Feedback and feedforward locomotor adaptations to ankle-foot load in people with incomplete spinal cord injury

Effects of gait training with weight support feedback walker on walker dependence, lower limb muscle activation, and gait ability in patients with incomplete spinal cord injury: A pilot randomized controlled trial

BACKGROUND

Spinal cord injury (SCI) is a devastating condition affecting an individual's life, particularly through lower extremity paralysis, which limits walking and daily activities.

OBJECTIVES

This study investigated the effects of weight support feedback walker (WSFW) gait training on walker dependence, lower limb muscle activation, and gait ability in patients with incomplete SCI (ISCI).

METHODS

Eleven subjects capable of walking > 20 m with and without a walker were randomly assigned to WSFW gait training (n = 6) or conventional walker (CW) gait training groups (n = 5). All subjects underwent standard physical therapy for 4 weeks. Additionally, the WSFW group participated in WSFW gait training, whereas the CW group participated in CW gait training conducted for 30 min per day, thrice per week, for 4 weeks. Walker dependence (the average force pressing WSFW with the user's arm during walker gait), lower extremity muscle activity (rectus femoris, biceps femoris, and medial gastrocnemius), and gait ability (gait elements: velocity, cadence, step length, and step length asymmetry) were measured to investigate the effects of training.

RESULTS

The WSFW group showed significant decrease in walker dependence compared to the CW group (P < 0.05). Some lower extremity muscle activation (left side biceps femoris) and velocity of the gait elements were increased in the WSFW group compared with those in the CW group (P < 0.05).

CONCLUSION

WSFW gait training could help patients with ISCI transfer their body weight to the paralyzed lower extremity. However, a randomized controlled trial with several subjects is essential to verify the effects of WSFW training.

Immediate effect of weight load on lower limb muscle activity and gait ability in patients with incomplete spinal cord injury during walker gait training

OBJECTIVE

Walkers are actively used to improve gait ability in patients with incomplete spinal cord injury (ISCI). This study aimed to investigate the immediate effect of weight load during walker gait training on lower limb muscle activity and gait ability in patients with ISCI using a dependence feedback walker (DFW).

DESIGN

A single group cross-sectional design.

SETTING

Local rehabilitation hospital.

PARTICIPANTS

Fourteen patients with ISCI (62.00 years, Onset duration: 20.57months).

INTERVENTIONS

The DFW was used to measure the change in lower limb muscle activity and gait ability on walker dependence during the 20-meter walk. Based on the initial measurement of walker dependence, three levels of walker dependence threshold were set (100%, 60%, and 20%). If the weight loaded on the walker exceeded the three threshold levels of walker dependence, auditory and visual feedback was generated.

OUTCOME MEASURES

During the 20-meter walk, changes in both lower limb muscle activity (rectus femoris, biceps femoris, medial gastrocnemius, tibialis anterior, and gluteus medius) and gait ability (velocity, cadence, and single-limb support phase) were measured by surface electromyography and 3-axis accelerometer.

RESULTS

The increase in lower limb muscle activation and improvement of gait ability were greater during 20% walker dependence gait training than during 100% walker dependence gait training (P < 0.05).

CONCLUSION

Reduction of walker dependence by extrinsic feedback generated via DFW during walker gait training may lead to increased lower limb muscle activity and improved gait. These results could be useful for successful self-gait training and improving walking independence in patients with ISCI.

