Little is known about the mechanism that improves the capabilities of athletes by high, intensity interval training (HIT). This study was conducted to determine the neurophysiological changes due to HIT. Changes in surface electromyography (SEMG) in well-trained endurance cyclists due to the training were identified. Seven subjects (maximal oxygen uptake [VO2max] 64.6 ±4.8 ml.kg-1.min-1, mean ± SD) undertook a 3 week training intervention, replacing ∼ 15% of their weekly endurance training with 6 sessions of laboratory-based HIT (8 × 5 min work bouts at 82% of PPO [∼85% VO2max], with 60 sec active recovery at 100 W). SEMG was used to assess neuromuscular changes before and after the 3 wk training program. During the first and sixth training session, SEMG was recorded. To determine the effects of the HIT program on performance, subjects performed a 40 km time trial (TT40) before and after the training intervention. The frequency of SEMG is a measure of the muscle fatigue and hence was used to identify the variation of the signal properties. Three weeks of intensified training decreased the mean power frequency of the SEMG signal during the latter stages of HIT (interval seven) 50.2 ± 5.1 to 47.5 ± 4.2 Hz (P < 0.05). The preliminary conclusions of these experiments suggest that high-intensity interval training enhanced endurance performance and reduced the fatiguing of the muscles. It is suggested that this was possibly due to recruitment of addition slow-twitch motor units.