Detecting temporal reversals in human locomotion

Effectiveness of backward gait training on body composition, cardiopulmonary fitness, inflammation, and metabolic marker in adults: A systematic review and meta-analysis

Walking is a fundamental physical activity with significant health implications. Backward gait training (BGT) has emerged as a novel approach with potential benefits, yet its effects in comparison to traditional forward gait training (FGT) remain uncertain. This systematic review and meta-analysis aimed to evaluate the effects of BGT on body composition, cardiopulmonary fitness, and inflammatory and metabolic markers in adults. A comprehensive search across electronic databases was conducted following the Preferred Publishing Items for Systematic Reviews and Meta-Analyses guidelines. Randomized clinical trials (RCTs) comparing BGT with FGT in adults were included. Methodological quality was assessed using the Cochrane risk-of-bias tool. The certainty of evidence was evaluated using the Grading of Recommendation, Assessment, Development, and Evaluation approach. The analysis included a total of 379 male participants across the studies. The meta-analysis demonstrated significant changes in body composition and inflammatory marker outcomes, which included waist-to-height ratio (standardized mean difference [SMD]-1.18, 95% confidence interval [CI]-1.89-0.48, I2 = 83%, P < 0.01), body mass index (SMD-0.55, 95% CI-0.77-0.32, I2= 0%, P < 0.01), and C-reactive protein (SMD-0.98, 95% CI-1.28-0.70, I2= 0%, P < 0.01). In addition, the qualitative review revealed potential enhancements in cardiopulmonary fitness and metabolic markers following BGT. While the results suggest potential benefits of BGT on body composition and inflammatory markers, the evidence remains limited and heterogeneous. Further robust research with diverse populations, longer intervention periods, and comprehensive outcome assessments is essential to elucidate the true impact of BGT and its utility for promoting overall health and well-being in adults.

The effect of backward walking observational training on gait parameters and balance in chronic stroke: randomized controlled study

BACKGROUND

Backward walking (BW) and action observation training may potentially help people at risk of falls. Moreover, action observation training could be a potential intervention to improve gait after a stroke.

AIM

We aimed to identify the effects of BW action observational training (BWOT) on gait parameters and balance in chronic stroke patients.

DESIGN

Randomized, controlled study.

SETTING

Rehabilitation center.

POPULATION

Twenty-four chronic stroke patients were randomly assigned to BWOT (N.=12) and landscape observational training (LOT) (N.=12) groups.

METHODS

The BWOT group performed BW after watching a video of a BW, while the LOT group performed BW training after watching a video of a landscape. Both groups received traditional therapy for 5 days per week and BWOT for 3 days a week for 4 weeks. The primary and secondary outcomes were gait and balance, respectively. Static balance was measured using the 5 Times Sit-To-Stand Test (5TSTS), the center of pressure (COP) displacement, and weight distribution (WD) of the affected side. Dynamic balance was measured using the activity-specific balance confidence (ABC) scale.

RESULTS

The BWOT group showed significant improvements in gait velocity (p=0.001, η²=0.470), step length (P=0.007, η²=0.313), stride lengths (P<0.002, η²=0.431), 5TSTS (P=0.021, η²=0.231), COP velocity (P=0.022, η²=0.226), length (P=0.001, η²=0.504), WD of the affected side (P=0.033, η²=0.193), and ABC score (P=0.023, η²=0.226) than the LOT group.

CONCLUSIONS

The 4-week BWOT training program significantly improved the gait parameters and static and dynamic balance in stroke patients.

CLINICAL REHABILITATION IMPACT

BWOT is an accessible and effective method of rehabilitation training that can also be applied to conventional therapy as a useful method for improving the gait and balance after stroke.

Forward and backward walking share the same motor modules and locomotor adaptation strategies

Forward and backward walking are remarkably similar motor behaviors to the extent that backward walking has been described as a time-reversed version of forward walking. However, because they display different muscle activity patterns, it has been questioned if forward and backward walking share common control strategies. To investigate this point, we used a split-belt treadmill experimental paradigm designed to elicit healthy individuals' motor adaptation by changing the speed of one of the treadmill belts, while keeping the speed of the other belt constant. We applied this experimental paradigm to both forward and backward walking. We analyzed several adaptation parameters including step symmetry, stability, and energy expenditure as well as the characteristics of the synergies of lower-limb muscles. We found that forward and backward walking share the same muscle synergy modules. We showed that these modules are marked by similar patterns of adaptation driven by stability and energy consumption minimization criteria, both relying on modulating the temporal activation of the muscle synergies. Our results provide evidence that forward and backward walking are governed by the same control and adaptation mechanisms.

