Dynamic asymmetry of phosphocreatine concentration and O2 uptake between the on‐ and off‐transients of moderate‐ and high‐intensity exercise in humans

Abstract

The on‐ and off‐transient (i.e. phase II) responses of pulmonary oxygen uptake (V̇O2) to moderate‐intensity exercise (i.e. below the lactate threshold, θ L) in humans has been shown to conform to both mono‐exponentiality and ‘on‐off’ symmetry, consistent with a system manifesting linear control dynamics. However above θ L the V̇O2 kinetics have been shown to be more complex: during high‐intensity exercise neither mono‐exponentiality nor ‘on‐off’ symmetry have been shown to appropriately characterise the V̇O2 response. Muscle [phosphocreatine] ([PCr]) responses to exercise, however, have been proposed to be dynamically linear with respect to work rate, and to demonstrate ‘on‐off’ symmetry at all work intenisties. We were therefore interested in examining the kinetic characteristics of the V̇O2 and [PCr] responses to moderate‐ and high‐intensity knee‐extensor exercise in order to improve our understanding of the factors involved in the putative phosphate‐linked control of muscle oxygen consumption. We estimated the dynamics of intramuscular [PCr] simultaneously with those of V̇O2 in nine healthy males who performed repeated bouts of both moderate‐ and high‐intensity square‐wave, knee‐extension exercise for 6 min, inside a whole‐body magnetic resonance spectroscopy (MRS) system. A transmit‐receive surface coil placed under the right quadriceps muscle allowed estimation of intramuscular [PCr]; V̇O2 was measured breath‐by‐breath using a custom‐designed turbine and a mass spectrometer system. For moderate exercise, the kinetics were well described by a simple mono‐exponential function (following a short cardiodynamic phase for V̇O2,), with time constants (τ) averaging: τV̇O2,on 35 ± 14 s (±s.d.), τ[PCr]on 33 ± 12 s, τV̇O2,off 50 ± 13 s and τ[PCr]off 51 ± 13 s. The kinetics for both V̇O2 and [PCr] were more complex for high‐intensity exercise. The fundamental phase expressing average τ values of τV̇O2,on 39 ± 4 s, τ[PCr]on 38 ± 11 s, τV̇O2,off 51 ± 6 s and τ[PCr]off 47 ± 11 s. An associated slow component was expressed in the on‐transient only for both V̇O2 and [PCr], and averaged 15.3 ± 5.4 and 13.9 ± 9.1 % of the fundamental amplitudes for V̇O2 and [PCr], respectively. In conclusion, the τ values of the fundamental component of [PCr] and V̇O2 dynamics cohere to within 10 %, during both the on‐ and off‐transients to a constant‐load work rate of both moderate‐ and high‐intensity exercise. On average, ≈90 % of the magnitude of the V̇O2 slow component during high‐intensity exercise is reflected within the exercising muscle by its [PCr] response.