A. St Clair Gibson

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans: summary and conclusions

It is hypothesised that physical activity is controlled by a central governor in the brain and that the human body functions as a complex system during exercise. Using feed forward control in response to afferent feedback from different physiological systems, the extent of skeletal muscle recruitment is controlled as part of a continuously altering pacing strategy, with the sensation of fatigue being the conscious interpretation of these homoeostatic, central governor control mechanisms.

Evidence for complex system integration and dynamic neural regulation of skeletal muscle recruitment during exercise in humans

A model is proposed in which the development of physical exhaustion is a relative rather than an absolute event and the sensation of fatigue is the sensory representation of the underlying neural integrative processes. Furthermore, activity is controlled as part of a pacing strategy involving active neural calculations in a “governor” region of the brain, which integrates internal sensory signals and information from the environment to produce a homoeostatically acceptable exercise intensity. The end point of the exercise bout is the controlling variable. This is an example of a complex, non-linear, dynamic system in which physiological systems interact to regulate activity before, during, and after the exercise bout.

Rectification and non-linear pre-processing of EMG signals for cortico-muscular analysis

Rectification of the electromyographic (EMG) signal is a commonly used pre-processing procedure that allows detection of significant coherence between EMG and measured cortical signals. However, despite its accepted and wide-spread use, no detailed analysis has been presented to offer insight into the precise function of rectification. We begin this paper with arguments based on single motor unit action potential (AP) trains to demonstrate that rectification effectively enhances the firing rate information of the signal. Enhancement is achieved by shifting the peak of the AP spectrum toward the lower firing rate frequencies, whilst maintaining the firing rate spectra. A similar result is obtained using the analytic envelope of the signal extracted using the Hilbert transform. This argument is extended to simulated EMG signals generated using a published EMG model. Detection of firing rate frequencies is obtained using phase randomised surrogate data, where the original EMG power spectrum exceeds the averaged rectified surrogate spectra at integer multiples of firing rate frequencies. Model simulations demonstrate that this technique accurately determines grouped firing rate frequencies. Extraction of grouped firing rate frequencies prior to coherency analyses may further aid interpretation of significant cortico-muscular coherence findings.

Effect of anticipation during unknown or unexpected exercise duration on rating of perceived exertion, affect, and physiological function

Objectives: To determine the effect of unknown exercise duration and an unexpected increase in exercise duration on rating of perceived exertion (RPE), affect, and running economy during treadmill running. Methods: Sixteen well trained male and female runners completed three bouts of treadmill running at 75% of their peak treadmill running speed. In the first trial, they were told to run for 20 minutes and were stopped at 20 minutes (20 MIN). In another trial, they were told to run for 10 minutes, but at 10 minutes were told to run for a further 10 minutes (10 MIN). In the final trial, they were not told for how long they would be running but were stopped after 20 minutes (unknown, UN). During each of the running bouts, RPE, oxygen consumption (ml/kg/min), heart rate (beats/min), stride frequency (min−1), affect scores (arbitrary units), and attentional focus (percentage associative thought scores) were recorded. Results: RPE increased significantly between 10 and 11 minutes in the 10 MIN compared with the 20 MIN and UN trials (p<0.05). The affect score decreased significantly between 10 and 11 minutes in the 10 MIN compared with the 20 MIN trial (p<0.05). Running economy, as measured by oxygen consumption, was significantly lower in the UN compared with the 20 MIN trial from 10 to 19 minutes (p<0.05). Conclusions: The change in RPE between 10 and 11 minutes in the 10 MIN trial suggests that RPE is not purely a measure of physical exertion, as treadmill speed was maintained at a constant pace both before and after the unexpected increase in exercise duration. The associated changes in affect score at similar times in the 10 MIN trial supports the hypothesis that RPE has an affective component.

A signalling role for muscle glycogen in the regulation of pace during prolonged exercise

Introduction: In this study we examined the pacing strategy and the end muscle glycogen contents in eight cyclists, once when they were carbohydrate loaded and once when they were non-loaded. Methods: Cyclists completed 2 hours of cycling at ∼73% of maximum oxygen consumption, which included five sprints at 100% of peak sustained power output every 20 minutes, followed immediately by a 1 hour time trial. Muscle biopsies were performed before and immediately after exercise, while blood samples were taken during the 2 hour steady state rides and immediately after exercise. Results: Carbohydrate loading improved mean power output during the 1 hour time trial (mean (SEM) 219 (17) v 233 (15) W; p<0.05) and enabled subjects to use significantly more muscle glycogen than during the trial following their normal diet. Significantly, the subjects, kept blind to all feedback except for time, started both time trials at similar workloads (∼30 W), but after 1 minute of cycling, the workload average 14 W higher throughout the loaded compared with the non-loaded time trial. There were no differences in subjects’ plasma glucose and lactate concentrations and heart rates in the carbohydrate loaded versus the non-loaded trial. Of the eight subjects, seven improved their time trial performance after carbohydrate loading. Finishing muscle glycogen concentrations in these seven subjects were remarkably similar in both trials (18 (3) v 20 (3) mmol/kg w/w), despite significantly different starting values and time trial performances (36.55 (1.47) v 38.14 (1.27) km/h; p<0.05). The intra-subject coefficient of variation (CV) for end glycogen content in these seven subjects was 10%, compared with an inter-subject CV of 43%. Conclusions: As seven subjects completed the time trials with the same end exercise muscle glycogen concentrations, diet induced changes in pacing strategies during the time trials in these subjects may have resulted from integrated feedback from the periphery, perhaps from glycogen content in exercising muscles.

Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.Methods. Twelve subjects (age 25.1+/-3.7 years, mass 70.1+/-8.2 kg, mean+/-SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.Results. There were no significant differences between ECC, CON and ISO peak torque output (211+/-63 vs 169+/-41 vs 177+/-61 Nm; ECC, CON, ISO) and IEMG activity (280+/-143 vs 305+/-146 vs 287+/-143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31+/-13% of initial values, similar to the final torque values in CON (58+/-15%), but significantly less than ECC (108.6+/-38.6%; P<0. 001) values. In CON and ECC, IEMG was maintained (95+/-27% and 93+/-21%; CON and ECC), whereas IEMG for ISO decreased to 38+/-13% of initial values. The greatest reduction in MPFS occurred in CON (69+/-10%) compared to ISO (78+/-9%; P<0.05) and ECC (93+/-6%; P<0.001).Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.

Exercising with reserve: evidence that the central nervous system regulates prolonged exercise performance

Objective: The purpose of this study was to measure the effects of an amphetamine (methylphenidate) on exercise performance at a fixed rating of perceived exertion of 16. Methods: Eight elite cyclists ingested 10 mg methylphenidate in a randomised, placebo-controlled crossover trial. Results: Compared with placebo, subjects receiving methylphenidate cycled for approximately 32% longer before power output fell to 70% of the starting value. At the equivalent time at which the placebo trial terminated, subjects receiving methylphenidate had significantly higher power outputs, oxygen consumptions, heart rates, ventilatory volumes and blood lactate concentrations although electromyographic activity remained unchanged. The ingestion of a centrally acting stimulant thus allowed subjects to exercise for longer at higher cardiorespiratory and metabolic stress indicating the presence of a muscular reserve in the natural state. Conclusions: This suggests that endurance performance is not only “limited” by mechanical failure of the exercising muscles (“peripheral fatigue”). Rather performance during prolonged endurance exercise under normal conditions is highly regulated by the central nervous system to ensure that whole-body homeostasis is protected and an emergency reserve is always present.

Non-random fluctuations in power output during self-paced exercise

Objectives: To analyse the power output measured during a self-paced 20-km cycling time trial, during which power output was free to vary, in order to assess the level and characteristics of the variability in power output that occurred during the exercise bout. Methods: Eleven well-trained cyclists performed a 20-km cycling time trial, during which power output was sampled every 200 m. Power spectrum analysis was performed on the power output data, and a fractal dimension was calculated for each trial using the Higuchi method. Results: In all subjects, power output was maintained throughout the trial until the final kilometre, when it increased significantly, indicating the presence of a global pacing strategy. The power spectrum revealed the presence of 1/f-like scaling of power output and multiple frequency peaks during each trial, with the values of the frequency peaks changing over the course of the trial. The fractal dimension (D-score) was similar for all subjects over the 20-km trial and ranged between 1.5 and 1.9. Conclusions: The presence of an end spurt in all subjects, 1/f-like scaling and multiple frequency peaks in the power output data indicate that the measured oscillations in power output during cycling exercise activity may not be system noise, but may rather be associated with system control mechanisms that are similar in different individuals.

Effects of supramaximal exercise on the electromyographic signal

Aim: To determine the neuromuscular recruitment characteristics during supramaximal exercise. Methods: Ten healthy subjects completed the Wingate anaerobic test (WAT) cycling protocol. Electromyographic (EMG) data and rate of fatigue were recorded throughout the cycling. Results: The mean (SD) rate of fatigue (decrease in power output) was 44.5 (8.6)%. No significant change was found in EMG amplitude. A significant decrease (p<0.01) in mean power frequency spectrum was found over the 30 second period. Conclusions: During WAT, mean power frequency spectrum was attenuated with no decline in EMG amplitude, which may be caused by an accumulation of metabolites in the periphery. However, it is also possible that the feedback loop from intramuscular metabolism to the central nervous system is unable, within the 30 second period of the WAT, to affect neural recruitment strategy.

The effect of a prophylactic dose of flurbiprofen on muscle soreness and sprinting performance in trained subjects

The aim of this study was to examine the effects of a prophylactic dose of a local, transcutaneously administered, non-steroidal anti-inflammatory drug on muscle soreness, muscle damage and sprinting performance in young trained males. Twenty-five subjects aged 19+/-3 years, actively participating in rugby union and field hockey, were familiarized with the test procedure and then divided at random into an experimental group (n = 13) and a control group (n = 12). The experimental group received two patches, each containing 40 mg flurbiprofen (TransAct LAT), 12 h before an exercise bout designed to produce delayed-onset soreness (DOMS). The control group received identical non-medicated placebo patches at the same time. Delayed-onset muscle soreness was induced by an exercise protocol consisting of drop jumps (seven sets of 10 repetitions). Serum creatine kinase activity, muscle soreness, muscle girth and acceleration in a maximal sprint over 30 m were measured before the induction of DOMS and at 12, 24, 48 and 72 h thereafter. Plasma lactate concentration was measured 3 min after the 30-m sprint tests. Subjects in both groups had significantly more pain at 24 and 48 h compared with at 12 and 72 h (P < 0.05; Friedman two-way analysis of variance). Thigh girth and serum creatine kinase did not change throughout the experiment. Although plasma lactate concentrations were elevated after the 30-m sprint, there were no differences between groups or as a result of DOMS. The greatest acceleration occurred between 5 and 10 m. This was not affected by the anti-inflammatory drug or DOMS. In conclusion, the aetiology of the DOMS induced in the trained subjects in this study seems to be independent of inflammatory processes or, more specifically, of increases in prostaglandin synthesis in the muscles.