Adaptive Functional Electrical Stimulation Delivers Stimulation Amplitudes Based on Real-Time Gait Biomechanics

Functional electrical stimulation (FES) is often used in poststroke gait rehabilitation to decrease foot drop and increase forward propulsion. However, not all stroke survivors experience clinically meaningful improvements in gait function following training with FES. The purpose of this work was to develop and validate a novel adaptive FES (AFES) system to improve dorsiflexor (DF) and plantarflexor (PF) stimulation timing and iteratively adjust the stimulation amplitude at each stride based on measured gait biomechanics. Stimulation timing was determined by a series of bilateral footswitches. Stimulation amplitude was calculated based on measured dorsiflexion angle and peak propulsive force, where increased foot drop and decreased paretic propulsion resulted in increased stimulation amplitudes. Ten individuals with chronic poststroke hemiparesis walked on an adaptive treadmill with adaptive FES for three 2-min trials. Stimulation was delivered at the correct time to the dorsiflexor muscles during 95% of strides while stimulation was delivered to the plantarflexor muscles at the correct time during 84% of strides. Stimulation amplitudes were correctly calculated and delivered for all except two strides out of nearly 3000. The adaptive FES system responds to real-time gait biomechanics as intended, and further individualization to subject-specific impairments and rehabilitation goals may lead to improved rehabilitation outcomes.

How Important is Position in Adaptive Treadmill Control?

To more closely mimic overground walking, researchers are developing adaptive treadmills (ATMs) that update belt speed in real-time based on user gait mechanics. Many existing ATM control schemes are solely based on position on the belt and do not respond to changes in gait mechanics, like propulsive forces, that result in increased overground walking speed. To target natural causal mechanisms to alter speed, we developed an ATM controller that adjusts speed via changes in position, step length, and propulsion. Gains on each input dictate the impact of the corresponding parameter on belt speed. The study objective was to determine the effect of modifying the position gain on self-selected walking speed, measures of propulsion, and step length. Twenty-two participants walked at their self-selected speed with four ATM controllers, each with a unique position gain. Walking speed, anterior and posterior ground reaction force peaks and impulses, net impulse, and step length were compared between conditions. Smaller position gains promoted more equivalent anterior and posterior impulses, resulting in a net impulse closer to zero (p = 0.0043), a characteristic of healthy gait. Walking speed, anterior and posterior ground reaction force peaks and impulses, and step length did not change between conditions (all p > 0.05). These results suggest that reducing the importance of position in the ATM controller may promote more balanced anterior and posterior impulses, possibly improving the efficacy of the ATM for gait rehabilitation by emphasizing changes in gait mechanics instead of position to naturally adjust speed.

Cognitive Training Improves Joint Stiffness Regulation and Function in ACLR Patients Compared to Healthy Controls

As cognitive function is critical for muscle coordination, cognitive training may also improve neuromuscular control strategy and knee function following an anterior cruciate ligament reconstruction (ACLR). The purpose of this case-control study was to examine the effects of cognitive training on joint stiffness regulation in response to negative visual stimuli and knee function following ACLR. A total of 20 ACLR patients and 20 healthy controls received four weeks of online cognitive training. Executive function, joint stiffness in response to emotionally evocative visual stimuli (neutral, fearful, knee injury related), and knee function outcomes before and after the intervention were compared. Both groups improved executive function following the intervention (p = 0.005). The ACLR group had greater mid-range stiffness in response to fearful (p = 0.024) and injury-related pictures (p = 0.017) than neutral contents before the intervention, while no post-intervention stiffness differences were observed among picture types. The ACLR group showed better single-legged hop for distance after cognitive training (p = 0.047), while the healthy group demonstrated no improvement. Cognitive training enhanced executive function, which may reduce joint stiffness dysregulation in response to emotionally arousing images and improve knee function in ACLR patients, presumably by facilitating neural processing necessary for neuromuscular control.

Adaptive treadmill control can be manipulated to increase propulsive impulse while maintaining walking speed

Adaptive treadmills (ATM) designed to promote increased propulsion may be an effective tool for gait training since propulsion is often impaired post-stroke. Our lab developed a novel ATM controller that adjusts belt speed via real-time changes in step length, propulsive impulse, and position. This study modified the relative importance of propulsion to step length in the controller to determine the effect of increased propulsive feedback gain on measures of propulsion and walking speed. Twenty-two participants completed five trials at their self-selected speed, each with a unique ATM controller. Walking speed, peak AGRF and PGRF, and AGRF, PGRF, and net impulse were compared between the modifications using one-way repeated measures ANOVAs at a significance level of 0.05. Participants chose similar walking speeds across all conditions (all p > 0.2730). There were no significant differences in peak AGRF (p = 0.1956) or PGRF (p = 0.5159) between conditions. AGRF impulse significantly increased as the gain on the propulsive impulse term was increased relative to the gain on step length (p < 0.0001) while PGRF and net impulse were similar across all conditions (p = 0.5487). Increasing the propulsive impulse gain essentially alters the treadmill environment by providing a controlled amount of resistance to increases in propulsive forces. Our findings demonstrate that the ATM can be modified to promote increased propulsive impulse while maintaining a consistent walking speed. Since increasing propulsion is a common goal of post-stroke gait training, these ATM modifications may improve the efficacy of the ATM for gait rehabilitation.

Adaptive treadmill walking encourages persistent propulsion

BACKGROUND

Adaptive treadmills allow real-time changes in walking speed by responding to changes in step length, propulsion, or position on the treadmill. The stride-to-stride variability, or persistence, of stride time during overground, fixed-speed, and adaptive treadmill walking has been studied, but persistence of propulsion during adaptive treadmill walking remains unknown. Because increased propulsion is often a goal of post-stroke rehabilitation, knowledge of the stride-to-stride variability may aid rehabilitation protocol design.

RESEARCH QUESTION

How do spatiotemporal and propulsive gait variables vary from stride to stride during adaptive treadmill walking, and how do they compare to fixed-speed treadmill walking?

METHODS

Eighteen young healthy subjects walked on an instrumented split-belt treadmill in the adaptive and fixed-speed modes for 10 minutes at their comfortable speed. Kinetic data was collected from the treadmill. Detrended fluctuation analysis was applied to the time series data. Shapiro-Wilk tests assessed normality and one-way repeated measures ANOVAs compared between adaptive, fixed-speed, and randomly shuffled conditions at a Bonferroni-corrected significance level of 0.0055.

