Expiratory Flow Limitation, Dynamic Hyperinflation, and Locomotor Power and Fatigue
Authors: Cunha Jonathan , Benedict Patricia , Domingo Antoinette , Kolkhorst Fred W , Rossiter Harry B , Cannon Daniel T 
Published: November 2023

Expiratory flow limitation (e.g. asthma, chronic obstructive pulmonary disease) results in dyspnea, dynamic hyperinflation leading to low inspiratory reserve volume, and premature exercise intolerance. Associated arterial desaturation or blunting of locomotor O2 delivery may exacerbate muscle fatigue (reduced power at a given motor activity). Unknown is whether expiratory flow limitation reduces locomotor power via limiting maximal evocable motor activity, exacerbating muscle fatigue, or both.

We hypothesized: 1) Imposed expiratory resistance reduces locomotor power and exercise tolerance. 2) Expiratory resistance reduces maximal evocable motor activity. 3) Reduced evocable motor activity is most closely related to dynamic hyperinflation and dyspnea.

To investigate, healthy volunteers (N=14, 23±3 yr, 170±8 cm, 68.7±12.5 kg) performed a series of cycling exercise tests with and without an imposed expiratory flow resistance (7 cm H2O·L·s−1). At baseline (unfatigued), simultaneous measurement of isokinetic crank power and electromyography (EMG) of the leg muscles during brief (5 s) efforts from low to maximum, established a linear EMG‐isokinetic power relationship. This allowed the total decline in maximal isokinetic power at intolerance during each experimental condition to be apportioned to: 1) the power equivalent from a reduction in maximum voluntary muscle activation (termed ‘activation fatigue’); and 2) the deficit in expected power at a given muscle activity (muscle fatigue). Additionally, we measured the difference between the required constant power task and the instantaneous maximal isokinetic power at intolerance (termed the ‘locomotor power reserve’). Under control or expiratory resistance, participants completed constant power cycling to intolerance terminated with a maximal 5 s isokinetic sprint. A third trial was completed without expiratory resistance, but terminated at a time equal to that achieved with expiratory resistance (isotime). Inspiratory capacity and dyspnea (Borg CR‐10) were measured each min.

Expiratory flow limitation reduced exercise tolerance (487±145 vs. 575±137 s; p<0.001; CIdiff −125, −52 s), resulted in a greater decline in inspiratory reserve volume (0.19±0.56 vs. 0.76±0.54 L; p<0.01; CIdiff −0.94, −0.20 L), and increased dyspnea (8.5±2 vs. 6.7±2.5; p<0.01; CIdiff 0.7, 3.0). Muscle fatigue was unaffected at isotime (19±34 vs. 17±34 W; p>0.05, CIdiff −20, 17 W). Conversely, activation fatigue was greater at isotime with expiratory resistance (102±76 vs. 127±71 W; p<0.05; CIdiff 1, 49 W), and was related to the reduction in inspiratory reserve volume (r=−0.64, p<0.05). As a result, locomotor power reserve was smaller at isotime with expiratory resistance (253±83 vs. 201±92 W; p<0.05; CIdiff −113, 10 W).

Dynamic hyperinflation and low inspiratory reserve volume exacerbated activation fatigue and reduced locomotor power reserve. Thus, imposed expiratory flow limitation initiates a cascade of abnormal lung mechanics and symptoms, which conflate to reduce exercise tolerance through limiting evocable motor activity.