Intentional on-line adaptation of stride length in human walking

Quantitative assessment of the gait improvement effect of LSVT BIG® using a wearable sensor in patients with Parkinson's disease

Background

The main effects of Lee Silvermann Voice Treatment-BIG® therapy (LSVT-BIG) on gait function are improvements in gait speed and stride length. Considering the mechanism of this improvement, LSVT-BIG may affect joint angles of the lower extremities. Therefore, further investigation of the effect of LSVT-BIG on gait function, especially joint angles, is needed.

Methods

Patients with Parkinson's disease (PD) who were eligible for LSVT-BIG were recruited. We measured the following items pre- and post-LSVT-BIG: MDS-Unified Parkinson's Disease Rating Scale (MDS-UPDRS), Functional Independence Measure (FIM), timed up and go test (TUG), and gait parameters using RehaGait®. Gait parameters included gait speed, stride duration and length, the standard deviation of stride duration and length, cadence, the ratio of the stance/swing phase, and the flexion and extension angles of the hip, knee, and ankle joints. Range of motion (ROM) was calculated as the difference of values between the maximum flexion and extension angles of each joint.

Results

Twenty-four participants completed the LSVT-BIG. Significant improvement was observed in the MDS-UPDRS (mean changes: Part I, -2.4 points; Part II, -3.5 points; Part III -8.9 points), TUG (-0.61 s), gait speed (+0.13 m/s), stride length (+0.12 m), flexion and extension angles and ROM of the hip joints (flexion, +2.0°; extension, +2.0; ROM, +4.0°). Enlargement in ROM of the hip joint was strongly correlated with increase in gait speed and stride length (r = 0.755, r = 0.804, respectively).

Conclusions

LSVT-BIG enlarged flexion and extension angles and ROM of the hip joint significantly. Change of ROM of the hip joint was directly related to the increase in stride length and gait speed observed in patients with PD after LSVT-BIG.

Increased temporal stride variability contributes to impaired gait coordination after stroke

Heightened motor variability is a prominent impairment after stroke. During walking, stroke survivors show increased spatial and temporal variability; however, the functional implications of increased gait variability are not well understood. Here, we determine the effect of gait variability on the coordination between lower limbs during overground walking in stroke survivors. Ambulatory stroke survivors and controls walked at a preferred pace. We measured stride length and stride time variability, and accuracy and consistency of anti-phase gait coordination with phase coordination index (PCI). Stroke survivors showed increased stride length variability, stride time variability, and PCI compared with controls. Stride time variability but not stride length variability predicted 43% of the variance in PCI in the stroke group. Stride time variability emerged as a significant predictor of error and consistency of phase. Despite impaired spatial and temporal gait variability following stroke, increased temporal variability contributes to disrupted accuracy and consistency of gait coordination. We provide novel evidence that decline in gait coordination after stroke is associated with exacerbated stride time variability, but not stride length variability. Temporal gait variability may be a robust indicator of the decline in locomotor function and an ideal target for motor interventions that promote stable walking after stroke.

Using Biofeedback to Reduce Step Length Asymmetry Impairs Dynamic Balance in People Poststroke

BACKGROUND

People poststroke often walk with a spatiotemporally asymmetric gait, due in part to sensorimotor impairments in the paretic lower extremity. Although reducing asymmetry is a common objective of rehabilitation, the effects of improving symmetry on balance are yet to be determined.

OBJECTIVE

We established the concurrent validity of whole-body angular momentum as a measure of balance, and we determined if reducing step length asymmetry would improve balance by decreasing whole-body angular momentum.

METHODS

We performed clinical balance assessments and measured whole-body angular momentum during walking using a full-body marker set in a sample of 36 people with chronic stroke. We then used a biofeedback-based approach to modify step length asymmetry in a subset of 15 of these individuals who had marked asymmetry and we measured the resulting changes in whole-body angular momentum.

RESULTS

When participants walked without biofeedback, whole-body angular momentum in the sagittal and frontal plane was negatively correlated with scores on the Berg Balance Scale and Functional Gait Assessment supporting the validity of whole-body angular momentum as an objective measure of dynamic balance. We also observed that when participants walked more symmetrically, their whole-body angular momentum in the sagittal plane increased rather than decreased.

CONCLUSIONS

Voluntary reductions of step length asymmetry in people poststroke resulted in reduced measures of dynamic balance. This is consistent with the idea that after stroke, individuals might have an implicit preference not to deviate from their natural asymmetry while walking because it could compromise their balance. Clinical Trials Number: NCT03916562.

Changes in lower limb biomechanics when following floor-projected foot placement visual cues for gait rehabilitation

BACKGROUND

Lately, the projection of foot placement visual cues onto the floor has been considered for use in gait rehabilitation. While promising, this approach needs further basic assessment to ensure proper uses.

RESEARCH QUESTION

Does following floor-projected foot placement visual cues of one's natural walking pattern induce gait mechanics changes immediately or after a practice period?

METHODS

Gait mechanics data from fifteen healthy individuals (7 female, 25.4 ± 5.0 years, 21.5 ± 1.68 kg/m²) was collected during normal walking without visual cues, and during two testing phases (immediate and after 45-60 min of practice) of walking with floor-projected visual cues depicting their normal spatial parameters. Magnitudes and variabilities of spatial gait parameters and sagittal plane lower limb kinematics and kinetics were compared between the three testing phases using repeated measures ANOVA and post-hoc paired t-tests.

