Critical power as a measure of physical work capacity and anaerobic threshold

Prediction of exhaustion time from heart rate drift

Beaury and Eclache (1978) proposed to extrapolate the drift of the heart rate up to maximal heart rate (Hrmax measured during an incremental maximal test) as a convenient way of estimation of the exhaustion tim (tlim) of an exercise at constant power (75 or 80% of Maximal Aerobic Power (MAP)). The purpose of this study was to evaluate this method of estimation of exhaustion time for a large range of power (60, 73, 86, 100 and 120% MAP). We compared the exercise duration calculated with this method (1limtheo) and the actual exhaustion time (tlim). The results showed that the subjects did not reach their maximal heart rate (Hrmax) at tlim and consequently that tlimtheo, calculated by extrapolation of heart rate drift, overestimated tlim, for all the loads in our study. The difference between tlimtheo and tlim (delta tlim expressed as a percentage of tlim) is significantly lower at 86% MAP than delta tlim at the other loads. It is likely that delta tlim is minimal around 80% MAP, i.e. the loads used in the study by Beaury and Eclache (1978). The values of heart rate (Hrlim), oxygen uptake (VO2lim) and oxygen puls (O2pulslim) measured at exhaustion suggested that the high level of energy cost is one of the main limiting factors at 86% MAP, in contrast with other loads.

A comparison of methods of estimating anaerobic work capacity

The hyperbolic relationship between power output (P) and time to exhaustion (t) is described as: (P-theta PA).t = W', where theta PA is the power asymptote (or 'critical power'), which theoretically represents the highest sustainable power output, and W' is a constant which represents a limited amount of work which can be performed above theta PA (or 'anaerobic capacity'). The purpose of this study was to determine the validity of W' as anaerobic capacity, using maximal oxygen deficit as the criterion measure. Thirteen women (mean (+/- SD) age 23 +/- 2 years, height 1.67 +/- 0.07 m and mass 60.8 +/- 8 kg) and 13 men (mean age 23 +/- 2 years, height 1.74 +/- 0.1 m and mass 71.3 +/- 14 kg) performed two sets of five bouts of high-intensity cycling exercise to exhaustion, each bout on a separate day and at a different relative power. Individual W' were calculated by nonlinear regression of time with power, with time as the dependent variable. Oxygen deficit was determined during each bout, and the mean of the values obtained from the power output which elicited the highest values was used as the criterion for anaerobic capacity. Mean (+/- SE) values for anaerobic capacity for women were 179 +/- 10 J.kg-1 (W') and 177 +/- 10 J.kg-1 (oxygen deficit) and for men, 224 +/- 10 J.kg-1 (W') and 235 +/- 9 J.kg-1 (oxygen deficit). The W' estimates and criterion measures were the same (F1,24 = 0.61, p = 0.44), and there was no method-gender interaction (F1,24 = 0.26, p = 0.62).(ABSTRACT TRUNCATED AT 250 WORDS)

The critical power concept. A review

The basis of the critical power concept is that there is a hyperbolic relationship between power output and the time that the power output can be sustained. The relationship can be described based on the results of a series of 3 to 7 or more timed all-out predicting trials. Theoretically, the power asymptote of the relationship, CP (critical power), can be sustained without fatigue; in fact, exhaustion occurs after about 30 to 60 minutes of exercise at CP. Nevertheless, CP is related to the fatigue threshold, the ventilatory and lactate thresholds, and maximum oxygen uptake (VO2max), and it provides a measure of aerobic fitness. The second parameter of the relationship, AWC (anaerobic work capacity), is related to work performed in a 30-second Wingate test, work in intermittent high-intensity exercise, and oxygen deficit, and it provides a measure of anaerobic capacity. The accuracy of the parameter estimates may be enhanced by careful selection of the power outputs for the predicting trials and by performing a greater number of trials. These parameters provide fitness measures which are mode-specific, combine energy production and mechanical efficiency in 1 variable, and do not require the use of expensive equipment or invasive procedures. However, the attractiveness of the critical power concept diminishes if too many predicting trials are required for generation of parameter estimates with a reasonable degree of accuracy.

