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

The ergogenic effects of transcranial direct current stimulation on exercise performance

The physical limits of the human performance have been the object of study for a considerable time. Most of the research has focused on the locomotor muscles, lungs and heart. As a consequence, much of the contemporary literature has ignored the importance of the brain in the regulation of exercise performance. With the introduction and development of new non-invasive devices, the knowledge regarding the behaviour of the central nervous system during exercise has advanced. A first step has been provided from studies involving neuroimaging techniques where the role of specific brain areas have been identified during isolated muscle or whole-body exercise. Furthermore, a new interesting approach has been provided by studies involving non-invasive techniques to manipulate specific brain areas. These techniques most commonly involve the use of an electrical or magnetic field crossing the brain. In this regard, there has been emerging literature demonstrating the possibility to influence exercise outcomes in healthy people following stimulation of specific brain areas. Specifically, transcranial direct current stimulation (tDCS) has been recently used prior to exercise in order to improve exercise performance under a wide range of exercise types. In this review article, we discuss the evidence provided from experimental studies involving tDCS. The aim of this review is to provide a critical analysis of the experimental studies investigating the application of tDCS prior to exercise and how it influences brain function and performance. Finally, we provide a critical opinion of the usage of tDCS for exercise enhancement. This will consequently progress the current knowledge base regarding the effect of tDCS on exercise and provides both a methodological and theoretical foundation on which future research can be based

Task‐specific strength increases after lower‐limb compound resistance training occurred in the absence of corticospinal changes in vastus lateralis

Neural adaptations subserving strength increases have been shown to be task‐specific, but responses and adaptation to lower‐limb compound exercises such as the squat are commonly assessed in a single‐limb isometric task. This two‐part study assessed neuromuscular responses to an acute bout (Study A) and 4 weeks (Study B) of squat resistance training at 80% of one‐repetition‐maximum, with measures taken during a task‐specific isometric squat (IS) and non‐specific isometric knee extension (KE). Eighteen healthy volunteers (25 ± 5 years) were randomised into either a training (n = 10) or a control (n = 8) group. Neural responses were evoked at the intracortical, corticospinal and spinal levels, and muscle thickness was assessed using ultrasound. The results of Study A showed that the acute bout of squat resistance training decreased maximum voluntary contraction (MVC) for up to 45 min post‐exercise (−23%, P < 0.001). From 15–45 min post‐exercise, spinally evoked responses were increased in both tasks (P = 0.008); however, no other evoked responses were affected (P ≥ 0.240). Study B demonstrated that following short‐term resistance training, participants improved their one repetition maximum squat (+35%, P < 0.001), which was reflected by a task‐specific increase in IS MVC (+49%, P = 0.001), but not KE (+1%, P = 0.882). However, no training‐induced changes were observed in muscle thickness (P = 0.468) or any evoked responses (P = 0.141). Adjustments in spinal motoneuronal excitability are evident after acute resistance training. After a period of short‐term training, there were no changes in the responses to central nervous system stimulation, which suggests that alterations in corticospinal properties of the vastus lateralis might not contribute to increases in strength

Improvement in Hemodynamic Responses to Metaboreflex Activation after One Year of Training in Spinal Cord Injured Humans

Spinal cord injured (SCI) individuals show an altered hemodynamic response to metaboreflex activation due to a reduced capacity to vasoconstrict the venous and arterial vessels below the level of the lesion. Exercise training was found to enhance circulating catecholamines and to improve cardiac preload and venous tone in response to exercise in SCI subjects. Therefore, training would result in enhanced diastolic function and capacity to vasoconstrict circulation. The aim of this study was to test the hypothesis that one year of training improves hemodynamic response to metaboreflex activation in these subjects. Nine SCI individuals were enrolled and underwent a metaboreflex activation test at the beginning of the study (T0) and after one year of training (T1). Hemodynamics were assessed by impedance cardiography and echocardiography at both T0 and T1. Results show that there was an increment in cardiac output response due to metaboreflex activity at T1 as compared to T0 (545.4 ± 683.9 mL · min(-1) versus 220.5 ± 745.4 mL · min(-1), P < 0.05). Moreover, ventricular filling rate response was higher at T1 than at T0. Similarly, end-diastolic volume response was increased after training. We concluded that a period of training can successfully improve hemodynamic response to muscle metaboreflex activation in SCI subjects