RESULTS

Compared to normal walking without foot placement visual cues, there was a statistically significant (p < 0.05) increase in stride length (maximum change of 0.01 ± 0.01 m), stance phase knee flexion (2.0 ± 2.5°), and swing phase hip flexion (1.2 ± 1.3°) in both immediate and post-practice testing phases, along with an increase in terminal stance hip (0.28 ± 0.38 %BW*Ht) and knee (0.25 ± 0.25 %BW*Ht) flexion moments in the immediate testing phase. All of these changes between testing phases were smaller than their corresponding normal gait smallest real differences (SRD). With the addition of visual cues, variability was statistically significantly decreased in spatial parameters and increased in knee flexion angle at heel strike and knee flexion moment in terminal stance.

SIGNIFICANCE

While biomechanical changes were observed, their magnitudes were small enough to suggest that floor-projected visual cues can be used in gait retraining without introducing unintended gait changes. Furthermore, the results suggested that lengthy practice periods are not necessary. The validity of these observations will, however, need to be confirmed in cases of severe impairments.

Individual Differences in Locomotor Function Predict the Capacity to Reduce Asymmetry and Modify the Energetic Cost of Walking Poststroke

Changes in the control of the lower extremities poststroke lead to persistent biomechanical asymmetries during walking. These asymmetries are associated with an increase in energetic cost, leading to the possibility that reducing asymmetry can improve walking economy. However, the influence of asymmetry on economy may depend on the direction and cause of asymmetry. For example, impairments with paretic limb advancement may result in shorter paretic steps, whereas deficits in paretic support or propulsion result in shorter nonparetic steps. Given differences in the underlying impairments responsible for step length asymmetry, the capacity to reduce asymmetry and the associated changes in energetic cost may not be consistent across this population. Here, we identified factors explaining individual differences in the capacity to voluntarily reduce step length asymmetry and modify energetic cost during walking. A total of 24 individuals poststroke walked on a treadmill, with visual feedback of their step lengths to aid explicit modification of asymmetry. We found that individuals who took longer paretic steps had a greater capacity to reduce asymmetry and were better able to transfer the effects of practice to overground walking than individuals who took shorter paretic steps. In addition, changes in metabolic cost depended on the direction of asymmetry, baseline cost of transport, and reductions in specific features of spatiotemporal asymmetry. These results demonstrate that many stroke survivors retain the residual capacity to voluntarily walk more symmetrically on a treadmill and overground. However, whether reductions in asymmetry reduce metabolic cost depends on individual differences in impairments affecting locomotor function.

EMG synchrony to assess impaired corticomotor control of locomotion after stroke

Adapting one's gait pattern requires a contribution from cortical motor commands. Evidence suggests that frequency-based analysis of electromyography (EMG) can be used to detect this cortical contribution. Specifically, increased EMG synchrony between synergistic muscles in the Piper frequency band has been linked to heightened corticomotor contribution to EMG. Stroke-related damage to cerebral motor pathways would be expected to diminish EMG Piper synchrony. The objective of this study is therefore to test the hypothesis that EMG Piper synchrony is diminished in the paretic leg relative to nonparetic and control legs, particularly during a long-step task of walking adaptability. Twenty adults with post-stroke hemiparesis and seventeen healthy controls participated in this study. EMG Piper synchrony increased more for the control legs compare to the paretic legs when taking a non-paretic long step (5.02±3.22% versus 0.86±2.62%), p<0.01) and when taking a paretic long step (2.04±1.98% versus 0.70±2.34%, p<0.05). A similar but non-significant trend was evident when comparing non-paretic and paretic legs. No statistically significant differences in EMG Piper synchrony were found between legs for typical walking. EMG Piper synchrony was positively associated with walking speed and step length within the stroke group. These findings support the assertion that EMG Piper synchrony indicates corticomotor contribution to walking.

Distinct β Band Oscillatory Networks Subserving Motor and Cognitive Control during Gait Adaptation

Everyday locomotion and obstacle avoidance requires effective gait adaptation in response to sensory cues. Many studies have shown that efficient motor actions are associated with μ rhythm (8-13 Hz) and β band (13-35 Hz) local field desynchronizations in sensorimotor and parietal cortex, whereas a number of cognitive task studies have reported higher behavioral accuracy to be associated with increases in β band power in prefrontal and sensory cortex. How these two distinct patterns of β band oscillations interplay during gait adaptation, however, has not been established. Here we recorded 108 channel EEG activity from 18 participants (10 males, 22-35 years old) attempting to walk on a treadmill in synchrony with a series of pacing cue tones, and quickly adapting their step rate and length to sudden shifts in pacing cue tempo. Independent component analysis parsed each participant's EEG data into maximally independent component (IC) source processes, which were then grouped across participants into distinct spatial/spectral clusters. Following cue tempo shifts, mean β band power was suppressed for IC sources in central midline and parietal regions, whereas mean β band power increased in IC sources in or near medial prefrontal and dorsolateral prefrontal cortex. In the right dorsolateral prefrontal cortex IC cluster, the β band power increase was stronger during (more effortful) step shortening than during step lengthening. These results thus show that two distinct patterns of β band activity modulation accompany gait adaptations: one likely serving movement initiation and execution; and the other, motor control and inhibition.

