Exercise intolerance and fatigue in chronic heart failure: is there a role for group III/IV afferent feedback?

Exercise intolerance and early fatiguability are hallmark symptoms of chronic heart failure. While the malfunction of the heart is certainly the leading cause of chronic heart failure, the patho-physiological mechanisms of exercise intolerance in these patients are more complex, multifactorial and only partially understood. Some evidence points towards a potential role of an exaggerated afferent feedback from group III/IV muscle afferents in the genesis of these symptoms. Overactivity of feedback from these muscle afferents may cause exercise intolerance with a double action: by inducing cardiovascular dysregulation, by reducing motor output and by facilitating the development of central and peripheral fatigue during exercise. Importantly, physical inactivity appears to affect the progression of the syndrome negatively, while physical training can partially counteract this condition. In the present review, the role played by group III/IV afferent feedback in cardiovascular regulation during exercise and exercise-induced muscle fatigue of healthy people and their potential role in inducing exercise intolerance in chronic heart failure patients will be summarised

Effects of different vibration frequencies, amplitudes and contraction levels on lower limb muscles during graded isometric contractions superimposed on whole body vibration stimulation

Background: Indirect vibration stimulation, i.e., whole body vibration or upper limb vibration, has been investigated increasingly as an exercise intervention for rehabilitation applications. However, there is a lack of evidence regarding the effects of graded isometric contractions superimposed on whole body vibration stimulation. Hence, the objective of this study was to quantify and analyse the effects of variations in the vibration parameters and contraction levels on the neuromuscular responses to isometric exercise superimposed on whole body vibration stimulation. Methods: In this study, we assessed the 'neuromuscular effects' of graded isometric contractions, of 20%, 40%, 60%, 80% and 100% of maximum voluntary contraction, superimposed on whole body vibration stimulation (V) and control (C), i.e., no-vibration in 12 healthy volunteers. Vibration stimuli tested were 30 Hz and 50 Hz frequencies and 0.5 mm and 1.5 mm amplitude. Surface electromyographic activity of the vastus lateralis, vastus medialis and biceps femoris were measured during V and C conditions with electromyographic root mean square and electromyographic mean frequency values used to quantify muscle activity and their fatigue levels, respectively. Results: Both the prime mover (vastus lateralis) and the antagonist (biceps femoris) displayed significantly higher (P < 0.05) electromyographic activity with the V than the C condition with varying percentage increases in EMG root-mean-square (EMGrms) values ranging from 20% to 200%. For both the vastus lateralis and biceps femoris, the increase in mean EMGrms values depended on the frequency, amplitude and muscle contraction level with 50 Hz-0.5 mm stimulation inducing the largest neuromuscular activity. Conclusions: These results show that the isometric contraction superimposed on vibration stimulation leads to higher neuromuscular activity compared to isometric contraction alone in the lower limbs. The combination of the vibration frequency with the amplitude and the muscle tension together grades the final neuromuscular output.Peer reviewe

筋収縮維持可能なmotor point追従刺激を用いた機能的電気刺激についての研究

　機能的電気刺激(以下FES)により連続的な筋収縮を行うと，FESによる筋収縮が弱化し運動誘発を起こしにくくなるという問題がある．これは臨床において，リハビリテーションを継続する時間やリハビリテーション中の安定的な運動補助に影響を与える可能性がある．　本研究は，FESによる筋収縮を持続的に誘発可能とすることを目的にしている．FESによる筋収縮の持続性向上のためNguyenらはSpatially distributed sequential stimulation (SDSS)を提案している． SDSSは下腿三頭筋において外側筋と内側筋とで刺激する筋肉を時間的に切り替える手法である．しかし，上腕二頭筋や上腕三頭筋，前脛骨筋といった筋肉は外側筋や内側筋が存在しないためSDSSを適用することはできず，SDSSは適用範囲に問題点かあると考えられる．　本研究は，FESによる筋収縮を持続的に誘発可能とするために多点表面電極を用いたMotor Pointの移動に応じた機能的電気刺激手法を提案する(以下Motor Point Tracking Stimulation: MPT)．Motor Pointとは一つの運動神経から複数の運等神経に分岐する分岐点で，Motor Pointの刺激によって筋収縮を生じやすくする位置のことである．Motor Pointに電気刺激することで筋収縮が小さいエネルギーで誘発できることが分かっている．その為Motor Pointの移動に依らずMotor Pointを刺激することで筋収縮を持続的に誘発できることが考えられる．　MPTの筋収縮の持続性を評価するためにMPTによる求心性収縮を継続した際の関節駆動域の時間変化について刺激位置を変化させない刺激及び異なった順序で刺激位置を変化させる刺激に対して比較した．結果としてMPTではほとんどの被験者において(5人7人中)比較対象と比べ関節駆動域の向上が確認できた．　これらの結果からMotor Pointに追従するように刺激位置を変化させることでFESによる筋収縮の持続性は改善することが確認できた．電気通信大学201

