Muscle oxygen kinetics at onset of intense dynamic exercise in humans

Abstract

The present study examined the onset and the rate of rise of muscle oxidation during intense exercise in humans and whether oxygen availability limits muscle oxygen uptake in the initial phase of intense exercise. Six subjects performed 3 min of intense one-legged knee-extensor exercise [65.3 +/- 3.7 (means +/- SE) W]. The femoral arteriovenous blood mean transit time (MTT) and time from femoral artery to muscle microcirculation was determined to allow for an examination of the oxygen uptake at capillary level. MTT was 15.3 +/- 1.8 s immediately before exercise, 10.4 +/- 0.7 s after 6 s of exercise, and 4.7 +/- 0.5 s at the end of exercise. Arterial venous O(2) difference (a-v(diff) O(2)) of 18 +/- 5 ml/l before the exercise was unchanged after 2 s, but it increased (P< 0.05) after 6 s of exercise to 43 +/- 10 ml/l and reached 146 +/- 4 ml/l at the end of exercise. Thigh oxygen uptake increased (P < 0.05) from 32 +/- 8 to 102 +/- 28 ml/min after 6 s of exercise and to 789 +/- 88 ml/min at the end of exercise. The time to reach half-peak a-v(diff) O(2) and thigh oxygen uptake was 13 +/- 2 and 25 +/- 3 s, respectively. The difference between thigh oxygen delivery (blood flow x arterial oxygen content) and thigh oxygen uptake increased (P < 0.05) after 6 s and returned to preexercise level after 14 s. The present data suggest that, at the onset of exercise, oxygen uptake of the exercising muscles increases after a delay of only a few seconds, and oxygen extraction peaks after approximately 50 s of exercise. The limited oxygen utilization in the initial phase of intense exercise is not caused by insufficient oxygen availability.

Interpreting V˙o 2 kinetics in heavy exercise revisited

“…Grassi et al (4) referred to the "early phase of the transient," in which during the first ϳ15 s O 2 extraction was not increased. Also, the data of Bangsbo et al (2) showed that O 2 delivery minus V O 2 increased only during the first ϳ15 s. As our laboratory has previously indicated (9,18), the excess delivery of O 2 within the first 15 s of exercise is to be expected because the muscle pump "indiscriminately" increases flow throughout the muscle and because substrates for oxidative metabolism increase (5). The main point here is that the first 15 s does not constitute the entire transient phase as implied by Burnley and Jones; another approximately five times the time constant (at least 100 s) occurs before the transient phase is complete.…”

“…Thus Tordi et al have no grounds to question the curve-fitting procedures used in previous studies in which subjects performed two to four transitions (2-6, 9, 11), since these studies used methods that were more sensitive than their own. Third, the charge that previous studies lacked statistical power is also incorrect because the sample sizes involved previously were equal or superior to those of Tordi et al Fourth, the primary time constant is unchanged as a result of prior heavy exercise irrespective of whether there are differences in baseline V O 2 (2,4,6,9,11).…”

“…[is] mediated primarily by local control mechanisms" and that "oxygen delivery to the tissues, not blood flow per se, is the controlled variable producing exercise hyperemia" (11), although myogenic, endothelial, and baroreflex factors can also influence blood flow. Studies that have measured O 2 extraction across exercising muscle show that this variable reaches an upper limit relatively early (40-60 s) and that further increases in muscle V O 2 are related to further increases in blood flow (2,8,10,19).…”

Simulating heart rate kinetics during incremental and interval training

“…The analysis of the heart rate response to exercise is a very important tool in the areas of physiology, cardiovascular health, prediction and rehabilitation, as it can detect hidden physiological responses or abnormalities and also provide important information regarding cardiovascular condition (see also [1,2,3,9,11,13,22]). In the area of fitness and sport it can also serve as a fundamental tool for the design of efficient training sessions.…”

“…Various models of the cardiovascular response to exercise have been proposed, mainly based on the statistical goodness of fit of the model curves to heart rate data, starting from the mono-exponential model for moderate constant exercise intensity [11] and continuing to the threephase model [2,3,9,13,18,20,22], which gives the heart rate response to constant intensity exercise by means of three exponential functions. Other models using first order lineal approximations have also been proposed (see [15] and the references therein); however, the implementation of these models is limited, as they can only be applied to exercises of constant intensity.…”