Optimal reaching subject to computational and physical constraints reveals structure of the sensorimotor control system.

Optimal feedback control provides an abstract framework describing the architecture of the sensorimotor system without prescribing implementation details such as what coordinate system to use, how feedback is incorporated, or how to accommodate changing task complexity. We investigate how such details are determined by computational and physical constraints by creating a model of the upper limb sensorimotor system in which all connection weights between neurons, feedback, and muscles are unknown. By optimizing these parameters with respect to an objective function, we find that the model exhibits a preference for an intrinsic (joint angle) coordinate representation of inputs and feedback and learns to calculate a weighted feedforward and feedback error. We further show that complex reaches around obstacles can be achieved by augmenting our model with a path-planner based on via points. The path-planner revealed "avoidance" neurons that encode directions to reach around obstacles and "placement" neurons that make fine-tuned adjustments to via point placement. Our results demonstrate the surprising capability of computationally constrained systems and highlight interesting characteristics of the sensorimotor system.

Closing the Loop: From Motor Neuroscience to Neurorehabilitation.

The fields of human motor control, motor learning, and neurorehabilitation have long been linked by the intuition that understanding how we move (and learn to move) leads to better rehabilitation. In reality, these fields have remained largely separate. Our knowledge of the neural control of movement has expanded, but principles that can directly impact rehabilitation efficacy remain somewhat sparse. This raises two important questions: What can basic studies of motor learning really tell us about rehabilitation, and are we asking the right questions to improve the lives of patients? This review aims to contextualize recent advances in computational and behavioral studies of human motor learning within the framework of neurorehabilitation. We also discuss our views of the current challenges facing rehabilitation and outline potential clinical applications from recent theoretical and basic studies of motor learning and control.

Joint-level coordination patterns for split-belt walking across different speed ratios.

Locomotion is a highly flexible process, requiring rapid changes to gait due to changes in the environment or goals. Here, we used a split-belt treadmill to examine how the central nervous system coordinates a novel gait pattern. Existing research has focused on summary measures, most often step lengths, when describing changes induced while walking on the split-belt treadmill and during subsequent aftereffects. Here, we asked how the nervous system adjusts individual joint motions and the coordination pattern of the legs when people walk with one leg moving at either 2×, 3×, or 4× the speed of the other leg. We found that relative to tied-belt walking, split-belt perturbations change the timing relationships between the legs while most joint angle peaks and range of motion change little. The kinematic changes over the course of adaptation (i.e., from the beginning to end of a single split-belt walking bout) were subtle, particularly when comparing individual joint motions. The magnitude of the belt speed differences impacted intralimb coordination but did not produce consistent differences in most other measures. Most significant changes in kinematics occurred in the fast leg. Overall, interlimb timing changes drove a large proportion of the differences observed between tied-belt and split-belt gaits. Thus, it appears that the central nervous system can produce novel gait patterns through changes in coordination between legs that lead to new configurations at significant time points. These patterns can use within-limb and within-joint patterns that closely resemble those of normal walking.NEW & NOTEWORTHY We studied how the nervous system coordinates limb movements during asymmetric gait. Using a split-belt treadmill, we found that most changes in motion occurred when comparing motions between limbs, rather than among joints within a limb. Individual joint patterns resembled speed-matched comparisons, but this meant that joint movements became asymmetric during split-belt walking. These findings demonstrate that the nervous system can use consistent joint motions that are reconfigured in time to achieve new gait patterns.

Dissociation between abnormal motor synergies and impaired reaching dexterity after stroke.