Stability of parameter estimates derived from the power/time relationship

The hyperbolic relationship between power output (P) and time to exhaustion (t) is described as t = W'/(P-theta PA). The purpose of this study was to determine the stability of estimations of estimates of theta PA and W', said to reflect maximal sustainable power and anaerobic capacity, respectively. Thirteen women and 13 men performed five bouts of cycling exercise to exhaustion. Individual theta PA and W' were calculated from the results of these five bouts (Trial 1). These procedures were repeated (Trial 2). For both sexes, Trial 2 estimates of theta PA were 5 to 6% higher than Trial 1 estimates, but they were highly correlated. Mean W' estimates were the same in Trials 1 and 2, with higher trial-to-trial correlations in the men than in the women.

Does critical swimming velocity represent exercise intensity at maximal lactate steady state?

The purpose of this investigation was to determine whether the critical swimming velocity (vcrit), which is employed in competitive swimming, corresponds to the exercise intensity at maximal lactate steady state. vcrit is defined as the swimming velocity which could theoretically be maintained forever without exhaustion and expression as the slope of a regression line between swimming distances covered and the corresponding times. A total of eight swimmers were instructed to swim two different distances (200 m and 400 m) at maximal effort and the time taken to swim each distance was measured. In the present study, vcrit is calculated as the slope of the line connecting the two times required to swim 200 m and 400 m. vcrit determined by this new simple method was correlated significantly with swimming velocity at 4 mmol.l-1 of blood lactate concentration (r = 0.914, P < 0.01) and mean velocity in the 400 m freestyle (r = 0.977, P < 0.01). In the maximal lactate steady-state test, the subjects were instructed to swim 1600 m (4 x 400 m) freestyle at three constant velocities (98%, 100% and 102% of vcrit). At 100% vcrit blood lactate concentration showed a steady-state level of approximately 3.2 mmol.l-1 from the first to the third stage and at 98% of vcrit lactate concentration had a tendency to decrease significantly at the fourth stage. On the other hand, at 102% of vcrit, blood lactate concentration increased progressively and those of the third and fourth stages were significantly higher than those at 100% of vcrit (P < 0.05). These data suggest that vcrit, which can be calculated by performing two timed, maximal effort swimming tests, may correspond to the exercise intensity at maximal lactate steady state.

Determination and validity of critical velocity as an index of swimming performance in the competitive swimmer

The purpose of this investigation was to test whether the concept of critical power used in previous studies could be applied to the field of competitive swimming as critical swimming velocity (vcrit). The vcrit, defined as the swimming velocity over a very long period of time without exhaustion, was expressed as the slope of a straight line between swimming distance (dlim) at each speed (with six predetermined speeds) and the duration (tlim). Nine trained college swimmers underwent tests in a swimming flume to measure vcrit at those velocities until the onset of fatigue. A regression analysis of dlim on tlim calculated for each swimmer showed linear relationships (r2 greater than 0.998, P less than 0.01), and the slope coefficient signifying vcrit ranged from 1.062 to 1.262 m.s-1 with a mean of 1.166 (SD 0.052) m.s-1. Maximal oxygen consumption (VO2max), oxygen consumption (VO2) at anaerobic threshold, and the swimming also velocity at the onset of blood lactate accumulation (vOBLA) were also determined during the incremental swimming test. The vcrit showed significant positive correlations with VO2 at anaerobic threshold (r = 0.818, P less than 0.01), vOBLA (r = 0.949, P less than 0.01) and mean velocity of 400 m freestyle (r = 0.864, P less than 0.01). These data suggested that vcrit could be adopted as an index of endurance performance in competitive swimmers.

