Maximal lactate steady state, critical power and EMG during cycling

Abstract.

We hypothesised that: (1) the maximal lactate steady state (MLSS), critical power (CP) and electromyographic fatigue threshold (EMG_FT) occur at the same power output in cycling exercise, and (2) exercise above the power output at MLSS (P-MLSS) results in continued increases in oxygen uptake (V˙O₂), blood lactate concentration ([La]) and integrated electromyogram (iEMG) with time. Eight healthy subjects [mean (SD) age 25 (3) years, body mass 72.1 (8.2) kg] performed a series of laboratory tests for the determination of MLSS, CP and EMG_FT. The CP was determined from four exhaustive trials of between 2 and 15 min duration. The MLSS was determined as the highest power output at which the increase in blood [La] was less than 1.0 mM across the last 20 min of a series of 30-min trials. The EMG_FT was determined from four trials of 2 min duration at different power outputs. The surface electromyogram was recorded continuously from the vastus lateralis muscle. The CP was significantly higher than the P-MLSS [242 (25) vs. 222 (23) W; P<0.05], although the two variables were strongly correlated (r=0.95; P<0.01). The EMG_FT could not be determined in 50% of the subjects. Blood [La], V˙O₂ and minute ventilation all increased significantly with time for exercise at power outputs above the P-MLSS. In conclusion, the P-MLSS, and not the CP, represents the upper limit of the heavy exercise domain in cycling. During exercise above the P-MLSS, there is no association between changes in iEMG and increases in V˙O₂ and blood [La] with time.