The slow component of O₂uptake is not accompanied by changes in muscle EMG during repeated bouts of heavy exercise in humans

Abstract

For exercise intensities that engender a sustained lactic acidosis (i.e. above the lactate threshold, LT), an additional increase in pulmonary O 2 uptake (V O 2 ) of delayed onset leads to a V O 2 which is higher than predicted from the V O 2 -work rate relationship for exercise performed in the sub-LT domain (Whipp & Mahler, 1980;Roston et al. 1987;Henson et al. 1989;Paterson & Whipp, 1991;Barstow et al. 1993). The physiological mechanism(s) underlying the V O 2 slow component (V O 2 SC) have yet to be clearly established. However, identifying the process(es) that increase the O 2 cost of performing heavy work would contribute significantly to our understanding of muscle energetics and the limitations to exercise tolerance in the heavy intensity exercise domain.Since Poole et al. (1994) demonstrated that the excess increase in pulmonary V O 2 observed during the slow component could, for the most part, be accounted for by an increase in leg V O 2 , considerable interest has been given to a mechanistic link between muscle fibre type and the V O 2 SC. Studies using isolated muscle preparations have shown that fast-twitch (i.e. type II) fibres are less efficient than slow-twitch (i.e. type I) fibres (Crow & Kushmerick, 1982;Kushmerick et al. 1992) and, therefore, would utilize more O 2 in order to regenerate the same amount of high-energy phosphates. Indeed, Barstow et al. (1996) 1. We hypothesized that either the recruitment of additional muscle motor units and/or the progressive recruitment of less efficient fast-twitch muscle fibres was the predominant contributor to the additional oxygen uptake (V O 2 ) observed during heavy exercise. Using surface electromyographic (EMG) techniques, we compared the V O 2 response with the integrated EMG (iEMG) and mean power frequency (MPF) response of the vastus lateralis with the V O 2 response during repeated bouts of moderate (below the lactate threshold, <LT) and heavy (above the lactate threshold, >LT) intensity cycle ergometer exercise.2. Seven male subjects (age 29 ± 7 years, mean ± S.D.) performed three transitions to a work rate (WR) corresponding to 90 % LT and two transitions to a work rate that would elicit a V O 2 corresponding to 50 % of the difference between peak V O 2 and the LT (i.e. ∆50 %, >LT 1 and >LT 2 ). 4. Integrated EMG and MPF remained relatively constant and at the same level throughout both >LT 1 and >LT 2 exercise and therefore were not associated with the V O 2 slow component.5. These data are consistent with the view that the increased O 2 cost (i.e. V O 2 slow component) associated with performing heavy exercise is coupled with a progressive increase in ATP requirements of the already recruited motor units rather than to changes in the recruitment pattern of slow versus fast-twitch motor units. Further, the lack of speeding of the kinetics of the primary V O 2 component with prior heavy exercise, thought to represent the initial muscle V O 2 response, are inconsistent with O 2 delivery being the limiting factor in V O 2 kinetics during heavy ...

Effects of prior very-heavy intensity exercise on indices of aerobic function and high-intensity exercise tolerance

“…4), as the absolute fundamental V O 2 asymptotic value was elevated by a similar amount (see also Refs. 10,28,31,38,50). And when sufficient time is allowed for the full recovery of V O 2 back to control baseline levels, there is therefore an increase in the fundamental gain (4, 9 -11, 15).…”

Thigh muscle activation distribution and pulmonary V̇o2kinetics during moderate, heavy, and very heavy intensity cycling exercise in humans

“…39) between the diversity of muscle T2 increase (reflecting the diversity of muscle recruitment; Refs. 2,4,25,36,39,40) and the magnitude of the V O 2 slow component. A progressively increasing diversity of recruitment would, if reliant on less-efficient fibers, i.e., fibers with poor P:O 2 or high energy cost of force production characteristics, result in a progressively increasing "O 2 cost" of the external power output and result in the expression of a V O 2 slow component.…”

“…Furthermore, Krustrup and colleagues (21,22) have shown in muscle biopsy samples of the quadriceps that PCr and glycogen depletion became increasingly more widespread during work rates engendering a V O 2 slow component. Investigations using surface electromyography (EMG), however, have not consistently corroborated this progressive recruitment hypothesis (40,41 (1,12,25,33,36) to investigate this suggestion. They showed a linear correlation between the magnitude of the V O 2 slow component (between 3 and 15 min of cycle ergometry) and T2 increases in muscles of the leg [which were both reduced by endurance training; Saunders et al (38)], consistent with the progressive recruitment hypothesis for the V O 2 slow component mechanism.…”

Human Physiology

“…Prior heavy exercise increases the amplitude of the primary oxygen uptake (l{),) response and reduces the amplitude of the Va, slow component during sub~equent heavy exercise with no change in the primary T-()2 time constant (Burnley et al 2000;Scheuermann et al 2001).…”