RESULTS

Stride time, stride length, step length, and braking impulse were persistent (α > 0.5) in the adaptive and fixed-speed conditions. Adaptive and fixed-speed were different from each other. Stride speed was persistent in the adaptive condition and anti-persistent (α < 0.5) in the fixed-speed condition. Peak propulsive force, peak braking force, and propulsive impulse were persistent in the adaptive condition but not the fixed-speed condition (α ≈ 0.5). Net impulse was non-persistent in the adaptive and fixed-speed conditions. All variables were non-persistent in the shuffled condition.

SIGNIFICANCE

During adaptive treadmill walking, increases in propulsive force and impulse persist for multiple strides. Persistence was stronger on the adaptive treadmill, where increased propulsion translates into increased walking speed. For post-stroke gait rehabilitation where increasing propulsion and speed are goals, the stronger persistence of adaptive treadmill walking may be beneficial.

Neuroticism and Extraversion Are Related to Changes in Postural Stability During Anatomically-Related Cognitive Tasks

The relationship between personality and postural stability has received little attention. This study addressed whether neuroticism and extraversion correlate with changes in postural stability while performing cognitive tasks related to brain regions selectively associated with neuroticism and extraversion. Thirty-two adults stood on a foam mat in tandem stance and completed a 2-back task and a weather prediction task (WPT). As predicted, higher neuroticism was related to increased dual task sway during the 2-back task, r = 0.40, p = 0.023, and lower extraversion was related to increased dual task sway during the WPT, r = -0.43, p = 0.013, suggesting that personality is related to postural stability in healthy young adults and that personality could be considered in the prediction and treatment of individuals with balance difficulties.

Combined user-driven treadmill control and functional electrical stimulation increases walking speeds poststroke

The variety of poststroke impairments and compensatory mechanisms necessitate adaptive and subject-specific approaches to locomotor rehabilitation. To implement subject-specific, adaptive training to treadmill-based gait training, we developed a user-driven treadmill (UDTM) control algorithm that adjusts the user's speed in real-time. This study examines the response of individuals poststroke to the combination of UDTM control and electrical stimulation of the paretic ankle musculature to augment forward propulsion during walking. Sixteen individuals poststroke performed a randomized series of walking tasks on an instrumented split-belt treadmill at their self-selected speeds 1) with fixed speed treadmill (FSTM) control only, 2) FSTM control and paretic limb functional electrical stimulation (FES), 3) UDTM control only, and 4) UDTM control and FES. With UDTM control and FES, participants selected speeds that were 0.13 m/s faster than their speeds with fixed speed control only. This instantaneous increase is comparable to the gains in SS speed seen after 12 weeks of training with FES and fast walking with fixed speed treadmill control by Kesar and colleagues (Δ = 0.18 m/s). However, we saw no significant differences in the corresponding push-off forces or trailing limb position. Since individuals can use a variety of strategies to change their walking speeds, it is likely that the differences among individual responses obscured trends in the group average changes in mechanics. Ultimately, the combination of UDTM control and functional electrical stimulation (FES) allows individuals to increase speeds after a short exposure and may be a beneficial addition to poststroke gait training programs.

Nonlinear net ankle quasi-stiffness reduces error and changes with speed but not load carried

BACKGROUND

Natural ankle quasi-stiffness (NAS) is a key metric used to personalize orthotic and prosthetic ankle-foot devices. NAS has traditionally been defined as the average slope (i.e. linear regression) of the net ankle moment vs. ankle angle curve during stance. However, NAS appears to have nonlinear characteristics. Characterizing nonlinear NAS across a wide range of tasks will enable us to incorporate these attributes into future orthotic and prosthetic ankle-foot device designs.

RESEARCH QUESTION

Does nonlinear NAS change across multiple intensities of walking, running, and load carriage tasks?

METHODS

This observational study examined 22 young, healthy individuals as they walked, ran, and walked while carrying a load at three intensities (speed or load). Linear, quadratic, and cubic regressions were done on the net ankle moment vs. ankle angle curve over three phases of stance: impact, loading, and push-off. RMSE between regressions and measured data were computed to determine regression accuracy, and multilevel linear models (MLMs) were used to determine significant differences between coefficients across intensities.

RESULTS

Quadratic and cubic regressions of NAS had significantly lower RMSE than linear NAS for all phases of stance. Because of diminishing reductions in RMSE between quadratic and cubic regressions, only quadratic regression coefficients were further analyzed. Most first (linear) and second (nonlinear) order coefficients of quadratic regressions exhibited clear trends with respect to changes in walking or running speed, but not to increases in load.

SIGNIFICANCE

This was the first study to our knowledge to thoroughly characterize nonlinear NAS across multiple gait tasks and intensities. This study provides an advanced understanding of the characteristics of nonlinear NAS for the design of future prosthetic and orthotic ankle-foot devices.

Negative Emotion and Joint-Stiffness Regulation Strategies After Anterior Cruciate Ligament Injury

CONTEXT

Fear of reinjury after an anterior cruciate ligament (ACL) reconstruction (ACLR) may be associated with persistent deficits in knee function and subsequent injury. However, the effects of negative emotion on neuromuscular-control strategies after an ACL injury have remained unclear.

OBJECTIVE

To identify how negative emotional stimuli affect neural processing in the brain and muscle coordination in patients after anterior cruciate ligament reconstruction compared with healthy control participants.

DESIGN

Case-control study.

SETTING

Neuromechanics laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twenty patients after unilateral anterior cruciate ligament reconstruction and 20 healthy recruits.

MAIN OUTCOME MEASURE(S)

Electrocortical θ (4-8 Hz) activity (event-related synchronization, % increased power relative to a nonactive baseline) at selected electrodes placed at the frontal (F3, Fz, F4) and parietal (P3, Pz, P4) cortices using electroencephalography, neurophysiological cardiac changes (beats/min), and subjective fear perceptions were measured, along with joint stiffness (Nm/°/kg) with and without an acoustic stimulus in response to 3 types of emotionally evocative images (neutral, fearful, and knee-injury pictures).