SIGNIFICANCE STATEMENT

Understanding brain dynamics supporting gait adaptation is crucial for understanding motor deficits in walking, such as those associated with aging, stroke, and Parkinson's. Only a few electromagnetic brain imaging studies have examined neural correlates of human upright walking. Here, application of independent component analysis to EEG data recorded during treadmill walking allowed us to uncover two distinct β band oscillatory cortical networks that are active during gait adaptation to shifts in the tempo of an auditory pacing cue: (8-13 Hz) μ rhythm and (13-35 Hz) β band power decreases in central and parietal cortex and (14-20 Hz) β band power increases in frontal brain areas. These results provide a fuller framework for electrophysiological studies of cortical gait control and its disorders.

Locomotor Adaptability Task Promotes Intense and Task-Appropriate Output From the Paretic Leg During Walking

OBJECTIVE

To test the hypothesis that participants with stroke will exhibit appropriate increase in muscle activation of the paretic leg when taking a long step with the nonparetic leg compared to during steady-state walking, with a consequent increase in biomechanical output and symmetry during the stance phase of the modified gait cycle.

DESIGN

Single-session observational study.

SETTING

Clinical research center in an outpatient hospital setting.

PARTICIPANTS

Adults with chronic poststroke hemiparesis (N=15).

INTERVENTIONS

Participants walked on an instrumented treadmill while kinetic, kinematic, and electromyogram data were recorded. Participants performed steady-state walking and a separate trial of the long-step adaptability task in which they were instructed to intermittently take a longer step with the nonparetic leg.

MAIN OUTCOME MEASURES

Forward progression, propulsive force, and neuromuscular activation during walking.

RESULTS

Participants performed the adaptability task successfully and demonstrated greater neuromuscular activation in appropriate paretic leg muscles, particularly increased activity in paretic plantarflexor muscles. Propulsion and forward progression by the paretic leg were also increased.

CONCLUSIONS

These findings support the assertion that the nonparetic long-step task may be effective for use in poststroke locomotor rehabilitation to engage the paretic leg and promote recovery of walking.

The combined effects of body weight support and gait speed on gait related muscle activity: a comparison between walking in the Lokomat exoskeleton and regular treadmill walking

Background

For the development of specialized training protocols for robot assisted gait training, it is important to understand how the use of exoskeletons alters locomotor task demands, and how the nature and magnitude of these changes depend on training parameters. Therefore, the present study assessed the combined effects of gait speed and body weight support (BWS) on muscle activity, and compared these between treadmill walking and walking in the Lokomat exoskeleton.

Methods

Ten healthy participants walked on a treadmill and in the Lokomat, with varying levels of BWS (0% and 50% of the participants’ body weight) and gait speed (0.8, 1.8, and 2.8 km/h), while temporal step characteristics and muscle activity from Erector Spinae, Gluteus Medius, Vastus Lateralis, Biceps Femoris, Gastrocnemius Medialis, and Tibialis Anterior muscles were recorded.

Results

The temporal structure of the stepping pattern was altered when participants walked in the Lokomat or when BWS was provided (i.e. the relative duration of the double support phase was reduced, and the single support phase prolonged), but these differences normalized as gait speed increased. Alternations in muscle activity were characterized by complex interactions between walking conditions and training parameters: Differences between treadmill walking and walking in the exoskeleton were most prominent at low gait speeds, and speed effects were attenuated when BWS was provided.

Conclusion

Walking in the Lokomat exoskeleton without movement guidance alters the temporal step regulation and the neuromuscular control of walking, although the nature and magnitude of these effects depend on complex interactions with gait speed and BWS. If normative neuromuscular control of gait is targeted during training, it is recommended that very low speeds and high levels of BWS should be avoided when possible.

Synchronous EMG activity in the piper frequency band reveals the corticospinal demand of walking tasks

Evidence indicates that the frequency-domain characteristics of surface electromyogram (EMG) signals are modulated according to the contributing sources of neural drive. Modulation of inter-muscular EMG synchrony within the Piper frequency band (30-60 Hz) during movement tasks has been linked to drive from the corticospinal tract. However, it is not known whether EMG synchrony is sufficiently sensitive to detect task-dependent differences in the corticospinal contribution to leg muscle activation during walking. We investigated this question in seventeen healthy older men and women. It was hypothesized that, relative to typical steady state walking, Piper band EMG synchrony of the triceps surae muscle group would be reduced for dual-task walking (because of competition for cortical resources), similar for fast walking (because walking speed is directed by an indirect locomotor pathway rather than by the corticospinal tract), and increased when taking a long step (because voluntary gait pattern modifications are directed by the corticospinal tract). Each of these hypotheses was confirmed. These findings support the use of frequency-domain analysis of EMG in future investigations into the corticospinal contribution to control of healthy and disordered human walking.

