The respiratory effects of two modes of passive exercise

We monitored gas exchange and muscle activity during two commonly used modes of passive leg exercise as a means of assessing the degree of passivity associated with these techniques. Additionally, we measured the rapid changes in ventilation at the start and end of the passive exercise to assess changes that occur in the fast exercise drive to breathe during passive exercise. We monitored seven subjects at rest and during 5 min of passive exercise using (1) cycling movements performed on a tandem bicycle and (2) leg extension movements performed in a chair apparatus. The increase in measured parameters from rest to passive exercise were all higher while using the tandem bicycle compared to the chair apparatus: ventilation [3.09 (0.63) versus 0.35 (0.38) l·min⁻¹], leg muscle electromyogram [8.3 (1.6) versus 1.6 (0.6) μV], carbon dioxide production [0.092 (0.018) versus −0.002 (0.001) l·min⁻¹] and oxygen consumption [0.158 (0.046) versus −0.008 (0.007) l·min⁻¹]. Passive exercise on the tandem bicycle was also associated with a significantly larger fast exercise drive to breathe at both the start and end of motion, compared to the chair apparatus [start: 5.59 (0.74) versus 3.08 (0.79) l·min⁻¹, end: 3.38 (0.79) versus 1.75 (0.54) l·min⁻¹]. The fast exercise drive to breathe at the end of exercise was significantly smaller (60%) than at the start for both modes. We conclude that passive exercise on an upright bicycle contains a significant active component that contributes both neural and metabolic influences toward the physiological adjustments observed. Furthermore, we conclude that adaptation occurs in the fast exercise drive to breathe accompanying passive limb movement.