Optimism persists when walking in unpredictable environments

Humans continuously modulate their control strategies during walking based on their ability to anticipate disturbances. However, how people adapt and use motor plans to create stable walking in unpredictable environments is not well understood. Our purpose was to investigate how people adapt motor plans when walking in a novel and unpredictable environment. We evaluated the whole-body center of mass (COM) trajectory of participants as they performed repetitions of a discrete goal-directed walking task during which a laterally-directed force field was applied to the COM. The force field was proportional in magnitude to forward walking velocity and randomly directed towards either the right or left each trial. We hypothesized that people would adapt a control strategy to reduce the COM lateral deviations created by the unpredictable force field. In support of our hypothesis, we found that with practice the magnitude of COM lateral deviation was reduced by 28% (force field left) and 44% (force field right). Participants adapted two distinct unilateral strategies, implemented regardless of if the force field was applied to the right or to the left, that collectively created a bilateral resistance to the unpredictable force field. These strategies included an anticipatory postural adjustment to resist against forces applied to the left, and a more lateral first step to resist against forces applied to the right. In addition, during catch trials when the force field was unexpectedly removed, participants exhibited trajectories similar to baseline trials. These findings were consistent with an impedance control strategy that provides a robust resistance to unpredictable perturbations. However, we also found evidence that participants made predictive adaptations in response to their immediate experience that persisted for three trials. Due to the unpredictable nature of the force field, this predictive strategy would sometimes result in greater lateral deviations when the prediction was incorrect. The presence of these competing control strategies may have long term benefits by allowing the nervous system to identify the best overall control strategy to use in a novel environment.

Properties of the surface electromyogram following traumatic spinal cord injury: a scoping review

Traumatic spinal cord injury (SCI) disrupts spinal and supraspinal pathways, and this process is reflected in changes in surface electromyography (sEMG). sEMG is an informative complement to current clinical testing and can capture the residual motor command in great detail-including in muscles below the level of injury with seemingly absent motor activities. In this comprehensive review, we sought to describe how the sEMG properties are changed after SCI. We conducted a systematic literature search followed by a narrative review focusing on sEMG analysis techniques and signal properties post-SCI. We found that early reports were mostly focused on the qualitative analysis of sEMG patterns and evolved to semi-quantitative scores and a more detailed amplitude-based quantification. Nonetheless, recent studies are still constrained to an amplitude-based analysis of the sEMG, and there are opportunities to more broadly characterize the time- and frequency-domain properties of the signal as well as to take fuller advantage of high-density EMG techniques. We recommend the incorporation of a broader range of signal properties into the neurophysiological assessment post-SCI and the development of a greater understanding of the relation between these sEMG properties and underlying physiology. Enhanced sEMG analysis could contribute to a more complete description of the effects of SCI on upper and lower motor neuron function and their interactions, and also assist in understanding the mechanisms of change following neuromodulation or exercise therapy.

Immediate effects of a single treadmill session with additional ankle loading on gait in children with hemiparetic cerebral palsy

BACKGROUND

Children with hemiparetic cerebral palsy are often characterized by reduced speed progression, shorter step length, and increased support base. These kinematic alterations result in inefficient gait.

OBJECTIVE

To assess the immediate effects of treadmill training with additional lower limb loading on kinematic gait parameters in children with Cerebral Palsy (CP).

METHODS

This cross-sectional, observational study, involved 20 children with hemiparetic CP that underwent single treadmill session with ankle loading. Kinematic gait data were collected by the Qualisys Motion Capture System during baseline (PRE), immediately after training (POST) and 5 minutes after post session (FOLLOW UP).

RESULTS

The results demonstrated increase in knee (p = 0.001) and hip (p = 0.005) range of motion, maximum knee (p <.0.001) and hip (p =.001) flexion in swing and paretic foot height during swing (p <0.001) when PRE x POST were compared.

CONCLUSION

Treadmill gait training with additional lower limb loading was a disturbance capable of modifying the locomotor strategy of these population. The increase in hip flexion during swing phase allowed higher paretic foot clearance which may favor the improvement of gait function.

Stepping responses to treadmill perturbations vary with severity of motor deficits in human SCI

