Muscle Fatigue Affects the Interpolated Twitch Technique When Assessed Using Electrically-Induced Contractions in Human and Rat Muscles

The interpolated twitch technique (ITT) is the gold standard to assess voluntary activation and central fatigue. Yet, its validity has been questioned. Here we studied how peripheral fatigue can affect the ITT. Repeated contractions at submaximal frequencies were produced by supramaximal electrical stimulations of the human adductor pollicis muscle in vivo and of isolated rat soleus fiber bundles; an extra stimulation pulse was given during contractions to induce a superimposed twitch. Human muscles fatigued by repeated 30-Hz stimulation trains (3 s on-1 s off) showed an ~80% reduction in the superimposed twitch force accompanied by a severely reduced EMG response (M-wave amplitude), which implies action potential failure. Subsequent experiments combined a less intense stimulation protocol (1.5 s on-3 s off) with ischemia to cause muscle fatigue, but which preserved M-wave amplitude. However, the superimposed twitch force still decreased markedly more than the potentiated twitch force; with ITT this would reflect increased "voluntary activation." In contrast, the superimposed twitch force was relatively spared when a similar protocol was performed in rat soleus bundles. Force relaxation was slowed by >150% in fatigued human muscles, whereas it was unchanged in rat soleus bundles. Accordingly, results similar to those in the human muscle were obtained when relaxation was slowed by cooling the rat soleus muscles. In conclusion, our data demonstrate that muscle fatigue can confound the quantification of central fatigue using the ITT

Modulators of actin-myosin dissociation: basis for muscle type functional differences during fatigue

The muscle types present with variable fatigue tolerance, in part due to the myosin isoform expressed. However, the critical steps that define 'fatigability' in vivo of fast vs slow myosin isoforms, at the molecular level, are not yet fully understood. We examined the modulation of the ATP-induced myosin sub-fragment 1 (S1) dissociation from pyrene-actin by inorganic phosphate (Pi), pH and temperature using a specially modified stopped-flow system that allowed fast kinetics measurements at physiological temperature. We contrasted the properties of rabbit psoas (fast) and bovine masseter (slow) myosins (obtained from samples collected from New Zealand rabbits and from a licensed abattoir, respectively, according to institutional and national ethics permits). To identify ATP cycling biochemical intermediates, we assessed ATP binding to a pre-equilibrated mixture of actomyosin and variable [ADP], pH (pH 7 vs pH 6.2) and Pi (zero, 15 or 30 added mM Pi) in a range of temperatures (5 to 45°C). Temperature and pH variations had little, if any, effect on the ADP dissociation constant (KADP) for fast S1 but for slow S1 KADP was weakened with increasing temperature or low pH. In the absence of ADP, the dissociation constant for phosphate (KPi) was weakened with increasing temperature for fast S1. In the presence of ADP, myosin type differences were revealed at the apparent phosphate affinity, depending on pH and temperature. Overall, the newly revealed kinetic differences between myosin types could help explain the in vivo observed muscle type functional differences at rest and during fatigue

Incubating Isolated Mouse EDL Muscles with Creatine Improves Force Production and Twitch Kinetics in Fatigue Due to Reduction in Ionic Strength

Creatine supplementation can improve performance during high intensity exercise in humans and improve muscle strength in certain myopathies. In this present study, we investigated the direct effects of acute creatine incubation on isolated mouse fast-twitch EDL muscles, and examined how these effects change with fatigue. muscle from mice aged 12–14 weeks was isolated and stimulated with field electrodes to measure force characteristics in 3 different states: (i) before fatigue; (ii) immediately after a fatigue protocol; and (iii) after recovery. These served as the control measurements for the muscle. The muscle was then incubated in a creatine solution and washed. The measurement of force characteristics in the 3 different states was then repeated. In un-fatigued muscle, creatine incubation increased the maximal tetanic force. In fatigued muscle, creatine treatment increased the force produced at all frequencies of stimulation. Incubation also increased the rate of twitch relaxation and twitch contraction in fatigued muscle. During repetitive fatiguing stimulation, creatine-treated muscles took 55.1±9.5% longer than control muscles to lose half of their original force. Measurement of weight changes showed that creatine incubation increased EDL muscle mass by 7%. sensitivity of contractile proteins as a result of ionic strength decreases following creatine incubation

Effects of membrane depolarization and changes in extracellular [K+] on the Ca2+ transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K+ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K+ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca2+ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} produced an increase in the resting [Ca2+]. Ca2+ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca2+ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca2+ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca2+ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} to 10 mM potentiated Ca2+ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[ {\text{K}}^{ + } ]_{\text{o}}$$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} on Ca2+ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} might be mediated by depolarization dependent changes in resting [Ca2+] and Ca2+ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{K}}_{\text{o}}^{ + }$$\end{document} on force generation

Attenuated Fatigue in Slow Twitch Skeletal Muscle during Isotonic Exercise in Rats with Chronic Heart Failure

