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(O(2))) 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(O(2)) 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 V(O(2)) 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(O(2)) 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(O(2)) 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(O(2)) 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 V(O(2))), with time constants (tau) averaging: tauV(O(2))(,on) 35 +/- 14 s (+/- S.D.), tauPCr(on) 33 +/- 12 s, tauV(O(2))(,off) 50 +/- 13 s and tauPCr(off) 51 +/- 13 s. The kinetics for both V(O(2)) and PCr were more complex for high-intensity exercise. The fundamental phase expressing average tau values of tauV(O(2))(,on) 39 +/- 4 s, tauPCr(on) 38 +/- 11 s, tauV(O(2))(,off) 51 +/- 6 s and tauPCr(off) 47 +/- 11 s. An associated slow component was expressed in the on-transient only for both V(O(2)) and PCr, and averaged 15.3 +/- 5.4 and 13.9 +/- 9.1 % of the fundamental amplitudes for V(O(2)) and PCr, respectively. In conclusion, the tau values of the fundamental component of PCr and V(O(2)) 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, approximately 90 % of the magnitude of the V(O(2)) slow component during high-intensity exercise is reflected within the exercising muscle by its PCr response.