations at the peripheral level of the neuromuscular system. Our findings would appear to represent the first step in muscle adaptation to training; therefore, there is potential for using WPHF for intrinsic foot muscle strengthening.

Electronic supplementary material

The online version of this article (10.1186/s13047-018-0258-1) contains supplementary material, which is available to authorized users.

Neuromuscular fatigue after low- and medium-frequency electrical stimulation in healthy adults

INTRODUCTION

In this study we investigated fatigue origins induced by low-frequency pulsed current (PC) and medium-frequency current (MF) neuromuscular electrical stimulation (NMES) after a clinical-like session.

METHODS

Eleven healthy men randomly underwent 2 NMES sessions, PC and MF, on quadriceps muscle (15-minute duration, 6 seconds on and 18 seconds off). Maximal voluntary contraction (MVC), central activation ratio (CAR), vastus lateralis electromyographic activity (EMG), and evoked contractile properties were determined before and after the sessions. Evoked torque and discomfort during the sessions were also measured.

RESULTS

Both currents produced decreases in MVC, EMG, and evoked contractile properties after the sessions. No difference was found between currents for all variables (P > 0.05). Evoked torque during sessions decreased (P < 0.05). No difference was observed in mean evoked torque and discomfort (P > 0.05).

DISCUSSION

Both currents induced similar neuromuscular fatigue. Clinicians can choose either PC or MF and expect similar treatment effects when the goal is to generate gains in muscle strength. Muscle Nerve 58: 293-299, 2018.

Neuromuscular Changes and Damage after Isoload versus Isokinetic Eccentric Exercise

PURPOSE

This study compared the effects of isoload (IL) and isokinetic (IK) knee extensor eccentric exercises on changes in muscle damage and neuromuscular parameters to test the hypothesis that the changes would be different after IL and IK exercises.

METHODS

Twenty-two young men were paired based on their strength and placed in the IL (N = 11) or the IK (N = 11) group. The IL group performed 15 sets of 10 eccentric contractions with a 150% of predetermined one-repetition maximum load. The IK group performed 15 sets of several maximal eccentric contractions matched set by set for the total amount of work and mean angular velocity with the IL group. Muscle damage markers (voluntary isometric peak torque, muscle soreness, and creatine kinase activity) and neuromuscular variables (e.g., voluntary activation, H-reflex, M-wave, and evoked torque) were measured before, immediately after, and 24, 48, 72, and 96 h postexercise.

RESULTS

Voluntary isometric peak torque decreased to the same extent (P = 0.94) in both groups immediately after (IL = -40.6% ± 13.8% vs IK = -42.4% ± 10.2%) to 96 h after the exercise (IL = -21.8% ± 28.5% vs IK = -26.7% ± 23.5%). Neither peak muscle soreness (IL = 48.1 ± 28.2 mm vs IK = 54.7 ± 28.9 mm, P = 0.57) nor creatine kinase activity (IL = 12,811 ± 22,654 U·L vs IK = 15,304 ± 24,739 U·L, P = 0.59) significantly differed between groups. H-reflex (IL = -23% vs IK = -35%) and M-wave (IL = -10% vs IK = -17%) significantly decreased immediately postexercise similarly between groups.

CONCLUSION

The changes in muscle damage and neuromuscular function after the exercise are similar between IL and IK, suggesting that resistance modality has little effects on acute muscle responses.

Central activation, metabolites, and calcium handling during fatigue with repeated maximal isometric contractions in human muscle

PURPOSE

To determine the roles of calcium (Ca²⁺) handling by sarcoplasmic reticulum (SR) and central activation impairment (i.e., central fatigue) during fatigue with repeated maximal voluntary isometric contractions (MVC) in human muscles.

METHODS

Contractile performance was assessed during 3 min of repeated MVCs (7-s contraction, 3-s rest, n = 17). In ten participants, in vitro SR Ca²⁺-handling, metabolites, and fibre-type composition were quantified in biopsy samples from quadriceps muscle, along with plasma venous [K⁺]. In 11 participants, central fatigue was compared using tetanic stimulation superimposed on MVC in quadriceps and adductor pollicis muscles.

