Why can a fall in muscle oxygen not be the trigger for exercise hyperpnoea?

1. Working muscles do increase their oxygen demand, which stimulates an increase in ventilation. 2. But the timing does not fit a simple oxygen trigger. 3. Hyperpnoea during exercise appears to occur before a drop in muscle oxygen levels can occur. 4. A cause cannot follow its effect, so something else must be driving the rise. 5. Neural factors are proposed instead, which fits the sudden rise as exercise begins and the sudden fall when it ends. 6. The second option contradicts that observed timing, since ventilation climbs at the start of exercise, not after it. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.6 Modifications in Respiratory Functions_