The effects of discharge variability on the contractile responses generated by the human leg muscles

Both recruitment and rate coding are well-known mechanisms by which the human nervous system grades muscle force. This thesis has utilised the mechanism of rate coding to generate optimised stimulation patterns that enhance contractile responses in both a fatigued and non-fatigued state. Human motoneurones are known to fire with significant discharge variability (irregularity) during voluntary contractions. Yet many of the mechanisms as to why they fire in this manner, are still unclear. This thesis has tested the hypotheses that integrating physiological variability into trains of stimuli could offer some advantages to the human neuromuscular system that have not yet been explored. We have stimulated single motor axons and multiple motor units with long, short and continuous trains of stimuli that integrate discharge variability. The results reported in this thesis highlight the benefits that discharge irregularity offers to improving contractile responses and reducing fatigue in human leg muscles. While the entirety of this research has been conducted in healthy human subjects, there is a potential for this to translate clinically. Functional electrical stimulation (FES) or neuromuscular stimulation is a well-known therapy utilised after stroke or spinal cord injury to assist in restoring motor function. It can help to reduce muscle atrophy, improve contractility and increase muscle strength by electrically exciting the muscles via surface electrodes directly over the muscle belly. Current FES stimulation patterns often incorporate high frequency, constant-interval stimuli that do not resemble the physiological patterns that are exhibited during normal voluntary contractions. This thesis has used novel stimulation patterns that emulate the firing of volitionally active motoneurones in an attempt to increase contractile responses and reduce the magnitude of muscular fatigue

Muscle fiber typology substantially influences time to recover from high-intensity exercise

Human fast-twitch muscle fi- bers generate high power in a short amount of time but are easily fatigued, whereas slow-twitch fibers are more fatigue resistant. The transfer of this knowledge to coaching is hampered by the invasive nature of the current evaluation of muscle typology by biopsies. Therefore, a noninvasive method was developed to estimate muscle typology through proton magnetic resonance spectroscopy in the gastrocnemius. The aim of this study was to investigate whether male subjects with an a priori-determined fast typology (FT) are character- ized by a more pronounced Wingate exercise-induced fatigue and delayed recovery compared with subjects with a slow typology (ST). Ten subjects with an estimated higher percentage of fast-twitch fibers and 10 subjects with an estimated higher percentage of slow-twitch fibers underwent the test protocol, consisting of three 30-s all-out Wingate tests. Recovery of knee extension torque was evaluated by maximal voluntary contraction combined with electrical stimulation up to 5 h after the Wingate tests. Although both groups delivered the same mean power across all Wingates, the power drop was higher in the FT group (—61%) compared with the ST group (—41%). The torque at maximal voluntary contraction had fully recovered in the ST group after 20 min, whereas the FT group had not yet recovered 5 h into recovery. This noninvasive estimation of muscle typology can predict the extent of fatigue and time to recover following repeated all-out exercise and may have applications as a tool to individualize training and recovery cycles. NEW & NOTEWORTHY A one-fits-all training regime is present in most sports, though the same training implies different stimuli in athletes with a distinct muscle typology. Individualization of training based on this muscle typology might be important to optimize per- formance and to lower the risk for accumulated fatigue and potentially injury. When conducting research, one should keep in mind that the muscle typology of participants influences the severity of fatigue and might therefore impact the results

