Oxygen deficit is not affected by the rate of transition from rest to submaximal exercise

Modeling energy expenditure and oxygen consumption in human exposure models: accounting for fatigue and EPOC

Human exposure and dose models often require a quantification of oxygen consumption for a simulated individual. Oxygen consumption is dependent on the modeled individual's physical activity level as described in an activity diary. Activity level is quantified via standardized values of metabolic equivalents of work (METS) for the activity being performed and converted into activity-specific oxygen consumption estimates. However, oxygen consumption remains elevated after a moderate- or high-intensity activity is completed. This effect, which is termed excess post-exercise oxygen consumption (EPOC), requires upward adjustment of the METS estimates that follow high-energy expenditure events, to model subsequent increased ventilation and intake dose rates. In addition, since an individual's capacity for work decreases during extended activity, methods are also required to adjust downward those METS estimates that exceed physiologically realistic limits over time. A unified method for simultaneously performing these adjustments is developed. The method simulates a cumulative oxygen deficit for each individual and uses it to impose appropriate time-dependent reductions in the METS time series and additions for EPOC. The relationships between the oxygen deficit and METS limits are nonlinear and are derived from published data on work capacity and oxygen consumption. These modifications result in improved modeling of ventilation patterns, and should improve intake dose estimates associated with exposure to airborne environmental contaminants.

VO(2) kinetics and the O(2) deficit in heavy exercise

The purpose of this study was to examine a new method for calculating the O(2) deficit that considered the O(2) uptake (VO(2)) kinetics during exercise as two separate phases in light of previous research in which it was shown that the traditional O(2) deficit calculation overestimated the recovery O(2) consumption (ROC). Eight subjects completed exercise transitions between unloaded cycling and 25% (heavy, H) or 50% (very heavy, VH) of the difference between the lactic acid threshold (LAT) and peak VO(2) for 8 min. The O(2) deficit, calculated in the traditional manner, was significantly greater than the measured ROC for both above-LAT exercises: 4.03 +/- 1.01 vs. 2.63 +/- 0.80 (SD) liters for VH and 2.36 +/- 0.91 vs. 1.74 +/- 0.63 liters for H for the O(2) deficit vs. ROC (P < 0.05). When the kinetics were viewed as two separate components with independent onsets, the calculated O(2) deficit (2.89 +/- 0.79 and 1.71 +/- 0.70 liters for VH and H, respectively) was not different from the measured ROC (P < 0.05). Subjects also performed the same work rate for only 3 min. These data, from bouts terminated before the slow component could contribute appreciably to the overall VO(2) response, show that the O(2) requirement during the transition is less than the final steady state for the work rate, as evidenced by symmetry between the O(2) deficit and ROC. This new method of calculating the O(2) deficit more closely reflects the expected O(2) deficit-ROC relationship (i.e., ROC >/= O(2) deficit). Therefore, estimation of the O(2) deficit during heavy exercise transitions should consider the slow component of VO(2) as an additional deficit component with delayed onset.

Oxygen uptake kinetics during exercise

The characteristics of oxygen uptake (VO2) kinetics differ with exercise intensity. When exercise is performed at a given work rate which is below lactate threshold (LT), VO2 increases exponentially to a steady-state level. Neither the slope of the increase in VO2 with respect to work rate nor the time constant of VO2 responses has been found to be a function of work rate within this domain, indicating a linear dynamic relationship between the VO2 and the work rate. However, some factors, such as physical training, age and pathological conditions can alter the VO2 kinetic responses at the onset of exercise. Regarding the control mechanism for exercise VO2 kinetics, 2 opposing hypotheses have been proposed. One of them suggests that the rate of the increase in VO2 at the onset of exercise is limited by the capacity of oxygen delivery to active muscle. The other suggests that the ability of the oxygen utilisation in exercising muscle acts as the rate-limiting step. This issue is still being debated. When exercise is performed at a work rate above LT, the VO2 kinetics become more complex. An additional component is developed after a few minutes of exercise. The slow component either delays the attainment of the steady-state VO2 or drives the VO2 to the maximum level, depending on exercise intensity. The magnitude of this slow component also depends on the duration of the exercise. The possible causes for the slow component of VO2 during heavy exercise include: (i) increases in blood lactate levels; (ii) increases in plasma epinephrine (adrenaline) levels; (iii) increased ventilatory work; (iv) elevation of body temperature; and (v) recruitment of type IIb fibres. Since 86% of the VO2 slow component is attributed to the exercising limbs, the major contributor is likely within the exercising muscle itself. During high intensity exercise an increase in the recruitment of low-efficiency type IIb fibres (the fibres involved in the slow component) can cause an increase in the oxygen cost of exercise. A change in the pattern of motor unit recruitment, and thus less activation of type IIb fibres, may also account for a large part of the reduction in the slow component of VO2 observed after physical training.