In this study, we investigated the responses to tread perturbations during human stepping on a treadmill. Our approach was to test the effects of perturbations to a single leg using a split-belt treadmill in healthy participants and in participants with varying severity of spinal cord injury (SCI). We recruited 11 people with incomplete SCI and 5 noninjured participants. As participants walked on an instrumented treadmill, the belt on one side was stopped or accelerated briefly during midstance to late stance. A majority of participants initiated an unnecessary swing when the treadmill was stopped in midstance, although the likelihood of initiating a step was decreased in participants with more severe SCI. Accelerating or decelerating one belt of the treadmill during stance altered the characteristics of swing. We observed delayed swing initiation when the belt was decelerated (i.e., the hip was in a more flexed position at time of swing) and advanced swing initiation with acceleration (i.e., hip extended at swing initiation). Furthermore, the timing and leg posture of heel strike appeared to remain constant, reflected by a sagittal plane hip angle at heel strike that remained the same regardless of the perturbation. In summary, our results supported the current understanding of the role of sensory feedback and central drive in the control of stepping in participants with incomplete SCI and noninjured participants. In particular, the observation of unnecessary swing during a stop perturbation highlights the interdependence of central and sensory drive in walking control. NEW & NOTEWORTHY Using a novel approach with a split-belt treadmill, we tested the effects of hip angle perturbations to a single leg in healthy participants and participants with varying severity of spinal cord injury (SCI). A majority of participants initiated an unnecessary swing when the treadmill was stopped in midstance, although the likelihood of initiating a step decreased with the severity of SCI. Our results demonstrated interdependence of central and sensory drive in walking control.

Robotic assisted gait as a tool for rehabilitation of individuals with spinal cord injury: a systematic review

BACKGROUND

Spinal cord injury (SCI) is characterized by a total or partial deficit of sensory and motor pathways. Impairments of this injury compromise muscle recruitment and motor planning, thus reducing functional capacity. SCI patients commonly present psychological, intestinal, urinary, osteomioarticular, tegumentary, cardiorespiratory and neural alterations that aggravate in chronic phase. One of the neurorehabilitation goals is the restoration of these abilities by favoring improvement in the quality of life and functional independence. Current literature highlights several benefits of robotic gait therapies in SCI individuals.

OBJECTIVES

The purpose of this study was to compare the robotic gait devices, and systematize the scientific evidences of these devices as a tool for rehabilitation of SCI individuals.

METHODS

A systematic review was carried out in which relevant articles were identified by searching the following databases: Cochrane Library, PubMed, PEDro and Capes Periodic. Two authors selected the articles which used a robotic device for rehabilitation of spinal cord injury.

RESULTS

Databases search found 2941 articles, 39 articles were included due to meet the inclusion criteria. The robotic devices presented distinct features, with increasing application in the last years. Studies have shown promising results regarding the reduction of pain perception and spasticity level; alteration of the proprioceptive capacity, sensitivity to temperature, vibration, pressure, reflex behavior, electrical activity at muscular and cortical level, classification of the injury level; increase in walking speed, step length and distance traveled; improvements in sitting posture, intestinal, cardiorespiratory, metabolic, tegmental and psychological functions.

CONCLUSIONS

This systematic review shows a significant progress encompassing robotic devices as an innovative and effective therapy for the rehabilitation of individuals with SCI.

Frontal-Plane Variability in Foot Orientation During Fatiguing Running Exercise in Individuals With Chronic Ankle Instability

CONTEXT

  Researchers have reported increased variability in frontal-plane movement at the ankle during jumping in individuals with chronic ankle instability (CAI), which may increase their risk of recurrent ankle sprain. It is not known if this behavior is present during running gait or how fatigue affects the amount of frontal-plane-movement variability in individuals with CAI.

OBJECTIVE

  To investigate the amount of roll-angle variability at the foot during a fatiguing exercise protocol in participants with CAI.

DESIGN

  Controlled laboratory study.

SETTING

  Motion-analysis research laboratory.

PATIENTS OR OTHER PARTICIPANTS

  A total of 18 volunteers with CAI (10 men, 8 women; age = 29.8 ± 9.2 years, height = 175.8 ± 11.2 cm, mass = 75.4 ± 10.7 kg) and 17 volunteers serving as controls (8 men, 9 women; age = 28.2 ± 6.3 years, height = 172.3 ± 10.6 cm, mass = 68.8 ± 12.9 kg).

INTERVENTION(S)

  Kinematic data for foot position were collected while participants performed a functional fatigue protocol based on shuttle runs.

MAIN OUTCOME MEASURE(S)

  Variability (ie, standard deviation) of the roll angle of the foot about the x-axis, corresponding to inversion-eversion, was measured at 2 discrete times: 50 milliseconds before foot strike and 65% of stance.

