Harold J Bell | PubMed Health

PubMed2003

CO2 does not affect passive exercise ventilatory decline.

Breathing increases abruptly at the start of passive exercise, stimulated by afferent feedback from the moving limbs, and declines toward a steady-state hyperpnea as exercise continues. This decline has been attributed to decreased arterial CO2 levels and adaptation in afferent feedback; however, the relative importance of these two mechanisms is unknown. To address this issue, we compared ventilatory responses to 5 min of passive leg extension exercise performed on 10 awake human subjects (6 men and 4 women) in isocapnic and poikilocapnic conditions. End-tidal Pco2 decreased significantly during poikilocapnic (Delta = -1.5 +/- 0.5 Torr, P < 0.001), but not isocapnic, passive exercise. Despite this difference, the ventilatory responses to passive exercise were not different between the two conditions. Using the fast changes in ventilation at the start (5.46 +/- 0.40 l/min, P < 0.001) and end (3.72 +/- 0.33 l/min, P < 0.001) of passive exercise as measures of the drive to breathe from afferent feedback, we found a decline of 68%. We conclude that the decline in ventilation during passive exercise is due to an adaptation in the afferent feedback from the moving limbs, not a decline in CO2 levels.

Adolescent↗

PubMed2002

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 x min(-1)], leg muscle electromyogram [8.3 (1.6) versus 1.6 (0.6) microV], carbon dioxide production [0.092 (0.018) versus -0.002 (0.001) l x min(-1)] and oxygen consumption [0.158 (0.046) versus -0.008 (0.007) l x 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 x min(-1), end: 3.38 (0.79) versus 1.75 (0.54) l x 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.

Adult↗