Abstract: BACKGROUND: Running efficiency is considered to be improved by increasing step frequency, which is a popular idea in long-distance running. However, this requires scientific evaluation and research in many aspects. 
OBJECTIVE: To compare the changes of heart rate, oxygen uptake, surface electromyography, and gait characteristics in individuals with preferred step frequency and varying step frequency under anaerobic threshold intensity, and to explore the effect of changing step frequency on runners, so as to provide scientific strategies for improving running efficiency.
METHODS: Twelve male marathon runners were enrolled in this trial. Before the beginning of the trial, each runner was tested in an incremental exercise test on the treadmill to obtain the values of ventilatory anaerobic threshold and anaerobic threshold speed. After the beginning of the trial, a random crossover design was used. The runners ran on the treadmill at the anaerobic threshold with preferred step frequency, preferred step frequency ±5%, and preferred step frequency ±10%. Paired t-test and Pearson correlation analysis were used to statistically analyze heart rate, oxygen uptake, surface electromyography, and gait characteristics, and to explore the changing laws of each indicator after changing the step frequency. The implementation of this study complied with the relevant ethical requirements of Beijing Sport University, and the subjects were fully informed of the trial process. 
RESULTS AND CONCLUSION: Heart rate and oxygen uptake: after changing the step frequency, the heart rate was increased significantly (P < 0.01), stride length changed significantly (P < 0.01), and oxygen uptake was increased significantly when preferred step frequency was -5% (P < 0.05) and ±10% (P < 0.01). There was a positive correlation between stride length and oxygen uptake (r=0.57, P < 0.01), and a negative correlation between step frequency/stride length and oxygen uptake (r=-0.42, P < 0.01). Surface electromyography: when preferred step frequency was +10%, the mean power frequency of biceps femoris, rectus femoris, and medial femoris was increased significantly (P < 0.05); when preferred step frequency was -5% and -10%, the mean power frequency of tibialis anterior muscle was increased (P < 0.05); and when preferred step frequency was +5%, the muscle contribution rate of rectus femoris was decreased significantly (P < 0.05). Gait characteristics: the ground contact time (P < 0.05) and the flight time (P < 0.01) were decreased while the ratio of ground contact time to flight time was increased (P < 0.05). When preferred step frequency was +5%, reduced knee joint extension range was 3o-4o (P < 0.05). When preferred step frequency was +10%, reduced knee joint extension range was 3o-4o (P < 0.05), reduced hip joint extension range was 3o (P < 0.05), and the range of the knee joint that swings forward was reduced by 4o (P < 0.01). As the step frequency was decreased, the ground contact time and the flight time were increased gradually, and the ratio of ground contact to flight time was decreased (P < 0.05). There was a negative correlation between ground contact time (r=-0.71, P < 0.01) and ground contact time/flight time ratio (r=-0.67, P < 0.01) and oxygen uptake, and a positive correlation between flight time and oxygen uptake (r=0.65, P < 0.01). Although no energy is saved immediately in individuals who increase the step frequency, which may require long-term training or adaptation, the energy expenditure is relatively lower in the study population when running with a shorter stride and at a larger ratio of step frequency/stride length. When running with a shortened stride, the ratio of ground contact to flight time is increased and the motion range of the hip and knee joint is reduced, which can make the body more stable and reduce the load on the knee joint.

Key words: anaerobic threshold, step frequency, energy consumption, surface electromyography, gait characteristics