Our results show that the relative phase in the final was more concentrated around 0° than in the preliminaries and the other combinations. This suggests that there was some interaction between the two sprinters in the final when they ran side-by-side, possibly resulting in phase locking.
A phase difference of 0° (in phase) is considered as the most stable point in the Haken-Kelso-Bunz (HKB; Haken et al., 1985) model of biological coordination, and the periodic movements of two individuals tend to stabilize in phase in interpersonal coordination tasks 22,23. In the time series of relative phases reported here, the tendency to converge to 0° in the final in which the two sprinters ran side-by-side was greater than in other control combinations, indicating that a synchronization may have occurred. Furthermore, the localization of the relative phase around 0° in the final was stronger than in the other control conditions. This indicates that the shift in the relative phase distribution from the preliminaries to the final was not simply speed-dependent (i.e., not due to increased speed), suggesting that an interaction between the two sprinters in the final may have caused phase locking.
Interpersonal synchronization has been reported for a variety of movements such as clapping 24, foot swinging 25, hand pendulum 26, rocking chair 23, postural movement 27–29, competitive tapping 30, and side-by-side walking 15,19,31,32. The present study supports that spontaneous interpersonal synchronization is a ubiquitous phenomenon that also occurs in non-laboratory sport competitive settings.
Importantly, growing evidence shows that auditory rhythmic stimulation can powerfully modulate and enhance human gait and running performance. Synchronization with auditory stimuli such as music or simple metronomes during exercise can improve contraction efficiency of active muscles 33, metabolic cost 34, stability of interlimb coordination 35–37, stride interval stability 38, and running performance 34,38,39. In particular, it has been reported that auditory stimulation synchronized with the runner's stride interval improved performance in 400 m, which is classified as a sprint race 39. When two sprinters run side by side each other footsteps might become auditory rhythmic stimulation influencing and enhancing performance. In the present study the footsteps tempo of Yamagata and Tada were very close in their stride interval, and each other footstep sounds might have become useful synchronized auditory stimuli that contributed to the historical records in the final.
Interpersonal synchronization and changes in performance might have also been supported by visual information. Interpersonal synchronization occurring through visual information has been shown to influence individual motor performance 28,29. In the case of Yamagata and Tada who ran side-by-side, synchronization between the sprinters may have occurred via visual information exchanged peripherally, which may have affected their running performance. Further research will be needed to clarify the exact nature of sensory information that could potentially modify and improve individual performance in maximum effort exercises such as a 100-m race.
4.2 Limitations and Future work
These were only case observations, and the phenomenon of synchronization among top sprinters has not been statistically proven. To scientifically elucidate this phenomenon, it is necessary to analyze the running performance of sprinters in various races in the future
In this analysis, we cannot deny the possibility that noise caused by camera oscillation may have distorted the peak time of the right ankle y-coordinate displacement and affected the relative phase. Although we tried to deal with such data drift and noise as much as possible, the above limitation cannot be eliminated in the analysis of videos shot in a natural environment. In the future, we will be able to evaluate the movements of actual athletes, unaffected by data drift or other factors that distort results from ordinary private video cameras by placing objects of defined size in the video and quantifying and eliminating the effects of camera movement.
It can also be noted that other factors (changes in arousal level, attention, and others) may have contributed to the two sprinters' historic records, independent of the interpersonal synchronization.
Furthermore, the findings of this study provide important suggestions for coaching athletes. Even in track and field running, which has been regarded as an individual sport, each runner is affected by interpersonal influences. Among these interpersonal influences, synchronization, which is difficult to perceive, can affect performance without the athlete’s conscious awareness. This study provides athletes and coaches with a new perspective of synchronization as an interpersonal influence. By clarifying the relationship between interpersonal synchrony and performance, it may be possible to improve performance through training and strategic use of synchronization.