RESULTS

The decline of peak MVC force with fatigue was similar for both muscles. Fatigue resistance correlated directly with % type I fibre area in quadriceps (r = 0.77, P = 0.009). The maximal rate of ryanodine-induced Ca²⁺-release and Ca²⁺-uptake fell by 31 ± 26 and 28 ± 13%, respectively. The tetanic force depression was correlated with the combined reduction of ATP and PCr, and increase of lactate (r = 0.77, P = 0.009). Plasma venous [K⁺] increased from 4.0 ± 0.3 to 5.4 ± 0.8 mM over 1-3-min exercise. Central fatigue occurred during the early contractions in the quadriceps in 7 out of 17 participants (central activation ratio fell from 0.98 ± 0.05 to 0.86 ± 0.11 at 1 min), but dwindled at exercise cessation. Central fatigue was seldom apparent in adductor pollicis.

CONCLUSIONS

Fatigue with repeated MVC in human limb muscles mainly involves peripheral aspects which include impaired SR Ca²⁺-handling and we speculate that anaerobic metabolite changes are involved. A faster early force loss in quadriceps muscle with some participants is attributed to central fatigue.

Low-Frequency Fatigue Assessed as Double to Single Twitch Ratio after Two Bouts of Eccentric Exercise of the Elbow Flexors

The aim of this study was to assess low-frequency fatigue as a double to single twitch ratio after repeated eccentric exercise of the elbow flexors. Maximal isometric torque, single and double twitch responses and low-frequency fatigue were assessed on the elbow flexors in 16 untrained male volunteers before, immediately after, 24 and 48 hours following two bouts of eccentric exercise consisted of 30 repetitions of lowering a dumbbell adjusted to ~75% of each individual's maximal isometric torque. Maximal isometric torque and electrically evoked responses decreased significantly in all measurements after the first bout of eccentric exercise (p < 0.05). In measurements performed at 24 and 48 hours after the second bout both maximal voluntary isometric torque and electrically evoked contractions were significantly higher than in measurements performed after the first bout (p < 0.05). Although low-frequency fatigue significantly increased up to 48 hours after each bout of eccentric exercise, its values at 24 and 48 hours after the second bout were significantly lower than at respective time points after the first bout (p < 0.05). Double to single twitch ratio could be used as a sensitive tool in the evaluation of muscle recovery and adaptation to repeated eccentric exercise.

Limitations in intense exercise performance of athletes - effect of speed endurance training on ion handling and fatigue development

Mechanisms underlying fatigue development and limitations for performance during intense exercise have been intensively studied during the past couple of decades. Fatigue development may involve several interacting factors and depends on type of exercise undertaken and training level of the individual. Intense exercise (½-6 min) causes major ionic perturbations (Ca²⁺ , Cl^- , H⁺ , K⁺ , lactate^- and Na⁺ ) that may reduce sarcolemmal excitability, Ca²⁺ release and force production of skeletal muscle. Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise. Regular speed endurance training (SET), i.e. exercise performed at intensities above that corresponding to maximum oxygen consumption (V̇O2, max ), enhances intense exercise performance. However, most of the studies that have provided mechanistic insight into the beneficial effects of SET have been conducted in untrained and recreationally active individuals, making extrapolation towards athletes' performance difficult. Nevertheless, recent studies indicate that only a few weeks of SET enhances intense exercise performance in highly trained individuals. In these studies, the enhanced performance was not associated with changes in V̇O2, max and muscle oxidative capacity, but rather with adaptations in muscle ion handling, including lowered interstitial concentrations of K⁺ during and in recovery from intense exercise, improved lactate^- -H⁺ transport and H⁺ regulation, and enhanced Ca²⁺ release function. The purpose of this Topical Review is to provide an overview of the effect of SET and to discuss potential mechanisms underlying enhancements in performance induced by SET in already well-trained individuals with special emphasis on ion handling in skeletal muscle.