Music and metronome cues produce different effects on gait spatiotemporal measures but not gait variability in healthy older adults

Rhythmic auditory cues including music and metronome beats have been used, sometimes interchangeably, to improve disordered gait arising from a range of clinical conditions. There has been limited investigation into whether there are optimal cue types. Different cue types have produced inconsistent effects across groups which differed in both age and clinical condition. The possible effect of normal ageing on response to different cue types has not been reported for gait. The aim of this study was to determine the effects of both rhythmic music and metronome cues on gait spatiotemporal measures (including variability) in healthy older people. Twelve women and seven men (>65 years) walked on an instrumented walkway at comfortable pace and then in time to each of rhythmic music and metronome cues at comfortable pace stepping frequency. Music but not metronome cues produced a significant increase in group mean gait velocity of 4.6 cm/s, due mostly to a significant increase in group mean stride length of 3.1cm. Both cue types produced a significant but small increase in cadence of 1 step/min. Mean spatio-temporal variability was low at baseline and did not increase with either cue type suggesting cues did not disrupt gait timing. Study findings suggest music and metronome cues may not be used interchangeably and cue type as well as frequency should be considered when evaluating effects of rhythmic auditory cueing on gait. Further work is required to determine whether optimal cue types and frequencies to improve walking in different clinical groups can be identified.

Reduced gait stability in high-functioning poststroke individuals

Falls during walking are a major cause of poststroke injury, and walking faster may decrease the ability to recover following a gait perturbation. We compared gait stability between high-functioning poststroke individuals and controls and evaluated the effect of gait speed on gait stability. Ten stroke subjects and ten age-matched controls walked on a self-paced treadmill at two speeds (matched/faster). Movement of the nonparetic/dominant leg was arrested unexpectedly at early swing. Poststroke individuals lowered the perturbed leg following perturbation (58% of cases) while controls maintained the leg elevated (49% of cases; P < 0.01). In poststroke individuals, double-support duration was restored later than in controls (4.6 ± 0.8 vs. 3.2 ± 0.3 strides; P < 0.007), and long-term phase shifts of arm and leg movements were larger and less coordinated on the paretic side. A moderate speed increase (~20%) enhanced the incidence of leg lowering in controls but not in stroke subjects. Faster walkers in both groups had a more coordinated response, limited to the nonparetic side in the stroke group. However, faster walkers were not more stable following perturbation. Our results suggest that gait perturbations can target basic control processes and identify neurological locomotor deficits in individuals with fall risk. Central regulation of body translation in space is involved in recovery of steady-state walking. Impaired descending control (stroke) decreases the ability of the motor system to recover from perturbations and regulate interlimb phase relationships, especially when changing gait speed. However, interlimb coordination may not be a major factor in the recovery of gait stability.

Stability of gait and interlimb coordination in older adults

Most falls in older adults occur when walking, specifically following a trip. This study investigated the short- and longer term responses of young (n = 24, 27.6 ± 4.5 yr) and older adults (n = 18, 69.1 ± 4.2 yr) to a trip during gait at comfortable speed and the role of interlimb coordination in recovery from tripping. Subjects walked on a self-paced treadmill when forward movement of their dominant leg was unexpectedly arrested for 250 ms. Recovery of center of mass (COM) movements and of double-support duration following perturbation was determined. In addition, the disruption and recovery of interlimb coordination of the arms and legs was evaluated. Although young and older subjects used similar lower limb strategies in response to the trip, older adults had less stable COM movement patterns before perturbation, had longer transient destabilization (>25%) after perturbation, required more gait cycles to recover double-support duration (older, 3.48 ± 0.7 cycles; young, 2.88 ± 0.4 cycles), and had larger phase shifts that persisted after perturbation (older, -83° to -90°; young, -39° to -42°). Older adults also had larger disruptions to interlimb coordination of the arms and legs. The timing of the initial disruption in coordination was correlated with the disturbance in gait stability only in young adults. In older adults, greater initial COM instability was related to greater longer term arm incoordination. These results suggest a relationship between interlimb coordination and gait stability, which may be associated with fall risk in older adults. Reduced coordination and gait stability suggest a need for stability-related functional training even in high-functioning older adults.

Walking speed, unilateral leg loading, and step symmetry in young adults

The purpose of this study was to examine the effects of gait speed and unilateral lower limb loading on step time and step length symmetry in healthy adults. Spatiotemporal gait data were collected from 22 healthy subjects (11 men, 11 women), using the GaitRite walkway, under four randomly sequenced test conditions: self-selected speed (SS), fast speed (F), self-selected speed with the right leg loaded (LSS), and the fastest attainable speed with the right leg loaded (LF). The symmetry index, calculated with the formula [((R-L)/0.5 × (R+L)) × 100], was used to quantify step time and step length symmetry. It was found that over-ground gait speed had no significant effects on the symmetry of step time or step length. Unilateral lower limb loading significantly increased step time asymmetry, with longer step time for the loaded leg. Step symmetry was further compromised and became more asymmetrical when, in addition to unilateral leg loading, subjects maximized their gait speed. This effect of fast speed with unilateral leg loading was particularly prominent in relation to step length, with its shortening in the unloaded leg and lengthening in the loaded leg. These observations in healthy subjects may serve as a reference for the assessment of gait symmetry in patients with unilateral lower limb pathologies.

