Effects of Auditory Stimuli on Beat-Synchronized Long-Distance Walking: Part B - High Resolution Vigor Response to Music, Metronome, and the Sound of Silence
In an initial phase of this study, a participant stepped in instructed synchrony to two types of auditory stimuli: either to the isochronous beat of a metronome - a presumed neutral stimulus or what might be considered a most rudimentary form of music - or to the beat of a broad-spectrum of single genre 4/4-meter music continuously over a 2-mile forested paved loop track. As in the prior companion study (Shay, 2021), improved resolution and attainment of steady state for walking step length and
... speed were realized by conducting the trials over this long-distance course. In the current study, a further improvement in resolution was realized by using music playlists each at a single BPM tempo, and by conducting the trials at all tempos (1 BPM spacings) in the resonance zone of 110-130 BPM for both auditory stimuli. Like the companion study which was conducted over a wider tempo range of 90-130 BPM with lower resolution playlists and wider tempo spacings, the current study also produced greater step length and speed on music at all 21 tempos examined. The music was more stimulating with an average improvement over metronome of about 7% throughout the tempo range. However, with the improved resolution capability of the current study, several unexpected results were obtained. Most prominent was the observations that mean step lengths and resultant mean speeds were significantly greater at some tempos than others producing spectral peaks and valleys, not just when stepping in synchrony to the music, but also when stepping in synchrony to the metronome. Confirming the studies of others, a strong speed peak was observed with music at 120 BPM (2 Hz), a known frequency of human resonance. In the current study, however, a very prominent peak was also observed at 120 BPM when stepping to the metronome. A related surprising observation was that the peaks for both stimuli were evenly spaced at tempo-cadences of 112, 116, 120, 124, and 128 BPM-SPM – all distant harmonics of 4. Additionally, the tempo location of peaks and valleys for music and metronome were similar – they generally mirrored each other. Although metronome beats have previously been assumed to be neutral or lacking in entrainment stimulation, the metronome beats appeared to induce significant vigor at least at some tempos. Based on those findings, a hypothesis was proposed for a second phase of this current study - walking in absence of any auditory or other repetitive stimulus over the same range of tempos should result in vigor response below that of the metronome. With the aid of accurate pedometers, the participant stepped multiple laps over the same 2-mile course at a variety of self-paced or micro-cued cadences without any auditory or other stimulus. The results provide compelling evidence in support of the hypothesis. The resulting speed-cadence regression curve produced without stimulus was parallel to and below the regression curves of music and metronome. The beat of the metronome does appear to be vigor entraining by some mechanism, and without an auditory stimulus, a reduced vigor response is obtained over the same tempo range that appears to have no structure as suggested by the absence of peaks at 120 BPM and other harmonic 4 tempos.The mechanism by which the metronome seemingly imparts vigor is unknown. Although metronome lacks the musical elements of groove, it is speculated that metronome vigor may be due to beat salience alone which is considered higher than that of most music. Another possibility is that conscious or unconscious mental rhythms are developed during walking while listening to the metronome over long distance providing metrical stimulation. These concepts and others will be examined in a subsequent investigation and companion paper, Part C. The 4 BPM spacing of peaks in the auditory speed spectra are compelling since they appear in both the metronome and music speed-cadence spectra. It is speculated that this phenomenon may be due to harmonics within the human motor system that are more responsive (more efficient for vigor entrainment or biomechanics) at those frequencies with prominence possibly centered around 120 BPM. Further investigations by others into the findings of this study are encouraged to better understand vigor entrainment of the human motor system and potential for advances in exercise performance and the gait rehabilitation of human movement disorders.