Backward Walking Induces Significantly Larger Upper-Mu-Rhythm Suppression Effects Than Forward Walking Does

Studies have compared the differences and similarities between backward walking and forward walking, and demonstrated the potential of backward walking for gait rehabilitation. However, current evidence supporting the benefits of backward walking over forward walking remains inconclusive. Considering the proven association between gait and the cerebral cortex, we used electroencephalograms (EEG) to differentiate the effects of backward walking and forward walking on cortical activities, by comparing the sensorimotor rhythm (8–12 Hz, also called mu rhythm) of EEG signals. A systematic signal procedure was used to eliminate the motion artifacts induced by walking to safeguard EEG signal fidelity. Statistical test results of our experimental data demonstrated that walking motions significantly suppressed mu rhythm. Moreover, backward walking exhibited significantly larger upper mu rhythm (10–12 Hz) suppression effects than forward walking did. This finding implies that backward walking induces more sensorimotor cortex activity than forward walking does, and provides a basis to support the potential benefits of backward walking over forward walking. By monitoring the upper mu rhythm throughout the rehabilitation process, medical experts can adaptively adjust the intensity and duration of each walking training session to improve the efficacy of a walking ability recovery program.

The Spinal Control of Backward Locomotion

Animal locomotion requires changing direction, from forward to backward. Here, we tested the hypothesis that sensorimotor circuits within the spinal cord generate backward locomotion and adjust it to task demands. We collected kinematic and electromyography (EMG) data during forward and backward locomotion at different treadmill speeds before and after complete spinal transection in six adult cats (three males and three females). After spinal transection, five/six cats performed backward locomotion, which required tonic somatosensory input in the form of perineal stimulation. One spinal cat performed forward locomotion but not backward locomotion while two others stepped backward but not forward. Spatiotemporal adjustments to increasing speed were similar in intact and spinal cats during backward locomotion and strategies were similar to forward locomotion, with shorter cycle and stance durations and longer stride lengths. Patterns of muscle activations, including muscle synergies, were similar for forward and backward locomotion in spinal cats. Indeed, we identified five muscle synergies that were similar during forward and backward locomotion. Lastly, spinal cats also stepped backward on a split-belt treadmill, with the left and right hindlimbs stepping at different speeds. Therefore, our results show that spinal sensorimotor circuits generate backward locomotion but require additional excitability compared with forward locomotion. Similar strategies for speed modulation and similar patterns of muscle activations and muscle synergies during forward and backward locomotion are consistent with a shared spinal locomotor network, with sensory feedback from the limbs controlling the direction.SIGNIFICANCE STATEMENT Animal locomotion requires changing direction, including forward, sideways and backward. This paper shows that the center controlling locomotion within the spinal cord can produce a backward pattern when instructed by sensory signals from the limbs. However, the spinal locomotor network requires greater excitability to produce backward locomotion compared with forward locomotion. The paper also shows that the spinal network controlling locomotion in the forward direction also controls locomotion in the backward direction.

Weight estimations with time-reversed point-light displays

Interpreting other’s actions is a very important ability not only in social life, but also in interactive sports. Previous experiments have demonstrated good estimation performances for the weight of lifted objects through point-light displays. The basis for these performances is commonly assigned to the concept of motor simulation regarding observed actions. In this study, we investigated the weak version of the motor simulation hypothesis which claims that the goal of an observed action strongly influences its understanding (Fogassi, Ferrari, Gesierich, Rozzi, Chersi, & Rizzolatti, 2005). Therefore, we conducted a weight judgement task with point-light displays and showed participants videos of a model lifting and lowering three different weights. The experimental manipulation consisted of a goal change of these actions by showing the videos normal and in a time-reversed order of sequence. The results show a systematic overestimation of weights for time-reversed lowering actions (thus looking like lifting actions) while weight estimations for time-reversed lifting actions did not differ from the original playback direction. The results are discussed in terms of motor simulation and different kinematic profiles of the presented actions.

Timing is everything: Dance aesthetics depend on the complexity of movement kinematics

What constitutes a beautiful action? Research into dance aesthetics has largely focussed on subjective features like familiarity with the observed movement, but has rarely studied objective features like speed or acceleration. We manipulated the kinematic complexity of observed actions by creating dance sequences that varied in movement timing, but not in movement trajectory. Dance-naïve participants rated the dance videos on speed, effort, reproducibility, and enjoyment. Using linear mixed-effects modeling, we show that faster, more predictable movement sequences with varied velocity profiles are judged to be more effortful, less reproducible, and more aesthetically pleasing than slower sequences with more uniform velocity profiles. Accordingly, dance aesthetics depend not only on which movements are being performed but on how movements are executed and linked into sequences. The aesthetics of movement timing may apply across culturally-specific dance styles and predict both preference for and perceived difficulty of dance, consistent with information theory and effort heuristic accounts of aesthetic appreciation.