RESULTS

Both groups had greater frontoparietal θ power with fearful pictures (Fz: 35.9% ± 29.4%; Pz: 81.4% ± 66.8%) than neutral pictures (Fz: 24.8% ± 29.7%, P = .002; Pz: 64.2 ± 54.7%, P = .024). The control group had greater heart-rate deceleration with fearful (-4.6 ± 1.4 beats/min) than neutral (-3.6 ± 1.3 beats/min, P < .001) pictures, whereas the ACLR group exhibited decreased heart rates with both the fearful (-4.6 ± 1.3 beats/min) and injury-related (-4.4 ± 1.5 beats/min) pictures compared with neutral pictures (-3.4 ± 1.4 beats/min, P < .001). Furthermore, during the acoustic startle condition, fearful pictures increased joint stiffness (Nm/°/kg) in the ACLR group at the midrange (0°-20°: 0.027 ± 0.02) and long range (0°-40°: 0.050 ± 0.02) compared with the neutral pictures (0°-20°: 0.017 ± 0.01, P = .024; 0°-40°: 0.043 ± 0.02, P = .014).

CONCLUSIONS

Negative visual stimuli simultaneously altered neural processing in the frontoparietal cortices and joint-stiffness regulation strategies in response to a sudden perturbation. The adverse effects of fear on neuromuscular control may indicate that psychological interventions should be incorporated in neuromuscular-control exercise programs after ACL injury.

Dynamic structure of variability in joint angles and center of mass position during user-driven treadmill walking

BACKGROUND

Overground locomotion exhibits greater movement variability and less dynamic stability compared to typical fixed-speed treadmill walking. To minimize the differences between treadmill and overground locomotion, researchers are developing user-driven treadmill systems that adjust the speed of the treadmill belts in real-time based on how fast the subject is trying to walk.

RESEARCH QUESTION

Does dynamic structure of variability, quantified by the Lyapunov exponent (LyE), of joint angles and center of mass (COM) position differ between a fixed-speed treadmill (FTM) and user-driven treadmill (UTM) for healthy subjects?

METHODS

Eleven healthy, adult subjects walked on a user-driven treadmill that updated its speed in real-time based on the subjects' propulsive forces, location, step length, and step time, and at a matched speed on a typical, fixed-speed treadmill for 1-minute. The LyE for flexion/extension joint angles and center of mass position were calculated.

RESULTS

Subjects exhibited higher LyE values of joint angles on the UTM compared to the FTM indicating that walking on the UTM may be more similar to overground locomotion. No change in COM LyE was observed between treadmill conditions indicating that subjects' balance was not significantly altered by this new training paradigm.

SIGNIFICANCE

The user-driven treadmill may be a more valuable rehabilitation tool for improving gait than fixed-speed treadmill training, as it may increase the effectiveness of transitioning learned behaviors to overground compared to fixed-speed treadmills.

Changes in gait mechanics and muscle activity with wedge height in an orthopaedic boot

BACKGROUND

Orthopaedic boots with wedging are commonly used in the treatment of individuals with Achilles tendon rupture to immobilize the foot in plantar flexion and approximate tendon ends.

RESEARCH QUESTION

To describe changes in muscle activity of the triceps surae and gait mechanics with the use of wedges in an orthopaedic boot immediately and after an accommodation period.

METHODS

Muscle activity of the triceps surae and gait parameters (vertical ground reaction force, knee extension power, gait speed) were collected using surface electromyography and motion capture in 12 healthy individuals. Participants walked in an instrumented orthopaedic boot with 0, 3, and 5 wedges tested in random order. Participants were provided a one hour accommodation period where time spent walking was collected. This was followed by a repeat assessment of triceps surae activity and gait.

RESULTS

Peak and integrated EMG in the medial gastrocnemius (p = 0.001, p < 0.001) and soleus (p = 0.010, p < 0.001) significantly decreased with increasing number of wedges. Peak and integrated EMG had a slight but non-significant decrease with increasing number of wedges in the lateral gastrocnemius (p = 0.151, p = 0.077). Vertical ground reaction force decreased (p = 0.019) and peak knee extension power increased (p = 0.003) with increasing number of wedges. There were no statistically significant differences in gait speed with wedges (p = 0.450). There were no significant changes in EMG or gait parameters from pre- to post-accommodation period.

SIGNIFICANCE

A combination of factors yield decreased triceps surae activity in individuals wearing an orthopaedic boot with wedges - decreasing loading on the immobilized limb and shifting power generation proximally.

The effect of stride length on lower extremity joint kinetics at various gait speeds

Robot-assisted training is a promising tool under development for improving walking function based on repetitive goal-oriented task practice. The challenges in developing the controllers for gait training devices that promote desired changes in gait is complicated by the limited understanding of the human response to robotic input. A possible method of controller formulation can be based on the principle of bio-inspiration, where a robot is controlled to apply the change in joint moment applied by human subjects when they achieve a gait feature of interest. However, it is currently unclear how lower extremity joint moments are modulated by even basic gait spatio-temporal parameters. In this study, we investigated how sagittal plane joint moments are affected by a factorial modulation of two important gait parameters: gait speed and stride length. We present the findings obtained from 20 healthy control subjects walking at various treadmill-imposed speeds and instructed to modulate stride length utilizing real-time visual feedback. Implementing a continuum analysis of inverse-dynamics derived joint moment profiles, we extracted the effects of gait speed and stride length on joint moment throughout the gait cycle. Moreover, we utilized a torque pulse approximation analysis to determine the timing and amplitude of torque pulses that approximate the difference in joint moment profiles between stride length conditions, at all gait speed conditions. Our results show that gait speed has a significant effect on the moment profiles in all joints considered, while stride length has more localized effects, with the main effect observed on the knee moment during stance, and smaller effects observed for the hip joint moment during swing and ankle moment during the loading response. Moreover, our study demonstrated that trailing limb angle, a parameter of interest in programs targeting propulsion at push-off, was significantly correlated with stride length. As such, our study has generated assistance strategies based on pulses of torque suitable for implementation via a wearable exoskeleton with the objective of modulating stride length, and other correlated variables such as trailing limb angle.

