Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans

The maximal oxygen uptake () during dynamic muscular exercise is commonly taken as a crucial determinant of the ability to sustain high-intensity exercise. Considerably less attention, however, has been given to the rate at which increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise.Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 exercise tests: initial ramp-incremental cycle ergometry to the limit of tolerance and subsequently, on different days, three bouts of square-wave exercise each at moderate, heavy, very heavy and severe intensities. Pulmonary gas exchange variables were determined breath by breath throughout exercise and recovery from the continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer).For moderate exercise, the kinetics were well described by a simple mono-exponential function, following a short cardiodynamic phase, with the on- and off-transients having similar time constants (τ1); i.e. τ1,on averaged 33 ± 16 s (± S.D.) and τ1,off 29 ± 6 s.The on-transient kinetics were more complex for heavy exercise. The inclusion of a second slow and delayed exponential component provided an adequate description of the response; i.e. τ1,on = 32 ± 17 s and τ2,on = 170 ± 49 s. The off-transient kinetics, however, remained mono-exponential (τ1,off = 42 ± 11 s).For very heavy exercise, the on-transient kinetics were also well described by a double exponential function (τ1,on = 34 ± 11 s and τ2,on = 163 ± 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. τ1,off = 33 ± 5 s and τ2,off = 460 ± 123 s).At the highest intensity (severe), the on-transient kinetics reverted to a mono-exponential profile (τ1,on = 34 ± 7 s), while the off-transient kinetics retained a two-component form (τ1,off = 35 ± 11 s and τ2,off = 539 ± 379 s).We therefore conclude that the kinetics of during dynamic muscular exercise are strikingly influenced by the exercise intensity, both with respect to model order and to dynamic asymmetries between the on- and off-transient responses

Comparação entre diferentes métodos de análise do componente lento do consumo de oxigênio: uma abordagem no domínio muito intenso de exercício Comparison between different methods of analysis of slow component of oxygen uptake: a view in severe exercise domain

O objetivo do presente estudo foi comparar, em domínio muito intenso de exercício, diferentes técnicas utilizadas para medir a amplitude do componente lento (CL) da cinética do consumo de oxigênio. Dez ciclistas treinados, do gênero masculino [média &plusmn; DP (idade: 25 &plusmn; 3,6 anos, massa corporal: 67,2 &plusmn; 4,5kg, altura: 174,8 &plusmn; 6,5cm e VO2max: 62,4 &plusmn; 3,1ml.kg¹.min¹)], realizaram duas idênticas transições de carga constante (intensidade de 75%delta: 75% da diferença entre o VO2 no limiar de lactato e o VO2max) em dias diferentes. O CL foi calculado a partir de diferentes métodos: (1) modelo biexponencial [VO2(t) = VO2base + A1 (1 e-(t-TA1/t1)) + A2 (1 e(tTA2/t2))], (2) intervalos predeterminados (o deltaVO26-2: diferença do VO2 entre o segundo e o sexto minuto de exercício e o deltaVO263: diferença do VO2 entre o terceiro e o sexto minuto de exercício) e (3) diferença entre o VO2 obtido no final do exercício e o valor obtido a partir de um ajuste monoexponencial do "componente primário" (tempo predeterminado de 120s) (CL6"CP"). Todos os métodos foram comparados entre si. Os resultados demonstraram significante subestimação do CL obtido pelo método de intervalos predeterminados (deltaVO26-2: 432 &plusmn; 126ml.min¹ e deltaVO263: 279 &plusmn; 88ml.min¹) quando comparado com o modelo biexponencial (676 &plusmn; 136ml.min¹) e ao CL6"CP" [(719 &plusmn; 265ml.min¹ (p < 0,05)]. Não houve diferenças significativas entre as outras comparações. Os resultados sugerem que a utilização de tempos predeterminados pode subestimar o CL quando comparado com o modelo biexponencial e com o CL6"CP".<br>The objective of the present study was to compare in severe exercise domain, different techniques used for measuring the amplitude of the slow component (SC) of oxygen uptake kinetics. Ten trained cyclists, male (age: 25 &plusmn; 3.6 years, body mass: 67.2 &plusmn; 4.5 kg, height: 174.8 &plusmn; 6.5 cm and VO2max: 62.4 &plusmn; 3.1 mL.kg¹.min¹), performed two identical bouts transitions at constant load [mean &plusmn; SD (intensity 75%delta: 75% of the difference between the VO2 lactate threshold and the VO2max)] in different days. The SC was calculated from different methods: (1) bi-exponential model [VO2(t) = VO2base + A1 (1 e(tTA1/t1)) + A2 (1 e(tTA2/t2))], (2) predetermined intervals (deltaVO262: difference between the second min VO2 and the end VO2; deltaVO263: difference between the third min VO2 and the end VO2) and (3) difference between the end VO2 and the value obtained from a mono-exponential adjustment of the "primary component" (predetermined time of 120 s) (SC6"PC"). All the methods were compared among themselves. The results showed a significant underestimation of the SC obtained by method of predetermined intervals (deltaVO262: 432 &plusmn; 126 ml.min¹ and deltaVO26-3: 279 &plusmn; 88 ml.min-1) when compared with bi-exponential model (676 &plusmn; 136 ml.min-1) and SC6-"PC" [(719 &plusmn; 265 ml.min-1 (p < 0.05)]. There was not significant difference among the other comparison. The results suggest that the use of predetermined time may underestimate the SC when compared with bi-exponential model and SC6"PC"