During isometric contractions, slow twitch soleus muscles (SOL) from rats with chronic heart failure (chf) are more fatigable than those of sham animals. However, a muscle normally shortens during activity and fatigue development is highly task dependent. Therefore, we examined the development of skeletal muscle fatigue during shortening (isotonic) contractions in chf and sham-operated rats. Six weeks following coronary artery ligation, infarcted animals were classified as failing (chf) if left ventricle end diastolic pressure was >15mmHg. During isoflurane anaesthesia, SOL with intact blood supply was stimulated (1s on 1s off) at 30Hz for 15 min and allowed to shorten isotonically against a constant afterload. Muscle temperature was maintained at 37°C. In resting muscle, maximum isometric force (Fmax) and the concentrations of ATP and CrP were not different in the two groups. During stimulation, Fmax and the concentrations declined in parallel sham and chf. Fatigue, which was evident as reduced shortening during stimulation, was also not different in the two groups. The isometric force decline was fitted to a bi-exponential decay equation. Both time constants increased transiently and returned to initial values after approximately 200 s of the fatigue protocol. This resulted in a transient rise in baseline tension between stimulations, although this effect which was less prominent in chf than sham. Myosin light chain 2s phosphorylation declined in both groups after 100 s of isotonic contractions, and remained at this level throughout 15 min of stimulation. In spite of higher energy demand during isotonic than isometric contractions, both shortening capacity and rate of isometric force decline were as well or better preserved in fatigued SOL from chf rats than in sham. This observation is in striking contrast to previous reports which have employed isometric contractions to induce fatigue

β-alanine supplementation improves in-vivo fresh and fatigued skeletal muscle relaxation speed

Purpose: In fresh muscle, supplementation with the rate-limiting precursor of carnosine, β-alanine (BA), results in a decline in muscle half-relaxation time (HRT) potentially via alterations to calcium (Ca2+) handling. Accumulation of hydrogen cation (H+) has been shown to impact Ca2+ signalling during muscular contraction, carnosine has the potential to serve as a cytoplasmic regulator of Ca2+ and H+ coupling, since it binds to both ions. The present study examined the effect of BA supplementation on intrinsic in-vivo isometric knee extensor force production and muscle contractility in both fresh and fatigued human skeletal muscle assessed during voluntary and electrically evoked (nerve and superficial muscle stimulation) contractions. Methods: Twenty-three males completed two experimental sessions, pre- and post- 28 day supplementation with 6.4 g.day−1 of BA (n=12) or placebo (PLA; n=11). Isometric force was recorded during a series of voluntary and electrically evoked knee extensor contractions. Results: BA supplementation had no effect on voluntary or electrically  evoked isometric force production, or twitch electromechanical delay and time-to-peak tension. There was a significant decline in muscle HRT in fresh and fatigued muscle conditions  during both resting (3±13%; 19±26%) and potentiated (1±15%; 2±20%) twitch contractions. Conclusions: The mechanism for reduced HRT in fresh and fatigued skeletal muscle following BA supplementation is unclear. Due to the importance of muscle relaxation on total energy consumption, especially during short, repeated contractions, BA supplementation may prove to be beneficial in minimising contractile slowing induced by fatigue. Trial registration The trial is registered with Clinicaltrials.gov, ID number NCT02819505

Age- and Gender-Related Changes in Contractile Properties of Non-Atrophied EDL Muscle

Background: In humans, ageing causes skeletal muscles to become atrophied, weak, and easily fatigued. In rodent studies, ageing has been associated with significant muscle atrophy and changes in the contractile properties of the muscles. However, it is not entirely clear whether these changes in contractile properties can occur before there is significant atrophy, and whether males and females are affected differently. Methods and Results: We investigated various contractile properties of whole isolated fast-twitch EDL muscles from adult (2–6 months-old) and aged (12–22 months-old) male and female mice. Atrophy was not present in the aged mice. Compared with adult mice, EDL muscles of aged mice had significantly lower specific force, longer tetanus relaxation times, and lower fatiguability. In the properties of absolute force and muscle relaxation times, females were affected by ageing to a greater extent than males. Additionally, EDL muscles from a separate group of male mice were subjected to eccentric contractions of 15 % strain, and larger force deficits were found in aged than in adult mice. Conclusion: Our findings provide further insight into the muscle atrophy, weakness and fatiguability experienced by the elderly. We have shown that even in the absence of muscle atrophy, there are definite alterations in the physiological properties of whole fast-twitch muscle from ageing mice, and for some of these properties the alterations are mor

An examination of a modified Yo-Yo test to measure intermittent running performance in rugby players

This is an Accepted Manuscript of an article published by Taylor & Francis in European Journal of Sport Science on 17/06/2018, available online: https://doi.org/10.1080/17461391.2018.1475509This study examined how starting each shuttle in the prone position altered the internal, external and perceptual responses to the Yo-Yo Intermittent Recovery Test Level 1. Using a randomized crossover design, 17 male rugby players completed the Yo-Yo IR1 and prone Yo-Yo IR1 on two separate occasions. External loads (via microtechnology), V ̇O2, heart rate (HR), rating of perceived exertion (RPE) were measured at 160, 280 and 440 m (sub-maximal) and when the test was terminated (peak). The pre-to-post change in blood lactate concentration (∆[La]b) was determined for both tests. All data were analysed using effect sizes and magnitude-based inferences. Between-trial differences (ES 90%CL) indicated total distance was most likely lower (-1.87 0.19), whereas other measures of peak external load were likely to very likely higher during the prone Yo-Yo IR1 (0.62-1.80). Sub-maximal RPE was likely to most likely higher (0.40-0.96) and peak RPE very likely higher (0.63 0.41) in the prone Yo-Yo IR1. The change in [La]b was likely higher after the prone Yo-Yo IRl. Mean HR was possibly lower at 440 m (-0.25 0.29) as was peak HR (-0.26 0.25) in the prone Yo-Yo IR1. "V" ̇E, "V" ̇O2 and "V" ̇CO2 were likely to very likely higher at 280 and 440 m (ES = 0.36-1.22), while peak values were possibly to likely higher (ES = 0.23-0.37) in the prone Yo-Yo IR1. Adopting a prone position during the Yo-Yo IR1 increases the internal, perceptual and external responses, placing greater emphasis on metabolically demanding actions typical of rugby