Oxygen Uptake Kinetics During Arm Cranking Within The Moderate Intensity Domain

Abstract

Recent studies have demonstrated that on-transient oxygen uptake (VO2) kinetics are slowed when constant-load exercise is initiated from the upper compared to lower region of the moderate intensity domain (below the lactate threshold, LT). These studies, focusing exclusively on lower limb exercise, attributed this intensity-dependent slowing of VO2 kinetics during moderate-intensity exercise to different muscle fiber recruitment patterns or altered oxygen transport dynamics. PURPOSE: To determine if pulmonary VO2 and heart rate (HR) kinetics are slowed during constant-load arm exercise (arm-cranking) performed in the upper (90%LT) compared to lower (45%LT) regions of the moderate intensity domain. METHODS: Seven healthy male subjects (26 ± 3 yr) performed 4-8 repetitions of arm-cranking exercise involving: 1) a single, 6-min step transition from 9 W to 90% of the estimated LT (AC90%); two continuous, equal, 6-min step transitions from 9 W to 45% LT (AC45%), and then to 90% LT (AC45-90%); and 3) the AC90% protocol superimposed upon low-intensity leg-cycling (AC90%+LC), where baseline HR >100 beats·min-1 (range: 100-120 beats·min-1). RESULTS: The time constants (t) of the VO2 and HR responses were longer (P<0.05) during AC45-90% (tVO2: 70 ± 5 s; tHR: 59 ± 4 s) compared to AC45% (tVO2: 54 ± 6 s; tHR: 47 ± 5 s) and AC90% (tVO2: 53 ± 4 s; tHR: 46 ± 3 s). The gain (DVO2/DW) and blood lactate concentration were also significantly greater (P<0.05) for AC45-90% than AC45%. During arm-cranking superimposed upon leg-cycling (AC90%+LC), HR kinetics were significantly slowed when compared to AC90% (mean difference for tHR: 9 ± 4 s, P<0.05) and AC45% (mean difference for tHR: 9 ± 5 s, P<0.05). Similar comparisons between conditions for tVO2 yielded no significant differences. CONCLUSION: During arm exercise performed within the moderate intensity domain, VO2 and HR kinetics do not conform to a dynamically linear model. Since HR, but not VO2 kinetics were slowed during AC90%+LC, factors other than convective oxygen delivery (e.g. those intrinsic to the exercising muscle) may explain the slower VO2 response in the upper reaches of the moderate intensity domain for arm exercise.