Neuroplastic changes in anterior cruciate ligament reconstruction patients from neuromechanical decoupling

The purpose of this study was to identify how the brain simultaneously perceives proprioceptive input during joint loading in anterior cruciate ligament reconstruction (ACLR) patients, when compared to healthy controls. Seventeen ACLR patients (ACLR) and seventeen controls (CONT) were tested for the somatosensory cortical activation using electroencephalography (EEG) while measuring knee laxity using a knee arthrometer. The relationship between cortical activation and joint laxity within group was also examined. The ACLR patients had increased cortical activation (36.4% ± 11.5%) in the somatosensory cortex during early loading (ERD1) to the injured limb compared to the CONT's matched limb (25.3% ± 13.2%, P = 0.013) as well as compared to the noninjured limb (25.1% ± 14.2%, P = 0.001). Higher somatosensory cortical activity during midloading (ERD2) to the ACLR knee positively correlated with knee laxity (mm) during early loading (LAX1, r = 0.530), midloading (LAX2, r = 0.506), total anterior loading (LAXA, r = 0.543), and total antero-posterior loading (LAXT, r = 0.501), while the noninjured limb revealed negative correlations between ERD1 and LAXA (r = -0.534) as well as between ERD2 and LAX2 (r = -0.565). ACLR patients demonstrate greater brain activation during joint loading in the injured knees when compared to healthy controls' matched knees as well as contralateral healthy knees, while the CONT group shows similar brain activation patterns during joint loading between limbs. These different neural activation strategies may indicate neuromechanical decoupling following an ACL reconstruction and evidence of altered sensorimotor perception and control of the knee (neuroplasticity), which may be critical to address after surgery for optimal neuromuscular control and patients' outcomes.

Walking speed changes in response to novel user-driven treadmill control

Implementing user-driven treadmill control in gait training programs for rehabilitation may be an effective means of enhancing motor learning and improving functional performance. This study aimed to determine the effect of a user-driven treadmill control scheme on walking speeds, anterior ground reaction forces (AGRF), and trailing limb angles (TLA) of healthy adults. Twenty-three participants completed a 10-m overground walking task to measure their overground self-selected (SS) walking speeds. Then, they walked at their SS and fastest comfortable walking speeds on an instrumented split-belt treadmill in its fixed speed and user-driven control modes. The user-driven treadmill controller combined inertial-force, gait parameter, and position based control to adjust the treadmill belt speed in real time. Walking speeds, peak AGRF, and TLA were compared among test conditions using paired t-tests (α = 0.05). Participants chose significantly faster SS and fast walking speeds in the user-driven mode than the fixed speed mode (p > 0.05). There was no significant difference between the overground SS walking speed and the SS speed from the user-driven trials (p < 0.05). Changes in AGRF and TLA were caused primarily by changes in walking speed, not the treadmill controller. Our findings show the user-driven treadmill controller allowed participants to select walking speeds faster than their chosen speeds on the fixed speed treadmill and similar to their overground speeds. Since user-driven treadmill walking increases cognitive activity and natural mobility, these results suggest user-driven treadmill control would be a beneficial addition to current gait training programs for rehabilitation.

A mathematical analysis to address the 6 degree-of-freedom segmental power imbalance

Segmental power is used in human movement analyses to indicate the source and net rate of energy transfer between the rigid bodies of biomechanical models. Segmental power calculations are performed using segment endpoint dynamics (kinetic method). A theoretically equivalent method is to measure the rate of change in a segment's mechanical energy state (kinematic method). However, these two methods have not produced experimentally equivalent results for segments proximal to the foot, with the difference in methods deemed the "power imbalance." In a 6 degree-of-freedom model, segments move independently, resulting in relative segment endpoint displacement and non-equivalent segment endpoint velocities at a joint. In the kinetic method, a segment's distal end translational velocity may be defined either at the anatomical end of the segment or at the location of the joint center (defined here as the proximal end of the adjacent distal segment). Our mathematical derivations revealed the power imbalance between the kinetic method using the anatomical definition and the kinematic method can be explained by power due to relative segment endpoint displacement. In this study, we tested this analytical prediction through experimental gait data from nine healthy subjects walking at a typical speed. The average absolute segmental power imbalance was reduced from 0.023 to 0.046 W/kg using the anatomical definition to ≤0.001 W/kg using the joint center definition in the kinetic method (95.56-98.39% reduction). Power due to relative segment endpoint displacement in segmental power analyses is substantial and should be considered in analyzing energetic flow into and between segments.

Dynamic structure of lower limb joint angles during walking post-stroke

BACKGROUND

Variability in joint kinematics is necessary for adaptability and response to everyday perturbations; however, intrinsic neuromotor changes secondary to stroke often cause abnormal movement patterns. How these abnormal movement patterns relate to joint kinematic variability and its influence on post-stroke walking impairments is not well understood.

OBJECTIVE

The purpose of this study was to evaluate the movement variability at the individual joint level in the paretic and non-paretic limbs of individuals post-stroke.

METHODS

Seven individuals with hemiparesis post-stroke walked on a treadmill for two minutes at their self-selected speed and the average speed of the six-minute walk test while kinematics were recorded using motion-capture. Variability in hip, knee, and ankle flexion/extension angles during walking were quantified with the Lyapunov exponent (LyE). Interlimb differences were evaluated.

RESULTS

The paretic side LyE was higher than the non-paretic side at both self-selected speed (Hip: 50%; Knee: 74%), and the average speed of the 6-min walk test (Hip: 15%; Knee: 93%).

CONCLUSION

Differences in joint kinematic variability between limbs of persons post-stroke supports further study of the source of non-paretic limb deviations as well as the clinical implications of joint kinematic variability in persons post-stroke. The development of bilaterally-targeted post-stroke gait interventions to address variability in both limbs may promote improved outcomes.

Toward goal-oriented robotic gait training: The effect of gait speed and stride length on lower extremity joint torques

Robot-assisted gait training is becoming increasingly common to support recovery of walking function after neurological injury. How to formulate controllers capable of promoting desired features in gait, i.e. goals, is complicated by the limited understanding of the human response to robotic input. A possible method to formulate controllers for goal-oriented gait training is based on the analysis of the joint torques applied by healthy subjects to modulate such goals. The objective of this work is to understand how sagittal plane joint torque is affected by two important gait parameters: gait speed (GS) and stride length (SL). We here present the results obtained from healthy subjects walking on a treadmill at different speeds, and asked to modulate stride length via visual feedback. Via principal component analysis, we extracted the global effects of the two factors on the peak-to-peak amplitude of joint torques. Next, we used a torque pulse approximation analysis to determine optimal timing and amplitude of torque pulses that approximate the SL-specific difference in joint torque profiles measured at different values of GS. Our results show a strong effect of GS on the torque profiles in all joints considered. In contrast, SL mostly affects the torque produced at the knee joint at early and late stance, with smaller effects on the hip and ankle joints. Our analysis generated a set of torque assistance profiles that will be experimentally tested using gait training robots.

