INTERMITTENT ISOMETRIC EXERCISE, A NEW MODEL TO STUDY VO2 SLOW COMPONENT

Abstract

Introduction During constant load bicycling at moderate intensities, oxygen uptake (VO2) stabilizes after 3 min. However at intensities above lactate threshold (LT) VO2 does not stabilize, but increases after 3 min at a slow rate until exhaustion. This increase in VO2 is defined as the VO2 slow component (VO2SC). Despite the fact that many studies have focused on this phenomenon over the past decade the underlying mechanisms have not been established. One reason may be the close temporal relationship with lactate production even if lactate per se probably plays a minor role in causing the VO2SC. Purpose The purpose of this study was to establish a new exercise model to study VO2SC with little contribution from anaerobic glycolysis. Methods Eight endurance trained (ET; VO2max 69.2 ± 1.8 mlkg−1 min−1 and MVC 1910 ± 90 N) and 7 strength trained subjects (ST; VO2max 51.3 ± 3.5 mlkg−1min−1 and MVC 2590 ± 130 N) performed intermittent (6s contraction and 4 s rest) isometric quadriceps contractions at 4 different constant loads. The loads were between 20 and 50% of MVC and the duration of exercise was 20 min or to exhaustion. Pulmonary VO2 and ventilation were measured with a Douglas bag system before, 5.5, 11.5 min into and just before cessation of exercise, along with blood lactate concentration (fingertip). Results At light loads (20–30% of MVC) no significant VO2SC was present in any of the subjects. At 35% a significant VO2SC was apparent in three strength trained subjects and these subjects became exhausted before 15 min at this load. The other subjects showed significant VO2SC at loads between 40 and 50% of MVC. The VO2SC appeared at lower relative workloads and was greater in ST compared with ET. Blood lactate concentration was 4.3 ± 1.1 (SD) mM at exhaustion. There was no correlation between increase in ventilation and VO2SC. Conclusion Intermittent isometric exercise is a feasible model to study VO2SC, the margin between load with no VO2SC and load with a large SC (> 5%/min) is very distinct and it is unrelated to blood acidosis.