Is the exhaustion time at maximal aerobic speed an index of aerobic endurance?

The velocity of the last stage of the Montreal Track Test (MTT) has been measured in fifteen well trained runners. This velocity (vMTT) was assumed to be close to maximal aerobic running speed. In three different sessions, the subjects ran up to exhaustion at velocities corresponding to 95, 100 and 105% vMTT. The exhaustion time at 100 % vMTT (tlim100) was assumed to be an estimation of the exhaustion time corresponding to maximal aerobic speed. The relationship between exhaustion time (tlim) and distance (Dlim) in the case of running exercises at constant velocity until exhaustion can be described by a linear relationship (Dlim = D + b*tlim). The slope of the relation corresponds to a velocity (vcrit) which can be sustained for a long time. The values of vcrit were calculated from the results of running exercises performed at 95, 100 and 105% of vMTT. The present study showed that tlim at 100% vMTT (tlim100) was negatively correlated with vMTT and vcrit but that D and ratio vcrit/vMTT were independent of vMTT. A theoretical study based on models previously proposed for oxygen kinetics during supramaximal exercises (exponential model and Margaria's model) demonstrates that this negative relationship between vMTT and tlim100 can be explained by the kinetics of the accumulation of oxygen deficit (O2 def). tlim100 should also depend on VO2max, maximal oxygen deficit (Max O2 def) and the relative importance of anaerobic energy when a VO2 plateau is reached. Moreover, the value of tlim100 largely depends on the accuracy of the assessment of vMTT. Consequently, the exhaustion times corresponding to the different estimations of maximal aerobic speed on a track or a treadmill cannot be considered as valid indices of aerobic endurance.

Critical velocity of continuous and intermittent running exercise. An example of the limits of the critical power concept.

The relationship between exhaustion time (tlim) and distance Dlim for running exercises at constant velocity until exhaustion can be described by a linear relationship (Dlim = a + b tlim) whose slope corresponds to a critical velocity. Seven runners participated to the study which compared the critical velocity of continuous versus intermittent running exercises. The critical velocity for continuous running (Vcritc) was calculated from the results (tlimc and Dlimc) of running exercises performed at 95 and 105% of the final velocity of the Montreal Track Test (vMTT). The intermittent running consisted of repetitions of running exercises performed at 95 and 105% vMTT during a time equal to half the value of the corresponding tlimc. The subjects recovered during a time equal to running time while jogging at a slow pace. The critical velocity for intermittent running (Vcriti) was calculated from the cumulated running distance (Dlimi) and cumulated running time (tlimi) corresponding to 95 and 105% vMTT. Vcriti was equal to Vcritc (4.56 +/- 0.444 m.s-1 vs 4.60 +/- 0.416 m.s-1). Nevertheless, in some subjects, the repetition numbers were very different for the intermittent running exercises at 95 and 105% vMTT. This paradoxical result could be explained by the fact that the value of Vcrit should be theoretically little sensitive to a large error in the value of tlim corresponding to a velocity slightly higher than critical velocity, for intermittent exercises as well as continuous exercises.

Variability in energy cost of running at the end of a triathlon and a marathon.

The aim of this study was to investigate the increase in energy cost of running occurring at the end of a triathlon and a marathon event and to link them to the metabolic and hormonal changes, as well as to variations in stride length. Seven subjects took part in 3 experimental situations: a 2 h 15 min triathlon (30 min swimming, 60 min cycling and 45 min running), a 2 h 15 min marathon (MR) were the last 45 min were run at the same speed as the triathlon run (TR), and a 45 min isolated run (IR) done at triathlon speed. The results show that energy cost during MR was higher than during TR (p < 0.01) (+ 8.9%). Similar observations were made for pulmonary ventilation (+ 7.9%) and heart rate (+ 6.3%). Moreover, the values were significantly greater than the values obtained during the IR. TR and MR lead to greater weight loss (p < 0.01) (2.4 +/- 0.3 kg) than IR (1 +/- 0.2 kg). The triathlon and the marathon produced a large decrease in plasma volume (respectively 19.6 +/- 1.4% and 12.9 +/- 1.1%) compared to IR (2 +/- 0.4%). Plasma renin activity was higher for the triathlon and the marathon than for the IR (p < 0.01). MR produces a significantly greater increase in plasma free fatty acids (F.F.A.) than TR (p < 0.05) and IR (p < 0.01). In addition, the F.F.A. at the end of TR were significantly higher than IR (p < 0.05). At the end of the trial the mean stride lengths for TR and IR were greater (+ 15%) (p < 0.01) than for MR. This study, carried out with subjects running overground, confirms the decrease in running efficiency previously shown at the end of a laboratory triathlon, and demonstrates that this decrease is lower than that occurring during a marathon.