Cellular Telephone Dialing Influences Kinematic and Spatiotemporal Gait Parameters in Healthy Adults

Gait speed is typically reduced when individuals simultaneously perform other tasks. However, the impact of dual tasking on kinetic and kinematic gait parameters is unclear because these vary with gait speed. The objective of this study was to identify whether dual tasking impacts gait in healthy adults when speed is constant. Twenty-two healthy adults dialed a cell phone during treadmill walking at a self-selected speed while kinetic, kinematic, and spatial parameters were recorded. Results indicated that dual tasking did not impact phone dialing speed, but increased stride width, peak knee flexion during stance, and peak plantarflexion, and decreased knee and ankle range of motion. Dual tasking appears to influence kinematic gait variables in a manner consistent with promotion of stability.

Robotic Assist-As-Needed as an Alternative to Therapist-Assisted Gait Rehabilitation

OBJECTIVE

Body Weight Supported Treadmill Training (BWSTT) with therapists' assistance is often used for gait rehabilitation post-stroke. However, this training method is labor-intensive, requiring at least one or as many as three therapists at once for manual assistance. Previously, we demonstrated that providing movement guidance using a performance-based robot-aided gait training (RAGT) that applies a compliant, assist-as-needed force-field improves gait pattern and functional walking ability in people post-stroke. In the current study, we compared the effects of assist-as-needed RAGT combined with functional electrical stimulation and visual feedback with BWSTT to determine if RAGT could serve as an alternative for locomotor training.

METHODS

Twelve stroke survivors were randomly assigned to one of the two groups, either receiving BWSTT with manual assistance or RAGT with functional electrical stimulation and visual feedback. All subjects received fifteen 40-minutes training sessions.

RESULTS

Clinical measures, kinematic data, and EMG data were collected before and immediately after the training for fifteen sessions. Subjects receiving RAGT demonstrated significant improvements in their self-selected over-ground walking speed, Functional Gait Assessment, Timed Up and Go scores, swing-phase peak knee flexion angle, and muscle coordination pattern. Subjects receiving BWSTT demonstrated significant improvements in the Six-minute walk test. However, there was an overall trend toward improvement in most measures with both interventions, thus there were no significant between-group differences in the improvements following training.

CONCLUSION

The current findings suggest that RAGT worked at least as well as BWSTT and thus may be used as an alternative rehabilitation method to improve gait pattern post-stroke as it requires less physical effort from the therapists compared to BWSTT.

Evaluation of measurements of propulsion used to reflect changes in walking speed in individuals poststroke

Recent rehabilitation approaches for individuals poststroke have focused on improving walking speed because it is a reliable measurement that is associated with quality of life. Previous studies have demonstrated that propulsion, the force used to propel the body forward, determines walking speed. However, there are several different ways of measuring propulsion and no studies have identified which measurement best reflects differences in walking speed. The primary purposes of this study were to determine for individuals poststroke, which measurement of propulsion (1) is most closely related to their self-selected walking speeds and (2) best reflects changes in walking speed within a session. Participants (N=43) with chronic poststroke hemiparesis walked at their self-selected and maximal walking speeds on a treadmill. Propulsive impulse, peak propulsive force, and mean propulsive value (propulsive impulse divided by duration) were analyzed. In addition, each participant׳s cadence was calculated. Pearson correlation coefficients were used to determine the relationships between different measurements of propulsion versus walking speed as well as changes in propulsion versus changes in walking speed. Stepwise linear regression was used to determine which measurement of propulsion best predicted walking speed and changes in walking speed. The results showed that all 3 measurements of propulsion were correlated to walking speed, with peak propulsive force showed the strongest correlation. Similarly, when participants increased their walking speeds, changes in peak propulsive forces showed the strongest correlation to changes in walking speed. In addition, multiplying each measurement by cadence improved the correlations. The present study suggests that measuring peak propulsive force and cadence may be most appropriate of the variables studied to characterize propulsion in individuals poststroke.

Passive-dynamic ankle-foot orthosis replicates soleus but not gastrocnemius muscle function during stance in gait: Insights for orthosis prescription

BACKGROUND

Passive-dynamic ankle-foot orthosis characteristics, including bending stiffness, should be customized for individuals. However, while conventions for customizing passive-dynamic ankle-foot orthosis characteristics are often described and implemented in clinical practice, there is little evidence to explain their biomechanical rationale.

OBJECTIVES

To develop and combine a model of a customized passive-dynamic ankle-foot orthosis with a healthy musculoskeletal model and use simulation tools to explore the influence of passive-dynamic ankle-foot orthosis bending stiffness on plantar flexor function during gait.

STUDY DESIGN

Dual case study.

METHODS

The customized passive-dynamic ankle-foot orthosis characteristics were integrated into a healthy musculoskeletal model available in OpenSim. Quasi-static forward dynamic simulations tracked experimental gait data under several passive-dynamic ankle-foot orthosis conditions. Predicted muscle activations were calculated through a computed muscle control optimization scheme.

RESULTS

Simulations predicted that the passive-dynamic ankle-foot orthoses substituted for soleus but not gastrocnemius function. Induced acceleration analyses revealed the passive-dynamic ankle-foot orthosis acts like a uniarticular plantar flexor by inducing knee extension accelerations, which are counterproductive to natural knee kinematics in early midstance.

CONCLUSION

These passive-dynamic ankle-foot orthoses can provide plantar flexion moments during mid and late stance to supplement insufficient plantar flexor strength. However, the passive-dynamic ankle-foot orthoses negatively influenced knee kinematics in early midstance.

CLINICAL RELEVANCE

Identifying the role of passive-dynamic ankle-foot orthosis stiffness during gait provides biomechanical rationale for how to customize passive-dynamic ankle-foot orthoses for patients. Furthermore, these findings can be used in the future as the basis for developing objective prescription models to help drive the customization of passive-dynamic ankle-foot orthosis characteristics.

