Viewpoint: Fatigue mechanisms determining exercise performance: integrative physiology is systems physiology

Cancer-Related Fatigue: Perception of Effort or Task Failure?

CONTEXT

Patient's rating of perceived effort (RPE) is used to assess central fatigue. Cancer-related fatigue (CRF) is believed to be of central origin. The increased RPE with a motor task, such as the Finger-Tapping Test (FTT), can easily be measured in the clinical setting.

OBJECTIVES

To correlate the FTT, RPE and the Brief Fatigue Inventory (BFI) rated fatigue severity in patients with cancer.

METHODS

Subjective fatigue was assessed in adult patients with cancer by the BFI. Participants performed a modified FTT with the index finger of the dominant hand: 15 seconds × 2, 30 seconds × 2, and 60 seconds × 2 with 1 minute of rest between each time trial. Rating of perceived effort at the end of task was measured by the Borg 10 scale.

EXCLUSIONS

Brain metastasis, history of brain radiation, Parkinson disease, Huntington Chorea, multiple sclerosis, delirium, and depression. Pearson correlation coefficients were used to describe the relationships between BFI, FTT, and Borg 10 scale.

RESULTS

Thirty patients participated. Mean age was 56.2. Sixteen were females (53.3%). The mean BFI mean was 4.1, median 4.4. Tapping rate did not correlate with fatigue severity. The RPE correlated with the mean BFI: r _s 0.438, P = .0155. These correlations persisted after adjustment for age.

CONCLUSION

An increased RPE in the absence of task failure suggests that the origin of CRF is central. The performance of an FTT with RPE helps to improve our understanding of fatigue in the clinical setting.

The short-term recovery of corticomotor responses in elbow flexors

Background

The recovery of neurophysiological parameters at various time intervals following fatiguing exercise has been investigated previously. However, the repetition of neuromuscular assessments during the recovery period may have interfered with the true corticomotor excitability responses. In this experiment, fatiguing contractions were combined with a single post-fatigue assessment at varying time points. Ten participants undertook 5 bouts of 60-s maximal voluntary contractions (MVC) of the elbow flexors, separated by 20 min. Before and after each 60-s fatiguing exercise (FAT), participants performed a series of 6-s contractions at 100, 75 and 50% of their MVC during which transcranial magnetic, transmastoid electrical and brachial plexus electrical stimuli were used to elicit motor evoked potentials (MEP), cervicomedullary motor evoked potentials (CMEP) and compound muscle action potentials (Mmax) in the biceps brachii muscle, respectively. Post-FAT measurements were randomly performed 0, 15, 30, 60, or 120 s after each FAT.

Results

MVC force declined to 65.1 ± 13.1% of baseline following FAT and then recovered to 82.7 ± 10.2% after 60 s. The MEP·Mmax⁻¹ ratio recorded at MVC increased to 151.1 ± 45.8% and then returned to baseline within 60 s. The supraspinal excitability (MEP·CMEP⁻¹) measured at MVC increased to 198.2 ± 47.2% and fully recovered after 30 s. The duration of post-MEP silent period recorded at MVC elongated by 23.4 ± 10.6% during FAT (all P < 0.05) but fully recovered after 15 s.

Conclusions

The current study represents the first accurate description of the time course and pattern of recovery for supraspinal and spinal excitability and inhibition following a short maximal fatiguing exercise in upper limb.

Physical exercise performance in temperate and warm environments is decreased by an impaired arterial baroreflex