Step length asymmetry is representative of compensatory mechanisms used in post-stroke hemiparetic walking

Post-stroke hemiparetic subjects walk with asymmetrical step lengths that are highly variable between subjects and may be indicative of the underlying impairments and compensatory mechanisms used. The goal of this study was to determine if post-stroke hemiparetic subjects grouped by step length asymmetry have similar abnormal walking biomechanics compared to non-impaired walkers. Kinematic and ground reaction force data were recorded from 55 hemiparetic subjects walking at their self-selected speed and 21 age and speed-matched non-impaired control subjects. Hemiparetic subjects were grouped by paretic step ratio, which was calculated as the paretic step-length divided by the sum of paretic and nonparetic step-lengths, into high (>0.535), symmetric (0.535-0.465) and low (<0.465) groups. Non-parametric Wilcoxin signed-rank tests were used to test for differences in joint kinetic measures between hemiparetic groups and speed-matched control subjects during late single-leg stance and pre-swing. The paretic leg ankle moment impulse was reduced in all hemiparetic subjects regardless of their paretic step ratio. The high group had increased nonparetic leg ankle plantarflexor and knee extensor moment impulses, the symmetric group had increased hip flexor moment impulses on both the paretic and nonparetic leg and the low group had no additional significant differences in joint moment impulses. These results suggest that the direction of asymmetry can be used to identify both the degree of paretic plantarflexor impairment and the compensatory mechanisms used by post-stroke hemiparetic subjects.

Comparing the efficacy of metronome beeps and stepping stones to adjust gait: steps to follow!

Acoustic metronomes and visual targets have been used in rehabilitation practice to improve pathological gait. In addition, they may be instrumental in evaluating and training instantaneous gait adjustments. The aim of this study was to compare the efficacy of two cue types in inducing gait adjustments, viz. acoustic temporal cues in the form of metronome beeps and visual spatial cues in the form of projected stepping stones. Twenty healthy elderly (aged 63.2 ± 3.6 years) were recruited to walk on an instrumented treadmill at preferred speed and cadence, paced by either metronome beeps or projected stepping stones. Gait adaptations were induced using two manipulations: by perturbing the sequence of cues and by imposing switches from one cueing type to the other. Responses to these manipulations were quantified in terms of step-length and step-time adjustments, the percentage correction achieved over subsequent steps, and the number of steps required to restore the relation between gait and the beeps or stepping stones. The results showed that perturbations in a sequence of stepping stones were overcome faster than those in a sequence of metronome beeps. In switching trials, switching from metronome beeps to stepping stones was achieved faster than vice versa, indicating that gait was influenced more strongly by the stepping stones than the metronome beeps. Together these results revealed that, in healthy elderly, the stepping stones induced gait adjustments more effectively than did the metronome beeps. Potential implications for the use of metronome beeps and stepping stones in gait rehabilitation practice are discussed.

Control of center of mass motion state through cuing and decoupling of spontaneous gait parameters in level walking

Can the center of mass (COM) motion state, i.e., its position and velocity relative to the base of support (BOS), which dictate gait stability, be predictably controlled by the global gait parameters of step length and gait speed, or by extension, cadence? The precise relationships among step length and gait speed, and the COM motion state are unknown, partially due to the interdependence between step length and gait speed and the difficulty in independent control of both parameters during spontaneous level walking. The purposes of this study were to utilize simultaneous audio-visual cuing to independently manipulate step length and gait speed, and to determine the extent to which the COM position and velocity can be subsequently controlled. Fifty-six young adults were trained at one of the three gait patterns in which both the step length and gait speed were targeted simultaneously. The results showed that the cuing could successfully "decouple" gait speed from step length. Although this approach did yield reliable control of the COM velocity through manipulation of gait speed (R(2)=0.97), the manipulation of step length yielded less precise control of COM position (R(2)=0.60). This latter control appears to require manipulation of an additional degree-of-freedom at the local segment level, such that the inclusion of trunk inclination with step length improved the prediction of COM position (R(2)=0.80).

Locomotor adaptation and aftereffects in patients with reduced somatosensory input due to peripheral neuropathy

We studied 12 peripheral neuropathy patients (PNP) and 13 age-matched controls with the "broken escalator" paradigm to see how somatosensory loss affects gait adaptation and the release and recovery ("braking") of the forward trunk overshoot observed during this locomotor aftereffect. Trunk displacement, foot contact signals, and leg electromyograms (EMGs) were recorded while subjects walked onto a stationary sled (BEFORE trials), onto the moving sled (MOVING or adaptation trials), and again onto the stationary sled (AFTER trials). PNP were unsteady during the MOVING trials, but this progressively improved, indicating some adaptation. During the after trials, 77% of control subjects displayed a trunk overshoot aftereffect but over half of the PNP (58%) did not. The PNP without a trunk aftereffect adapted to the MOVING trials by increasing distance traveled; subsequently this was expressed as increased distance traveled during the aftereffect rather than as a trunk overshoot. This clear separation in consequent aftereffects was not seen in the normal controls suggesting that, as a result of somatosensory loss, some PNP use distinctive strategies to negotiate the moving sled, in turn resulting in a distinct aftereffects. In addition, PNP displayed earlier than normal anticipatory leg EMG activity during the first after trial. Although proprioceptive inputs are not critical for the emergence or termination of the aftereffect, somatosensory loss induces profound changes in motor adaptation and anticipation. Our study has found individual differences in adaptive motor performance, indicative that PNP adopt different feed-forward gait compensatory strategies in response to peripheral sensory loss.