Sensitivity of occipito-temporal cortex, premotor and Broca's areas to visible speech gestures in a familiar language

When looking at a speaking person, the analysis of facial kinematics contributes to language discrimination and to the decoding of the time flow of visual speech. To disentangle these two factors, we investigated behavioural and fMRI responses to familiar and unfamiliar languages when observing speech gestures with natural or reversed kinematics. Twenty Italian volunteers viewed silent video-clips of speech shown as recorded (Forward, biological motion) or reversed in time (Backward, non-biological motion), in Italian (familiar language) or Arabic (non-familiar language). fMRI revealed that language (Italian/Arabic) and time-rendering (Forward/Backward) modulated distinct areas in the ventral occipito-temporal cortex, suggesting that visual speech analysis begins in this region, earlier than previously thought. Left premotor ventral (superior subdivision) and dorsal areas were preferentially activated with the familiar language independently of time-rendering, challenging the view that the role of these regions in speech processing is purely articulatory. The left premotor ventral region in the frontal operculum, thought to include part of the Broca's area, responded to the natural familiar language, consistent with the hypothesis of motor simulation of speech gestures.

Dynamic motor imagery mentally simulates uncommon real locomotion better than static motor imagery both in young adults and elderly

A new form of Motor Imagery (MI), called dynamic Motor Imagery (dMI) has recently been proposed. The dMI adds to conventional static Motor Imagery (sMI) the presence of simultaneous actual movements partially replicating those mentally represented. In a previous research conducted on young participants, dMI showed to be temporally closer than sMI in replicating the real performance for some specific locomotor conditions. In this study, we evaluated if there is any influence of the ageing on dMI. Thirty healthy participants were enrolled: 15 young adults (27.1±3.8 y.o.) and 15 older adults (65.9±9.6y.o.). The performance time and the number of steps needed to either walk to a target (placed at 10m from participants) or to imagine walking to it, were assessed. Parameters were measured for sMI, dMI and real locomotion (RL) in three different locomotor conditions: forward walking (FW), backward walking (BW), and lateral walking (LW). Temporal performances of sMI and dMI did not differ between RL in the FW, even if significantly different to each other (p = 0.0002). No significant differences were found for dMI with respect to RL for LW (p = 0.140) and BW (p = 0.438), while sMI was significantly lower than RL in LW (p<0.001). The p-value of main effect of age on participants’ temporal performances was p = 0.055. The interaction between age and other factors such as the type of locomotion (p = 0.358) or the motor condition (p = 0.614) or third level interaction (p = 0.349) were not statistically significant. Despite a slight slowdown in the performance of elderly compared to young participants, the temporal and spatial accuracy was better in dMI than sMI in both groups. Motor imagery processes may be strengthened by the feedback generated through dMI, and this effect appears to be unaffected by age.

Dilation and Constriction of Subjective Time Based on Observed Walking Speed

The physical properties of events are known to modulate perceived time. This study tested the effect of different quantitative (walking speed) and qualitative (walking-forward vs. walking-backward) features of observed motion on time perception in three complementary experiments. Participants were tested in the temporal discrimination (bisection) task, in which they were asked to categorize durations of walking animations as "short" or "long." We predicted the faster observed walking to speed up temporal integration and thereby to shift the point of subjective equality leftward, and this effect to increase monotonically with increasing walking speed. To this end, we tested participants with two different ranges of walking speeds in Experiment 1 and 2 and observed a parametric effect of walking speed on perceived time irrespective of the direction of walking (forward vs. rewound forward walking). Experiment 3 contained a more plausible backward walking animation compared to the rewound walking animation used in Experiments 1 and 2 (as validated based on independent subjective ratings). The effect of walking-speed and the lack of the effect of walking direction on perceived time were replicated in Experiment 3. Our results suggest a strong link between the speed but not the direction of perceived biological motion and subjective time.