How Spinal Neural Networks Reduce Discrepancies between Motor Intention and Motor Realization

This paper attempts a rational, step-by-step reconstruction of many aspects of the mammalian neural circuitry known to be involved in the spinal cord's regulation of opposing muscles acting on skeletal segments. Mathematical analyses and local circuit simulations based on neural membrane equations are used to clarify the behavioral function of five fundamental cell types, their complex connectivities, and their physiological actions. These cell types are: α-MNs, γ-MNs, IaINs, IbINs, and Renshaw cells. It is shown that many of the complexities of spinal circuitry are necessary to ensure near invariant realization of motor intentions when descending signals of two basic types independently vary over large ranges of magnitude and rate of change. Because these two types of signal afford independent control, or Factorization, of muscle LEngth and muscle TEnsion, our construction was named the FLETE model (Bullock and Grossberg, 1988b, 1989). The present paper significantly extends the range of experimental data encompassed by this evolving model.National Science Foundation (IRI-87-16960, IRI-90-24877); Instituto Tecnológico y de Estudios Superiores de Monterre

Optimising the prescription of training for post-activation potentiation in rugby league players

Maximum lower-body muscular strength and power are key determinants of successful performance in rugby league (Baker & Newton, 2009; Johnston, Gabbett, & Jenkins, 2014). Due to large amounts of concurrent energy system training and the congested fixture schedule throughout the competitive season there is limited time available for strength training (McLellan, Lovell, & Gass, 2011; Moreira, Kempton, Aoki, Sirotic, & Coutts, 2015). Strength and conditioning coaches are therefore challenged to prescribe appropriate training modalities which aim to maintain highly developed levels of strength and power.Complex training is a mixed resistance training modality which aims to address strength and power during a single training session by alternating heavy resistance exercise, set for set, with explosive plyometric exercise (Docherty, Robbins, & Hodgson, 2004). This training modality is underpinned by post-activation potentiation (PAP) which refers to the acute augmentation of force and power production following a heavy load conditioning activity (Hodgson, Docherty, & Robbins, 2005; Tillin & Bishop, 2009). Since PAP and fatigue are simultaneously induced, an appropriate recovery interval is required to enhance explosive performance when the muscle has partially recovered from fatigue but is still potentiated (Docherty et al., 2004) which may limit its practical application.This thesis aimed to investigate methods of eliciting PAP at shorter recovery intervals to enhance its practical applicability for strength and conditioning coaches. The main aims of this thesis were:1. To determine any differences in the PAP response between the hex-bar deadlift (HBD) and back squat (BS) exercises and identify the optimal recovery interval required for PAP to manifest.2. To examine if moderately loaded HBD and BS exercises combined with accommodating resistance elicit PAP at shorter recovery intervals.3. To examine the difference in the magnitude of the PAP response between stronger, more experienced and weaker, less experienced athletes.4. To examine muscle activation as a result of the PAP response using surface electromyography.5. To investigate the chronic adaptations to muscle architecture and athletic performance following two complex training interventions over a 6-week mesocycle

Central and peripheral manipulations of perceived exertion and endurance performance