Does walking in a virtual environment induce unstable gait? An examination of vertical ground reaction forces

Virtual reality (VR) can induce postural instability in standing and walking, as quantified with kinematic parameters. This study examines the effect of a VR environment on kinetic gait parameters. Ten healthy volunteers walked on an instrumented treadmill in a VR environment and a non-VR environment. In the VR environment, a corridor with colored vertical stripes comprising the walls was projected onto a concave screen placed in front of the treadmill. The speed of the moving image was perceptually equivalent to the speed of the treadmill, creating an illusion that subjects walked through the corridor. Vertical ground reaction forces were sampled. Kinetic parameters that reflect gait stability (weight acceptance peak force, weight acceptance rate, push-off peak force and push-off rate) were compared between the VR and non-VR environments. Subjects walked in the VR environment with increased magnitudes and rates of weight acceptance force and with increased rates of push-off force. Variability in weight acceptance rates and peak forces, and variability in push-off peak forces, were also increased in the VR environment. The gait deviations reflect a compensatory response to visual stimulation that occurs in the VR environment, suggesting that walking in a VR environment may induce gait instability in healthy subjects.

Relationship between step length asymmetry and walking performance in subjects with chronic hemiparesis

OBJECTIVE

To understand the relationship between step length asymmetry and hemiparetic walking performance.

DESIGN

Descriptive.

SETTING

Gait analysis laboratory.

PARTICIPANTS

Convenience sample of 49 subjects with chronic hemiparesis.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Subjects walked at their self-selected walking speed over both an instrumented mat and forceplates to collect spatiotemporal parameters and ground reaction forces, respectively. Step length asymmetry was quantified by using a step length ratio (SLR) defined as paretic step length divided by nonparetic step length. Paretic leg propulsion, self-selected walking speed, hemiparetic severity (assessed by Brunnstrom stages of motor recovery), and some spatiotemporal walking parameters quantified the hemiparetic walking performance. Paretic leg propulsion was quantified by the paretic propulsion (P(P)) ratio, calculated as the percentage contribution of paretic leg to the total propulsive impulse.

RESULTS

Significant negative correlation (r=-.78) was revealed between SLR and P(P), indicating that subjects generating less propulsive force with the paretic leg walked asymmetrically with longer paretic steps than nonparetic steps. SLR and self-selected walking speed revealed a weaker correlation (r=-.35), whereas hemiparetic severity correlated strongly with SLR (rho=-.53).

CONCLUSIONS

Step length asymmetry is related to propulsive force generation during hemiparetic walking. Subjects generating least paretic propulsion walk with relatively longer paretic steps. This suggests that one of the mechanisms for the longer paretic step may be the relatively greater compensatory nonparetic leg propulsion. Further, those with more severe hemiparesis (those dependent on abnormal flexor and extensor synergies) walk with the longest paretic steps relative to nonparetic. Finally, our results indicated that asymmetrical step lengths may not necessarily limit the self-selected walking speed, likely due to other compensatory mechanisms.

The advantage of cyclic over discrete movements remains evident following changes in load and amplitude

Previous studies suggested that the advantage in speed accuracy trade-off of cyclic over discrete aiming tasks with the upper limbs may be associated with the operation of spinal neural oscillators, as in locomotion. Similar to the locomotor rhythm that is fairly robust and can accommodate changes in loading or stride length, we predicted that cyclic aiming tasks would be equally resistant to changes in load or amplitude, thereby preserving the advantage over discrete tasks. To test the hypothesis, cyclic and discrete aiming movements were performed with and without loading of the hand. Furthermore a "complex" condition was introduced in which the distance between the targets that the participants moved to alternated between 2.5 and 5 cm. In all cases, two target sizes were used to test spatial accuracy and to be able to calculate the Index of Performance (IP). Findings revealed that even though part of the advantage of the cyclic over the discrete regime was lost during the complex movement pattern and with addition of weight, the former remained superior to the latter. Furthermore, adding weight did not change the oscillation frequency in the cyclic movements. It is concluded that the superiority of cyclic movements over discrete ones is fairly robust, consistent with the high degree of flexibility that is typically observed in neural oscillators.

Effects of step length on stepping responses used to arrest a forward fall

This study investigated effects of step length on the stepping response used to arrest an impending forward fall. Twelve healthy young (mean age 22, S.D. 3.3 years) males participated by recovering balance with a single step following a forward lean-and-release. Participants were instructed to step to one of three floor targets representing small, natural, and large step lengths. The effect of step length was examined on the primary outcome variables: pushoff time, liftoff and landing time, swing duration, balance recovery time, landing impulse, and center of mass (COM) characteristics. Pushoff and liftoff times were not affected by step length, although swing phase duration, landing and recovery times and the anterior-posterior (AP) impulse at landing increased with increasing step length. The results support the idea of an invariant step preparation phase. Given that our participants naturally chose not to utilize a step as short as they were capable of employing, healthy young individuals do not minimize recovery time nor strength requirements when selecting their step length.