Contribution of Paretic and Nonparetic Limb Peak Propulsive Forces to Changes in Walking Speed in Individuals Poststroke

BACKGROUND

Recent rehabilitation efforts after stroke often focus on increasing walking speed because it is associated with quality of life. For individuals poststroke, propulsive force generated from the paretic limb has been shown to be correlated to walking speed. However, little is known about the relative contribution of the paretic versus the nonparetic propulsive forces to changes in walking speed.

OBJECTIVE

The primary purpose of this study was to determine the contribution of propulsive force generated from each limb to changes in walking speed during speed modulation within a session and as a result of a 12-week training program.

METHODS

Gait analysis was performed as participants (N = 38) with chronic poststroke hemiparesis walked at their self-selected and faster walking speeds on a treadmill before and after a 12-week gait retraining program.

RESULTS

Prior to training, stroke survivors increased nonparetic propulsive forces as the primary mechanism to change walking speed during speed modulation within a session. Following gait training, the paretic limb played a larger role during speed modulation within a session. In addition, the increases in paretic propulsive forces observed following gait training contributed to the increases in the self-selected walking speeds seen following training.

CONCLUSIONS

Gait retraining in the chronic phase of stroke recovery facilitates paretic limb neuromotor recovery and reduces the reliance on the nonparetic limb's generation of propulsive force to increase walking speed. These findings support gait rehabilitation efforts directed toward improving the paretic limb's ability to generate propulsive force.

Baseline predictors of treatment gains in peak propulsive force in individuals poststroke

Background

Current rehabilitation for individuals poststroke focuses on increasing walking speed because it is an indicator of community walking ability and quality of life. Propulsive force generated from the paretic limb is critical to walking speed and may reflect actual neural recovery that restores the affected neural systems. A wide variation across individuals in the improvements in paretic propulsive force was observed following an intervention that targeted paretic propulsive force. This study aimed to determine if specific baseline characteristics can be used to predict patients who would respond to the intervention.

Methods

Participants (N = 19) with chronic poststroke hemiparesis walked at their self-selected and maximal walking speeds on a treadmill before and after a 12-week gait training program. Propulsive forces from the paretic limb were analyzed. Pearson correlation coefficient was used to determine the relationships between (1) treatment gains in walking speed and propulsive force following intervention, and (2) treatment gains in propulsive force and baseline propulsive forces.

Results

Treatment gains in self-selected walking speed were correlated to treatment gains in paretic propulsive force following intervention. In addition, changes in paretic propulsive force between self-selected and maximal walking speeds at baseline were strongly correlated to treatment gains in paretic propulsive force.

Conclusions

The capacity to modulate paretic propulsive force, rather than the absolute propulsive force during self-selected or maximal walking speed, predicted treatment gains in propulsive force following the intervention. Findings from this research could help to inform clinicians and researchers to target the appropriate patient population for rehabilitation interventions.

Mechanisms used to increase peak propulsive force following 12-weeks of gait training in individuals poststroke

Current rehabilitation efforts for individuals poststroke focus on increasing walking speed because it is a predictor of community ambulation and participation. Greater propulsive force is required to increase walking speed. Previous studies have identified that trailing limb angle (TLA) and ankle moment are key factors to increases in propulsive force during gait. However, no studies have determined the relative contribution of these two factors to increase propulsive force following intervention. The purpose of this study was to quantify the relative contribution of ankle moment and TLA to increases in propulsive force following 12-weeks of gait training for individuals poststroke. Forty-five participants were assigned to 1 of 3 training groups: training at self-selected speeds (SS), at fastest comfortable speeds (Fast), and Fast with functional electrical stimulation (FastFES). For participants who gained paretic propulsive force following training, a biomechanical-based model previously developed for individuals poststroke was used to calculate the relative contributions of ankle moment and TLA. A two-way, mixed-model design, analysis of covariance adjusted for baseline walking speed was performed to analyze changes in TLA and ankle moment across groups. The model showed that TLA was the major contributor to increases in propulsive force following training. Although the paretic TLA increased from pre-training to post-training, no differences were observed between groups. In contrast, increases in paretic ankle moment were observed only in the FastFES group. Our findings suggested that specific targeting may be needed to increase ankle moment.

Changes in Post-Stroke Gait Biomechanics Induced by One Session of Gait Training

The objective of this study was to determine whether one session of targeted locomotor training can induce measurable improvements in the post-stroke gait impairments. Thirteen individuals with chronic post-stroke hemiparesis participated in one locomotor training session combining fast treadmill training and functional electrical stimulation (FES) of ankle dorsi- and plantar-flexor muscles. Three dimensional gait analysis was performed to assess within-session changes (after versus before training) in gait biomechanics at the subject's self-selected speed without FES. Our results showed that one session of locomotor training resulted in significant improvements in peak anterior ground reaction force (AGRF) and AGRF integral for the paretic leg. Additionally, individual subject data showed that a majority of study participants demonstrated improvements in the primary outcome variables following the training session. This study demonstrates, for the first time, that a single session of intense, targeted post-stroke locomotor retraining can induce significant improvements in post-stroke gait biomechanics. We posit that the within-session changes induced by a single exposure to gait training can be used to predict whether an individual is responsive to a particular gait intervention, and aid with the development of individualized gait retraining strategies. Future studies are needed to determine whether these single-session improvements in biomechanics are accompanied by short-term changes in corticospinal excitability, and whether single-session responses can serve as predictors for the longer-term effects of the intervention with other targeted gait interventions.

Short-term Performance-based Error-augmentation versus Error-reduction Robotic Gait Training for Individuals with Chronic Stroke: A Pilot Study

The success of locomotion training with robotic exoskeletons requires identifying control algorithms that effectively retrain gait patterns in neurologically impaired individuals. Here we report how the two training paradigms, performance-based error-augmentation versus error-reduction, modified walking patterns in four chronic post-stroke individuals as a proof-of-concept for future locomotion training following stroke. Stroke subjects were instructed to match a prescribed walking pattern template derived from neurologically intact individuals. Target templates based on the spatial paths of lateral ankle malleolus positions during walking were created for each subject. Robotic forces were applied that either decreased (error-reduction) or increased (error-augmentation) the deviation between subjects' instantaneous malleolus positions and their target template. Subjects' performance was quantified by the amount of deviation between their actual and target malleolus paths. After the error-reduction training, S1 showed a malleolus path with reduced deviation from the target template by 16%. In contrast, S4 had a malleolus path further away from the template with increased deviation by 12%. After the error-augmentation training, S2 had a malleolus path greatly approximating the template with reduced deviation by 58% whereas S3 walked with higher steps than his baseline with increased deviation by 37%. These findings suggest that an error-reduction force field has minimal effects on modifying subject's gait patterns whereas an error-augmentation force field may promote a malleolus path either approximating or exceeding the target walking template. Future investigation will need to evaluate the long-term training effects on over-ground walking and functional capacity.