Most patients with stroke experience motor deficits, usually referred to collectively as hemiparesis. Although hemiparesis is one of the most common and clinically recognizable motor abnormalities, it remains undercharacterized in terms of its behavioral subcomponents and their interactions. Hemiparesis comprises both negative and positive motor signs. Negative signs consist of weakness and loss of motor control (dexterity), whereas positive signs consist of spasticity, abnormal resting posture, and intrusive movement synergies (abnormal muscle co-activations during voluntary movement). How positive and negative signs interact, and whether a common mechanism generates them, remains poorly understood. Here, we used a planar, arm-supported reaching task to assess poststroke arm dexterity loss, which we compared with the Fugl-Meyer stroke scale; a measure primarily reflecting abnormal synergies. We examined 53 patients with hemiparesis after a first-time ischemic stroke. Reaching kinematics were markedly more impaired in patients with subacute (<3 mo) compared to chronic (>6 mo) stroke even for similar Fugl-Meyer scores. This suggests a dissociation between abnormal synergies (reflected in the Fugl-Meyer scale) and loss of dexterity, which in turn suggests different underlying mechanisms. Moreover, dynamometry suggested that Fugl-Meyer scores capture weakness as well as abnormal synergies, in line with these two deficits sharing a neural substrate. These findings have two important implications: First, clinical studies that test for efficacy of rehabilitation interventions should specify which component of hemiparesis they are targeting and how they propose to measure it. Metrics used widely for this purpose may not always be chosen appropriately. For example, as we show here, the Fugl-Meyer score may capture some hemiparesis components (abnormal synergies and weakness) but not others (loss of dexterity). Second, there may be an opportunity to design rehabilitation interventions to address specific subcomponents of hemiparesis.NEW & NOTEWORTHY Motor impairment is common after stroke and comprises reduced dexterity, weakness, and abnormal muscle synergies. Here we report that, when matched on an established synergy and weakness scale (Fugl-Meyer), patients with subacute stroke have worse reaching dexterity than chronic ones. This result suggests that the components of hemiparesis are dissociable and have separable mechanisms and, thus, may require distinct assessments and treatments.

Reinforcement Signaling Can Be Used to Reduce Elements of Cerebellar Reaching Ataxia.

Damage to the cerebellum causes a disabling movement disorder called ataxia, which is characterized by poorly coordinated movement. Arm ataxia causes dysmetria (over- or under-shooting of targets) with many corrective movements. As a result, people with cerebellar damage exhibit reaching movements with highly irregular and prolonged movement paths. Cerebellar patients are also impaired in error-based motor learning, which may impede rehabilitation interventions. However, we have recently shown that cerebellar patients can learn a simple reaching task using a binary reinforcement paradigm, in which feedback is based on participants' mean performance. Here, we present a pilot study that examined whether patients with cerebellar damage can use this reinforcement training to learn a more complex motor task-to decrease the path length of their reaches. We compared binary reinforcement training to a control condition of massed practice without reinforcement feedback. In both conditions, participants made target-directed reaches in 3-dimensional space while vision of their movement was occluded. In the reinforcement training condition, reaches with a path length below participants' mean were reinforced with an auditory stimulus at reach endpoint. We found that patients were able to use reinforcement signaling to significantly reduce their reach paths. Massed practice produced no systematic change in patients' reach performance. Overall, our results suggest that binary reinforcement training can improve reaching movements in patients with cerebellar damage and the benefit cannot be attributed solely to repetition or reduced visual control.

Patients with Cerebellar Ataxia Do Not Benefit from Limb Weights.

Patients with cerebellar ataxia are sometimes treated by the addition of mass to the limbs, though this practice has received limited study. Recent work suggests that adding mass to the limbs might have predictable effects on the pattern of cerebellar dysmetria (i.e., over or undershooting) that depends on a hypothesized mismatch between the actual limb inertia and the brain's estimate of limb inertia. Based on this model, we predicted that addition of mass would only be effective in reducing dysmetria in hypometric patients. Cerebellar patients were challenged with making a single-joint, single degree of freedom reaching movement while various limb masses were tested. In this task, some single-jointed reaches were improved by adding masses that were optimized in a patient-specific manner. However, this improvement did not translate to multi-joint movements. In multi-joint movements, the "best" patient-specific masses (as determined in a single-joint task) generally exacerbated subjects' reaching errors. This finding raises questions as to the merits of adding limb weights as a therapy to mitigate the effects of dysmetria.

Individualized feedback to change multiple gait deficits in chronic stroke.

