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, θL) 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 θL 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.

Pulmonary oxygen uptake and muscle deoxygenation kinetics during recovery in trained and untrained male adolescents

“…However, at the off-transition there is a paucity of data regarding the agreement between muscle oxygen consumption and pulmonary oxygen uptake kinetics. Good agreement between muscle phosphocreatine and pulmonary oxygen uptake kinetics during recovery from exercise has been demonstrated (Rossiter et al 2002). However, pulmonary oxygen uptake kinetics was recently shown to be significantly longer than, and unrelated to, direct measurements of muscle oxygen consumption kinetics during the recovery from exercise (Krustrup et al 2009).…”

“…This hypothesis was based in part on the findings of similar studies in adults (Fukuoka et al 2002;Fukuoka et al 2006), and the assumption that pulmonary oxygen uptake kinetics reflects muscle oxygen consumption kinetics (Grassi et al 1996;Koga et al 2005;Krustrup et al 2009;Rossiter et al 2002), which has been shown to be faster during recovery in the trained state (as indicated by muscle phosphocreatine kinetics, Forbes et al 2008;Takahashi et al 1995;Yoshida 2002). Given the distinction in training status between our two groups, it is therefore perhaps surprising that there was no difference in the off-kinetics of pulmonary oxygen uptake between the trained and untrained adolescents.…”

“…moderate-intensity exercise), the kinetics of pulmonary oxygen uptake proceed via a time-course that can be effectively modelled as a single exponential function with a time delay reflecting the muscle-to-lung transit time (Ozyener et al 2001). The kinetics of the pulmonary oxygen uptake response at the on-and offset of exercise is of interest because they are generally interpreted to represent the kinetics of muscle oxygen consumption kinetics (Barstow et al 1990;Grassi et al 1996;Koga et al 2005;Krustrup et al 2009;Rossiter et al 2002), though how strong this relationship is at the offset of exercise has recently been questioned (Krustrup et al 2009). …”

“…The recovery of pulmonary oxygen uptake from exercise to baseline levels is considered to reflect the recovery of muscle phosphocreatine to pre-exercise levels (Barker et al 2008;Rossiter et al 2002).…”

“…In the adult population, training has been shown to result in both faster muscle phosphocreatine (Forbes et al 2008;Takahashi et al 1995;Yoshida 2002) and pulmonary oxygen uptake (Fukuoka et al 2002;Fukuoka et al 2006) kinetics during recovery from moderate-intensity exercise, suggestive of enhanced oxidative capacity of the exercising muscle (Barker et al 2008;Paganini et al 1997;Rossiter et al 2002). The favourable adaptations to oxygen delivery, mitochondrial density and oxidative enzyme activity and thus aerobic fitness that appropriate training permits (Holloszy and Coyle 1984;Murias et al 2010;Phillips et al 1995;Russell et al 2002) are likely to explain the faster recovery kinetics relative to the untrained state.…”