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

The on- and off-transient (i.e. phase II) responses of pulmonary oxygen uptake (V-O2) to moderate-intensity exercise (i.e. below the lactate threshold, theta(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 theta(L) the V-O2, 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 1 1, 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 V-O2 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 V-O2, 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]; V-O2 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 for V-O2), with time constants (tau) averaging: tauV(O2,on) 35 +/- 14 s (+/- S.D.), tau[PCr](on) 33 +/- 12 s, tauV(O2),(off) 50 +/- 13 s and tau[PCr](off) 51 +/- 13 s. The kinetics for both V-O2 and [PCr] were more complex for high-intensity exercise. The fundamental phase expressing average tau values of 39 +/- 4 s, tau[PCr](on) 38 +/- 11 s, tauV(O2),(off) 51 +/- 6 s and tau[PCr](off) 47 +/- 11 s. An associated slow component was expressed in the on-transient only for both V-O2 and [PCr], and averaged 15.3 +/- 5.4 and 13.9 +/- 9.1% of the fundamental amplitudes for 17, and [PCr], respectively. In conclusion, the tau values of the fundamental component of [PCr] and V-O2 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, similar to90% of the magnitude of the V-O2 slow component during high-intensity exercise is reflected within the exercising muscle by its [PCr] response.