Physiological responses of young and elderly men to prolonged exercise at critical power

The critical power (CP) of a muscle group or individual may represent the highest rate of work which can be performed for an extended period. We investigated this concept in young (n = 13, 24.5 years) and elderly (n = 12, 70.7 years) active men by first determining CP and then comparing responses elicited by 24 min of cycle exercise at power outputs (omega) corresponding to CP. Values from the final 2 min of the 24-min ride were expressed relative to maximal values established in a ramp test. CP for the elderly was only 65% that for the young, but on a relative basis, it was significantly higher both in terms of omega (67 vs 62% of omega max) and oxygen consumption (VO2) (91.5 vs 85.2% of maximum oxygen consumption). There were no group differences in relative values for ventilation (VE), heart rate or respiratory exchange ratio (R). During the 24-min ride, VO2 and R achieved a plateau in both groups, while VE, blood lactate and arterial PCO2 continued to change in the young. It was concluded that CP can be determined in active elderly men, but that CP may not represent a true non-fatiguing work rate in either young or elderly men.

The relationship between anaerobic threshold and electromyographic fatigue threshold in college women

The purpose of this study was to investigate the relationship between anaerobic threshold (Th(an)) and muscle fatigue threshold (EMGFT) as estimated from electromyographic (EMG) data taken from the quadriceps muscles (vastus lateralis) during exercise on a cycle ergometer. The subjects in this study were 20 female college students, including highly trained endurance athletes and untrained sedentary individuals, whose fitness levels derived from their maximal oxygen consumption ranged from 24.9 to 62.2 ml.kg-1.min-1. The rate of increase in integrated EMG (iEMG) activity as a function of time (iEMG slope) was calculated at each of four constant power outputs (350, 300, 250, 200 W), sufficiently high to bring about muscle fatigue. The iEMG slopes so obtained were plotted against the exercise intensities imposed, resulting in linear plots which were extrapolated to zero slope to give an intercept on the power axis which was in turn interpreted as the highest exercise intensity sustainable without electromyographic evidence of neuromuscular fatigue (EMGFT). The Th(an) was estimated from gas exchange parameters during an incremental exercise test on the same cycle ergometer. The mean results indicated that oxygen uptake (VO2) at Than was 1.39 l.min-1, SD 0.44 and VO2 at EMGFT was 1.33 l.min-1, SD 0.57. There was no significant difference between these mean values (P greater than 0.05) and there was a highly significant correlation between VO2 at Than and VO2 at EMGFT (r = 0.823, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Blood lactate in trained cyclists during cycle ergometry at critical power

The purposes of this investigation were to determine the validity of critical power (CP) as a measure of the work rate that can be maintained for a very long time without fatigue and to determine whether this corresponded with the maximal lactate steady-state (lass,max). Eight highly trained endurance cyclists (maximal oxygen uptake 74.1 ml.kg-1.min-1, SD 5.3) completed four cycle ergometer tests to exhaustion at pre-determined work rates (360, 425, 480 and 520 W). From these four co-ordinates of work and time to fatigue the regression of work limit on time limit was calculated for each individual (CP). The cyclists were then asked to exercise at their CP for 30 min. If CP could not be maintained, the resistance was reduced minimally to allow the subject to complete the test and maintain a blood lactate plateau. Capillary blood was sampled at 0,5,10,20 and 30 min into exercise for the analysis of lactate. Six of the eight cyclists were unable to maintain CP for 30 min without fatigue. In these subjects, the mean power attained was 6.4% below that estimated by CP. Mean blood lactates (n = 8) reached a steady-state (8.9 mmol.l-1 SD 1.6) during the last 20 min of exercise indicating that CP slightly overestimated lass,max, Individual blood lactates during the last 20 min of exercise were more closely related to the gamma-intercept of the CP curve (r = 0.78, P less than 0.05) than either CP (0.34, NS) or mean power output (r = 0.42, NS).