Perception of effort, defined as “the conscious sensation of hard, heavy and strenuous exercise”, is known to regulate endurance performance and human behaviour. Perception of effort has recently been shown to be exacerbated by mental exertion and is also known to be a main feature of fatigue. However, to date, not only its neurophysiology but also how manipulations of perceived exertion might impact endurance performance remain poorly understood. The main aim of this thesis was to investigate how manipulations of perceived exertion might impact endurance performance. This thesis is divided in two parts: central and peripheral manipulations of perceived exertion. In each part, three experimental chapters aimed to get a better insight in the neurophysiology of perceived exertion and its impact on endurance performance. In the first part (central manipulations), we firstly investigated the impact of exacerbating perceived exertion via mental exertion involving the response inhibition process on self-paced running endurance performance. This study demonstrated that as with time to exhaustion tests, time trial performance is impaired following mental exertion leading to mental fatigue. Secondly, we investigated whether mental exertion leading to mental fatigue could alter the rate of central fatigue development during constant load whole-body exercise. This study demonstrated that the exacerbated perception of effort in presence of mental fatigue does not reflect an altered rate of central fatigue development, but is likely to be due to i) an impaired central motor command and/or ii) an alteration of the central processing of the corollary discharge. Thirdly, we investigated whether mental exertion could impact the repeatability of maximal voluntary contraction of the knee extensors. We found that contrary to submaximal exercise, force production capacity is not altered by mental exertion. Finally, these three studies demonstrated that i) mental exertion negatively impacts submaximal exercise but not maximal exercise and that ii) mental fatigue differs from central fatigue. In the second part (peripheral manipulation), we firstly developed and tested the reliability of a new endurance exercise model non-limited by the cardiorespiratory system (one leg dynamic exercise), which will be of benefits for future researches aiming to manipulate feedback from group III-IV muscle afferents. Secondly, we described neuromuscular alterations induced by this exercise and tested a new methodology to indirectly measure feedback from group III-IV muscle afferents. This study demonstrated that one leg dynamic exercise induced central and peripheral fatigue and also a decrease in spinal excitability associated with an increase in cortical excitability. Furthermore, this study also suggests that monitoring cardiovascular responses during muscle occlusion might be a suitable tool to indirectly measure feedback from group III-IV muscle afferents. Thirdly, we tested the corollary discharge and afferent feedback model of perceived exertion with electromyostimulation. This study demonstrated for the first time that for the same force output, perception of effort generation is independent of muscle afferents and reflects the magnitude of the central motor command (manipulated by electromyostimulation). All together, these findings provide further evidence in support of the corollary discharge model of perceived exertion, and provide a new exercise model to investigate and manipulate perception of effort. This thesis, when integrating both experimental parts, provides new insight on how perception of effort regulates endurance performance. Specifically, it demonstrates how muscle fatigue is a contributor of the continuous increase in perception of effort during endurance exercise, but also that other contributors play a role in this increase in perception of effort. Indeed, we demonstrated for the first time that i) perception of effort alterations in the presence of mental fatigue is independent of any alterations of the neuromuscular system, and ii) muscle afferents does not directly impact perception of effort, but may influence it indirectly via their role in motor control

Neuromuscular fatigue in healthy muscle: Underlying factors and adaptation mechanisms

AbstractObjectivesThis review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue.MethodOne hundred and eighty-two articles indexed in PubMed (1954–2010) have been considered.ResultsNeuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units.ConclusionFatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production

Neuromuscular fatigue, muscle temperature and hypoxia: an integrative approach.