Predominant cause of prolonged low-frequency force depression changes during recovery after in situ fatiguing stimulation of rat fast-twitch muscle

To investigate time-dependent changes in sarcoplasmic reticulum (SR) Ca2+ release and myofibrillar (my-) Ca2+ sensitivity during recovery from prolonged low-frequency force depression (PLFFD), rat gastrocnemius muscles were electrically stimulated in situ. After 0 h (R0), 0.5 h (R0.5), 2 h (R2), 6 h (R6), or 12 h of recovery, the superficial gastrocnemius muscles were excised and used for biochemical and skinned fiber analyses. At R0, R0.5, R2, and R6, the ratio of force at 1 Hz to that at 50 Hz was decreased in the skinned fibers. The ratio of depolarization-induced force to the maximum Ca2+-activated force (depol/Ca2+ force ratio) was utilized as an indicator of SR Ca2+ release. At R0, both the depol/Ca2+ force ratio and my-Ca2+ sensitivity were decreased. At R0.5 and R2, my-Ca2+ sensitivity was recovered, while the depol/Ca2+ force ratio remained depressed. At R6, my-Ca2+ sensitivity was decreased again, whereas the depol/Ca2+ force ratio was nearly restored. Western blot analyses demonstrated that decreased my-Ca2+ sensitivity at R6 and reduced depol/Ca2+ force ratio at R0, R0.5, and R2 were accompanied by depressions in S-glutathionylated troponin I and increases in dephosphorylated ryanodine receptor 1, respectively. These results indicate that, in the early stage of recovery, reduced SR Ca2+ release plays a primary role in the etiology of PLFFD, whereas decreased my-Ca2+ sensitivity is involved in the late stage, and suggest that S-glutathionylation of troponin I and dephosphorylation of ryanodine receptor 1 contribute, at least partly, to fatiguing contraction-induced alterations in my-Ca2+ sensitivity and SR Ca2+ release, respectively.

Mechanisms of force depression caused by different types of physical exercise studied by direct electrical stimulation of human quadriceps muscle

Purpose

Force production frequently remains depressed for several hours or even days after various types of strenuous physical exercise. We hypothesized that the pattern of force changes during the first hour after exercise can be used to reveal muscular mechanisms likely to underlie the decline in muscle performance during exercise as well as factors involved in the triggering the prolonged force depression after exercise.

Methods

Nine groups of recreationally active male volunteers performed one of the following types of exercise: single prolonged or repeated short maximum voluntary contractions (MVCs); single or repeated all-out cycling bouts; repeated drop jumps. The isometric force of the right quadriceps muscle was measured during stimulation with brief 20 and 100 Hz trains of electrical pulses given before and at regular intervals for 60 min after exercise.

Results

All exercises resulted in a prolonged force depression, which was more marked at 20 Hz than at 100 Hz. Short-lasting (≤2 min) MVC and all-out cycling exercises showed an initial force recovery (peak after ~ 5 min) followed by a secondary force depression. The repeated drop jumps, which involve eccentric contractions, resulted in a stable force depression with the 20 Hz force being markedly more decreased after 100 than 10 jumps.

Conclusions

In accordance with our hypothesis, the results propose at least three different mechanisms that influence force production after exercise: (1) a transiently recovering process followed by (2) a prolonged force depression after metabolically demanding exercise, and (3) a stable force depression after mechanically demanding contractions.

Different Effect of Local and General Fatigue on Knee Joint Stiffness

PURPOSE

This study aimed to investigate the influence of locally and generally induced fatigue on the stiffness properties of the knee joint.

METHODS

Twenty-two male (24.9 ± 4.5 yr, 1.78 ± 0.06 m, 75.4 ± 6.4 kg, 23.9 ± 1.8 kg·m) and 18 female (21.1 ± 1.5 yr, 1.66 ± 0.05 m, 63.4 ± 6.5 kg, 22.9 ± 2.5 kg·m) amateur athletes participated. Peak torque (PT) of the knee extensor musculature, muscle stiffness (MS) of the vastus lateralis, and musculoarticular stiffness (MAS) of the knee joint were assessed pre- and postlocally and generally induced fatigue (undertaken on two separate days with a 1-wk interval).

RESULTS

Males were characterized by higher values of MAS, relaxed and contracted MS, normalized PT (PT/body mass), and normalized MAS (MAS/external load) irrespective of time point (P < 0.05).