BACKGROUND: Walking deficits in people post-stroke are often multiple and idiosyncratic in nature. Limited patient and therapist resources necessitate prioritization of deficits such that some may be left unaddressed. More efficient delivery of therapy may alleviate this challenge. Here, we look to determine the utility of a novel principal component-based visual feedback system that targets multiple, patient-specific features of gait in people post-stroke. METHODS: Ten individuals with stroke received two sessions of visual feedback to attain a walking goal. This goal consisted of bilateral knee and hip joint angles of a typical 'healthy' walking pattern. The feedback system uses principal component analysis (PCA) to algorithmically weight each of the input features so that participants received one stream of performance feedback. In the first session, participants had to explore different patterns to achieve the goal, and in the second session they were informed of the goal walking pattern. Ten healthy, age-matched individuals received the same paradigm, but with a hemiparetic goal (i.e. to produce the pattern of an exemplar stroke participant). This was to distinguish the extent to which performance limitations in stroke were due neurological injury or the PCA based visual feedback itself. RESULTS: Principal component-based visual feedback can differentially bias multiple features of walking toward a prescribed goal. On average, individuals with stroke typically improved performance via increased paretic knee and hip flexion, and did not perform better with explicit instruction. In contrast, healthy people performed better (i.e. could produce the desired exemplar stroke pattern) in both sessions, and were best with explicit instruction. Importantly, the feedback for stroke participants accommodated a heterogeneous set of walking deficits by individually weighting each feature based on baseline walking. CONCLUSIONS: People with and without stroke are able to use this novel visual feedback to train multiple, specific features of gait. Important for stroke, the PCA feedback allowed for targeting of patient-specific deficits. This feedback is flexible to any feature of walking in any plane of movement, thus providing a potential tool for therapists to simultaneously target multiple aberrant features of gait.

Motor Learning Enhances Use-Dependent Plasticity.

Motor behaviors are shaped not only by current sensory signals but also by the history of recent experiences. For instance, repeated movements toward a particular target bias the subsequent movements toward that target direction. This process, called use-dependent plasticity (UDP), is considered a basic and goal-independent way of forming motor memories. Most studies consider movement history as the critical component that leads to UDP (Classen et al., 1998; Verstynen and Sabes, 2011). However, the effects of learning (i.e., improved performance) on UDP during movement repetition have not been investigated. Here, we used transcranial magnetic stimulation in two experiments to assess plasticity changes occurring in the primary motor cortex after individuals repeated reinforced and nonreinforced actions. The first experiment assessed whether learning a skill task modulates UDP. We found that a group that successfully learned the skill task showed greater UDP than a group that did not accumulate learning, but made comparable repeated actions. The second experiment aimed to understand the role of reinforcement learning in UDP while controlling for reward magnitude and action kinematics. We found that providing subjects with a binary reward without visual feedback of the cursor led to increased UDP effects. Subjects in the group that received comparable reward not associated with their actions maintained the previously induced UDP. Our findings illustrate how reinforcing consistent actions strengthens use-dependent memories and provide insight into operant mechanisms that modulate plastic changes in the motor cortex.SIGNIFICANCE STATEMENT Performing consistent motor actions induces use-dependent plastic changes in the motor cortex. This plasticity reflects one of the basic forms of human motor learning. Past studies assumed that this form of learning is exclusively affected by repetition of actions. However, here we showed that success-based reinforcement signals could affect the human use-dependent plasticity (UDP) process. Our results indicate that learning augments and interacts with UDP. This effect is important to the understanding of the interplay between the different forms of motor learning and suggests that reinforcement is not only important to learning new behaviors, but can shape our subsequent behavior via its interaction with UDP.

Movement and perception recalibrate differently across multiple days of locomotor learning.

Learning a new movement through error-based adaptation leads to recalibration of movement and altered perception of that movement. Although presumed to be closely related, the relationship between adaptation-based motor and perceptual changes is not well understood. Here we investigated the changes in motor behavior and leg speed perception over 5 days of split-belt treadmill adaptation. We specifically wanted to know if changes in the perceptual domain would demonstrate savings-like behavior (i.e., less recalibration with more practice) and if these changes would parallel the savings observed in the motor domain. We found that the recalibration of leg speed perception decreased across days of training, indicating savings-like behavior in this domain. However, we observed that the magnitude of savings across days was different between motor and perceptual domains. These findings suggest a degree of independence between the motor and perceptual processes that occur with locomotor adaptation. NEW & NOTEWORTHY Error-based adaptation learning drives changes in movement and perception of movement. Are these changes across domains linked or simply coincidental? Here, we studied changes in movement and perception across 5 days of repeated locomotor adaptation. Savings-like behavior in the motor and perceptual domains developed with different magnitudes and over different timescales, leading us to conclude that motor and perceptual processes operate at least somewhat independently during locomotor adaptation.

Accelerating locomotor savings in learning: compressing four training days to one.

