Intersample fluctuations in phosphocreatine concentration determined by 31P-magnetic resonance spectroscopy and parameter estimation of metabolic responses to exercise in humans

1. The ATP turnover rate during constant-load exercise is often estimated from the initial rate of change of phosphocreatine concentration ([PCr]) using P-31-magnetic resonance spectroscopy (MRS). However, the phase and amplitude characteristics of the sample-to-sample fluctuations can markedly influence this estimation (its well as that for the time constant (tau) of the [PCr] change) and confound its physiological interpretation especially fur small amplitude responses. 2. This influence was investigated in six healthy males who per formed repeated constant-load quadriceps exercise of a moderate intensity in a whole-body MRX system. A transmitreceive surface coil was placed under the right quadriceps, allowing determination of intramuscular [PCr]; pulmonary oxygen uptake (V-O2) was simultaneously determined, breath-by breath, using a mass spectrometer and a turbine volume measuring module. 3. The probability density functions (PDF) of [PCr] and V-O2 fluctuations were determined for each test during the steady states of r est and exercise and the PDF was then fitted to a Gaussian function. The standard deviation of the [PCr] and V-O2 fluctuations at rest and during exercise (s(r) and s(w) respectively) and the peak centres of the distributions (xc(r), and xc(w)) were determined, as were the skewness (gamma (1)) and kurtosis (gamma (2)) coefficients. 4. There was no difference between s(r) and s(w) for [PCr] relative to the resting control baseline (s(r) =1.554%Delta (S.D. = 0.44), s(W) =1.514%Delta (S.D. = 0.35)) or the PDF peak centres (xc(r) = -0.013%Delta (S.D. = 0.09), xc(W) -0.197%Delta (S.D. = 0.18)). The standard deviation and peal; centre of the 'noise' in V-O2 also did not vary between rest and exercise (s(r) = 0.0427 1 min(-1) (S.D. = 0.0104), s(W) = 0.0640 1 min(-1) (S.D. = 0.0292); xc(r) = -0.0051 1 min(-1) (S.D. = 0.0069), xc(W) 0.0022 1 min(-1) (S.D. = 0.0034)). 5. Our results demonstrate that the intersample 'noise' associated with [PCr] determination by P-31-MRS may be characterised as a stochastic Gaussian process that is uncorrelated with work rate, as previously described for V-O2. This 'noise' can significantly affect the estimation of tau [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 t 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.