The Influence of Lung Function and Respiratory Muscle Strength on Quadriceps Muscle Fatigability in COPD Patients Under Long-term Oxygen Therapy.

BACKGROUND: This research investigates quadriceps muscle fatigability (MF) in chronic obstructive pulmonary disease (COPD) patients with chronic respiratory failure (CRF) at different levels of lung obstruction [severe obstruction (SO)=FEV1 <50% and >30% versus very severe obstruction (VSO)=FEV1 ≤30%]. It explores the relationships between quadriceps MF and lung function, respiratory muscles, and oxygenation status. METHODS: A post hoc cross-over analysis in 45 COPD patients (20 SO and 25 VSO) undergoing long-term oxygen therapy was performed. Delta change in quadriceps maximum voluntary contraction (MVC) (absolute value and percentage) before and after a constant workload was calculated. Associations between quadriceps MF and lung function, respiratory muscles, and gas exchange were examined using Pearson's correlation and multivariate linear regression analysis. RESULTS: SO patients experience a more substantial reduction in MVC compared to VSO (-15.15±9.13% vs -9.29±8.90%, p=0.0357), despite comparable resting MVC. Dyspnea is more pronounced in VSO at the beginning and end of the exercise. Correlations were found between MF and maximal inspiratory pressure (MIP) (r=-0.4412, p=0.0056), maximal expiratory pressure (MEP) (r=-0.3561, p=0.0282), and a tendency for FEV1% (r=-0.2931, p=0.0507). The regression model (R2=0.4719) indicates that lower MIP and FEV1 and high total lung capacity are significant factors in reducing quadriceps muscle fatigability after a fatiguing task. CONCLUSION: COPD patients with more severe pulmonary obstruction and hyperinflation and lower respiratory muscle strength have lower quadriceps MF but higher dyspnea both at rest and during exercise.

Upper-limb interval versus constant-load exercise in patients with COPD: a physiological crossover study.

Objective: Upper-limb exercise is recommended for patients with COPD, albeit there are limited data concerning the optimal modality to implement. We compared interval (INT-EX) to continuous (CONT-EX) upper-limb exercise in terms of exercise tolerance, ventilatory and metabolic responses when both conditions were sustained at an equivalent work rate. Methods: 26 stable COPD patients undertook three upper-limb exercise sessions to initially establish peak work rate (PWR) via an incremental exercise test and subsequently two equivalent work rate tests to the limit tolerance in balanced order: 1) INT-EX consisting of 30-s work at 100% PWR interspersed with 30-s work at 40% of PWR; and 2) CONT-EX at 70% PWR. Results: 20 patients (76.9%) had longer tolerance during INT-EX, while six out of 26 (23.1%) exhibited longer tolerance during CONT-EX. The average endurance time was 434.1±184.7 and 315.7±128.7 s for INT-EX and CONT-EX, respectively. During INT-EX at isotime (i.e. when work completed was the same between INT-EX and CONT-EX), the majority of patients manifested lower oxygen uptake, minute ventilation, pulmonary hyperinflation, heart rate, symptoms and higher CO2 blood concentration. Patients with longer INT-EX had a lower comorbidity score (Cumulative Illness Rating Scale: 1.58±0.30 versus 1.88±0.29, p=0.0395) and better-preserved lung function (forced vital capacity 84.7±15.31% versus 67.67±20.56%, p=0.0367; forced expiratory volume in 1 s 57.15±14.59 versus 44.67±12.99% predicted, p=0.0725) compared to patients with longer CONT-EX. Conclusion: INT-EX is more sustainable than CONT-EX for the majority of COPD patients with moderate obstruction, leading to lower dynamic hyperinflation and symptoms at isotime. Further studies need to define the benefits of its application during pulmonary rehabilitation.

Concurrent metaboreflex activation increases chronotropic and ventilatory responses to passive leg movement without sex-related differences.