LOCALLY INDUCED FATIGUE

Contracted MS increased more (P < 0.01) and normalized PT decreased more (P = 0.03) in males than in females postfatigue. Significant increases occurred in MAS in females (P = 0.01); relaxed MS (males, P < 0.001; females, P < 0.001), contracted MS (males, P < 0.001; females, P = 0.04), and normalized MAS (males, P = 0.001; females, P = 0.01) in both sexes; and normalized contracted MS (contracted MS/external load) in males (P < 0.001). Normalized PT decreased significantly in males (P < 0.01) postfatigue.

GENERALLY INDUCED FATIGUE

Contracted MS (P = 0.01) and MAS (P = 0.05) decreased significantly in males post-fatigue.

CONCLUSION

The stiffness properties of the knee joint are influenced by locally and generally induced fatigue, with different responses being observed in males and females.

The slow component of pulmonary O2 uptake accompanies peripheral muscle fatigue during high-intensity exercise

During constant-power output (PO) exercise above lactate threshold (LT), pulmonary O2 uptake (V̇o2p) features a developing slow component (V̇o2pSC). This progressive increase in O2 cost of exercise is suggested to be related to the effects of muscle fatigue development. We hypothesized that peripheral muscle fatigue as assessed by contractile impairment would be associated with the V̇o2pSC. Eleven healthy men were recruited to perform four constant-PO tests at an intensity corresponding to ∼Δ60 (very heavy, VH) where Δ is 60% of the difference between LT and peak V̇o2p. The VH exercise was completed for each of 3, 8, 13, and 18 min (i.e., VH3, VH8, VH13, VH18) with each preceded by 3 min of cycling at 20 W. Peripheral muscle fatigue was assessed via pre- vs. postexercise measurements of quadriceps torque in response to brief trains of electrical stimulation delivered at low (10 Hz) and high (50 Hz) frequencies. During exercise, breath-by-breath V̇o2p was measured by mass spectrometry and volume turbine. The magnitude of V̇o2pSC increased ( P < 0.05) from 224 ± 81 ml/min at VH3 to 520 ± 119, 625 ± 134, and 678 ± 156 ml/min at VH8, VH13, and VH18, respectively. The ratio of the low-to-high frequency (10/50 Hz) response was reduced ( P < 0.05) at VH3 (−12 ± 9%) and further reduced ( P < 0.05) at VH8 (−25 ± 11%), VH13 (−42 ± 19%), and VH18 (−46 ± 16%), mirroring the temporal pattern of V̇o2pSC development. The reduction in 10/50 Hz ratio was correlated ( P < 0.001, r2 = 0.69) with V̇o2pSC amplitude. The temporal and quantitative association of decrements in muscle torque production and V̇o2pSC suggest a common physiological mechanism between skeletal muscle fatigue and loss of muscle efficiency.

Fatigue associated with prolonged graded running

Scientific experiments on running mainly consider level running. However, the magnitude and etiology of fatigue depend on the exercise under consideration, particularly the predominant type of contraction, which differs between level, uphill, and downhill running. The purpose of this review is to comprehensively summarize the neurophysiological and biomechanical changes due to fatigue in graded running. When comparing prolonged hilly running (i.e., a combination of uphill and downhill running) to level running, it is found that (1) the general shape of the neuromuscular fatigue-exercise duration curve as well as the etiology of fatigue in knee extensor and plantar flexor muscles are similar and (2) the biomechanical consequences are also relatively comparable, suggesting that duration rather than elevation changes affects neuromuscular function and running patterns. However, 'pure' uphill or downhill running has several fatigue-related intrinsic features compared with the level running. Downhill running induces severe lower limb tissue damage, indirectly evidenced by massive increases in plasma creatine kinase/myoglobin concentration or inflammatory markers. In addition, low-frequency fatigue (i.e., excitation-contraction coupling failure) is systematically observed after downhill running, although it has also been found in high-intensity uphill running for different reasons. Indeed, low-frequency fatigue in downhill running is attributed to mechanical stress at the interface sarcoplasmic reticulum/T-tubule, while the inorganic phosphate accumulation probably plays a central role in intense uphill running. Other fatigue-related specificities of graded running such as strategies to minimize the deleterious effects of downhill running on muscle function, the difference of energy cost versus heat storage or muscle activity changes in downhill, level, and uphill running are also discussed.