The Effect of Transcranial Direct Current Stimulation on Exercise Performance

The physical limits of the human being have been the object of study for a considerable time. Human and exercise physiology, in combination with multiple other related disciplines, studied the function of the organs and their relationship during exercise. When studying the mechanisms causing the limits of the human body, most of the research has focused on the locomotor muscles, lungs and heart. Therefore, it is not surprising that the limit of the performance has predominantly been explained at a "peripheral" level. Many studies have successfully demonstrated how performance can be improved (or not) by manipulating a "peripheral" parameter. However, in most cases it is the brain that regulates and integrates these physiological functions, and much of the contemporary literature has ignored its potential role in exercise performance. This may be because moderating brain function is fraught with difficulty, and challenging to measure. However, with the recent introduction and development of new non-invasive devices, the knowledge regarding the behaviour of the central nervous system during exercise can be advanced. Transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) are two such methods. These methods can transiently moderate the activity of a targeted brain area, potentially altering the regulation of a particular physiological (or psychological) system, and consequently eliciting a change in exercise performance. Despite the promising theory, there is little or no experimental data regarding the potential to moderate neurophysiological mechanisms through tDCS to improve exercise performance. Consequently, the experiments performed as part of this thesis investigated the capacity for tDCS to alter physical performance. The ability of tDCS as a targeted and selective intervention at the brain level provides the unique opportunity to reduce many methodological constraints that might limit or confound understanding regarding some of the key physiological mechanisms during exercise. Therefore, the primary aim of this thesis was to investigate how tDCS may moderate both central and peripheral neurophysiological mechanisms, and how this may effect various exercise tasks. The first study investigated the effect of a well-documented analgesic tDCS montage on exercise-induced muscle pain. This study demonstrated for the first time, that although anodal tDCS of the motor cortex (M1) reduces pain in a cold pressor task, it does not elicit any reduction in exercise-induced muscle pain and consequently has no effect on exercise performance. As reductions in exercise-induced pain have previously been documented to improve performance, probably the lack of effect was due to either the M1 having a limited processing role in exercise-induced pain, or that the cathodal stimulation of the prefrontal cortex negated any positive impact of anodal M1 stimulation. Given the lack of guidelines for tDCS electrode montage for exercise, the second study examined the effect of different electrode montages on isometric performance and the neuromuscular response of knee extensor muscle. Given that the anode increases excitability and the cathode decreases excitability, the placement of these has the potential to elicit significant effects on exercise performance. The results showed that exercise performance improved only when an extrachepalic tDCS montage was applied to the M1, but in the absence of changes to the measured neuromuscular parameters. These results suggest that tDCS can have a positive effect on single limb submaximal exercise, but not on maximal muscle contraction. The improvement in performance was probably the consequence of the reduction in perceived exertion for a given load. This is the first experiment showing an improvement in exercise performance on single joint exercise of the lower limbs following tDCS. The results suggest that the extrachepalic set-up is recommended for exercise studies in order to avoid any potential negative effect of the cathodal electrode. Previous studies investigating tDCS have shown its potential to alter autonomic activity, and in some circumstances reduce the cardiovascular response during exercise. Considering the emerging studies and applications of tDCS on exercise and the potential benefits of tDCS in the treatment of cardiovascular diseases, the third study monitored multiple cardiovascular variables following tDCS in a group of healthy volunteers. Using more advanced techniques and methods compared to previous research, including the post exercise ischemia technique and transthoracic bioimpedance, the results suggest that tDCS administration has no significant effect on the cardiovascular response in healthy individuals. The final study sought to apply the findings obtained in the study 2 to whole body exercise. The same extrachepalic set up was applied over both the motor cortices, with both anodal and cathodal stimulation conditions. The neuromuscular response and cycling performance was also monitored. Following anodal tDCS, time to exhaustion and motor cortex excitability of lower limbs increased. Interestingly, cathodal stimulation did not induce any change in cycling performance or neuromuscular response. This study demonstrated for the first time the ability of anodal tDCS to improve performance of a constant load cycling task, and highlights the inability of cathodal tDCS to decrease cortical activation during muscle contraction. Taken together, the experiments performed as part of this thesis provide new insights on how brain stimulation influences exercise performance, with notable findings regarding the role of M1 excitability and perception of effort. Furthermore, considering the lack of knowledge regarding the use of tDCS on exercise, these findings will help further understanding of how to apply tDCS in exercise science. This consequently improves the knowledge base regarding the effect of tDCS on exercise and provides both a methodological and theoretical foundation on which future research can be based

Leg vein pressure pulser (LVPP): a mechatronic device for spinal cord injured patient standing in for the ineffectiveness of paralyzed leg muscles to pump blood from leg veins towards heart

Paraplegic subject often experience an impairment of the cardiovascular function. One of the most important factors in the occurrence of this situation is the drastic reduction in venous return from the muscle at rest, due to the absence of the function of muscle pump that is present in the able-bodied subjects. It is reasonable to assume that an external mechanical force applied on the lower limbs of paraplegic subjects might produce a positive effect on cardiovascular condition. Accordingly, the research aimed to identify and develop a robotic device to restore the cardiovascular function in paraplegics. To test this hypothesis we performed two experimental studies involving healthy subjects where two automatic robotic able to apply an ascendant external pressure to lower limbs were tested. Hemodynamic response was monitored beat-by-beat by means of impedance cardiography and Doppler ultrasound was used to measure cardiac volumes. Results show that the application of robotic mechanical actuators pneumatic is able to generate cycles of compression and decompression on venous structures of the lower limbs similar to what normally happens in the skeletal muscle when subjects are walking. Especially when actuators simulated muscle pattern activation similar to mechanism of walking we were able to further increase cardiac output and end diastolic volume. In conclusion our preliminary finding suggest that is possible to compensate partially the lack of venous return in spinal cord injury population thus increasing the quality of life, reducing the risk of cardiovascular disease and then extend their life expectancy

