The purpose of the current study was to examine how passive assistance, provided by an exoskeletal device would lead to a change in the dynamics of inter-limb coordination, which could be demonstrated with a reduction in duration of coordination and synchrony of gait. Our results provided evidence in favor of both primary hypotheses—walking with the device showed inter-limb coordination patterns that were of shorter duration and lower regularity than walking without the device. This indicated that assistance from the exoskeleton had potential to make gait more exploratory rather than more restricted. Further, this was achieved without any associated differences in the linear measures (Table 2).
No differences were found in average step length, width, and time between the two groups. The participants walking at their PWS, adapted to the exoskeleton without requiring spatio-temporal adjustments. Walking with a unilateral passive spring-loaded exoskeleton was not a complicated task. Additional exoskeleton studies in healthy13,29 and pathological populations12,17 do not show changes in gait patterns during treadmill walking. Although the design of exoskeletons with unilateral or bilateral placement may have effects on step width, typically it has been found that step width is less likely to be affected by the exoskeleton30,31. This may also be due to the requirement of staying at the center of the treadmill (both anteroposterior and mediolaterally) at constant speed
In our work with stroke survivors, we found that walking with this device for five days changed spatiotemporal patterns, specifically step time, length and width and double support time18. The differences with results here could be due to healthy participants in this study who would be generally more adaptive, had less exposure to the EXO device, and were walking on a treadmill instead of over ground.
Determination of the EMB is an important step towards reconstructing the time series to maximize the information that can be extracted from a complex nonlinear system. This can be further visualized through state space reconstruction (Fig. 4). Walking with the device did not change how many dimensions were needed to maximize the information extracted from the dynamical system. In healthy young individuals, state-space reconstruction of normal treadmill walking showed a tear-drop shape (Fig. 4A), and this structure was maintained even when people walked with the exoskeleton (Fig. 4B).
The radius, however, was larger in the EXO group—to assess the coordination duration between the limbs, a larger radius was need while the recurrence rate was kept to 2.5%. This means that two trajectories were further apart for the EXO group. It can be speculated that human coordination has certain constraints that makes it healthy. A passive exoskeleton assistance may remove these constraints, making people to be more explorative. However, this does not mean that smaller radius causes the duration of coordination to be shorter.
There were no differences in the %DET between the EXO and the NO EXO group. If coordination patterns were highly deterministic as in robotic gait, %DET would have been close to 100%. In this study, we found that %DET was approximately 90% in both groups, demonstrating that inter-limb coordination in healthy young participants was characterized by a high degree of determinism. Specifically, the chances of finding recurrent points forming diagonal lines in the recurrence plot was high, around 90%. A similar level of determinism has been shown in healthy humans when coordination between different physiological systems such as breathing and walking were considered26. When the sensorimotor systems are affected, such as in Parkinson’s disease32 and hypovestibular disorders33, recurrence patterns were demonstrated to be less deterministic in comparison to healthy. In postural tasks, %DET has been shown to be consistently high in healthy participants and were affected by specific tasks such as maintaining balance during high frequency oscillations of the support surface27. In that study, %DET was reduced significantly only when oscillations crossed a specific threshold and became too difficult. In our study, it was shown that addition of a unilateral exoskeletal device did not affect the determinism of recurrence points on the diagonal lines. Therefore, our task of walking at PWS being a simple task and the participants being healthy essentially led to both groups being similarly deterministic.
Walking with the exoskeleton led to a reduced average time that the two limbs were coordinated at PWS compared to those that did not wear the exoskeleton. Theoretically, robotic gait is likely to produce very long mean diagonal lengths because each limb produces periodic sinusoidal movements that repeat perfectly. Contrarily, there is an inherent variability in healthy human dynamical systems34 and such variability is characterized in the recurrence plots (A). When healthy young individuals walked with their limb loaded unilaterally, their inter- and intra-limb coordination (continuous relative phase and cross correlation) were affected such that inter-limb coordination measures reduced24. In our theoretical model, duration of coordination was reduced when walking with the device. Specifically, in Fig. 1, exoskeletal assisted gait guided the system towards the right side the model. This was possibly due to reduced coordination between the limbs allowing for greater exploration, instead of tighter coordination that potentially restricted adaptive flexibility. Alternatively, it is possible that the weight of exoskeleton influenced reduced coordination and teasing out the effect of the device weight would require further investigations.
