Variability in Hopping is Largely Individual and Reduced by a Metronome

Understanding human movement deepens our knowledge of motor control, which informs the development of more effective rehabilitation techniques and assistive tech-nologies. To investigate the variability in repetitive hopping, we carried out a sequence of experiments. Our study centers on the idea that when subjects are allowed to choose their hopping characteristics, such as frequency and height, it will result in observable differences in several parameters. These include the ground reaction force (both in pattern and peak value), the hopping time, and the duty factor, which varies across subjects and also for each individual. Additionally, we found that the introduction of a metronome as an external control will lead to a decrease in the intra-subject variability. The rationale is that the metronome's consistent rhythm imposes external pacing, potentially leading to more uniform hopping characteristics compared to when subjects hop without such rhythmic guidance. In our experiments, we also directed the subjects to hop at four different frequencies. Our results revealed insignificant variability from 100% to 150% of Preferred Hopping Frequency (PHF), while both (inter-subject) variation in strategies among participants in adjusting the frequency and (intra-subject) variability in hopping metrics of each subject were observed at 75% PHF compared to higher frequencies.