Influence of gait speed on stability: recovery from anterior slips and compensatory stepping

Falls precipitated by slipping are a major health concern, with the majority of all slip-related falls occurring during gait. Recent evidence shows that a faster and/or more anteriorly positioned center of mass (COM) is more stable against backward balance loss, and that compensatory stepping is the key to recovering stability upon balance loss. The purposes of this paper were to determine whether walking speed affected gait stability for backward balance loss at slip onset and touchdown of compensatory stepping, and whether compensatory stepping response resembled the regular gait pattern. Forty-seven young subjects were slipped unexpectedly either at a self-selected fast, natural or slow speed. Speed-related differences in stability at slip onset and touchdown of the subsequent compensatory step were analyzed using the COM position-velocity state. The results indicate that gait speed highly correlated with stability against backward balance loss at slip onset. The low COM velocity of the slow group was not sufficiently compensated for by a more anteriorly positioned COM associated with a shorter step length at slip onset. At touchdown of the compensatory step, the speed-related differences in stability diminished, due to the continued advantage of anterior COM positioning from a short compensatory step retained by the slow group, coupled with an increase in COM velocity. Compensatory step length and relative COM position altered as a function of gait speed, indicating the motor program for gait regulation may play a role in modulating the compensatory step.

Step characteristics during obstacle avoidance in hemiplegic stroke

Whereas several animal studies have indicated the important role of the motor cortex in the control of voluntary gait modifications, little is known about the effects of cortical lesions on gait adaptability in humans. Obstacle avoidance tasks provide an adequate paradigm to study the adaptability of the stepping pattern under controlled, experimental conditions. In the present study, an exploratory assessment was made of the failure rate, the preferred stepping strategies (step lengthening vs step shortening), and the spatiotemporal stride characteristics (percentage increases in stride length, duration, and velocity of the crossing and postcrossing strides) during obstacle avoidance in 11 hemiplegic stroke patients and seven healthy controls. Patients were less successful in avoiding obstacles than controls (14% failure rate vs 0.5% in controls), independent of whether the affected or the unaffected leg led the obstacle avoidance. The number of failed trials increased systematically when the available response time became shorter. During successful trials, lengthening of the step was generally preferred over shortening. This bias towards step lengthening was more pronounced in stroke patients (step lengthening in 91% of the trials vs 75% in controls), irrespective of the side of obstacle presentation. For both groups, overall strategy preference did not adhere to a principle of minimal foot displacement, since step lengthening was used even if it would be more spatially efficient to shorten the step. No statistically significant group differences were found for the increases in length, duration, and velocity of the crossing and postcrossing strides. However, for a subgroup of more slowly walking patients, large percentage increases were found in crossing stride length, duration, and velocity. Similar results were obtained for the postcrossing stride, indicating that, for this subgroup of patients, restoration of the normal walking cadence was more difficult. Overall, no systematic differences were found between the affected and the unaffected leg in stroke patients with respect to failure rates, stepping strategies, or spatiotemporal measures of obstacle avoidance. The present findings suggest that the ability to adequately modify the stepping pattern in response to imposed spatiotemporal constraints is impaired in persons with stroke, especially when modifications have to be performed under time pressure. In addition, the stepping strategies employed by subjects with stroke are different from those found in controls, possibly to reduce the complexity of the avoidance maneuver and to enhance safety. Finally, unilateral cortical damage results in an impaired ability to avoid obstacles on both sides of the body, suggesting that the reduced ability of stroke patients to negotiate obstacles may be related to problems of a more general coordinative nature.

The effect of velocity on the strategies used during gait termination

The purpose of this study was to examine the changes that occur in gait termination (GT) as cadence increases. Five healthy volunteers walked at three cadences in time to a metronome under the following conditions: normal walking, planned stopping and unplanned stopping. Electromyography and force plate data were collected at 1000 Hz. Peak posterior ground reaction force (deceleration force) was greatest for the lead limb in unplanned stopping and, in general, deceleration force increased as cadence increased. Duration of muscle activity relative to the time to peak loading was unchanged by cadence and was always longest in the lead limb during planned stopping. As cadence increases, the signal to stop must occur earlier during stance phase if a person is to stop in the subsequent step.

Gait adjustments in response to an obstacle are faster than voluntary reactions

It has been reported that obstacle avoidance reactions during gait have very short latencies. This raises the question whether the cortex can be involved, as it is in voluntary reactions. In this study, latencies of obstacle avoidance (OA) reactions were determined and related to latencies of voluntary stride modifications and simple reaction times (SRT) of hand and foot. Twenty-five healthy young adults participated in this study. While they were walking on the treadmill, an obstacle suddenly fell in front of their left leg. The first reaction to the obstacle was the moment at which the differentiated acceleration curve of the foot deviated from the control signal. Latencies of OA reactions were 122 ms (SD 14 ms) on average. Two very different avoidance reactions (lengthening and shortening of the stride) were noticed, but there was no avoidance strategy effect on OA latencies. OA latencies were significantly shorter as compared to latencies of voluntary stride modifications and simple reaction times of hand and foot. The short OA latencies could not only be explained from the dynamic nature of the task. It is suggested that subcortical pathways might be involved in obstacle avoidance.

