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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 θLthe V̇O2kinetics 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 V̇O2response. 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 V̇O2and [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 V̇O2in 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]; V̇O2was 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,on35 ± 14 s (±s.d.), τ[PCr]on33 ± 12 s, τV̇O2,off50 ± 13 s and τ[PCr]off51 ± 13 s. The kinetics for both V̇O2and [PCr] were more complex for high‐intensity exercise. The fundamental phase expressing average τ values of τV̇O2,on39 ± 4 s, τ[PCr]on38 ± 11 s, τV̇O2,off51 ± 6 s and τ[PCr]off47 ± 11 s. An associated slow component was expressed in the on‐transient only for both V̇O2and [PCr], and averaged 15.3 ± 5.4 and 13.9 ± 9.1 % of the fundamental amplitudes for V̇O2and [PCr], respectively. In conclusion, the τ values of the fundamental component of [PCr] and V̇O2dynamics 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 V̇O2slow component during high‐intensity exercise is reflected within the exercising muscle by its [PCr] response.

1The ATP turnover rate during constant‐load exercise is often estimated from the initial rate of change of phosphocreatine concentration ([PCr]) using 31P‐magnetic resonance spectroscopy (MRS). However, the phase and amplitude characteristics of the sample‐to‐sample fluctuations can markedly influence this estimation (as well as that for the time constant (τ) of the [PCr] change) and confound its physiological interpretation especially for small amplitude responses.2This influence was investigated in six healthy males who performed repeated constant‐load quadriceps exercise of a moderate intensity in a whole‐body MRS system. A transmit‐ receive surface coil was placed under the right quadriceps, allowing determination of intramuscular [PCr]; pulmonary oxygen uptake (V̇O2) was simultaneously determined, breath‐by‐breath, using a mass spectrometer and a turbine volume measuring module.3The probability density functions (PDF) of [PCr] and V̇O2fluctuations were determined for each test during the steady states of rest and exercise and the PDF was then fitted to a Gaussian function. The standard deviation of the [PCr] and V̇O2fluctuations at rest and during exercise (srand sw, respectively) and the peak centres of the distributions (xcrand xcw) were determined, as were the skewness (γ1) and kurtosis (γ2) coefficients.4There was no difference between srand swfor [PCr] relative to the resting control baseline (sr= 1.554%Δ (s.d.= 0.44), sw= 1.514%Δ (s.d.= 0.35)) or the PDF peak centres (xcr=−0.013 %Δ (s.d.= 0.09), xcw−0.197 %Δ (s.d.= 0.18)). The standard deviation and peak centre of the ‘noise’ in V̇O2also did not vary between rest and exercise (sr= 0.0427l min−1(s.d.= 0.0104), sw= 0.0640l min−1(s.d.= 0.0292); xcr=−0.0051 l min−1(s.d.= 0.0069), xcw0.0022 l min−1(s.d.= 0.0034)).5Our results demonstrate that the intersample ‘noise’ associated with [PCr] determination by 31P‐MRS may be characterised as a stochastic Gaussian process that is uncorrelated with work rate, as previously described for V̇O2. This ‘noise’ can significantly affect the estimation of τ[PCr] and especially the initial rate of change of [PCr], i.e. the fluctuations can lead to variations in estimation of the initial rate of change of [PCr] of more than twofold, if the inherent ‘noise’ is not accounted for. This ‘error’ may be significantly reduced in such cases if the initial rate of change is estimated from the time constant and amplitude of the response.

1In the non‐steady state of moderate intensity exercise, pulmonary O2uptake (V̇p,O2) is temporally dissociated from muscle O2consumption (V̇m,O2) due to the influence of the intervening venous blood volume and the contribution of body O2stores to ATP synthesis. A monoexponential model of V̇p,O2without a delay term, therefore, implies an obligatory slowing of V̇p,O2kinetics in comparison to V̇m,O2.2During moderate exercise, an association of V̇m,O2and [phosphocreatine] ([PCr]) kinetics is a necessary consequence of the control of muscular oxidative phosphorylation mediated by some function of [PCr]. It has also been suggested that the kinetics of V̇p,O2will be expressed with a time constant within 10 % of that of V̇m,O2.3V̇p,O2and intramuscular [PCr] kinetics were investigated simultaneously during moderate exercise of a large muscle mass in a whole‐body NMR spectrometer. Six healthy males performed prone constant‐load quadriceps exercise. A transmit‐receive coil under the right quadriceps allowed determination of intramuscular [PCr]; V̇p,O2was measured breath‐by‐breath, in concert with [PCr], using a turbine and a mass spectrometer system.4Intramuscular [PCr] decreased monoexponentially with no delay in response to exercise. The mean of the time constants (τPCr) was 35 s (range, 20–64 s) for the six subjects.5Two temporal phases were evident in the V̇p,O2response. When the entire V̇p,O2response was modelled to be exponential with no delay, its time constant (τ′V̇p,O2) was longer in all subjects (group mean = 62 s; range, 52–92 s) than that of [PCr], reflecting the energy contribution of the O2stores.6Restricting the V̇p,O2model fit to phase II resulted in matching kinetics for V̇p,O2(group mean τV̇p,O2= 36 s; range, 20–68 s) and [PCr], for all subjects.7We conclude that during moderate intensity exercise the phase II τV̇p,O2provides a good estimate of τPCrand by implication that of V̇m,O2(τV̇m,O2).