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 (V̇O2) to moderate-intensity exercise (i.e. below the lactate threshold, θL) 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 θL the V̇O2 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 V̇O2 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 V̇O2 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 V̇O2 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]; V̇O2 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 V̇O2,), with time constants (τ) averaging: τV̇O2,on 35 ± 14 s (± s.d.), τ[PCr]on 33 ± 12 s, τV̇O2,off 50 ± 13 s and τ[PCr]off 51 ± 13 s. The kinetics for both V̇O2 and [PCr] were more complex for high-intensity exercise. The fundamental phase expressing average τ values of τV̇O2,on 39 ± 4 s, τ[PCr]on 38 ± 11 s, τV̇O2,off 51 ± 6 s and τ[PCr]off 47 ± 11 s. An associated slow component was expressed in the on-transient only for both V̇O2 and [PCr], and averaged 15.3 ± 5.4 and 13.9 ± 9.1 % of the fundamental amplitudes for V̇O2 and [PCr], respectively. In conclusion, the τ values of the fundamental component of [PCr] and V̇O2 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, ≈90 % of the magnitude of the V̇O2 slow component during high-intensity exercise is reflected within the exercising muscle by its [PCr] response

Central and peripheral adjustments during high-intensity exercise following cold water immersion

We investigated the acute effects of cold water immersion (CWI) or passive recovery (PAS) on physiological responses during high-intensity interval training (HIIT)

Exceeding a “critical” muscle Pi: implications for V˙O2 and metabolite slow components, muscle fatigue and the power–duration relationship

Purpose: The consequences of the assumption that the additional ATP usage, underlying the slow component of oxygen consumption (V˙O2) and metabolite on-kinetics, starts when cytosolic inorganic phosphate (Pi) exceeds a certain “critical” Pi concentration, and muscle work terminates because of fatigue when Pi exceeds a certain, higher, “peak” Pi concentration are investigated. Methods: A previously developed computer model of the myocyte bioenergetic system is used. Results: Simulated time courses of muscle V˙O2, cytosolic ADP, pH, PCr and Pi at various ATP usage activities agreed well with experimental data. Computer simulations resulted in a hyperbolic power–duration relationship, with critical power (CP) as an asymptote. CP was increased, and phase II V˙O2 on-kinetics was accelerated, by progressive increase in oxygen tension (hyperoxia). Conclusions: Pi is a major factor responsible for the slow component of the V˙O2 and metabolite on-kinetics, fatigue-related muscle work termination and hyperbolic power–duration relationship. The successful generation of experimental system properties suggests that the additional ATP usage, underlying the slow component, indeed starts when cytosolic Pi exceeds a “critical” Pi concentration, and muscle work terminates when Pi exceeds a “peak” Pi concentration. The contribution of other factors, such as cytosolic acidification, or glycogen depletion and central fatigue should not be excluded. Thus, a detailed quantitative unifying mechanism underlying various phenomena related to skeletal muscle fatigue and exercise tolerance is offered that was absent in the literature. This mechanism is driven by reciprocal stimulation of Pi increase and additional ATP usage when “critical” Pi is exceeded