Acquiring new movements requires the capacity of the nervous system to remember previously experienced motor patterns. The phenomenon of faster relearning after initial learning is termed "savings." Here we studied how savings of a novel walking pattern develops over several days of practice and how this process can be accelerated. We introduced participants to a split-belt treadmill adaptation paradigm for 30 min for 5 consecutive days. By training day 5, participants were able to produce near-perfect performance when switching between split and tied-belt environments. We found that this was due to their ability to shift specific elements of their stepping pattern to account for the split treadmill speeds from day to day. We also applied a state-space model to further characterize multiday locomotor savings. We then explored methods of achieving comparable savings with less total training time. We studied people training only on day 1, with either one extended split-belt exposure or alternating four times between split-belt and tied-belt conditions rapidly in succession. Both of these single-day training groups were tested again on day 5. Experiencing four abbreviated exposures on day 1 improved the performance on day 5 compared with one extended exposure on day 1. Moreover, this abbreviated group performed similarly to the group that trained for 4 consecutive days before testing on day 5, despite only having one-quarter of the total training time. These results demonstrate that we can leverage training structure to achieve a high degree of performance while minimizing training sessions. NEW & NOTEWORTHY Learning a new movement requires repetition. Here, we demonstrate how to more efficiently train an adapted walking pattern. By compressing split-belt treadmill training delivered over 4 days to four abbreviated bouts of training delivered on the first day of training, we were able to induce equivalent savings over a 5-day span. These results suggest that we can manipulate the delivery of training to most efficiently drive multiday learning of a novel walking pattern.

Making Sense of Cerebellar Contributions to Perceptual and Motor Adaptation.

The cerebellum is thought to adapt movements to changes in the environment in order to update an implicit understanding of the association between our motor commands and their sensory consequences. This trial-by-trial motor recalibration in response to external perturbations is frequently impaired in people with cerebellar damage. In healthy people, adaptation to motor perturbations is also known to induce a form of sensory perceptual recalibration. For instance, hand-reaching adaptation tasks produce transient changes in the sense of hand position, and walking adaptation tasks can lead to changes in perceived leg speed. Though such motor adaptation tasks are heavily dependent on the cerebellum, it is not yet understood how the cerebellum is associated with these accompanying sensory recalibration processes. Here we asked if the cerebellum is required for the recalibration of leg-speed perception that normally occurs alongside locomotor adaptation, as well as how ataxia severity is related to sensorimotor recalibration deficits in patients with cerebellar damage. Cerebellar patients performed a speed-matching task to assess perception of leg speed before and after walking on a split-belt treadmill, which has two belts driving each leg at a different speed. Healthy participants update their perception of leg speed following split-belt walking such that the "fast" leg during adaptation feels slower afterwards, whereas cerebellar patients have significant deficits in this sensory perceptual recalibration. Furthermore, our analysis demonstrates that ataxia severity is a crucial factor for both the sensory and motor adaptation impairments that affect patients with cerebellar damage.

The cerebellum contributes to proprioception during motion.

Proprioception, the sense of limb position and motion, is essential for generating accurate movements. Limb position sense has typically been studied under static conditions (i.e., the fixed position of a limb in space), with less known about dynamic position sense (i.e., limb position during movement). Here we investigated how a person's estimate of hand position varies when using spatial or temporal information to judge the unseen hand's location during reaching. We assessed the acuity of dynamic position sense in two directions, orthogonal to hand movement, which only requires spatial information, and in line with hand movement, which has both spatial and temporal components. Our results showed that people have better proprioceptive acuity in the orthogonal condition where only spatial information is used. We then assessed whether cerebellar damage impairs proprioceptive acuity in both tasks during passive and active movement. Cerebellar patients showed reduced acuity in both tasks and in both movement conditions relative to age-matched controls. However, patients' deficits were most apparent when judgments of active movement relied on temporal information. Furthermore, both cerebellar patient and control performance correlated with the trial-to-trial variability of their active movements: subjects are worse at the proprioceptive tasks when movements are variable. Our results suggest that, during active movements, proprioceptive acuity may be reliant on the motor system's ability to predict motor output. Therefore, the resultant proprioceptive deficits occurring after cerebellar damage may be related to a more general impairment in movement prediction.NEW & NOTEWORTHY We assessed limb position sense during movement in patients with cerebellar damage and found deficits in proprioceptive acuity during both passive and active movement. The effect of cerebellar damage was most apparent when individuals relied on both timing and spatial information during active movement. Thus proprioceptive acuity during active movements may be reliant on the motor system's ability to predict motor output.

Proprioceptive Localization Deficits in People With Cerebellar Damage.