Natural Translating Locomotion Modulates Cortical Activity at Action Observation

The present study verified if the translational component of locomotion modulated cortical activity recorded at action observation. Previous studies focusing on visual processing of biological motion mainly presented point light walker that were fixed on a spot, thus removing the net translation toward a goal that yet remains a critical feature of locomotor behavior. We hypothesized that if biological motion recognition relies on the transformation of seeing in doing and its expected sensory consequences, a significant effect of translation compared to centered displays on sensorimotor cortical activity is expected. To this aim, we explored whether EEG activity in the theta (4–8 Hz), alpha (8–12 Hz), beta 1 (14–20 Hz) and beta 2 (20–32 Hz) frequency bands exhibited selectivity as participants viewed four types of stimuli: a centered walker, a centered scrambled, a translating walker and a translating scrambled. We found higher theta synchronizations for observed stimulus with familiar shape. Higher power decreases in the beta 1 and beta 2 bands, indicating a stronger motor resonance was elicited by translating compared to centered stimuli. Finally, beta bands modulation in Superior Parietal areas showed that the translational component of locomotion induced greater motor resonance than human shape. Using a Multinomial Logistic Regression classifier we found that Dorsal-Parietal and Inferior-Frontal regions of interest (ROIs), constituting the core of action-observation system, were the only areas capable to discriminate all the four conditions, as reflected by beta activities. Our findings suggest that the embodiment elicited by an observed scenario is strongly mediated by horizontal body displacement.

Low perceptual sensitivity to altered video speed in viewing a soccer match

When watching videos, our sense of reality is continuously challenged. How much can a fundamental dimension of experience such as visual flow be modified before breaking the perception of real time? Here we found a remarkable indifference to speed manipulations applied to a popular video content, a soccer match. In a condition that mimicked real-life TV watching, none of 100 naïve observers spontaneously noticed speed alterations up/down to 12%, even when asked to report motion anomalies, and showed very low sensitivity to video speed changes (Just Noticeable Difference, JND = 18%). When tested with a constant-stimuli speed discrimination task, JND was still high, though much reduced (9%). The presence of the original voice-over with compensation for pitch did not affect perceptual performance. Thus, our results document a rather broad tolerance to speed manipulations in video viewing, even under attentive scrutiny. This finding may have important implications. For example, it can validate video compression strategies based on sub-threshold temporal squeezing. This way, a soccer match can last only 80 min and still be perceived as natural. More generally, knowing the boundaries of natural speed perception may help to optimize the flow of artificial visual stimuli which increasingly surround us.

The dynamic motor imagery of locomotion is task-dependent in patients with stroke

PURPOSE

Recently, Motor Imagery (MI) has been associated with the execution of movements miming in part the mentally represented action (dynamic MI, dMI). Preliminary studies have reported as dMI may improve trainings in sport, with imagery timing close to the physical execution one. This study was aimed to investigate time and spatial parameters of dMI with actual locomotion in people with stroke.

METHODS

Twelve patients (stroke group, SG) were compared with twelve healthy elderly (elderly group, EG) and twenty young adults (young group, YG). Subjects performed mental representations of different walking (forward, FW; lateral, LW, backward, BW), accompanied or not by movements imitating walking (dMI and static MI, sMI). Then, they performed actual locomotion (AL). Outcome measures were related to the time and the number of steps spent for completing the tasks for all the given locomotor conditions.

RESULTS

Significant differences were found in patients with respect to healthy subjects, with time in sMI significantly shorter than in dMI (p < 0.004) and AL (p < 0.002), but not between dMI and AL in FW (p = 0.806). In patients, times obtained in sMI and dMI was significantly shorter with respect to those of AL in LW and BW. Patients performed imagery tasks with similar times in all locomotion. Healthy groups did not reveal differences among tasks in BW, while significant differences were found in LW. Analogous results were found in terms of number of performed steps.

CONCLUSIONS

In patients with stroke, a spatiotemporal functional equivalence with AL was found only for dMI, and not for sMI, in forward walking. This could be due to familiarity with this task. These results might have implications for the rehabilitative techniques based on MI.

Different performances in static and dynamic imagery and real locomotion. An exploratory trial

Motor imagery (MI) is a mental representation of an action without its physical execution. Recently, the simultaneous movement of the body has been added to the mental simulation. This refers to dynamic motor imagery (dMI). This study was aimed at analyzing the temporal features for static and dMI in different locomotor conditions (natural walking, NW, light running, LR, lateral walking, LW, backward walking, BW), and whether these performances were more related to all the given conditions or present only in walking. We have been also evaluated the steps performed in the dMI in comparison with the ones performed by real locomotion. 20 healthy participants (29.3 ± 5.1 years old) were asked to move towards a visualized target located at 10 mt. In dMI, no significant temporal differences respect the actual locomotion were found for all the given tasks (NW: p = 0.058, LR: p = 0.636, BW: p = 0.096; LW: p = 0,487). Significant temporal differences between static imagery and actual movements were found for LR (p < 0.001) and LW (p < 0.001), due to an underestimation of time needed to achieve the target in imagined locomotion. Significant differences in terms of number of steps among tasks were found for LW (p < 0.001) and BW (p = 0.036), whereas neither in NW (p = 0.124) nor LR (p = 0.391) between dMI and real locomotion. Our results confirmed that motor imagery is a task-dependent process, with walking being temporally closer than other locomotor conditions. Moreover, the time records of dMI are nearer to the ones of actual locomotion respect than the ones of static motor imagery.