RESULTS

  No differences in roll-angle range or variability were observed between limbs in either group. At 65% of stance, we found a main effect for time, whereby both groups demonstrated decreased roll-angle ranges at the end of the fatigue protocol ( P = .01). A between-groups effect in the roll-angle variability at 65% of stance was noted ( P = .04), with the CAI group exhibiting higher levels of variability. No between-groups differences were observed at 50 milliseconds before foot strike.

CONCLUSIONS

  Chronic ankle instability is a complex, multifactorial condition that can affect patients in diverse ways. Identifying excessive foot-position variability in particular situations could potentially inform targeted rehabilitation programs.

Feed-Forwardness of Spinal Networks in Posture and Locomotion

We present a new perspective on the concept of feed-forward compared to feedback mechanisms for motor control. We propose that conceptually all sensory information in real time provided to the brain and spinal cord can be viewed as a feed-forward phenomenon. We also propose that the spinal cord continually adapts to a broad array of ongoing sensory information that is used to adjust the probability of making timely and predictable decisions of selected networks that will execute a given response. One interpretation of the term feedback historically entails responses with short delays. We propose that feed-forward mechanisms, however, range in timeframes of milliseconds to an evolutionary perspective, that is, "evolutionary learning." Continuously adapting events enable a high level of automaticity within the sensorimotor networks that mediate "planned" motor tasks. We emphasize that either a very small or a very large proportion of motor responses can be under some level of conscious vs automatic control. Furthermore, we make a case that a major component of automaticity of the neural control of movement in vertebrates is located within spinal cord networks. Even without brain input, the spinal cord routinely uses feed-forward processing of sensory information, particularly proprioceptive and cutaneous, to continuously make fundamental decisions that define motor responses. In effect, these spinal networks may be largely responsible for executing coordinated sensorimotor tasks, even those under normal "conscious" control.

Effects of spinal cord injury-induced changes in muscle activation on foot drag in a computational rat ankle model

Spinal cord injury (SCI) can lead to changes in muscle activation patterns and atrophy of affected muscles. Moderate levels of SCI are typically associated with foot drag during the swing phase of locomotion. Foot drag is often used to assess locomotor recovery, but the causes remain unclear. We hypothesized that foot drag results from inappropriate muscle coordination preventing flexion at the stance-to-swing transition. To test this hypothesis and to assess the relative contributions of neural and muscular changes on foot drag, we developed a two-dimensional, one degree of freedom ankle musculoskeletal model with gastrocnemius and tibialis anterior muscles. Anatomical data collected from sham-injured and incomplete SCI (iSCI) female Long-Evans rats as well as physiological data from the literature were used to implement an open-loop muscle dynamics model. Muscle insertion point motion was calculated with imposed ankle trajectories from kinematic analysis of treadmill walking in sham-injured and iSCI animals. Relative gastrocnemius deactivation and tibialis anterior activation onset times were varied within physiologically relevant ranges based on simplified locomotor electromyogram profiles. No-atrophy and moderate muscle atrophy as well as normal and injured muscle activation profiles were also simulated. Positive moments coinciding with the transition from stance to swing phase were defined as foot swing and negative moments as foot drag. Whereas decreases in activation delay caused by delayed gastrocnemius deactivation promote foot drag, all other changes associated with iSCI facilitate foot swing. Our results suggest that even small changes in the ability to precisely deactivate the gastrocnemius could result in foot drag after iSCI.

Metabolic cost of lateral stabilization during walking in people with incomplete spinal cord injury