Group III/IV muscle afferents limit the intramuscular metabolic perturbation during whole body exercise in humans

KEY POINTS

The purpose of this study was to determine the role of group III/IV muscle afferents in limiting the endurance exercise-induced metabolic perturbation assayed in muscle biopsy samples taken from locomotor muscle. Lumbar intrathecal fentanyl was used to attenuate the central projection of μ-opioid receptor-sensitive locomotor muscle afferents during a 5 km cycling time trial. The findings suggest that the central projection of group III/IV muscle afferent feedback constrains voluntary neural 'drive' to working locomotor muscle and limits the exercise-induced intramuscular metabolic perturbation. Therefore, the CNS might regulate the degree of metabolic perturbation within locomotor muscle and thereby limit peripheral fatigue. It appears that the group III/IV muscle afferents are an important neural link in this regulatory mechanism, which probably serves to protect locomotor muscle from the potentially severe functional impairment as a consequence of severe intramuscular metabolic disturbance.

ABSTRACT

To investigate the role of metabo- and mechanosensitive group III/IV muscle afferents in limiting the intramuscular metabolic perturbation during whole body endurance exercise, eight subjects performed 5 km cycling time trials under control conditions (CTRL) and with lumbar intrathecal fentanyl impairing lower limb muscle afferent feedback (FENT). Vastus lateralis muscle biopsies were obtained before and immediately after exercise. Motoneuronal output was estimated through vastus lateralis surface electromyography (EMG). Exercise-induced changes in intramuscular metabolites were determined using liquid and gas chromatography-mass spectrometry. Quadriceps fatigue was quantified by pre- to post-exercise changes in potentiated quadriceps twitch torque (ΔQTsingle ) evoked by electrical femoral nerve stimulation. Although motoneuronal output was 21 ± 12% higher during FENT compared to CTRL (P < 0.05), time to complete the time trial was similar (∼8.8 min). Compared to CTRL, power output during FENT was 10 ± 4% higher in the first half of the time trial, but 11 ± 5% lower in the second half (both P < 0.01). The exercise-induced increase in intramuscular inorganic phosphate, H(+) , adenosine diphosphate, lactate and phosphocreatine depletion was 55 ± 30, 62 ± 18, 129 ± 63, 47 ± 14 (P < 0.001) and 27 ± 14% (P < 0.01) greater in FENT than CTRL. ΔQTsingle was greater following FENT than CTRL (-52 ± 2 vs -31 ± 1%, P < 0.001) and this difference was positively correlated with the difference in inorganic phosphate (r(2)  = 0.79; P < 0.01) and H(+) (r(2)  = 0.92; P < 0.01). In conclusion, during whole body exercise, group III/IV muscle afferents provide feedback to the CNS which, in turn, constrains motoneuronal output to the active skeletal muscle. This regulatory mechanism limits the exercise-induced intramuscular metabolic perturbation, preventing an abnormal homeostatic challenge and excessive peripheral fatigue.

The increase in hydric volume is associated to contractile impairment in the calf after the world's most extreme mountain ultra-marathon

Background

Studies have recently focused on the effect of running a mountain ultra-marathon (MUM) and their results show muscular inflammation, damage and force loss. However, the link between peripheral oedema and muscle force loss is not really established. We tested the hypothesis that, after a MUM, lower leg muscles’ swelling could be associated with muscle force loss. The knee extensor (KE) and the plantar flexor (PF) muscles’ contractile function was measured by supramaximal electrical stimulations, potentiated low- and high-frequency doublets (PS10 and PS100) of the KE and the PF were measured by transcutaneous electrical nerve stimulation and bioimpedance was used to assess body composition in the runners (n = 11) before (Pre) and after (Post) the MUM and compared with the controls (n = 8).

Results

The maximal voluntary contraction of the KE and the PF significantly decreased by 20 % Post-MUM in the runners. Hydration of the non-fat mass (NF-Hyd) and extracellular water volume (Ve) were increased by 12 % Post-MUM (p < 0.001) in the runners. Calf circumference (+2 %, p < 0.05) was also increased. Significant relationships were found for percentage increases in Ve and NF-Hyd with percentage decrease in PS10 of the PF (r = −0.68 and r = −0.70, p < 0.05) and with percentage increase of calf circumference (r = 0.72 and r = 0.73, p < 0.05) in the runners.

