Author(s): BROWN, J., HALL, L., BOAT, R., WILLIAMS, N., JOHNSON, M., SHARPE, G., Institution: NOTTINGHAM TRENT UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1351

INTRODUCTION:
Exercise-induced leg muscle fatigue increases dyspnoea (specifically the sensation of Respiratory Effort) in a subsequent respiratory challenge (1). This effect could be attributed to either central or peripheral fatigue (or both). This study aimed to isolate the impact of peripheral fatigue on dyspnoea by inducing leg muscle fatigue via electrical stimulation. As dyspnoea is a multidimensional sensation, and air hunger (AH), is commonly experienced during physical activity (PA) by those with pulmonary conditions (2), the present study investigated whether leg muscle peripheral (and not central) fatigue affects Respiratory Effort and AH.
METHODS:
Fifteen participants completed four separate experimental sessions. Each session comprised one of two breathing challenges (Respiratory Effort or AH), with (QFATIGUE) or without (CON) quadriceps fatigue (30 mins of neuromuscular electrical stimulation (NMES). The Respiratory Effort challenge involved progressive inspiratory flow resistive loading (two minutes each of 20, 35, 50, 65% of maximum inspiratory mouth pressure (MIP)). The AH challenge was carbon dioxide (CO2) rebreathing to the limit of tolerance. Respiratory Effort and AH intensity were rated using a modified Borg Scale (0-10). The multidimensional dyspnoea profile (MDP) was administered post challenge. Cardiorespiratory data was measured continuously. Quadriceps fatigue was evaluated by changes in potentiated twitch force (ΔQtw,pot; peripheral fatigue), and voluntary muscle activation (ΔVA; central fatigue).
RESULTS:
NMES successfully caused peripheral fatigue (ΔQtw,pot;- 24 + 8%), without reducing VA (0.5 ± 3.3%). During resistive loading of 65% MIP, Respiratory Effort was 12.3% higher in QFATIGUE (7.3 ± 1.9) than CON (6.5 ± 1.5) (P < 0.05). The immediate perception MDP scores were 12.4% higher in QFATIGUE (24.4 ± 9) than CON (21.7 ± 8.2, P < 0.05). During CO2 rebreathing AH was 44.4% higher in QFATIGUE than CON at 50% (QFATIGUE: 2.6 ± 1.2; CON: 1.8 ± 0.9, P < 0.01) and 21.7% higher at 75% (QFATIGUE: 5.6 ± 1.8; CON: 4.6 ± 1.7, P < 0.05) of the challenge duration despite no differences in PETCO2. CO2 rebreathing duration was not different between conditions (P > 0.05).
CONCLUSION:
Peripheral limb muscle fatigue increases perceptions of Respiratory Effort and AH independent of central fatigue. An increase in type III/IV afferent feedback may elevate these perceptions during PA through an increase in corollary discharge from both cortical (Respiratory Effort) and medullary (AH) motor activity. Pulmonary disease often reduces PA which subsequently increases skeletal muscle fatiguability (3). Our data suggest that peripheral muscle fatigue may further exacerbate exertional dyspnoea which may in turn cause a downward spiral in PA (4).

References:
1. Grippo et al. (2003) 2. Banzett et al. (2021) 3. Mador et al. (2009) 4. Brunton et al. (2021)