Influence of stretch-shortening cycle on mechanical behaviour of triceps surae during hopping

Six subjects performed a first series of vertical plantar flexions and a second series of vertical rebounds, both involving muscle triceps surae exclusively. Vertical displacements, vertical forces and ankle angles were recorded during the entire work period of 60 seconds per series. In addition, expired gases were collected during the test and recovery for determination of the energy expenditure. Triceps surae was mechanically modelled with a contractile component and with an elastic component. Mechanical behaviour and work of the different muscle components were determined in both series. The net muscular efficiency calculated from the work performed by the centre of gravity was 17.5 +/- 3.0% (mean +/- SD) in plantar flexions and 29.9 +/- 4.8% in vertical rebounds. The net muscle efficiency calculated from the work performed by the contractile component was 17.4 +/- 2.9% in plantar flexions and 16.1 +/- 1.4% in vertical rebounds. These results suggest that the muscular efficiency differences do not reflect muscle contractile component efficiency but essentially the storage and recoil of elastic energy. This is supported by the relationship (P less than 0.01) found in vertical rebounds between the extra work and the elastic component work. A detailed observation of the mechanical behaviour of muscle mechanical components showed that the strategy to maximize the elastic work depends also on the force-velocity characteristics of the movement and that the eccentric-concentric work of the contractile component does not always correspond respectively to the ankle extension-flexion.

Asymmetries of oxygen uptake transients at the on- and offset of heavy exercise in humans

1. At work rates which do not result in a sustained increase in blood lactate ([L-]), oxygen uptake (VO2) approaches the steady state with first-order kinetics. However, when [L-] is increased, at least two kinetic components are required to characterize the VO2 response dynamics. The purpose of the present investigation was to determine whether these more-complex kinetics are best represented as: (a) two components which operate throughout the exercise or (b) a delayed slow component which is consequent to the lactic acidaemia and which does not influence the early development of the O2 deficit. 2. Six healthy subjects underwent an incremental ramp test on a cycle ergometer, to the limit of tolerance, for determination of the maximum VO2 (micro VO2) and and estimation [symbol: see text] of the threshold for lactic acidaemia (theta L) non-invasively. Subjects then performed, on different days, two to four repetitions of square-wave exercise from a baseline of unloaded pedalling ('O' Watts (W)) to work rates (WR) less than theta L (90% theta L) and greater than theta L (half-way between theta L and micro VO2). Ventilatory and pulmonary gas exchange variables were determined breath-by-breath. For each subject, the VO2 transitions were averaged prior to fitting a least-squares algorithm to the on- and off-transient responses. 3. The less than theta L test resulted in a mono-exponential VO2 response, with a time constant of 31.3 and 31.5 s for the on- and off-transients, respectively. 4. The VO2 responses to the greater than theta L test were fitted to three competing models: (a) a single exponential for the entire period; (b) a double exponential for the entire period; and (c) an initial single exponential with a subsequent phase of delayed onset. Model (c) yielded a significantly lower residual mean-squares error than methods (a) and (b), with a time constant for the initial component of 40.2 s for the on-transient and 32.9 s for the off-transient and a subsequent phase of VO2 increase for the on-transient which averaged 230 ml min-1. The delta VO2/delta WR for the early kinetics of the greater than theta L test were not different from the less than theta L test (9.6 and 9.5 ml min-1 W-1, respectively). 5. These data suggest that the slow phase of the greater than theta L VO2 kinetics is a delayed-onset process. This being the case, the O2 deficit during heavy exercise, as conventionally estimated, would be overestimated.

Blood lactate during submaximal exercises. Comparison between intermittent incremental exercises and isolated exercises

Values of oxygen consumption, carbon dioxide production, ventilation and blood lactate concentration were determined in eight active male subjects during the minute following submaximal square-wave exercise on a treadmill under two sets of conditions. Square-wave exercise was (1) integrated in a series of intermittent incremental exercises of 4-min duration separated by 1-min rest periods; (2) isolated, of 4- and 12-min duration, and of intensity corresponding to each of the intermittent incremental periods of exercise. For square-wave exercise of the same duration (4 min) and intensity, no significant differences in the above-mentioned parameters were noted between intermittent incremental exercise and isolated exercise. Only at high work rate (greater than 92% maximal oxygen uptake), were blood lactate levels in three subjects slightly higher after 12-min of isolated exercise than after the 4-min periods of isolated exercise. Examination of these results suggests that (1) 80-90% of the blood lactate concentration observed under our experimental conditions results from the accumulation of lactate in the blood during the period of oxygen deficit; (2) therefore the blood lactate concentration/exercise intensity relationship, for the most part, appears to represent the lactate accumulated early in the periods of intermittent incremental exercise.