Barry W. Scheuermann

The Effects of Caffeine on the Kinetics of O2Uptake, CO2Production and Expiratory Ventilation in Humans During the On-Transient of Moderate and Heavy Intensity Exercise

In order to test the hypothesis that glycogen sparing observed early during exercise following caffeine ingestion was a consequence of tighter metabolic control reflected in faster VO2 kinetics, we examined the effect of caffeine ingestion on oxygen uptake (VO2), carbon dioxide production (VCO2) and expiratory ventilation (VE) kinetics at the onset of both moderate (MOD) and heavy (HVY) intensity exercise. Male subjects (n= 10) were assigned to either a MOD (50 % VO2,max, n= 5) or HVY (80 % VO2,max, n= 5) exercise condition. Constant‐load cycle ergometer exercise was performed as a step function from loadless cycling 1 h after ingestion of either dextrose (placebo, PLAC) or caffeine (CAFF; 6 mg (kg body mass)−1). Alveolar gas exchange was measured breath‐by‐breath. A 2‐ or 3‐component exponential model, fitted through the entire exercise transient, was used to analyse gas exchange and ventilatory data for the determination of total lag time (TLT: the time taken to attain 63 % of the total exponential increase). Caffeine had no effect on TLT for VO2 kinetics at either exercise intensity (MOD: 36 ± 14 s (PLAC) and 41 ± 10 s (CAFF); HVY: 99 ± 30 s (PLAC) and 103 ± 26 (CAFF) (mean ±s.d.)). TLT for VE was increased with caffeine at both exercise intensities (MOD: 50 ± 20 s (PLAC) and 59 ± 21 s (CAFF); HVY: 168 ± 35 s (PLAC) and 203 ± 48 s (CAFF)) and for VCO2 during MOD only (MOD: 47 ± 14 s (PLAC) and 53 ± 17 s (CAFF); HVY: 65 ± 13 s (PLAC) and 69 ± 17 s (CAFF)). Contrary to our hypothesis, the metabolic effects of caffeine did not alter the on‐transient VO2 kinetics in moderate or heavy exercise. VCO2 kinetics were slowed by a reduction in CO2 stores reflected in pre‐exercise and exercise end‐tidal CO2 pressure (PET,CO2) and plasma PCO2 which, we propose, contributed to slowed VE kinetics.

Breathing patterns during slow and fast ramp exercise in man

Breathing frequency (fb), tidal volume (VT), and respiratory timing during slow (SR, 8 W min−1) and fast (FR, 65 W min−1) ramp exercise to exhaustion on a cycle ergometer was examined in seven healthy male subjects. Expiratory ventilation (V̇E), pulmonary gas exchange (V̇O2 and V̇CO2) and end‐tidal gas tensions (PET,O2 and PET,CO2) were determined using breath‐by‐breath techniques. Arterialized venous blood was sampled from a dorsal hand vein at 2 min intervals during SR and 30 s intervals during FR and analysed for arterial plasma PCO2 (Pa,CO2). PET,CO2 increased with increasing work rates (WRs) below the ventilatory threshold (VT); at WRs ≥ 90 % V̇O2,max, PET,CO2 was reduced (P < 0.05) below 0 W values in SR but not in FR. fb and VT were similar for SR and FR at all submaximal WRs, resulting in a similar V̇E. At exhaustion V̇E was similar but fb was higher (P < 0.05) and VT was lower (P < 0.05) in SR (fb, 51 ± 10 breaths min−1; VT, 2590 ± 590 ml) than in FR (fb, 42 ± 8 breaths min−1; VT, 3050 ± 470 ml). The time of expiration (TE) decreased with increasing WR, but there was no difference between SR and FR. The time of inspiration (TI) decreased at exercise intensities ≥ VT; at exhaustion, TI was shorter (P < 0.05) during SR (0.512 ± 0.097 s) than during FR (0.753 ± 0.100 s). The TI to total breath duration (TI/TTot) and the inspiratory flow (VT/TI) were similar during SR and FR at all submaximal exercise intensities; at V̇O2 max, TI/TTot was lower (P < 0.05) and VT/TI was higher (P < 0.05) during SR (TI/TTot, 0.473 ± 0.030; VT/TI, 5.092 ± 0.377 1 s−1) than during FR (TI/TTot, 0.567 ± 0.050; VT/TI, 4.117 ± 0.635 1 s−1). These results suggest that during progressive exercise, breathing pattern and respiratory timing may be determined, at least at submaximal work rates, independently of alveolar and arterial PCO2.