Disparate Mechanisms of Fatigability in Response to Prolonged Running versus Cycling of Matched Intensity and Duration

Introduction: Running and cycling represent two of the most common forms of endurance exercise. However, a direct comparison of the neuromuscular consequences of these two modalities after prolonged exercise has never been made. The aim of this study was to compare the alterations in neuromuscular function induced by matched-intensity and duration cycling and running exercise. Methods: During separate visits, 17 endurance-trained male participants performed 3 h of cycling and running at 105% of the gas exchange threshold. Neuromuscular assessments were taken are preexercise, midexercise, and postexercise, including knee extensor maximal voluntary contractions (MVC), voluntary activation (VA), high- and low-frequency doublets (Db100 and Db10, respectively), potentiated twitches (Q(tw,pot)), motor evoked potentials (MEP), and thoracic motor evoked potentials (TMEP). Results: After exercise, MVC was similarly reduced by similar to 25% after both running and cycling. However, reductions in VA were greater after running (-16% +/- 10%) than cycling (-10% +/- 5%; P < 0.05). Similarly, reductions in TMEP were greater after running (-78% +/- 24%) than cycling (-15% +/- 60%; P = 0.01). In contrast, reductions in Db100 (running vs cycling, -6% +/- 21% vs -13% +/- 6%) and Db10:100 (running vs cycling, -6% +/- 16% vs -19% +/- 13%) were greater for cycling than running (P <= 0.04). Conclusions: Despite similar decrements in the knee extensor MVC after running and cycling, the mechanisms responsible for force loss differed. Running-based endurance exercise is associated with greater impairments in nervous system function, particularly at the spinal level, whereas cycling-based exercise elicits greater impairments in contractile function. Differences in the mechanical and metabolic demands imposed on the quadriceps could explain the disparate mechanisms of neuromuscular impairment after these two exercise modalities.