Conclusions

The present study suggests that increases in circumference and in hydric volume are associated to contractile impairment in the calf in ultra-marathon runners.

Effects of cold water immersion and active recovery on hemodynamics and recovery of muscle strength following resistance exercise

Cold water immersion (CWI) and active recovery (ACT) are frequently used as postexercise recovery strategies. However, the physiological effects of CWI and ACT after resistance exercise are not well characterized. We examined the effects of CWI and ACT on cardiac output (Q̇), muscle oxygenation (SmO2), blood volume (tHb), muscle temperature (Tmuscle), and isometric strength after resistance exercise. On separate days, 10 men performed resistance exercise, followed by 10 min CWI at 10°C or 10 min ACT (low-intensity cycling). Q̇ (7.9 ± 2.7 l) and Tmuscle (2.2 ± 0.8°C) increased, whereas SmO2 (-21.5 ± 8.8%) and tHb (-10.1 ± 7.7 μM) decreased after exercise (P < 0.05). During CWI, Q̇ (-1.1 ± 0.7 l) and Tmuscle (-6.6 ± 5.3°C) decreased, while tHb (121 ± 77 μM) increased (P < 0.05). In the hour after CWI, Q̇ and Tmuscle remained low, while tHb also decreased (P < 0.05). By contrast, during ACT, Q̇ (3.9 ± 2.3 l), Tmuscle (2.2 ± 0.5°C), SmO2 (17.1 ± 5.7%), and tHb (91 ± 66 μM) all increased (P < 0.05). In the hour after ACT, Tmuscle, and tHb remained high (P < 0.05). Peak isometric strength during 10-s maximum voluntary contractions (MVCs) did not change significantly after CWI, whereas it decreased after ACT (-30 to -45 Nm; P < 0.05). Muscle deoxygenation time during MVCs increased after ACT (P < 0.05), but not after CWI. Muscle reoxygenation time after MVCs tended to increase after CWI (P = 0.052). These findings suggest first that hemodynamics and muscle temperature after resistance exercise are dependent on ambient temperature and metabolic demands with skeletal muscle, and second, that recovery of strength after resistance exercise is independent of changes in hemodynamics and muscle temperature.

Skeletal muscle contractility and fatigability in adults with cystic fibrosis

BACKGROUND

Recent discovery of cystic fibrosis transmembrane conductance regulator expression in human skeletal muscle suggests that CF patients may have intrinsic skeletal muscle abnormalities potentially leading to functional impairments. The aim of the present study was to determine whether CF patients with mild to moderate lung disease have altered skeletal muscle contractility and greater muscle fatigability compared to healthy controls.

METHODS

Thirty adults (15 CF and 15 controls) performed a quadriceps neuromuscular evaluation using single and paired femoral nerve magnetic stimulations. Electromyographic and mechanical parameters during voluntary and magnetically-evoked contractions were recorded at rest, during and after a fatiguing isometric task. Quadriceps cross-sectional area was determined by magnetic resonance imaging.

RESULTS

Some indexes of muscle contractility tended to be reduced at rest in CF compared to controls (e.g., mechanical response to doublets stimulation at 100 Hz: 74±30 Nm vs. 97±28 Nm, P=0.06) but all tendencies disappeared when expressed relative to quadriceps cross-sectional area (P>0.5 for all parameters). CF and controls had similar alterations in muscle contractility with fatigue, similar endurance and post exercise recovery.

CONCLUSIONS

We found similar skeletal muscle endurance and fatigability in CF adults and controls and only trends for reduced muscle strength in CF which disappeared when normalized to muscle cross-sectional area. These results indicate small quantitative (reduced muscle mass) rather than qualitative (intrinsic skeletal muscle abnormalities) muscle alterations in CF with mild to moderate lung disease.

