Ventilatory and gas-exchange responses to incremental exercise performed with reduced muscle glycogen content

SummaryThis study examined the relationship between minute ventilation (V˙E), CO2 production (V˙CO2), and blood lactate concentration ([La−]) during incremental exercise performed with reduced muscle glycogen stores. Nine untrained female subjects (25.3 ± 4.2 year) performed incremental cycling in a normal glycogen (NG) state and under conditions of reduced muscle glycogen (RG) content. To reduce muscle glycogen stores, subjects cycled to exhaustion (124 ± 33 min) at a power output corresponding to their gas-exchange anaerobic threshold. Peak oxygen uptake (V˙O2peak) was unchanged with glycogen reduction, even though subjects achieved a significantly lower maximal power output in the RG state (p < 0.05). Peak blood [La−] decreased significantly by 37% in the RG state (p < 0.001). At any percentage of V˙O2peak, O2 uptake and V˙E were similar for both treatment conditions, whereas V˙CO2 and respiratory exchange ratio values were lower during the RG trial than under NG conditions. Therefore, V˙E/V˙CO2 tended to be higher and end-tidal CO2 partial pressure tended to be lower during exercise performed in the RG state. V˙E was significantly correlated with V˙CO2 under both treatment conditions (NG: r = 0.99, p < 0.01; RG: r = 0.99, p < 0.01). However, the slope of the V˙E–V˙CO2 relationship was significantly elevated during the RG trial (p < 0.01). V˙E during exercise was similar under both treatment conditions, even though V˙CO2 and blood [La−] were lower during the RG trial compared to the NG trial. This suggests that factors other than CO2 delivery to the lung and metabolic acidosis play an important role in regulating V˙E during exercise.