PURPOSE: Anaerobic threshold (AT) has been defined as the level of exercise at which a change from aerobic to partially anaerobic energy supply is supposed to occur. Even though lactate [La] accumulation is expected at the work loads above AT, accumulation rate differs between individuals. Beside the alteration of metabolic pathways, progressive recruitment of fast twitch fibers may explain the changes in [La] accumulation rate. In this study we aimed to investigate the relationship between AT and mean frequency (MF) and Root Mean Square (RMS) parameters of surface electromyogram (SEMG) during incremental cycling. METHODS: 8 male subjects aged 23.5 ± 1.4 (mean ± SE) underwent one minute incremental cycling exercise to determine peak VO2. Predetermined loads are added every minute up to voluntarily exhaustion. Anaerobic threshold was calculated according to V-slope method. The SEMG activities were recorded by bipolar surface electrodes from the left rectus femoris (RF), vastus medialis (VM), gastrocnemius lateralis (GL) and gastrocnemius medialis (GM). RMS and MF data were averaged for every work load. RMS values were normalized with VO2 at the initial, AT and maximal level of exercise for each individual. Repeated measures and paired-t test were used to analyze the SEMG data. RESULTS: Peak values obtained during the exhausting work load (281 ± 15 W) for ventilatory, metabolic and cardiovascular parameters were VE, 156.6 ± 9 l/min; VO2, 3.59 ± 0.2 l/min (50.01 ± 3.17 ml.kg−1.min−1); VCO2, 4.27 ± 0.21 l/min; HR, 187 ± 3 beats/min. Changes in MF and RMS/VO2 values for initial, AT and maximal steps of exercise are given in Figures 1 and 2.(FIGURE 1 and 2 Caption not available. pls. insert.)FigureFigureCONCLUSIONS: Presence of high and low MF may indicate the different contractile and excitation characteristics of muscle fibers together with motor unit type. Since the MF value did not increase during the incremental exercise (with the exception of VM AT value), it is possible to argue the absence of fiber recruitment with various firing characteristics. The significant elevation of RMS/VO2 in VM and RF beyond AT may indicate the addition of new motor units with similar contractile properties. This may be an important factor to discuss the possible causes of [La] accumulation.