The present study aimed to investigate whether running performance in different environments is dependent on intact arterial baroreceptor reflexes. We also assessed the exercise-induced cardiovascular and thermoregulatory responses in animals lacking arterial baroafferent signals. To accomplish these goals, male Wistar rats were subjected to sinoaortic denervation (SAD) or sham surgery (SHAM) and had a catheter implanted into the ascending aorta to record arterial pressure and a telemetry sensor implanted in the abdominal cavity to record core temperature. After recovering from these surgeries, the animals were subjected to constant- or incremental-speed exercises performed until the voluntary interruption of effort under temperate (25° C) and warm (35° C) conditions. During the constant-speed exercises, the running time until the rats were fatigued was shorter in SAD rats in both environments. Although the core temperature was not significantly different between the groups, tail skin temperature was higher in SAD rats under temperate conditions. The denervated rats also displayed exaggerated increases in blood pressure and double product compared with the SHAM rats; in particular, in the warm environment, these exaggerated cardiovascular responses in the SAD rats persisted until they were fatigued. These SAD-mediated changes occurred in parallel with increased variability in the very low and low components of the systolic arterial pressure power spectrum. The running performance was also affected by SAD during the incremental-speed exercises, with the maximal speed attained being decreased by approximately 20% in both environments. Furthermore, at the maximal power output tolerated during the incremental exercises, the mean arterial pressure, heart rate and double product were exaggerated in the SAD relative to SHAM rats. In conclusion, the chronic absence of the arterial baroafferents accelerates exercise fatigue in temperate and warm environments. Our findings also suggest that an augmented cardiovascular strain accounted for the early interruption of exercise in the SAD rats.

Acute supra-therapeutic oral terbutaline administration has no ergogenic effect in non-asthmatic athletes

This study aimed to investigate the effects on a possible improvement in aerobic and anaerobic performance of oral terbutaline (TER) at a supra-therapeutic dose in 7 healthy competitive male athletes. On day 1, ventilatory threshold, maximum oxygen uptake [Formula: see text] and corresponding power output were measured and used to determine the exercise load on days 2 and 3. On days 2 and 3, 8 mg of TER or placebo were orally administered in a double-blind process to athletes who rested for 3 h, and then performed a battery of tests including a force-velocity exercise test, running sprint and a maximal endurance cycling test at Δ50 % (50 % between VT and [Formula: see text]). Lactatemia, anaerobic parameters and endurance performance ([Formula: see text] and time until exhaustion) were raised during the corresponding tests. We found that TER administration did not improve any of the parameters of aerobic performance (p > 0.05). In addition, no change in [Formula: see text] kinetic parameters was found with TER compared to placebo (p > 0.05). Moreover, no enhancement of the force-velocity relationship was observed during sprint exercises after TER intake (p > 0.05) and, on the contrary, maximal strength decreased significantly after TER intake (p < 0.05) but maximal power remained unchanged (p > 0.05). In conclusion, oral acute administration of TER at a supra-therapeutic dose seems to be without any relevant ergogenic effect on anaerobic and aerobic performances in healthy athletes. However, all participants experienced adverse side effects such as tremors.

Impairment of maximal aerobic power with moderate hypoxia in endurance athletes: do skeletal muscle mitochondria play a role?

This study investigates the role of central vs. peripheral factors in the limitation of maximal oxygen uptake (V̇o2max) with moderate hypoxia [inspired fraction (FiO2) =14.5%]. Fifteen endurance-trained athletes performed maximal cycle incremental tests to assess V̇o2max, maximal cardiac output (Q̇max), and maximal arteriovenous oxygen (a-vO2) difference in normoxia and hypoxia. Muscle biopsies of vastus lateralis were taken 1 wk before the cycling tests to evaluate maximal muscle oxidative capacity (V̇max) and sensitivity of mitochondrial respiration to ADP ( Km) on permeabilized muscle fibers in situ. Those athletes exhibiting the largest reduction of V̇o2max in moderate hypoxia (Severe Loss group: −18 ± 2%) suffered from significant reductions in Q̇max (−4 ± 1%) and maximal a-vO2 difference (−14 ± 2%). Athletes who well tolerated hypoxia, as attested by a significantly smaller drop of V̇o2max with hypoxia (Moderate Loss group: −7 ± 1%), also display a blunted Q̇max (−9 ± 2%) but, conversely, were able to maintain maximal a-vO2 difference (+1 ± 2%). Though V̇max was similar in the two experimental groups, the smallest reduction of V̇o2max with moderate hypoxia was observed in those athletes presenting the lowest apparent Km for ADP in the presence of creatine ( Km+Cr). In already-trained athletes with high muscular oxidative capacities, the qualitative, rather than quantitative, aspects of the mitochondrial function may constitute a limiting factor to aerobic ATP turnover when exercising at low FiO2, presumably through the functional coupling between the mitochondrial creatine kinase and ATP production. This study suggests a potential role for peripheral factors, including the alteration of cellular homeostasis in active muscles, in determining the tolerance to hypoxia in maximally exercising endurance-trained athletes.