Walking stability during cell phone use in healthy adults

The number of falls and/or accidental injuries associated with cellular phone use during walking is growing rapidly. Understanding the effects of concurrent cell phone use on human gait may help develop safety guidelines for pedestrians. It was shown previously that older adults had more pronounced dual-task interferences than younger adults when concurrent cognitive task required visual information processing. Thus, cell phone use might have greater impact on walking stability in older than in younger adults. This study examined gait stability and variability during a cell phone dialing task (phone) and two classic cognitive tasks, the Paced Auditory Serial Addition Test (PASAT) and Symbol Digit Modalities Test (SDMT). Nine older and seven younger healthy adults walked on a treadmill at four different conditions: walking only, PASAT, phone, and SDMT. We computed short-term local divergence exponent (LDE) of the trunk motion (local stability), dynamic margins of stability (MOS), step spatiotemporal measures, and kinematic variability. Older and younger adults had similar values of short-term LDE during all conditions, indicating that local stability was not affected by the dual-task. Compared to walking only, older and younger adults walked with significantly greater average mediolateral MOS during phone and SDMT conditions but significantly less ankle angle variability during all dual-tasks and less knee angle variability during PASAT. The current findings demonstrate that healthy adults may try to control foot placement and joint kinematics during cell phone use or another cognitive task with a visual component to ensure sufficient dynamic margins of stability and maintain local stability.

Mechanisms to increase propulsive force for individuals poststroke

Background

Propulsive force generation is critical to walking speed. Trialing limb angle and ankle moment are major contributors to increases in propulsive force during gait. For able-bodied individuals, trailing limb angle contributes twice as much as ankle moment to increases in propulsive force during speed modulation. The aim of this study was to quantify the relative contribution of ankle moment and trailing limb angle to increases in propulsive force for individuals poststroke.

Methods

A biomechanical-based model previously developed for able-bodied individuals was evaluated and enhanced for individuals poststroke. Gait analysis was performed as subjects (N = 24) with chronic poststroke hemiparesis walked at their self-selected and fast walking speeds on a treadmill.

Results

Both trailing limb angle and ankle moment increased during speed modulation. In the paretic limb, the contribution from trailing limb angle versus ankle moment to increases in propulsive force is 74% and 17%. In the non-paretic limb, the contribution from trailing limb angle versus ankle moment to increases in propulsive force is 67% and 22%.

Conclusions

Individuals poststroke increase propulsive force mainly by changing trailing limb angle in both the paretic and non-paretic limbs. This strategy may contribute to the inefficiency in poststroke walking patterns. Future work is needed to examine whether these characteristics can be modified via intervention.

Change in knee contact force with simulated change in body weight

The relationship between obesity, weight gain and progression of knee osteoarthritis is well supported, suggesting that excessive joint loading may be a mechanism responsible for cartilage deterioration. Examining the influence of weight gain on joint compressive forces is difficult, as both muscles and ground reaction forces can have a significant impact on the forces experienced during gait. While previous studies have examined the relationship between body weight and knee forces, these studies have used models that were not validated using experimental data. Therefore, the objective of this study was to evaluate the relationship between changes in body weight and changes in knee joint contact forces for an individual's gait pattern using musculoskeletal modeling that is validated against known internal compressive forces. Optimal weighting constants were determined for three subjects to generate valid predictions of knee contact forces (KCFs) using in vivo data collection with instrumented total knee arthroplasty. A total of five simulations per walking trial were generated for each subject, from 80% to 120% body weight in 10% increments, resulting in 50 total simulations. The change in peak KCF with respect to body weight was found to be constant and subject-specific, predominantly determined by the peak force during the baseline condition at 100% body weight. This relationship may be further altered by any change in kinematics or body mass distribution that may occur as a result of a change in body weight or exercise program.

The relative contribution of ankle moment and trailing limb angle to propulsive force during gait

A major factor for increasing walking speed is the ability to increase propulsive force. Although propulsive force has been shown to be related to ankle moment and trailing limb angle, the relative contribution of each factor to propulsive force has never been determined. The primary purpose of this study was to quantify the relative contribution of ankle moment and trailing limb angle to propulsive force for able-bodied individuals walking at different speeds. Twenty able-bodied individuals walked at their self-selected and 120% of self-selected walking speed on the treadmill. Kinematic data were collected using an 8-camera motion-capture system. A model describing the relationship between ankle moment, trailing limb angle and propulsive force was obtained through quasi-static analysis. Our main findings were that ankle moment and trailing limb angle each contributes linearly to propulsive force, and that the change in trailing limb angle contributes almost as twice as much as the change in ankle moment to the increase in propulsive force during speed modulation for able-bodied individuals. Able-bodied individuals preferentially modulate trailing limb angle more than ankle moment to increase propulsive force. Future work will determine if this control strategy can be applied to individuals poststroke.

Assist-as-Needed Robot-Aided Gait Training Improves Walking Function in Individuals Following Stroke

A novel robot-aided assist-as-needed gait training paradigm has been developed recently. This paradigm encourages subjects' active participation during training. Previous pilot studies demonstrated that assist-as-needed robot-aided gait training (RAGT) improves treadmill walking performance post-stroke. However, it is not known if there is an over-ground transfer of the training effects from RAGT on treadmill or long-term retention of the effects. The purpose of the current study was to examine the effects of assist-as-needed RAGT on over-ground walking pattern post-stroke. Nine stroke subjects received RAGT with visual feedback of each subject's instantaneous ankle malleolus position relative to a target template for 15 40-minute sessions. Clinical evaluations and gait analyses were performed before, immediately after, and 6 months post-training. Stroke subjects demonstrated significant improvements and some long-term retention of the improvements in their self-selected over-ground walking speed, Dynamic Gait Index, Timed Up and Go, peak knee flexion angle during swing phase and total hip joint excursion over the whole gait cycle for their affected leg . These preliminary results demonstrate that subjects improved their over-ground walking pattern and some clinical gait measures post-training suggesting that assist-as-needed RAGT including visual feedback may be an effective approach to improve over-ground walking pattern post-stroke.