Older women strongly prefer stride lengthening to shortening in avoiding obstacles

In the present study the obstacle avoidance strategy during treadmill walking was investigated in ten young (aged 19-32) and ten older females (aged 65-78). Minimisation of displacement of the foot from its original landing position has been proposed to be the main criterion for the selection of alternate foot placement. Each participant performed 60 obstacle avoidance trials. Foot-obstacle configurations were varied in order to obtain both lengthening and shortening avoidance reactions. For each trial it was calculated how much lengthening and how much shortening of the stride was required minimally for successful avoidance. The difference between required lengthening and required shortening was expressed as a percentage of the control stride length and was used as a measure of minimal displacement. The behavior of young females was in agreement with the minimal displacement criterion. The older females, however, exhibited a strong preference for stride lengthening, even in situations in which stride shortening would be highly favorable. The explanation for the long step strategy preference of the older females is discussed in terms of age-related changes in decision-making, differences between young and older persons in the unobstructed gait pattern, and safety considerations.

Acceleration patterns of the head and pelvis when walking on level and irregular surfaces

The aim of this study was to evaluate acceleration patterns at the head and pelvis while subjects walked on a level and an irregular walking surface, to develop an understanding of how the postural control system responds to challenging walking conditions. Thirty young, healthy subjects walked on a level corridor and on artificial grass underlain with foam and wooden blocks placed in an arbitrary manner. Temporo-spatial gait parameters and acceleration patterns at the head and pelvis were measured. The results revealed that when walking on the irregular surface, subjects were able to maintain their velocity, but adopted a slower and more variable cadence and a significantly longer stride length. The magnitude of pelvis accelerations increased, however head accelerations were not affected by the walking surface. When considered as an overall pattern of movement, these findings suggest that one of the primary objectives of the postural control system when walking on irregular surfaces is head control, and that subjects adapt their stepping pattern on irregular surfaces to ensure that the head remains stable.

Stride variability in human gait: the effect of stride frequency and stride length

This study focused on spatial and temporal variability of the stride in human gait. We determined the role of stride frequency (F) and stride length (L) on those parameters. Eight healthy subjects walked on a treadmill using 25 different FL combinations (0.95<L<1.5 m, and 0.8<F<1.26 Hz). The results showed that spatial and temporal variabilities tend to increase in concert with respect to change in stride parameters. In addition, stride variability was found (1) to be minimal at F=1 Hz; and (2) to increase with smaller L. During additional trials, subjects walked freely at various speeds. Although it is generally hypothesized that freely chosen behaviors are optimal in terms of variability, our data show that this is not always the case in human gait.

Task-induced modulation of motor evoked potentials in upper-leg muscles during human gait: a TMS study

The aim of this study was to determine the relative involvement of the corticospinal (CS) pathway in voluntarily controlled walking compared to unconstrained walking. In the voluntarily controlled walking condition, subjects had to walk at the same speed as in unconstrained walking with a mechanical constraint, which is known to affect specifically the upper-leg muscles. The motor cortex was activated transcranially using a focal magnetic stimulation coil in order to elicit motor evoked potentials (MEPs) in the rectus femoris (RF) and the biceps femoris (BF). The magnetic stimulation was delivered at the end of the swing (at 90% of the cycle duration), when the EMG backgrounds were similar in the two experimental conditions. For each subject in each condition, MEPs were measured for several stimulus intensities in order to establish the input/output (I/O) curve (MEPs amplitude plotted against stimulus strength). The results showed a significant increase in the MEPs amplitude of both the RF and BF in voluntarily controlled walking compared to unconstrained walking, which is the first evidence of cofacilitation of MEPs in antagonist upper-leg muscles during human gait. In conclusion, although a lot of studies have emphasized a privileged input of the corticospinal pathway to the distal lower-leg muscles, this study shows that, if a locomotory task requires fine control of the proximal upper-leg muscles, a selective facilitation of MEPs is observed in these muscles.

Speed dependence of averaged EMG profiles in walking

Electromyogram (EMG) profiles strongly depend on walking speed and, in pathological gait, patients do not usually walk at normal speeds. EMG data was collected from 14 muscles in two groups of healthy young subjects who walked at five different speeds ranging from 0.75 to 1.75 ms(-1). We found that average EMG profiles varied in a predictable way with speed. The average EMG profile for each muscle at any speed could be estimated in a simple way from two functions, one constant and one proportionally increasing with walking speed. By taking into account the similarity among profiles within functional groups, the number of basic functions could be reduced further. Any average EMG profile among the 14 leg muscles studied at all speeds in the measured range could be predicted from six constant and ten speed-dependent basic patterns. These results can be interpreted in terms of a central pattern generator for human walking.

Spatially constrained locomotion under informational conflict

This study investigates the informational based that supports intentional adaptation of locomotion to spatial environmental constraints. A virtual reality setup was used to present subjects with targets providing normal as well as abnormal optical expansion during locomotor pointing (i.e. positioning of a foot on a visible target on the floor during walking). The manipulation dissociated two variables providing temporal information about time-to-passage (TTP): TTP(beta alpha) which encompasses target expansion, and TTP(alpha) which is independent of target expansion. While a previous study showed TTP(alpha) to be sufficient, the present results reveal that TTP(beta alpha) may be used when it is available. This finding indicates that both variables play a role that varies according to the circumstances. Furthermore, the present results provide evidence of the operation of a security principle for action in conflicting situations.