Real world exposures to physiologically and/or psychologically stressful environments are often multifactorial. For example, high-altitude typically combines exposure to hypobaric hypoxia, solar radiation and cold ambient temperatures, while sea level thermal stress is often combined with supplementary or transient stressors such as rain, solar radiation and wind. In such complex environments, the effect of one stressor on performance may be subject to change, simply due to the presence of another independent stressor. Such differential influences can occur in three basic forms; additive, antagonistic and synergistic, each term defining a fundamental concept of inter-parameter interactions. As well as the natural occurrence of stressors in combination, understanding interactions is fundamental to experimentally modelling how multiple physiological strains integrate in their influence on or regulation of - exercise intensity. In this thesis the current literature on neuromuscular fatigue and the influence of thermal and hypoxic stress is reviewed (Chapter 1). This is followed by an outline of the methodological developments used in the subsequent experiments (Chapter 2). In the first experimental study (Chapter 3) a novel approach was adopted to investigate the combined effect of muscle cooling and hypoxia on neuromuscular fatigue in humans. The results showed that the neuromuscular system s maximal force generating capacity declined by 8.1 and 13.9% during independent cold and hypoxic stress compared to control. Force generation decreased by 21.4% during combined hypoxic-cold compared to control, closely matching the additive value of hypoxia and cold individually (22%). This was also reflected in the measurement of mechanical fatigue (electromechanical ratio), demonstrating an additive response during combined hypoxic-cold. From this study, it was concluded that when moderate hypoxia and cold environmental temperatures are combined during low intensity exercise, the level of fatigue increases additively with no interaction between these stressors. Before conducting a more complex investigation on combined stressors, a better understanding of the role of muscle temperature on central fatigue - i.e. voluntary muscle activation via the afferent signalling pathways was sought. The focus of Chapter 4 was to quantify the relationship between muscle temperature and voluntary muscle activation (central fatigue) across a wide range of temperatures. The primary finding was that different muscle temperatures can induce significant changes in voluntary activation (0.5% reduction per-degree-centigrade increase in muscle temperature) when neural drive is sustained for a prolonged effort (e.g. 120-s); however this effect is not exhibited during efforts that are brief in duration (e.g. 3-s). To further explore this finding, Chapter 5 investigated the effect of metaboreceptive feedback at two different muscle temperatures, using post-exercise muscle ischemia, on voluntary activation of a remote muscle group. The results showed that at the same perceived mental effort, peripheral limb discomfort was significantly higher with increasing muscle temperature (2% increase per-degree-centigrade increase). However any influence of increased muscle temperature on leg muscle metaboreceptive feedback did not appear to inhibit voluntary muscle activation - i.e. central control - of a remote muscle group, as represented by an equal force output and voluntary activation in the thermoneutral, contralateral leg. In Chapter 6, the psycho-sensory effects of changes in muscle temperature on central fatigue during dynamic exercise were investigated. During sustained dynamic exercise, fatigue development appeared to occur at a faster rate in hot muscle (4% increase per-degree-centigrade increase) leading to a nullification of the beneficial effects of increased muscle temperature on peak power output after a period of ~60-s maximal exercise. In support of previous studies using isometric exercise (Chapter 4 and 6), participants reported significantly higher muscular pain and discomfort in hot muscle compared to cooler muscle during dynamic exercise (2 and 1% increase per-degree-centigrade increase respectively), however this did not result in a lower power output. From Chapters 4, 5 and 6 it was concluded that in addition to faster rates of metabolite accumulation due to cardiovascular strain, it is possible that a direct sensitisation of the metaboreceptive group III and IV muscle afferents occurs in warmer muscle. This likely contributes to the reduction in voluntary muscle activation during exercise in the heat, while it may attenuate central fatigue in the cold. It was also interpreted that muscle afferents may have a similar signalling role to cutaneous sensory afferents; the latter of which are recognised for their role in providing thermal feedback to the cognitive-behavioural centres of the brain and aiding exercise regulation under thermal stress. The impact of body core and active muscle temperature on voluntary muscle activation represented a similar ratio (5 to 1 respectively) to the temperature manipulated (single leg) to non-temperature manipulated mass (rest of body) in Chapters 4, 5 and 6. This indicates that voluntary muscle activation may also be regulated based on a central meta-representation of total body heat content i.e. the summed firing rates of all activated thermoreceptors in the brain, skin, muscle, viscera and spine. Building on the initial findings of Chapter 3, Chapter 7 investigated the causative factors behind the expression of different interaction types during exposure to multi-stressor environments. This was achieved by studying the interaction between thermal stress and hypoxia on the rate of peripheral and central fatigue development during a high intensity bout of knee extension exercise to exhaustion. The results showed that during combined exposure to moderate hypoxia and mild cold, the reductions in time to exhaustion were additive of the relative effects of hypoxia and cold independently. This differs from the findings in Chapter 3, in which fatigue was additive of the absolute effects of cold and hypoxia. In contrast, combining moderate hypoxia with severe heat stress resulted in a significant antagonistic interaction on both the absolute and relative reductions in time to exhaustion i.e. the combined effect being significantly less than the sum of the individual effects. Based on the results in Chapter 7, a quantitative paradigm for understanding of systematic integration of multifactorial stressors was proposed. This is, that the interaction type between stressors is influenced by the impact magnitude of the individual stressors effect on exercise capacity, whereby the greater the stressors impact, the greater the probability that one stressor will be cancelled out by the other. This is the first study to experimentally model the overarching principles characterising the presence of simultaneous physiological strains, suggesting multifactorial integration be subject to the worst strain takes precedence when the individual strains are severe