The goal of this clinical trial is to evaluate the effect of a 4-week perturbation-based balance training with and without targeted Neuromuscular electrical stimulation on neuromechanic gait fall risk predictor and reactive balance.

The aim of this study is to investigate whether a 4-weeks perturbation-based balance training can improve kinematic and spatiotemporal parameters of reactive balance control, and kinematic and neuromuscular gait parameters, described as early fall risk predictors, in persons with stroke. Additionally, a second aim is to determine whether an impairment-oriented intervention aimed to correct the gait patterns during the proposed walking perturbation training, using a targeted neuromuscular electrical stimulation (NMES) applied to the rectus femoris and tibialis anterior muscles, could enhance the potential benefits of the proposed training protocol among stroke population.

Background. Although falls are multi-factorial, muscle weakness, gait deficits and impaired compensatory stepping responses are key contributing factors for falls in persons with stroke. From a control mechanism perspective, it is postulated that stepping strategies may be represented by the muscle activity pattern triggered via long-loop brain stem responses with cortical modulation. However, due to the sensorimotor impairments, commonly observed in stroke population, components of muscle responses (early and late) are delayed and smaller in amplitude, which affect the motor responses to unexpected balance disturbances. Perturbation-based balance training is a novel therapeutic strategy that incorporates exposure to repeated support surface disturbances simulating environmental perturbations like slips and trips to evoke rapid reactive balance reactions. Additionally, it has been described that reactive balance control and sensorimotor adaptation are dependent on severity of motor impairment. Thus, development of therapeutic interventions aiming to reduce training times and/or facilitate the inclusion of persons with moderate to severe sensorimotor impairments are crucial for the implementation of reactive balance training protocols into clinical practice.

In the present proposal, the investigators aim to investigate whether a 4-weeks perturbation-based balance training can improve kinematic and spatiotemporal parameters of reactive balance control, and kinematic and neuromuscular gait parameters, described as early fall risk predictors, in persons with stroke. Additionally, the investigators aim to determine whether an impairment-oriented intervention aimed to correct the gait patterns during the proposed walking perturbation training, using a targeted neuromuscular electrical stimulation (NMES) applied to the rectus femoris and tibialis anterior muscles, could enhance the potential benefits of the proposed training protocol among stroke population.

Methods. The study employs a primary two-arm randomized, controlled design to examine effects of a 4-weeks perturbation-based balance training with and without targeted NMES on reactive balance control and early gait fall risk predictors in stroke population. Forty participants will be randomly assigned (1:1) to the NMES (n=20) or No-NMES group (N=20). Participants will be asked to walk over a six by two meters computer-controlled movable platform at a self-selected speed. Slip-like perturbations will be induced by the device software that moves the platform 12 inches forward at 0.46 m/s with an acceleration of 9.4 m/s2. For each training session, participants will be asked to walk over the computer-controlled movable platform for six blocks of ten walking trials, among which there will be four perturbation trials per block. In total, all the participants will experience 24 perturbation trials per training session. Participants will be asked to come to the Laboratory two times per week, so each participant will complete 8 perturbation-based balance training sessions.

The following outcome measures will be assessed in the present research proposal: Perturbation outcomes (fall or recovery), Center of mass (CoM) stability, and limb support of the slipping (paretic) leg. All these outcomes measures will be assessed after an externally-induced balance perturbation before and after the 4-weeks perturbation-based balance training. On the other hand, step-to-step transition and altered neuromusuclar patterns, both described as early fall risk predictor and assessed through force platform and electromyography (EMG) respectively, will be assessed during a 5 minutes treadmill walking test at a self-selected speed before and after the training.

Expected result. The investigators expect that once the proposed training protocol is finished, participants will show less laboratory falls, and higher CoM stability and limb support values after experience a gait "slip-like" disturbance compared to a baseline assessment. Similarly, participants will show improved values in step-to-step transition (active mode transition) and less deviations from normal values of lower limb EMG patterns, compared to a baseline evaluation. Regarding the second aim, the investigators expect that once the proposed training protocol is finished, the group of persons with stroke that will perform the perturbation-based balance training with the gait pattern corrected (NMES group) will show less laboratory falls, and higher CoM stability and limb support values after experience a gait "slip-like" disturbance, and improved values in step-to-step transition (active mode transition) with less deviations from healthy participants´ lower limb EMG patterns, during an instrumented gait assessment, compared to the No NMES group.

The investigators believe that the description of changes on kinematic, spatiotemporal and neuromuscular parameters after a reactive balance training protocol could contribute to a better understanding of the mechanisms behind compensatory strategies aimed to regain postural control in persons with stroke. Furthermore, the investigators believe that the results of this project may be useful in developing new and effective therapeutic strategies to reduce falls among stroke population and may contribute to facilitate the translation of perturbation-based balance training into clinical practice.

Inclusion Criteria:

Healthy participants will be included if they passed a cognitive test (> 26/30 on Montreal Cognitive Assessment Scale) and finish a six-minute walk test independently (to ensure that these individuals can walk without balance or gait impairments)

Exclusion Criteria:

Healthy individuals will be excluded if they self-reported any neurological, musculoskeletal, or other systemic disorders that would affect the participant's locomotion.