Relation between lactic acid steady-state and muscle oxygenation in elite cyclists
AbstractBackground & Purpose. In cycling, several physiological values are taken into account to evaluate the performance level of each athlete and/or to plan his training program. One of these values is the maximal power output corresponding to the lactic acid steady-state. To find this power value, it is necessary to test the athlete by an incremental protocol to collect all the physiological parameters necessary to program a subsequent square-wave test, during which it is possible to measure directly this power output. This procedure takes a long time. Bellotti et al. showed that the lactic acid steady-state may be determined by measuring the deoxygenated haemoglobin by NIRS technique in healthy subjects (Bellotti et al., 2013: Med Sci Sports Exerc. 45(6): pp1208-16). We wanted to verify if it is possible, in elite cyclists, to detect the power output corresponding to the lactic acid steady-state using the NIRS technique during a single incremental test.Methods. The experiment was carried on 15 male, elite cyclists participating in international U23 races: 212 years, 1.760.08 m, 66.16.9 kg (avgs, n=15). We used an incremental protocol on a stationary bike (Monark 818 equipped with SRM system, Figure 1 left) consisting on five loads of 360 s each, starting from 0.85 w kg-1 followed by 1w kg-1 increment until the last load at 4.85 w kg-1. Total and oxygenated haemoglobin (tHb and HbO2) were continuously measured in the vastus lateralis of the left quadriceps muscle by NIRS technique (Figure 1 right, NIMO, NIROX, Brescia, Italy), and lactic acid concentration (AL) was measured by Accusport (Bishop, 2011: Sport Med, 22, pp525-530) at 180 s and the end of the last three loads defined as Wlow = 2.970.12 w kg-1, Wmedium =3.930.11 w kg-1 and Whigh = 4.900.14 w kg-1 (avgs, n=15). Pedalling cadence was 1.5÷1.6 Hz. We calculated the time variation of AL (AL/t) and the time rate of tHb and HbO2 (tHb/t and HbO2/t) by a linear interpolation of the NIRS data during the time interval between 180 s and the end of Wlow, Wmedium and Whigh. Twelve participants completed the entire protocol whereas three stopped between 300 and 320 s of Whigh.Results and Discussion. Figure 2 shows a typical experimental record. The subject pedals against the incremental load (continuous line) while muscular oxygenation (dotted line) and AL (point and continuous line) were measured during the last three loads. During exercise at Wlow, all the participants showed an increase both of tHb (upper trace) and HbO2 (intermediate trace) due to peripheral vasodilatation (Grassi et al. 1999: J Appl Physiol; 87, pp348-355). The values of AL at 180 s of Wlow (3.10.8 mM; avgs, n=15) were always greater than AL at the end of Wlow and seemed to be too high for elite athletes. This result may be due to early lactate production; at the end of Wlow, AL decreased to a value compatible with the mechanical power at that moment (1.70.5 mM; avgs, n=15). During exercise at Wmedium the participants reached the AL steady-state: there was no difference between AL measured at 180 s and 360 s (2.30.6 vs 2.70.7 mM; avgs, n=15) and AL/t is 1.792.19 µM s-1 (avgs, n=15). NIRS measurements showed that the muscle oxygenation was about constant: tHb/t and HbO2/t were - 0.00240.0081 µM s-1and -0.00610.0124 µM s-1 respectively (avgs, n=15). Figure 3 shows AL/ttHb/t and HbO2/t plotted as a function of power output; note that the linear function of AL/t (continuous line) and HbO2/t (dotted line) reach zero at the same point very near to a Wmedium. According to Grassi (1999), during and incremental exercise, the onset of lactic acid accumulation is related the beginning of the haemoglobin desaturation. During exercise at Whigh we measured a sharp increase of AL and a decrease of muscle oxygenation: AL/t was 12.768.11 µM s-1 (avgs, n=15) and tHb/t and HbO2/t were -0.00970.0165 µMs-1; and -0.01500.0151 µMs-1 respectively; (avgs, n=15).Figure 4 shows AL/t as a function of HbO2/t. The linear function through all the experimental data passed very close to the axis origin, showing a strong relation between the AL steady-state and the HbO2 steady-state. This point of coincidence is very near to Wmedium.Conclusion. We measured the AL steady-state at 3.75 w kg-1 and the muscular oxygenation steady-state at 3.74 w kg-1. This suggest that it is possible to determine the power output corresponding to the AL steady-state during in incremental protocol, only measuring the muscular oxygenation by NIRS technique.
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