Zusammenhang zwischen Muskelfaseraktivierung, Sauerstoffkinetik und Levelling-Off der Sauerstoffaufnahme

Introduction) The oxygen uptake (V̇O2)-plateau is defined as a flattening of the V̇O2-workload-relationship in the severe intensity domain. It is the only valid criterion for the diagnosis of maximum oxygen uptake (V̇O2max), which represents the maximum rate of aerobic energy contribution and is one of the most important measurements in exercise physiology. However, depending on the V̇O2-plateau definition and the exercise protocol only 30-70% of the subjects show a plateau at V̇O2max. The incidence of a V̇O2-plateau has been assigned to differences in anaerobic capacity. However, studies on interrelationships between the occurrence of a V̇O2-plateau and measurements of anaerobic capacity showed inconsistent results. This is potentially caused by the fact that exercise tolerance in the severe intensity domain is not only affected by anaerobic capacity but also by V̇O2-kinetics. Differences in V̇O2-kinetics have been related to muscle fibre activation. Furthermore, it has been shown that muscle fibre activation and V̇O2-kinetics is speeded up following an intensive warm-up exercise. Thus, the aim of the present study was to check the effect of muscle fibre activation and V̇O2-kinetics on the incidence of a V̇O2-plateau. For this purpose we tested the following hypotheses: 1) Subjects with a V̇O2-plateau show a faster ramp test V̇O2-kinetics, which is caused in elevated muscle fibre activation. 2) An intensive warm-up exercise leads to elevated muscle fibre activation, a faster ramp test V̇O2-kinetics and an increase in V̇O2-plateau incidence. Methods) To test these hypotheses a cross-sectional and experimental study de-sign was used. This was done by comparing subjects with and without a V̇O2-plateau (hypothesis 1) and by manipulating the muscle fibre activation and V̇O2-kinetics due to an intensive warm-up exercise (hypothesis 2). Additionally, a simulation model was developed to verify and extend the empiric findings. In total, five studies with 9 to 20 male subjects each were performed. Warm-up intensity, recovery duration and incremental rate of the ramp tests were systematically varied between the studies. All exercise tests were performed on a cycle ergometer. V̇O2 was measured using a portable breath-by-breath device. Muscle fibre activation was recorded from Vastus lateralis, Vastus medialis and Gastrocnemius medialis using a surface electromyography system (EMG). V̇O2- and EMG-kinetics were calculated using linear and non-linear regression analyses. The slope of the V̇O2-workload-relationship of the final 50 W was used to quantify the V̇O2-plateau. To simulate the effect of V̇O2-kinetics on the V̇O2-plateau incidence, the simulation model of Wilcox et al. (2016) was added by V̇O2-demand and V̇O2-deficit accumulation. Results) Hypothesis 1: Ten out of the 24 tested subjects of the studies 2-4 showed a plateau in the incremental ramp test. The plateauing subjects had faster ramp test V̇O2-kinetics. More specifically, they showed a shorter delay at the beginning of the ramp test (MRT: 43.3 ± 8.6 vs. 52.8 ± 7.1 s; p = 0.007) and a steeper increase of V̇O2 in the submaximal intensity domain (∆V̇O2/∆P: 10.1 ± 0.2 vs. 9.2 ± 0.5 ml min-1 W-1; p < 0.001). The faster ramp test kinetics was accompanied by lower V̇O2-deficit accumulation up to two minutes before ramp test termination (2.24 ± 0.40 vs. 2.78 ± 0.33l; p = 0.001). However, at ramp test termination the accumulated V̇O2-deficit did not differ between the subjects with and without a V̇O2-plateau (4.34 ± 0.43 vs. 4.54 ± 0.60 l; p = 0.342). Also the EMG-kinetics did not differ between the plateauing and non-plateauing subjects (∆EMG/∆P: 0.56 ± 0.18 vs. 0.56 ± 0.26% W-1; p = 0.940). Hypothesis 2: In none of the five studies a systematically speeding of V̇O2- and EMG-Ramp test kinetics or an increase in V̇O2-plateau incidence were found. Thus, in the studies 1-4 there was no change in ∆V̇O2/∆P (all p > 0.05) and in study 5 ∆V̇O2/∆P was even reduced by the intensive warm-up exercise (10.3 ± 0.7 vs. 9.4 ± 0.7 ml min-1 W-1; p < 0.001). The ∆EMG/∆P and the V̇O2-plateau inci-dence did not differ between the not-primed and the primed condition in all studies (all p > 0.05). Simulation approach: The simulation model confirmed hypothesis 1 also. Thus, the probability of a V̇O2-plateau at a given anaerobic capacity and V̇O2max increases with a faster V̇O2-kinetics. With respect to hypothesis 2, the simulation model indicates that the speeding of V̇O2-kinetics is of insufficient magnitude to induce a systematic increase of ∆V̇O2/∆P and V̇O2-plateau incidence. Conclusion: V̇O2-kinetics but not muscle fibre activation is mayor predictor of the V̇O2-plateau occurrence. However, an intensive warm-up exercise does not systematically increase muscle fibre activation, V̇O2-kinetics and the V̇O2-plateau incidence in a ramp test. This is probably caused in a rather small effect of warm-up exercise on ramp test V̇O2-kinetics, which is accompanied by warm-up induced reduction of anaerobic capacity.