The effects of training on the metabolic and respiratory profile of high-intensity cycle ergometer exercise

The tolerable work duration (t) for high-intensity cycling is well described as a hyperbolic function of power (W): W = (W'.t-1) + Wa, where Wa is the upper limit for sustainable power (lying between maximum W and the threshold for sustained blood [lactate] increase, theta lac), and W' is a constant which defines the amount of work which can be performed greater than Wa. As training increases the tolerable duration of high-intensity cycling, we explored whether this reflected an alteration of Wa, W' or both. Before and after a 7-week regimen of intense interval cycle-training by healthy males, we estimated ( ) theta lac and determined maximum O2 uptake (mu VO2); Wa; W'; and the temporal profiles of pulmonary gas exchange, blood gas, acid-base and metabolic response to constant-load cycling at and above Wa. Although training increased theta lac (24%), mu VO2 (15%) and Wa (15%), W' was unaffected. For exercise at Wa, a steady state was attained for VO2, [lactate] and pH both pre- and post-training, despite blood [norepinephrine] and [epinephrine] ([NE], [E]) and rectal temperature continuing to rise. For exercise greater than Wa, there was a progressive increase in VO2 (resulting in mu VO2 at fatigue), [lactate], [NE], [E] and rectal temperature, and a progressive decrease for pH. We conclude that the increased endurance capacity for high-intensity exercise following training reflects an increased W asymptote of the W-t relationship with no effect on its curvature; consequently, there is no appreciable change in the amount of work which can be performed above Wa.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between the 4 mmol running velocity, the time-distance relationship and the Léger-Boucher's test

The relationship between distance and best time is roughly linear for distances between 1500 and 5000 m. The slope of this relationship has the dimension of a velocity (Vlim) which can be sustained during a long time. The individual time-distance relationships and the resulting Vlim have been studied in 32 subjects practicing different athletic activities by measuring exhaustion time for 2 to 4 constant-velocity running exercises performed to exhaustion. The velocity corresponding to 4 mmol.l-1 of blood lactate (V4 mmol) has been compared with Vlim. As maximal oxygen uptake is a major factor determining V4 mmol, Vlim and V4 mmol have also been correlated with the result of a field test which is assumed to measure maximal aerobic power (Léger-Boucher's test). This test consists in running until exhaustion at a velocity which increases every two minutes. The higher the velocity at exhaustion (Vléger) is, the higher the maximal oxygen uptake is assumed. Both Vlim and Vléger were very well correlated with V4 mmol (r greater than 0.90) and the average value of Vlim was almost equal to the average value of V4 mmol (13.89 vs 13.71 km.h-1). However, it was not possible to estimate V4 mmol accurately from the values of Vlim or Vléger because the standard errors of estimates were too large.

Comparison between a 30-s all-out test and a time-work test on a cycle ergometer

The relationship between the amount of work (Wlim) performed at the end of constant-power exhausting exercise and exhaustion time (tlim) has been studied for supramaximal exercise [105%, 120%, 135% and 150% of the individual maximal aerobic power, (MAP)] performed on a Monark cycle ergometer in nine men. The Wlim--tlim relationship was described by a linear relationship (Wlim = a + b . tlim). Intercept a was roughly equivalent to the work produced during a 1-min exercise performed at MAP. Slope b was equal to 79% of MAP. Intercept a has been correlated with the total amount of work (AW) performed during a 30-s all-out test supposed to assess anaerobic capacity. Intercept a was significantly (p less than 0.05) correlated with AW. The anaerobic capacity was not depleted at the end of the all-out test, as the mechanical power at the 30th s of this test was approximately equal to twice MAP. However, AW was significantly higher than intercept a. It was likely that the value of intercept a was an underestimation of the maximal anaerobic capacity because of the inertia of the aerobic metabolism. Indeed, an exponential model of the Wlim--tlim relationship, which takes the interia of the aerobic metabolism into account, shows that a linear approximation of the Wlim--tlim relationship yields a systematic underestimation of the anaerobic capacity. Consequently, intercept a of the Wlim--tlim relationship is not a more accurate estimation of the anaerobic capacity than the AW performed during a 30-s all-out test.(ABSTRACT TRUNCATED AT 250 WORDS)