It has been hypothesized that an important function of the cerebellum is predicting the state of the body during movement. Yet, the extent of cerebellar involvement in perception of limb state (i.e., proprioception, specifically limb position sense) has yet to be determined. Here, we investigated whether patients with cerebellar damage have deficits when trying to locate their hand in space (i.e., proprioceptive localization), which is highly important for everyday movements. By comparing performance during passive robot-controlled and active self-made multi-joint movements, we were able to determine that some cerebellar patients show improved precision during active movement (i.e., active benefit), comparable to controls, whereas other patients have reduced active benefit. Importantly, the differences in patient performance are not explained by patient diagnosis or clinical ratings of impairment. Furthermore, a subsequent experiment confirmed that active deficits in proprioceptive localization occur during both single-joint and multi-joint movements. As such, it is unlikely that localization deficits can be explained by the multi-joint coordination deficits occurring after cerebellar damage. Our results suggest that cerebellar damage may cause varied impairments to different elements of proprioceptive sense. It follows that proprioceptive localization should be adequately accounted for in clinical testing and rehabilitation of people with cerebellar damage.

Effective reinforcement learning following cerebellar damage requires a balance between exploration and motor noise.

Reinforcement and error-based processes are essential for motor learning, with the cerebellum thought to be required only for the error-based mechanism. Here we examined learning and retention of a reaching skill under both processes. Control subjects learned similarly from reinforcement and error-based feedback, but showed much better retention under reinforcement. To apply reinforcement to cerebellar patients, we developed a closed-loop reinforcement schedule in which task difficulty was controlled based on recent performance. This schedule produced substantial learning in cerebellar patients and controls. Cerebellar patients varied in their learning under reinforcement but fully retained what was learned. In contrast, they showed complete lack of retention in error-based learning. We developed a mechanistic model of the reinforcement task and found that learning depended on a balance between exploration variability and motor noise. While the cerebellar and control groups had similar exploration variability, the patients had greater motor noise and hence learned less. Our results suggest that cerebellar damage indirectly impairs reinforcement learning by increasing motor noise, but does not interfere with the reinforcement mechanism itself. Therefore, reinforcement can be used to learn and retain novel skills, but optimal reinforcement learning requires a balance between exploration variability and motor noise.

Motor learning in childhood reveals distinct mechanisms for memory retention and re-learning.

Adults can easily learn and access multiple versions of the same motor skill adapted for different conditions (e.g., walking in water, sand, snow). Following even a single session of adaptation, adults exhibit clear day-to-day retention and faster re-learning of the adapted pattern. Here, we studied the retention and re-learning of an adapted walking pattern in children aged 6-17 yr. We found that all children, regardless of age, showed adult-like patterns of retention of the adapted walking pattern. In contrast, children under 12 yr of age did not re-learn faster on the next day after washout had occurred-they behaved as if they had never adapted their walking before. Re-learning could be improved in younger children when the adaptation time on day 1 was increased to allow more practice at the plateau of the adapted pattern, but never to adult-like levels. These results show that the ability to store a separate, adapted version of the same general motor pattern does not fully develop until adolescence, and furthermore, that the mechanisms underlying the retention and rapid re-learning of adapted motor patterns are distinct.

A dual-learning paradigm can simultaneously train multiple characteristics of walking.

Impairments in human motor patterns are complex: what is often observed as a single global deficit (e.g., limping when walking) is actually the sum of several distinct abnormalities. Motor adaptation can be useful to teach patients more normal motor patterns, yet conventional training paradigms focus on individual features of a movement, leaving others unaddressed. It is known that under certain conditions, distinct movement components can be simultaneously adapted without interference. These previous "dual-learning" studies focused solely on short, planar reaching movements, yet it is unknown whether these findings can generalize to a more complex behavior like walking. Here we asked whether a dual-learning paradigm, incorporating two distinct motor adaptation tasks, can be used to simultaneously train multiple components of the walking pattern. We developed a joint-angle learning task that provided biased visual feedback of sagittal joint angles to increase peak knee or hip flexion during the swing phase of walking. Healthy, young participants performed this task independently or concurrently with another locomotor adaptation task, split-belt treadmill adaptation, where subjects adapted their step length symmetry. We found that participants were able to successfully adapt both components of the walking pattern simultaneously, without interference, and at the same rate as adapting either component independently. This leads us to the interesting possibility that combining rehabilitation modalities within a single training session could be used to help alleviate multiple deficits at once in patients with complex gait impairments.

Blocking trial-by-trial error correction does not interfere with motor learning in human walking.

