The slow component of O2 uptake is not accompanied by changes in muscle EMG during repeated bouts of heavy exercise in humans

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)over dot(O2)) observed during heavy exercise. Using surface electromyographic (EMG) techniques, we compared the (V)over dot(O2) response with the integrated EMG (iEMG) and mean power frequency (MPF) response of the vastus lateralis with the (V)over dot(O2) 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)over dot(O2), corresponding to 50% of the difference between peak (V)over dot(O2) and the LT (i.e. Delta 50%, >LT1 and >LT2). 3. The (V)over dot(O2) slow component was significantly reduced by:prior heavy intensity exercise (>LT1, 410 +/- 196 ml min(-1); >LT2, 230 +/- 191 ml min(-1)). The time constant (tau), amplitude (A) and gain ((V)over dot(O2)/Delta WR) of the primary (V)over dot(O2) response (phase II) were not affected by prior heavy exercise when a three-component, exponential model was used to describe the (V)over dot(O2), response. 4. Integrated EMG and MPF remained relatively constant and at the same level throughout both >LT1 and >LT2 exercise and therefore were not associated with the (V)over dot(O2) slow component. 5. These data are consistent with the view that the increased O-2 cost (i.e. (V)over dot(O2) 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)over dot(O2) component with prior heavy exercise, thought to represent the initial muscle (V)over dot(O2) response, are inconsistent with O-2 delivery being: the limiting factor in (V)over dot(O2) kinetics during heavy exercise.