Characteristics and coupling of cardiac and locomotor rhythms during treadmill walking tasks

Studying the variability of physiological subsystems (e.g., cardiac and locomotor control systems) has been insightful in understanding how functional and dysfunctional patterns emerge within their behaviors. The coupling of these subsystems (termed cardiolocomotor coupling) is believed to be important to maintain healthy functioning in the diverse conditions in which individuals must operate. Aging and pathology result in alterations to both the patterns of individual systems, as well as to how those systems couple to each other. By examining cardiac and locomotor rhythms concurrently during treadmill walking, it is possible to ascertain how these two rhythms relate to each other in different populations (i.e., younger and older adults) and with varying task constraints (i.e., a gait synchronization task or fast walking task). The purpose of this research was to simultaneously document the characteristics of cardiac and gait rhythms in younger (18-35 yrs) and older (63-80 yrs) healthy adults while walking and to establish the extent to which changes in these systems are coupled when gait is constrained. This study consisted of two repeated-measures experiments that participants completed on two separate days. Both experiments consisted of three 15-minute phases. During the first (baseline) and third (retention) phases of both experiments, participants walked with no cues or specific instructions at their preferred walking speed. During the second phase, participants were asked to synchronize their step falls to the timing of a visual cue (experiment 1) or to walk at 125% of their preferred walking speed (experiment 2). Fifty-one healthy adults (26 older, 67.67±4.70 yrs, 1.72±0.09 m, 70.13±14.30 kg; 25 younger, 24.57±4.29 yrs, 1.76±0.09 m, 73.34±15.35 kg) participated in this study. Participants’ cardiac rhythms (R-R interval time series) and locomotor rhythms (stride interval, step width, and step length time series) were measured while walking on a treadmill. Characteristics of variability in cardiac and locomotor rhythms and the coupling between cardiac and gait rhythms were measured. Results revealed that younger and older healthy adults alter gait patterns similarly when presented with a gait synchronization or fast walking task and that these tasks also alter cardiac patterns. Likewise, both groups exhibited enhanced cardiolocomotor coupling when tasked with a step timing constraint or increased speed during treadmill walking. Combined, these findings suggest that walking tasks likely alter both locomotor and cardiac dynamics and the coupling of physiological subsystems could be insightful in understanding the diverse effects aging and pathology have on individuals

Fractal Gait Patterns Are Retained after Entrainment to a Fractal Stimulus Fractal Gait Patterns Are Retained after Entrainment to a Fractal Stimulus

Abstract Previous work has shown that fractal patterns in gait can be altered by entraining to a fractal stimulus. However, little is understood about how long those patterns are retained or which factors may influence stronger entrainment or retention. In experiment one, participants walked on a treadmill for 45 continuous minutes, which was separated into three phases. The first 15 minutes (pre-synchronization phase) consisted of walking without a fractal stimulus, the second 15 minutes consisted of walking while entraining to a fractal visual stimulus (synchronization phase), and the last 15 minutes (postsynchronization phase) consisted of walking without the stimulus to determine if the patterns adopted from the stimulus were retained. Fractal gait patterns were strengthened during the synchronization phase and were retained in the postsynchronization phase. In experiment two, similar methods were used to compare a continuous fractal stimulus to a discrete fractal stimulus to determine which stimulus type led to more persistent fractal gait patterns in the synchronization and post-synchronization (i.e., retention) phases. Both stimulus types led to equally persistent patterns in the synchronization phase, but only the discrete fractal stimulus led to retention of the patterns. The results add to the growing body of literature showing that fractal gait patterns can be manipulated in a predictable manner. Further, our results add to the literature by showing that the newly adopted gait patterns are retained for up to 15 minutes after entrainment and showed that a discrete visual stimulus is a better method to influence retention

Fractal gait patterns are retained after entrainment to a fractal stimulus.

Previous work has shown that fractal patterns in gait can be altered by entraining to a fractal stimulus. However, little is understood about how long those patterns are retained or which factors may influence stronger entrainment or retention. In experiment one, participants walked on a treadmill for 45 continuous minutes, which was separated into three phases. The first 15 minutes (pre-synchronization phase) consisted of walking without a fractal stimulus, the second 15 minutes consisted of walking while entraining to a fractal visual stimulus (synchronization phase), and the last 15 minutes (post-synchronization phase) consisted of walking without the stimulus to determine if the patterns adopted from the stimulus were retained. Fractal gait patterns were strengthened during the synchronization phase and were retained in the post-synchronization phase. In experiment two, similar methods were used to compare a continuous fractal stimulus to a discrete fractal stimulus to determine which stimulus type led to more persistent fractal gait patterns in the synchronization and post-synchronization (i.e., retention) phases. Both stimulus types led to equally persistent patterns in the synchronization phase, but only the discrete fractal stimulus led to retention of the patterns. The results add to the growing body of literature showing that fractal gait patterns can be manipulated in a predictable manner. Further, our results add to the literature by showing that the newly adopted gait patterns are retained for up to 15 minutes after entrainment and showed that a discrete visual stimulus is a better method to influence retention

Synchronization phase time series for the metronome and stride intervals in Experiment 1.

<p>The fractal pattern of the metronome time series that prescribed the gait patterns is depicted in blue and the actual stride interval time series during the synchronization phase is depicted in red. Although the stride interval time series had greater variability magnitude, similar underlying structure is observed in both time series.</p

Time series of the stimulus and stride intervals in Experiment 1.

<p>The fractal time series used to drive the metronome (A) and one participant's stride interval time series before, during, and after synchronizing with the metronome (B). The mean, standard deviation, and DFA α for each phase is presented. DFA α increased the synchronization phase and remained elevated during the post-synchronization phase.</p

Mean, standard deviation, and DFA α of the stride interval time series in Experiment 1.

<p>A significant decrease in mean (A) and increase in standard deviation (B) was observed during the synchronization phase. The dashed gray line indicates the mean (1.00 sec) and standard deviation (0.07 sec) of the fractal stimulus that was used during the synchronization phase. Error bars represent standard error. Asterisks indicate the sync phase was significantly different relative to the pre- and post-sync phases for mean and standard deviation. A significant increase in DFA α (C) was observed in the synchronization phase, which was retained in the post-synchronization phase. Follow-up analyses showed that the post-synchronization elevated values were not only due to immediate retention. Rather, all three 5 minute epochs in the post-synchronization phase exhibited an elevated DFA α value. The dashed gray line indicates the DFA α value (0.98) of the fractal stimulus that was used during the synchronization phase. Asterisks indicate the sync and post-sync phases were significantly elevated relative to the pre-sync phase, and that the post-sync 1–5, 6–10, and 11–15 phases were not different from each other.</p

Time series of the stimulus and stride intervals in Experiment 2.

<p>The fractal time series used to drive both stimuli (A) and one participant's stride interval time series before, during, and after synchronizing with the discrete stimulus (B) and the continuous stimulus (C). DFA α increased in the synchronization phase with both stimuli, but only remained elevated in the post-synchronization phase when the discrete stimulus was employed.</p

Schematic of the experimental setup in Experiment 2.

<p>While treadmill walking at a self-selected speed, the participants synchronized their heel-strike of each limb with the appearance of a corresponding virtual footprint in the virtual environment that was projected on a screen (A) that consisted of either a discrete (B) or continuous virtual stimulus (C). Both virtual environments contained a moving ground plane, providing optic flow of the environment that closely mimicked the treadmill speed.</p