In pathology such as COPD26, coupling between physiological time series such as breathing and walking became more rigid with longer mean lengths in comparison to healthy participants. This was believed to indicate the reduced coupling between physiological systems in healthy participants allowed for greater variability and essentially stemmed from a need to be more flexible and adaptable. Intuitively, that the changes induced with the passive exoskeleton maybe an indication of more flexible and adaptive behavior that such an artificial system allows. This may provide a window of opportunity to make unhealthy, restrictive gait such as in stroke, more adaptive and flexible.
Further, the exoskeleton-assistance reduced the synchrony between the two limbs during walking at PWS. Jordan et al. (2006) showed that when people walk at PWS, they have the lowest α-value (using detrended fluctuation analysis) in comparison to non-preferred speeds (higher/lower than PWS) where strength of long-range correlations increase35. This has been understood to be the result of an increase in dynamical constraints and therefore, a reduction in the available degrees of freedom as we move away from PWS. In our study, it appeared that adding a spring-loaded, unilateral, passive device enabled the participants to increase these degrees of freedom and walk with a greater explorative ability. Similar to the addition of an exoskeleton, adding sensory stimuli such as vibratory insole tactors36 or auditory feedback37,38 changed dynamical patterns during walking. Taking these results further, we have shown that adding assistive constraints led to changes in inter-limb coordination dynamics specifically to the synchrony of these coordination patterns such that walking became less repeatable and more adaptive.
Our finding indicated that a passive exoskeleton induced more exploration by changing the coordination between the limbs. Such changes could be very important for those patients whose movements are constricted. A passive exoskeleton reducing the recurrent gait patterns (less duration and synchrony) can be an indication of error-based learning39,40. That is, there is a potential that the exoskeleton assistance may remove coordination constraints, allowing you to explore the task and environment more, so the person can learn something new or different by making errors. In many pathological cases such as neurological deficits or sensorimotor deficits, abnormal coordination was shown4,11,41. A passive exoskeleton has the potential to break such restrictive abnormal coordination patterns, allowing patients to learn healthy coordination, helping patients restore their healthy inter-limb coordination. Future work could assess such coordination dynamics during inter-limb coordination tasks such as split-belt adaptation.
In this study, some limitations were identified. One of the limitations was a between-subjects design. A within-subjects approach may provide more information about change in intra-person behaviors. Although it appears everyone adapted to the device, there were no measurements of adaptation. Within-subjects test may also show how a person adapts to an assistive device. Also, treadmill walking is known to alter gait compared to overground walking42. Examining inter-limb coordination on overground walking using nonlinear tools may reveal different effect of the exoskeleton assistance.
For the exoskeleton that was used for this study, special shoes had to be worn, which may have affected the way people walk. However, participants were given five minutes familiarization trial to get adjusted to walking in this device. Additionally, there was no objective way to know the tightness of the tendon. In a related study18, the number of ratchet clicks on the disc at the waist was used to have a generic measure of the exotendon tightness. The EXO group only had an exoskeleton on the one side. It is still unknown whether the reduced duration and synchrony of inter-limb coordination was due to the weight of the device or asymmetry between the limbs. To answer these questions, our future direction should focus on how unilateral and bilateral limb loading changes the specific coordination measurements.
The current investigation focused on how a passive exoskeleton assistance affected the duration and the synchrony of inter-limb gait coordination. It was found that walking with passive assistance decreased the duration and the synchrony of coordination between the limbs. This could indicate that patient populations with abnormal inter-limb coordination could utilize such assistive devices, the assistance could disrupt the abnormal coordination between the limbs, allowing the wearers to explore the environment, and possibly help them to restore healthy coordination.