Intersample fluctuations in phosphocreatine concentration determined by P-31-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 f rom the initial rate of change of phosphocreatine concentration ([PCr]) usi ng P-31-magnetic resonance spectroscopy (MRS). However, the phase and ampli tude characteristics of the sample-to-sample fluctuations can markedly infl uence 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 repe ated constant-load quadriceps exercise of a moderate intensity in a whole-b ody MRX system. A transmitreceive surface coil was placed under the right q uadriceps, 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 w ere 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%Delt a (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; cent re 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 [PC r] 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 e specially the initial rate of change of [PCr], i.e. the fluctuations can le ad to variations in estimation of the initial rate of change of [PCr] of mo re than t twofold, if the inherent 'noise' is not accounted for. This 'erro r' may be significantly reduced in such cases if the initial rate of change is estimated from the time constant and amplitude of the response.