Changes in motor unit behavior following isometric fatigue of the first dorsal interosseous muscle

The neuromuscular strategies employed to compensate for fatigue-induced muscle force deficits are not clearly understood. This study utilizes surface electromyography (sEMG) together with recordings of a population of individual motor unit action potentials (MUAPs) to investigate potential compensatory alterations in motor unit (MU) behavior immediately following a sustained fatiguing contraction and after a recovery period. EMG activity was recorded during abduction of the first dorsal interosseous in 12 subjects at 20% maximum voluntary contraction (MVC), before and directly after a 30% MVC fatiguing contraction to task failure, with additional 20% MVC contractions following a 10-min rest. The amplitude, duration and mean firing rate (MFR) of MUAPs extracted with a sEMG decomposition system were analyzed, together with sEMG root-mean-square (RMS) amplitude and median frequency (MPF). MUAP duration and amplitude increased immediately postfatigue and were correlated with changes to sEMG MPF and RMS, respectively. After 10 min, MUAP duration and sEMG MPF recovered to prefatigue values but MUAP amplitude and sEMG RMS remained elevated. MU MFR and recruitment thresholds decreased postfatigue and recovered following rest. The increase in MUAP and sEMG amplitude likely reflects recruitment of larger MUs, while recruitment compression is an additional compensatory strategy directly postfatigue. Recovery of MU MFR in parallel with MUAP duration suggests a possible role for metabolically sensitive afferents in MFR depression postfatigue. This study provides insight into fatigue-induced neuromuscular changes by examining the properties of a large population of concurrently recorded single MUs and outlines possible compensatory strategies involving alterations in MU recruitment and MFR.

Neuromuscular adjustments of the knee extensors and plantar flexors following match-play tennis in the heat

Objectives

This study tested the hypothesis that impairments in lower limb maximal strength and voluntary activation (VA) are exacerbated following match-play tennis in hot compared with cool conditions.

Methods

Torque and VA were evaluated during brief (5 s) and sustained (20 s) maximal voluntary isometric contractions of the knee extensors (KE) and plantar flexors (PF) in 12 male tennis players before (pre) and after (post, 24 h and 48 h) ∼115 min of play in hot (∼37°C) and cool (∼22°C) conditions.

Results

Rectal temperature was higher following play in hot than in cool (∼39.2 vs ∼38.5°C; p<0.05). Torque production decreased from prematch to postmatch during the brief and sustained contractions in hot (KE: ∼22%; PF: ∼13%) and cool (KE: ∼9%, PF: ∼7%) (p<0.05). KE strength losses in hot were greater than in cool (p<0.05) and persisted for 24 h (p<0.05). Postmatch brief and sustained KE VA was lower in hot than in cool (p<0.05), in which VA was maintained. PF VA was maintained throughout the protocol. Peak twitch torque and maximum rates of torque development and relaxation in the KE and PF were equally reduced postmatch relative to prematch in hot and cool conditions (p<0.05), and were restored near baseline within 24 h.

Conclusions

Neuromuscular system integrity of the lower limbs is compromised immediately following match-play tennis in hot and cool conditions due to the development of peripheral fatigue. The larger and persistent KE strength losses observed under heat stress are associated with greater levels of central fatigue especially during sustained contractions.

Antioxidant treatments do not improve force recovery after fatiguing stimulation of mouse skeletal muscle fibres

The contractile performance of skeletal muscle declines during intense activities, i.e. fatigue develops. Fatigued muscle can enter a state of prolonged low-frequency force depression (PLFFD). PLFFD can be due to decreased tetanic free cytosolic [Ca(2+) ] ([Ca(2+) ]i ) and/or decreased myofibrillar Ca(2+) sensitivity. Increases in reactive oxygen and nitrogen species (ROS/RNS) may contribute to fatigue-induced force reductions. We studied whether pharmacological ROS/RNS inhibition delays fatigue and/or counteracts the development of PLFFD. Mechanically isolated mouse fast-twitch fibres were fatigued by sixty 150 ms, 70 Hz tetani given every 1 s. Experiments were performed in standard Tyrode solution (control) or in the presence of: NADPH oxidase (NOX) 2 inhibitor (gp91ds-tat); NOX4 inhibitor (GKT137831); mitochondria-targeted antioxidant (SS-31); nitric oxide synthase (NOS) inhibitor (l-NAME); the general antioxidant N-acetylcysteine (NAC); a cocktail of SS-31, l-NAME and NAC. Spatially and temporally averaged [Ca(2+) ]i and peak force were reduced by ∼20% and ∼70% at the end of fatiguing stimulation, respectively, with no marked differences between groups. PLFFD was similar in all groups, with 30 Hz force being decreased by ∼60% at 30 min of recovery. PLFFD was mostly due to decreased tetanic [Ca(2+) ]i in control fibres and in the presence of NOX2 or NOX4 inhibitors. Conversely, in fibres exposed to SS-31 or the anti ROS/RNS cocktail, tetanic [Ca(2+) ]i was not decreased during recovery so PLFFD was only caused by decreased myofibrillar Ca(2+) sensitivity. The cocktail also increased resting [Ca(2+) ]i and ultimately caused cell death. In conclusion, ROS/RNS-neutralizing compounds did not counteract the force decline during or after induction of fatigue.

Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes

PURPOSE

The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes.

METHODS

Fourteen highly trained male triathletes (VO2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period.

RESULTS

Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group.

CONCLUSION

In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise.

Time course of central and peripheral alterations after isometric neuromuscular electrical stimulation-induced muscle damage

Isometric contractions induced by neuromuscular electrostimulation (NMES) have been shown to result in a prolonged force decrease but the time course of the potential central and peripheral factors have never been investigated. This study examined the specific time course of central and peripheral factors after isometric NMES-induced muscle damage. Twenty-five young healthy men were subjected to an NMES exercise consisting of 40 contractions for both legs. Changes in maximal voluntary contraction force of the knee extensors (MVC), peak evoked force during double stimulations at 10 Hz (Db₁₀) and 100 Hz (Db₁₀₀), its ratio (10∶100), voluntary activation, muscle soreness and plasma creatine kinase activity were assessed before, immediately after and throughout four days after NMES session. Changes in knee extensors volume and T₂ relaxation time were also assessed at two (D2) and four (D4) days post-exercise. MVC decreased by 29% immediately after NMES session and was still 19% lower than the baseline value at D4. The decrease in Db₁₀ was higher than in Db₁₀₀ immediately and one day post-exercise resulting in a decrease (−12%) in the 10∶100 ratio. On the contrary, voluntary activation significantly decreased at D2 (−5%) and was still depressed at D4 (−5%). Muscle soreness and plasma creatine kinase activity increased after NMES and peaked at D2 and D4, respectively. T₂ was also increased at D2 (6%) and D4 (9%). Additionally, changes in MVC and peripheral factors (e.g., Db₁₀₀) were correlated on the full recovery period, while a significant correlation was found between changes in MVC and VA only from D2 to D4. The decrease in MVC recorded immediately after the NMES session was mainly due to peripheral changes while both central and peripheral contributions were involved in the prolonged force reduction. Interestingly, the chronological events differ from what has been reported so far for voluntary exercise-induced muscle damage.

Cold water immersion enhances recovery of submaximal muscle function after resistance exercise

We investigated the effect of cold water immersion (CWI) on the recovery of muscle function and physiological responses after high-intensity resistance exercise. Using a randomized, cross-over design, 10 physically active men performed high-intensity resistance exercise followed by one of two recovery interventions: 1) 10 min of CWI at 10°C or 2) 10 min of active recovery (low-intensity cycling). After the recovery interventions, maximal muscle function was assessed after 2 and 4 h by measuring jump height and isometric squat strength. Submaximal muscle function was assessed after 6 h by measuring the average load lifted during 6 sets of 10 squats at 80% of 1 repetition maximum. Intramuscular temperature (1 cm) was also recorded, and venous blood samples were analyzed for markers of metabolism, vasoconstriction, and muscle damage. CWI did not enhance recovery of maximal muscle function. However, during the final three sets of the submaximal muscle function test, participants lifted a greater load (P < 0.05, Cohen's effect size: 1.3, 38%) after CWI compared with active recovery. During CWI, muscle temperature decreased ∼7°C below postexercise values and remained below preexercise values for another 35 min. Venous blood O2 saturation decreased below preexercise values for 1.5 h after CWI. Serum endothelin-1 concentration did not change after CWI, whereas it decreased after active recovery. Plasma myoglobin concentration was lower, whereas plasma IL-6 concentration was higher after CWI compared with active recovery. These results suggest that CWI after resistance exercise allows athletes to complete more work during subsequent training sessions, which could enhance long-term training adaptations.