title: Dynamic asymmetry of phosphocreatine concentration and O(2) uptake between the on- and off-transients of moderate- and high-intensity exercise in humans.
abstract: 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.), tau[PCr](on) 33 +/- 12 s, tauV(O(2))(,off) 50 +/- 13 s and tau[PCr](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, tau[PCr](on) 38 +/- 11 s, tauV(O(2))(,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(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.
authors.affiliation: Department of Physiology, St George's Hospital Medical School, Cranmer Terrace, Tooting, London, UK.
authors.forename: H B
authors.initials: HB
authors.lastname: Rossiter
authors.forename: S A
authors.initials: SA
authors.lastname: Ward
authors.forename: J M
authors.initials: JM
authors.lastname: Kowalchuk
authors.forename: F A
authors.initials: FA
authors.lastname: Howe
authors.forename: J R
authors.initials: JR
authors.lastname: Griffiths
authors.forename: B J
authors.initials: BJ
authors.lastname: Whipp
mesh_terms: D000328:Adult; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000737:chemistry / Q000378:metabolism; D010101:Oxygen Consumption / Q000502:physiology*; D010725:Phosphocreatine / Q000378:metabolism*; D011659:Pulmonary Gas Exchange / Q000502:physiology
chemical_list: D010725:Phosphocreatine