How long did it last? You would better ask a human

In the future, human-like robots will live among people to provide company and help carrying out tasks in cooperation with humans. These interactions require that robots understand not only human actions, but also the way in which we perceive the world. Human perception heavily relies on the time dimension, especially when it comes to processing visual motion. Critically, human time perception for dynamic events is often inaccurate. Robots interacting with humans may want to see the world and tell time the way humans do: if so, they must incorporate human-like fallacy. Observers asked to judge the duration of brief scenes are prone to errors: perceived duration often does not match the physical duration of the event. Several kinds of temporal distortions have been described in the specialized literature. Here we review the topic with a special emphasis on our work dealing with time perception of animate actors versus inanimate actors. This work shows the existence of specialized time bases for different categories of targets. The time base used by the human brain to process visual motion appears to be calibrated against the specific predictions regarding the motion of human figures in case of animate motion, while it can be calibrated against the predictions of motion of passive objects in case of inanimate motion. Human perception of time appears to be strictly linked with the mechanisms used to control movements. Thus, neural time can be entrained by external cues in a similar manner for both perceptual judgments of elapsed time and in motor control tasks. One possible strategy could be to implement in humanoids a unique architecture for dealing with time, which would apply the same specialized mechanisms to both perception and action, similarly to humans. This shared implementation might render the humanoids more acceptable to humans, thus facilitating reciprocal interactions.

Similar muscles contribute to horizontal and vertical acceleration of center of mass in forward and backward walking: implications for neural control

Leg kinematics during backward walking (BW) are very similar to the time-reversed kinematics during forward walking (FW). This suggests that the underlying muscle activation pattern could originate from a simple time reversal, as well. Experimental electromyography studies have confirmed that this is the case for some muscles. Furthermore, it has been hypothesized that muscles showing a time reversal should also exhibit a reversal in function [from accelerating the body center of mass (COM) to decelerating]. However, this has not yet been verified in simulation studies. In the present study, forward simulations were used to study the effects of muscles on the acceleration of COM in FW and BW. We found that a reversal in function was indeed present in the muscle control of the horizontal movement of COM (e.g., tibialis anterior and gastrocnemius). In contrast, muscles' antigravity contributions maintained their function for both directions of movement. An important outcome of the present study is therefore that similar muscles can be used to achieve opposite functional demands at the level of control of the COM when walking direction is reversed. However, some muscles showed direction-specific contributions (i.e., dorsiflexors). We concluded that the changes in muscle contributions imply that a simple time reversal would be insufficient to produce BW from FW. We therefore propose that BW utilizes extra elements, presumably supraspinal, in addition to a common spinal drive. These additions are needed for propulsion and require a partial reconfiguration of lower level common networks.

The perception of visible speech: estimation of speech rate and detection of time reversals

Four experiments investigated the perception of visible speech. Experiment 1 addressed the perception of speech rate. Observers were shown video-clips of the lower face of actors speaking at their spontaneous rate. Then, they were shown muted versions of the video-clips, which were either accelerated or decelerated. The task (scaling) was to compare visually the speech rate of the stimulus to the spontaneous rate of the actor being shown. Rate estimates were accurate when the video-clips were shown in the normal direction (forward mode). In contrast, speech rate was underestimated when the video-clips were shown in reverse (backward mode). Experiments 2-4 (2AFC) investigated how accurately one discriminates forward and backward speech movements. Unlike in Experiment 1, observers were never exposed to the sound track of the video-clips. Performance was well above chance when playback mode was crossed with rate modulation, and the number of repetitions of the stimuli allowed some amount of speechreading to take place in forward mode (Experiment 2). In Experiment 3, speechreading was made much more difficult by using a different and larger set of muted video-clips. Yet, accuracy decreased only slightly with respect to Experiment 2. Thus, kinematic rather then speechreading cues are most important for discriminating movement direction. Performance worsened, but remained above chance level when the same stimuli of Experiment 3 were rotated upside down (Experiment 4). We argue that the results are in keeping with the hypothesis that visual perception taps into implicit motor competence. Thus, lawful instances of biological movements (forward stimuli) are processed differently from backward stimuli representing movements that the observer cannot perform.