Effects of prior exercise on oxygen uptake and phosphocreatine kinetics during high-intensity knee-extension exercise in humans

Abstract

1. A prior bout of high-intensity square-wave exercise can increase the temporal adaptation of pulmonary oxygen uptake (.V(O2)) to a subsequent bout of high-intensity exercise. The mechanisms controlling this adaptation, however, are poorly understood. 2. We therefore determined the dynamics of intramuscular [phosphocreatine] ([PCr]) simultaneously with those of .V(O2) in seven males who performed two consecutive bouts of high-intensity square-wave, knee-extensor exercise in the prone position for 6 min with a 6 min rest interval. A magnetic resonance spectroscopy (MRS) transmit-receive surface coil under the quadriceps muscle allowed estimation of [PCr]; .V(O2) was measured breath-by-breath using a custom-designed turbine and a mass spectrometer system. 3. The .V(O2) kinetics of the second exercise bout were altered compared with the first such that (a) not only was the instantaneous rate of .V(O2) change (at a given level of .V(O2)) greater but the phase II tau was also reduced - averaging 46.6 +/- 6.0 s (bout 1) and 40.7 +/- 8.4 s (bout 2) (mean +/- S.D.) and (b) the magnitude of the later slow component was reduced. 4. This was associated with a reduction of, on average, 16.1% in the total exercise-induced [PCr] decrement over the 6 min of the exercise, of which 4.0% was due to a reduction in the slow component of [PCr]. There was no discernable alteration in the initial rate of [PCr] change. The prior exercise, therefore, changed the multi-compartment behaviour towards that of functionally first-order dynamics. 5. These observations demonstrate that the .V(O2) responses relative to the work rate input for high-intensity exercise are non-linear, as are, it appears, the putative phosphate-linked controllers for which [PCr] serves as a surrogate.

Figure 1. An example of a ‘stack plot’ of the time-sequence spectra during a high-intensity rest- exercise-rest-exercise-rest protocol (subject 1)

[PCr], 0 p.p.m.; [P_i], −5 p.p.m.; γ, α and β[ATP], approximately 2.5, 7.5 and 16 p.p.m.

Figure 2. The relative concentrations of [PCr] during repeated high-intensity exercise in subject 1

A is fitted with model A (i.e. the whole 360 s response is fitted with a single exponential). B shows the fit to the same data using the model B a posteriori approach (see text) to identify the exponential region. The arrows indicate the ‘fitting window’. The residuals to the fitted responses are shown below.

Figure 3. The response of to repeated high-intensity exercise in subject 1

A is fitted with model A (i.e. the whole 360 s response is fitted with a single exponential). B shows the fit to the same data using the model B a posteriori approach (see text) to identify the exponential region. The arrows indicate the ‘fitting window’. The residuals to the fitted responses are shown below.

Figure 4. Example of time-aligned averaged responses of (A) and [PCr] (B) to two repeats of high-intensity knee-extensor exercise in subject 1 lying prone in the bore of the whole-body NMR spectrometer

The dotted vertical lines indicate the onset and cessation of exercise.

Figure 5. The initial rate of change and phase II time constants of for two repeated high-intensity bouts

A, the rate of change of (at a given , β) is significantly increased from the first to the second bout of repeated high-intensity exercise in seven subjects. B, as a result of the unchanged steady-state amplitude of in the two repeats, the time constant (τ) is also significantly reduced.

Figure 6. The steady-state amplitude of [PCr] from two repeated high-intensity bouts

Δ[PCr]_ss is reduced from bout 1 to bout 2 in repeated high-intensity exercise in seven subjects.

Figure 7. The interaction of the kinetics of muscle O₂ consumption and [PCr] during repeated high-intensity exercise bouts

A model illustrating the interaction of amplitude, rate of change and the time constant of response of muscle O₂ consumption (upper panel) and intramuscular [PCr] (lower panel) in two repetitions of high-intensity knee-extensor exercise in humans.