Previous studies in animal models showed that exercise-induced metabolites accumulation may sensitize the mechanoreflex-induced response. The aim of this study was to assess whether the magnitude of the central hemodynamic and ventilatory adjustments evoked by isolated stimulation of the mechanoreceptors in humans are influenced by the prior accumulation of metabolic byproducts in the muscle. 10 males and 10 females performed two exercise bouts consisting of 5-min of intermittent isometric knee-extensions performed 10% above the previously determined critical force. Post-exercise, the subjects recovered for 5 min either with a suprasystolic circulatory occlusion applied to the exercised quadriceps (PECO) or under freely-perfused conditions (CON). Afterwards, 1-min of continuous passive leg movement was performed. Central hemodynamics, pulmonary data, and electromyography from exercising/passively-moved leg were recorded throughout the trial. Root mean square of successive differences (RMSSD, index of vagal tone) was also calculated. Δpeak responses of heart rate (ΔHR) and ventilation ([Formula: see text]) to passive leg movement were higher in PECO compared to CON (ΔHR: 6 ± 5 vs 2 ± 4 bpm, p = 0.01; 3.9 ± 3.4 vs 1.9 ± 1.7 L min-1, p = 0.02). Δpeak of mean arterial pressure (ΔMAP) was significantly different between conditions (5 ± 3 vs - 3 ± 3 mmHg, p < 0.01). Changes in RMSSD with passive leg movement were different between PECO and CON (p < 0.01), with a decrease only in the former (39 ± 18 to 32 ± 15 ms, p = 0.04). No difference was found in all the other measured variables between conditions (p > 0.05). These findings suggest that mechanoreflex-mediated increases in HR and [Formula: see text] are sensitized by metabolites accumulation. These responses were not influenced by biological sex.

Prior Involvement of Central Motor Drive Does Not Impact Performance and Neuromuscular Fatigue in a Subsequent Endurance Task.

PURPOSE: This study evaluated whether central motor drive during fatiguing exercise plays a role in determining performance and the development of neuromuscular fatigue during a subsequent endurance task. METHODS: On separate days, 10 males completed three constant-load (80% peak power output), single-leg knee-extension trials to task failure in a randomized fashion. One trial was performed without preexisting quadriceps fatigue (CON), and two trials were performed with preexisting quadriceps fatigue induced either by voluntary (VOL; involving central motor drive) or electrically evoked (EVO; without central motor drive) quadriceps contractions (~20% maximal voluntary contraction (MVC)). Neuromuscular fatigue was assessed via pre-post changes in MVC, voluntary activation (VA), and quadriceps potentiated twitch force ( Qtw,pot ). Cardiorespiratory responses and rating of perceived exertion were also collected throughout the sessions. The two prefatiguing protocols were matched for peripheral fatigue and stopped when Qtw,pot declined by ~35%. RESULTS: Time to exhaustion was shorter in EVO (4.3 ± 1.3 min) and VOL (4.7 ± 1.5 min) compared with CON (10.8 ± 3.6 min, P < 0.01) with no difference between EVO and VOL. ΔMVC (EVO: -47% ± 8%, VOL: -45% ± 8%, CON: -53% ± 8%), Δ Qtw,pot (EVO: -65% ± 7%, VOL: -59% ± 14%, CON: -64% ± 9%), and ΔVA (EVO: -9% ± 7%, VOL: -8% ± 5%, CON: -7% ± 5%) at the end of the dynamic task were not different between conditions (all P > 0.05). Compared with EVO (10.6 ± 1.7) and CON (6.8 ± 0.8), rating of perceived exertion was higher ( P = 0.05) at the beginning of VOL (12.2 ± 1.0). CONCLUSIONS: These results suggest that central motor drive involvement during prior exercise plays a negligible role on the subsequent endurance performance. Therefore, our findings indicate that peripheral fatigue-mediated impairments are the primary determinants of high-intensity single-leg endurance performance.