Movements can be learned implicitly in response to new environmental demands or explicitly through instruction and strategy. The former is often studied in an environment that perturbs movement so that people learn to correct the errors and store a new motor pattern. Here, we demonstrate in human walking that implicit learning of foot placement occurs even when an explicit strategy is used to block changes in foot placement during the learning process. We studied people learning a new walking pattern on a split-belt treadmill with and without an explicit strategy through instruction on where to step. When there is no instruction, subjects implicitly learn to place one foot in front of the other to minimize step-length asymmetry during split-belt walking, and the learned pattern is maintained when the belts are returned to the same speed, i.e., postlearning. When instruction is provided, we block expression of the new foot-placement pattern that would otherwise naturally develop from adaptation. Despite this appearance of no learning in foot placement, subjects show similar postlearning effects as those who were not given any instruction. Thus locomotor adaptation is not dependent on a change in action during learning but instead can be driven entirely by an unexpressed internal recalibration of the desired movement.

Age-related forgetting in locomotor adaptation.

The healthy aging process affects the ability to learn and remember new facts and tasks. Prior work has shown that motor learning can be adversely affected by non-motor deficits, such as time. Here we investigated how age, and a dual task influence the learning and forgetting of a new walking pattern. We studied healthy younger (<30 yo) and older adults (>50 yo) as they alternated between 5-min bouts of split-belt treadmill walking and resting. Older subjects learned a new walking pattern at the same rate as younger subjects, but forgot some of the new pattern during the rest breaks. We tested if forgetting was due to reliance on a cognitive strategy that was not fully engaged after rest breaks. When older subjects performed a dual cognitive task to reduce strategic control of split-belt walking, their adaptation rate slowed, but they still forgot much of the new pattern during the rest breaks. Our results demonstrate that the healthy aging process is one component that weakens motor memories during rest breaks and that this phenomenon cannot be explained solely by reliance on a conscious strategy in older adults.

Two ways to save a newly learned motor pattern.

Savings, or faster relearning after initial learning, demonstrates humans' remarkable ability to retain learned movements amid changing environments. This is important within the context of locomotion, as the ability of the nervous system to "remember" how to walk in specific environments enables us to navigate changing terrains and progressively improve gait patterns with rehabilitation. Here, we used a split-belt treadmill to study precisely how people save newly learned walking patterns. In Experiment 1, we investigated savings by systematically varying the learning and unlearning environments. Savings was predominantly influenced by 1) previous exposure to similar abrupt changes in the environment and 2) the amount of exposure to the new environment. Relearning was fastest when these two factors coincided, and we did not observe savings after the environment was introduced gradually during initial learning. In Experiment 2, we then studied whether people store explicit information about different walking environments that mirrors savings of a new walking pattern. Like savings, we found that previous exposure to abrupt changes in the environment also drove the ability to recall a previously experienced walking environment accurately. Crucially, the information recalled was extrinsic information about the learning environment (i.e., treadmill speeds) and not intrinsic information about the walking pattern itself. We conclude that simply learning a new walking pattern is not enough for long-term savings; rather, savings of a learned walking pattern involves recall of the environment or extended training at the learned state.

A marching-walking hybrid induces step length adaptation and transfers to natural walking.

Walking is highly adaptable to new demands and environments. We have previously studied adaptation of locomotor patterns via a split-belt treadmill, where subjects learn to walk with one foot moving faster than the other. Subjects learn to adapt their walking pattern by changing the location (spatial) and time (temporal) of foot placement. Here we asked whether we can induce adaptation of a specific walking pattern when one limb does not "walk" but instead marches in place (i.e., marching-walking hybrid). The marching leg's movement is limited during the stance phase, and thus certain sensory signals important for walking may be reduced. We hypothesized that this would produce a spatial-temporal strategy different from that of normal split-belt adaptation. Healthy subjects performed two experiments to determine whether they could adapt their spatial-temporal pattern of step lengths during the marching-walking hybrid and whether the learning transfers to over ground walking. Results showed that the hybrid group did adapt their step lengths, but the time course of adaptation and deadaption was slower than that for the split-belt group. We also observed that the hybrid group utilized a mostly spatial strategy whereas the split-belt group utilized both spatial and temporal strategies. Surprisingly, we found no significant difference between the hybrid and split-belt groups in over ground transfer. Moreover, the hybrid group retained more of the learned pattern when they returned to the treadmill. These findings suggest that physical rehabilitation with this marching-walking paradigm on conventional treadmills may produce changes in symmetry comparable to what is observed during split-belt training.