On- and Off-transient Oxygen Uptake Kinetics Across Exercise Intensity Domains in Female African-America and Caucasian

Abstract

The oxygen uptake (VO2) kinetic response to a step increase in work rate (WR) can be typically characterized by a mono exponential function during moderate (M) exercise and by a double exponential function during heavy (H) and very heavy (VH) exercise. During recovery from M intensity, the VO2 kinetic response decreases mono exponentially to a steady state. However, during recovery from H and VH exercise, the slow component that typically is evident during the on-transient, may or may not manifest in the off-transient. The characterization of the response to a step increase in WR has been found to provide useful indices of fitness level or degree of cardiorespiratory dysfunction. Since most studies have been conducted predominantly in male subjects, to date there is limited VO2 kinetics information in females. PURPOSE: To examine the relationship between the on- and off-transient VO2 kinetics at three distinct exercise intensities in African American (n=7; 23±3 yr) and Caucasian (n=6; 21±2 yr) females. METHODS: All subjects performed a maximal exercise test and three sets of square-wave exercise tests at M, H, and VH intensities on a cycle ergometer. RESULTS: In both groups, VO2 kinetics during M were well described by a single exponential with the on- and off-transients having similar time constants (African American: ôirc;1,on = 39±13 and 39±15 s; Caucasian: ôirc;1,off = 53±10 and41±4 s;), while H required a double exponential function (African American: & oC1,on = 47±11 and ôirc;2,on = 289±63 s and oC1, off = 46±6 and ô2,off = 259±120 s; Caucasian: oC1, on = 41±12 and ô2,on = 277± 81 s and oC1, off = 40±3 and ô2,off = 215± 133 s). The on-and off-transient VO2 kinetic response for VH, was different between groups. While both the on- and off-transients were well described by a double exponential function in Caucasians (oC1, on = 43±15 and ô2,on = 215±36 s and ô1,off = 42±7 and ô2,off = 223±64 s), the on-transient reverted to a mono-exponential profile (ôirc;1,on = 44±10 s), but the off-transient retained its two-exponential profile (ô1,off = 51±10 and ô2, off = 243±93 s) in African Americans. CONCLUSION: Caucasian and African American women have similar VO2 kinetic responses to M and H exercise. However, the on- and off- VO2 kinetic response to VH exercise remained symmetric in Caucasians but not in African Americans.