Changes in VO2 Kinetics, Efficiency and Muscle Activity after a Prolonged Road Cycling Race

Abstract

Increasing evidence suggests that the recruitment of additional muscle motor units and/or the progressive recruitment of less efficient fast-twitch fibres play a role in the aetiology of the oxygen uptake (VO2) slow component. However, little is known on the effects of prolonged exercise on VO2 kinetics, efficiency and muscle activity (EMG). PURPOSE: To determine if a prolonged road cycling race led to important changes in VO kinetics, efficiency and EMG. METHODS: Nine trained cyclists (age: 40.0±7.6 yrs; training amount: 11778±5044 km·yr−1) performed maximally over a hilly road race (140 km; total elevation: 1750 m; duration: 277.1 ±22.7 min). Prior and immediately after the race, they performed a 6-min exercise on a Monark ergometer (model 810E, Monark-Crescent AB, Varberg, Sweden) at the same constant intensity (258±37 W; >ventilatory threshold) and pedaling cadence (80 rpm). Expired gas for calculation of cycling gross efficiency was collected continuously (Cosmed K4b2, Rome, Italy). The VO2 response was fit with a model including two exponential functions (primary and slow component phases). EMG signals were taken from the vastus lateralis (Bagnoli 8-EMG system, Delsys, Boston, MA) and analyzed to determine root mean square (RMS) and mean power frequency (MPF). RESULTS: From prior to after the race, a tendency toward higher amplitude values of the primary phase (26.7±11.5 vs. 34.9±5.6 ml·min−1. kg−1, p=0.06) was found with no change in time delay (1.8±0.3 vs. 1.4±0.7 s) and time constant (26.4±17.8 vs. 23.6±7.4 s). There was no change in the slow component phase (amplitude: 9.1±5.3 vs. 8.6±3.3 ml·min-1.kg−1; time delay: 118.2±35.7 vs. 147.3±35.6 s; time constant: 189.5±89.4 vs. 168.5±79.5 s) after the race. From prior to after the race, a tendency (p=0.13) toward a reduced gross efficiency (22.4±3.7 vs. 19.9±0.6%, −11%) was shown. Mean RMS (1.14±0.50 vs. 1.53±0.64mV, p<0.01) and mean RMS/VO2(−32%, p<0.05) were lower after than prior the race while mean MPF was unchanged (127.2±7.7 vs. 127.0±15.4 Hz, p<0.05). CONCLUSION: It is assumed that the glycogen depletion in the type I muscles fibres and the shift in substrates induce the observed excess in oxygen consumption and the lower gross and neuromuscular efficiencies observed. However, it did not appear to be directly related to the recruitment pattern of motor units.