Disassembly of Large Composite-Rich Installations

Considering the demanufacturing of large infrastructures (as wind blades and aircrafts) rich in composite materials, the most impacting step in terms of costs is disassembly. Different routes could be followed for dismantling and transportation and several factors influence the final result (as the technology used, the logistic and the administrative issues). For this reason, it is fundamental to understand which solution has to be followed to reduce the impact of decommissioning on the overall recycling and reusing cost. This work, after the formalization of the different possible disassembly scenarios, proposes a Decision Support System (DSS) for disassembly of large composite-rich installations, that has been designed and implemented for the identification of the most promising disassembly strategy, according to the process costs minimization. The mathematical models constituting the core of this tool are detailed and the DSS is applied to disassembly of onshore wind blades, underling the importance of similar systems to optimize demanufacturing costs

Reduced rate of force development under fatigued conditions is associated to the decline in force complexity in adult males

Purpose: This study aimed to verify whether the slowing of muscle contraction quickness, typically observed in states of fatigue, may worsen force control by decreasing the rate with which force fluctuations are modulated. Therefore, we investigated the relationship between rate of force development (RFD), and force fluctuations' magnitude (Coefficient of variation, CoV) and complexity (Approximate Entropy, ApEn; Detrended fluctuation analysis, DFAα). Methods: Fourteen participants performed intermittent ballistic isometric contractions of the plantar dorsiflexors at 70% of maximal voluntary force until task failure (under 60% twice). Results: Indices of RFD (RFDpeak, RFD50, RFD100, and RFD150) decreased over time by approximately 46, 32, 44, and 39%, respectively (p all ≤ 0.007). DFAα increased by 10% (p &lt; 0.001), and CoV increased by 15% (p &lt; 0.001), indicating decreased force complexity along with increased force fluctuations, respectively. ApEn decreased by just over a quarter (28%, p &lt; 0.001). The linear hierarchical models showed negative associations between RFDpeak and DFAα (β = - 3.6 10-4, p &lt; 0.001), CoV (β = - 1.8 10-3, p &lt; 0.001), while ApEn showed a positive association (β = 8.2 × 10-5, p &lt; 0.001). Conclusion: The results suggest that exercise-induced reductions in contraction speed, lead to smoother force complexity and diminished force control due to slower adjustments around the target force. The fatigued state resulted in worsened force producing capacity and overall force control

Ischemic preconditioning of the muscle reduces the metaboreflex response of the knee extensors

Purpose: This study investigated the effect of ischemic preconditioning (IP) on metaboreflex activation following dynamic leg extension exercise in a group of healthy participants. Method: Seventeen healthy participants were recruited. IP and SHAM treatments (3 × 5 min cuff occlusion at 220 mmHg or 20 mmHg, respectively) were administered in a randomized order to the upper part of exercising leg’s thigh only. Muscle pain intensity (MP) and pain pressure threshold (PPT) were monitored while administrating IP and SHAM treatments. After 3 min of leg extension exercise at 70% of the maximal workload, a post-exercise muscle ischemia (PEMI) was performed to monitor the discharge group III/IV muscle afferents via metaboreflex activation. Hemodynamics were continuously recorded. MP was monitored during exercise and PEMI. Results: IP significantly reduced mean arterial pressure compared to SHAM during metaboreflex activation (mean ± SD, 109.52 ± 7.25 vs. 102.36 ± 7.89 mmHg) which was probably the consequence of a reduced end diastolic volume (mean ± SD, 113.09 ± 14.25 vs. 102.42 ± 9.38 ml). MP was significantly higher during the IP compared to SHAM treatment, while no significant differences in PPT were found. MP did not change during exercise, but it was significantly lower during the PEMI following IP (5.10 ± 1.29 vs. 4.00 ± 1.54). Conclusion: Our study demonstrated that IP reduces hemodynamic response during metaboreflex activation, while no effect on MP and PPT were found. The reduction in hemodynamic response was likely the consequence of a blunted venous return

A comparison of different methods to analyse data collected during time-to-exhaustion tests

Despite their widespread use in exercise physiology, time-to-exhaustion (TTE) tests present an often-overlooked challenge to researchers, which is how to computationally deal with between- and within-subject differences in exercise duration. We aimed to verify the best analysis method to overcome this problem