The slow component of O(2) uptake is not accompanied by changes in muscle EMG during repeated bouts of heavy exercise in humans

Abstract

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 (VO2) observed during heavy exercise. Using surface electromyographic (EMG) techniques, we compared the VO2 response with the integrated EMG (iEMG) and mean power frequency (MPF) response of the vastus lateralis with the VO2 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 VO2 corresponding to 50 % of the difference between peak VO2 and the LT (i.e. Delta50 %, > LT1 and > LT2). 3. The VO2 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 (DeltaVO2/DeltaWR) of the primary VO2 response (phase II) were not affected by prior heavy exercise when a three-component, exponential model was used to describe the V2 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 VO2 slow component. 5. These data are consistent with the view that the increased O2 cost (i.e. VO2 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 VO2 component with prior heavy exercise, thought to represent the initial muscle VO2 response, are inconsistent with O2 delivery being the limiting factor in V > O2 kinetics during heavy exercise.

Figure 1. Parameters of the exponential models used to describe V˙_O2 kinetics from moderate (below the lactate threshold, <LT) and heavy intensity (>LT) exercise

Upper panel, during the on-transition; lower panel, during recovery. Parameters correspond to those in eqns (1) and (2) (exercise) and (3) and (4) (recovery).

Figure 2. On-transient responses to a moderate increase in work rate intensity

An example of the on-transient responses for O₂ uptake (A), integrated EMG (B) and mean power frequency (C) for an individual subject to a step increase in work rate of moderate intensity below the lactate threshold. The group mean O₂ response for a step increase in work rate to <LT and during repeated bouts of exercise above the LT (>LT₁, >LT₂) are presented in Figs 4 and 5, respectively.

Figure 3. On-transient responses during repeated bouts of heavy intensity exercise

An example of the on-transient responses for O₂ uptake (A and B), integrated EMG (C and D) and mean power frequency (E and F) for an individual subject during repeated bouts of constant load, heavy intensity exercise above the lactate threshold. The initial bout of heavy intensity exercise (>LT₁) is shown in the left panels while the subsequent bout (>LT₂) is presented in the right panels.

Figure 4. Group mean responses to a moderate step increase in work rate

The group mean response for O₂ uptake (A), integrated EMG (B) and mean power frequency (C) during a step increase in work rate of moderate intensity below the lactate threshold.

Figure 5. Group mean responses to repeated bouts of heavy intensity exercise

The group mean responses to the on-transient for O₂ uptake (A), integrated EMG, normalized to end-exercise values (B), and mean power frequency (C) during repeated bouts of constant load, heavy intensity exercise above the lactate threshold. The initial exercise bout (>LT₁) is represented by the filled circles (•) while the subsequent exercise bout (>LT₂) is represented by the open circles (○). When the differences in baseline O₂ uptake are accounted for, the kinetics of the primary exponential responses were similar for >LT₁ and >LT₂ (see Table 3).

Figure 6. Amplitude and time constant for the primary fast component during repeated bouts of heavy excerise

The relationship between the amplitudes (A₁′; panel A), and the time constants (τ₁; panel B) of the primary fast component during the initial heavy intensity exercise bout (>LT₁) and the subsequent heavy intensity exercise bout (>LT₂). The dotted line is the line of identity.