People with incomplete spinal cord injury (iSCI) expend considerable energy to walk, which can lead to rapid fatigue and limit community ambulation. Selecting locomotor patterns that enhance lateral stability may contribute to this population's elevated cost of transport. The goal of the current study was to quantify the metabolic energy demands of maintaining lateral stability during gait in people with iSCI. To quantify this metabolic cost, we observed ten individuals with iSCI walking with and without external lateral stabilization. We hypothesized that with external lateral stabilization, people with iSCI would adapt their gait by decreasing step width, which would correspond with a substantial decrease in cost of transport. Our findings support this hypothesis. Subjects significantly (p<0.05) decreased step width by 22%, step width variability by 18%, and minimum lateral margin of stability by 25% when they walked with external lateral stabilization compared to unassisted walking. Metabolic cost of transport also decreased significantly (p<0.05) by 10% with external lateral stabilization. These findings suggest that this population is capable of adapting their gait to meet changing demands placed on balance. The percent reduction in cost of transport when walking with external lateral stabilization was strongly correlated with functional impairment level as assessed by subjects' scores on the Berg Balance Scale (r=0.778) and lower extremity motor score (r=0.728). These relationships suggest that as functional balance and strength decrease, the amount of metabolic energy used to maintain lateral stability during gait will increase.

Effects of traumatic brain injury on locomotor adaptation

BACKGROUND AND PURPOSE

Locomotor adaptation is a form of short-term learning that enables gait modifications and reduces movement errors when the environment changes. This adaptation is critical for community ambulation for example, when walking on different surfaces. While many individuals with traumatic brain injury (TBI) recover basic ambulation, less is known about recovery of more complex locomotor skills, like adaptation. The purpose of this study was to investigate how TBI affects locomotor adaptation.

METHODS

Fourteen adults with TBI and 11 nondisabled comparison participants walked for 15 minutes on a split-belt treadmill with 1 belt moving at 0.7 m/s, and the other at 1.4 m/s. Subsequently, aftereffects were assessed and de-adapted during 15 minutes of tied-belt walking (both belts at 0.7 m/s).

RESULTS

Participants with TBI showed greater asymmetry in interlimb coordination on split-belts than the comparison group. Those with TBI did not adapt back to baseline symmetry, and some individuals did not store significant aftereffects. Greater asymmetry on split-belts and smaller aftereffects were associated with greater ataxia.

DISCUSSION

Participants with TBI were more perturbed by walking on split-belts and showed some impairment in adaptation. This suggests a reduced ability to learn a new form of coordination to compensate for environmental changes. Multiple interacting factors, including cerebellar damage and impairments in higher-level cognitive processes, may influence adaptation post-TBI.

CONCLUSIONS

Gait adaptation to novel environment demands is impaired in persons with chronic TBI and may be an important skill to target in rehabilitation.

VIDEO ABSTRACT AVAILABLE

(See Video, Supplemental Digital Content 1, http://links.lww.com/JNPT/A74) for more insights from the authors.

Evidence for a time-invariant phase variable in human ankle control

Human locomotion is a rhythmic task in which patterns of muscle activity are modulated by state-dependent feedback to accommodate perturbations. Two popular theories have been proposed for the underlying embodiment of phase in the human pattern generator: a time-dependent internal representation or a time-invariant feedback representation (i.e., reflex mechanisms). In either case the neuromuscular system must update or represent the phase of locomotor patterns based on the system state, which can include measurements of hundreds of variables. However, a much simpler representation of phase has emerged in recent designs for legged robots, which control joint patterns as functions of a single monotonic mechanical variable, termed a phase variable. We propose that human joint patterns may similarly depend on a physical phase variable, specifically the heel-to-toe movement of the Center of Pressure under the foot. We found that when the ankle is unexpectedly rotated to a position it would have encountered later in the step, the Center of Pressure also shifts forward to the corresponding later position, and the remaining portion of the gait pattern ensues. This phase shift suggests that the progression of the stance ankle is controlled by a biomechanical phase variable, motivating future investigations of phase variables in human locomotor control.

Joint-specific changes in locomotor complexity in the absence of muscle atrophy following incomplete spinal cord injury

Background

Following incomplete spinal cord injury (iSCI), descending drive is impaired, possibly leading to a decrease in the complexity of gait. To test the hypothesis that iSCI impairs gait coordination and decreases locomotor complexity, we collected 3D joint angle kinematics and muscle parameters of rats with a sham or an incomplete spinal cord injury.

Methods

12 adult, female, Long-Evans rats, 6 sham and 6 mild-moderate T8 iSCI, were tested 4 weeks following injury. The Basso Beattie Bresnahan locomotor score was used to verify injury severity. Animals had reflective markers placed on the bony prominences of their limb joints and were filmed in 3D while walking on a treadmill. Joint angles and segment motion were analyzed quantitatively, and complexity of joint angle trajectory and overall gait were calculated using permutation entropy and principal component analysis, respectively. Following treadmill testing, the animals were euthanized and hindlimb muscles removed. Excised muscles were tested for mass, density, fiber length, pennation angle, and relaxed sarcomere length.

Results

Muscle parameters were similar between groups with no evidence of muscle atrophy. The animals showed overextension of the ankle, which was compensated for by a decreased range of motion at the knee. Left-right coordination was altered, leading to left and right knee movements that are entirely out of phase, with one joint moving while the other is stationary. Movement patterns remained symmetric. Permutation entropy measures indicated changes in complexity on a joint specific basis, with the largest changes at the ankle. No significant difference was seen using principal component analysis. Rats were able to achieve stable weight bearing locomotion at reasonable speeds on the treadmill despite these deficiencies.

Conclusions

Decrease in supraspinal control following iSCI causes a loss of complexity of ankle kinematics. This loss can be entirely due to loss of supraspinal control in the absence of muscle atrophy and may be quantified using permutation entropy. Joint-specific differences in kinematic complexity may be attributed to different sources of motor control. This work indicates the importance of the ankle for rehabilitation interventions following spinal cord injury.

Robotic loading during treadmill training enhances locomotor recovery in rats spinally transected as neonates

Loading on the limbs has a powerful influence on locomotion. In the present study, we examined whether robotic-enhanced loading during treadmill training improved locomotor recovery in rats that were spinally transected as neonates. A robotic device applied a force on the ankle of the hindlimb while the rats performed bipedal stepping on a treadmill. The robotic force enhanced loading during the stance phase of the step cycle. One group of spinally transected rats received 4 wk of bipedal treadmill training with robotic loading while another group received 4 wk of bipedal treadmill training but without robotic loading. The two groups exhibited similar stepping performance during baseline tests of bipedal treadmill stepping. However, after 4 wk, the spinally transected rats that received bipedal treadmill training with robotic loading performed significantly more weight-bearing steps than the bipedal treadmill training only group. Bipedal treadmill training with robotic loading enhanced the ankle trajectory and ankle velocity during the step cycle. Based on immunohistochemical analyses, the expression of the presynaptic marker, synaptophysin, was significantly greater in the ventral horn of the lumbar spinal cord of the rats that received bipedal treadmill training with robotic loading. These findings suggested that robotic loading during bipedal treadmill training improved the ability of the lumbar spinal cord to generate stepping. The results have implications for the use of robotic-enhanced gait training therapies that encourage motor learning after spinal cord injury.

Locomotor adaptation to a soleus EMG-controlled antagonistic exoskeleton

Locomotor adaptation in humans is not well understood. To provide insight into the neural reorganization that occurs following a significant disruption to one's learned neuromuscular map relating a given motor command to its resulting muscular action, we tied the mechanical action of a robotic exoskeleton to the electromyography (EMG) profile of the soleus muscle during walking. The powered exoskeleton produced an ankle dorsiflexion torque proportional to soleus muscle recruitment thus limiting the soleus' plantar flexion torque capability. We hypothesized that neurologically intact subjects would alter muscle activation patterns in response to the antagonistic exoskeleton by decreasing soleus recruitment. Subjects practiced walking with the exoskeleton for two 30-min sessions. The initial response to the perturbation was to "fight" the resistive exoskeleton by increasing soleus activation. By the end of training, subjects had significantly reduced soleus recruitment resulting in a gait pattern with almost no ankle push-off. In addition, there was a trend for subjects to reduce gastrocnemius recruitment in proportion to the soleus even though only the soleus EMG was used to control the exoskeleton. The results from this study demonstrate the ability of the nervous system to recalibrate locomotor output in response to substantial changes in the mechanical output of the soleus muscle and associated sensory feedback. This study provides further evidence that the human locomotor system of intact individuals is highly flexible and able to adapt to achieve effective locomotion in response to a broad range of neuromuscular perturbations.