Dynamic instability during post-stroke hemiparetic walking

Falls and fall-related injuries cause extremely costly and potentially fatal health problems in people post-stroke. However, there is no global indicator of walking instability for detecting which individuals will have increased risk of falls. The purposes of this study were to directly quantify walking stability in stroke survivors and neurologically intact controls and to determine which stability measures would reveal the changes in walking stability following stroke. This study thus provided an initial step to establish objective measures for identifying potential fallers. Nine post-stroke individuals and nine controls walked on a treadmill at four different speeds. We computed short-term local divergence exponent (LDE) and maximum Floquet multiplier (maxFM) of the trunk motion, average and variability of dynamic margins of stability (MOS) and step spatiotemporal measures. Post-stroke individuals demonstrated larger short-term LDE (p = 0.002) and maxFM (p = 0.041) in the mediolateral (ML) direction compared to the controls but remained orbitally stable (maxFM < 1). In addition, post-stroke individuals walked with greater average step width (p = 0.003) but similar average ML MOS (p = 0.154) compared to the controls. Post-stroke individuals also exhibited greater variability in all MOS and step measures (all p < 0.005). Our findings indicate that post-stroke individuals walked with greater local and orbital instability and gait variability than neurologically intact controls. The results suggest that short-term LDE of ML trunk motion and the variability of MOS and step spatiotemporal measures detect the changes in walking stability associated with stroke. These stability measures may have the potential for identifying those post-stroke individuals at increased risk of falls.

Frontal plane compensatory strategies associated with self-selected walking speed in individuals post-stroke

BACKGROUND

Approximately two out of three individuals post-stroke experience walking impairments. Frontal plane compensatory strategies (i.e. pelvic hiking and circumduction) are observed in post-stroke gait in part to achieve foot clearance in response to reduced knee flexion and ankle dorsiflexion. The objective of this study was to investigate the relationship between self-selected walking speed and the kinematic patterns related to paretic foot clearance during post-stroke walking.

METHODS

Gait analysis was performed at self-selected walking speed for 21 individuals post-stroke. Four kinematic variables were calculated during the swing phase of the paretic limb: peak pelvic tilt (pelvic hiking), peak hip abduction (circumduction), peak knee flexion, and peak ankle dorsiflexion. Paretic joint angles were analyzed across self-selected walking speed as well as between functionally relevant ambulation categories (Household <0.4m/s, Limited Community 0.4-0.8m/s, Community >0.8m/s).

FINDINGS

While all subjects exhibited similar foot clearance, slower walkers exhibited greater peak pelvic hiking and less knee flexion, ankle dorsiflexion, and circumduction compared to faster walkers (P<.05). Additionally, four of the fastest walkers compensated for poor knee flexion and ankle dorsiflexion through large amounts of circumduction.

INTERPRETATION

These findings suggest that improved gait performance after stroke, as measured by self-selected walking speed, is not necessarily always accomplished through gait patterns that more closely resemble healthy gait for all variables. It appears the ability to walk fast is achieved by either sufficient ankle dorsiflexion and knee flexion to achieve foot clearance or the employment of circumduction to overcome a deficit in either ankle dorsiflexion or knee flexion.

Interlimb symmetry of dynamic knee joint stiffness and co-contraction is maintained in early stage knee osteoarthritis

Individuals with knee OA often exhibit greater co-contraction of antagonistic muscle groups surrounding the affected joint which may lead to increases in dynamic joint stiffness. These detrimental changes in the symptomatic limb may also exist in the contralateral limb, thus contributing to its risk of developing knee osteoarthritis. The purpose of this study is to investigate the interlimb symmetry of dynamic knee joint stiffness and muscular co-contraction in knee osteoarthritis. Muscular co-contraction and dynamic knee joint stiffness were assessed in 17 subjects with mild to moderate unilateral medial compartment knee osteoarthritis and 17 healthy control subjects while walking at a controlled speed (1.0m/s). Paired and independent t-tests determined whether significant differences exist between groups (p<0.05). There were no significant differences in dynamic joint stiffness or co-contraction between the OA symptomatic and OA contralateral group (p=0.247, p=0.874, respectively) or between the OA contralateral and healthy group (p=0.635, p=0.078, respectively). There was no significant difference in stiffness between the OA symptomatic and healthy group (p=0.600); however, there was a slight trend toward enhanced co-contraction in the symptomatic knees compared to the healthy group (p=0.051). Subjects with mild to moderate knee osteoarthritis maintain symmetric control strategies during gait.

Muscle volume as a predictor of maximum force generating ability in the plantar flexors post-stroke

INTRODUCTION

Post-stroke muscle weakness is commonly thought to be the result of a combination of decreased voluntary activation and decreased maximum force generating ability (MFGA). We assessed the ability of muscle volumes obtained using MRI to estimate the MFGA of the plantar flexor muscle group in individuals post-stroke.

METHODS

MRI was used to measure muscle volume of the plantar flexor muscle group in 17 individuals with post-stroke hemiparesis. A modified burst superimposition test was used to measure force of volitional contraction and predict the MFGA of the plantar flexors.

RESULTS

While muscle volume obtained by means of MRI provided information on the overall size of muscle, it overestimated the force generating ability of the paretic plantar flexors.

CONCLUSIONS

Results suggest that MRI-derived muscle volume underestimates the functional impairment in individuals post-stroke. Interestingly, the central activation ratio had a strong relationship with the maximum volitional force of contraction.

Changes in the activation and function of the ankle plantar flexor muscles due to gait retraining in chronic stroke survivors

Background

A common goal of persons post-stroke is to regain community ambulation. The plantar flexor muscles play an important role in propulsion generation and swing initiation as previous musculoskeletal simulations have shown. The purpose of this study was to demonstrate that simulation results quantifying changes in plantar flexor activation and function in individuals post-stroke were consistent with (1) the purpose of an intervention designed to enhance plantar flexor function and (2) expected muscle function during gait based on previous literature.

Methods

Three-dimensional, forward dynamic simulations were created to determine the changes in model activation and function of the paretic ankle plantar flexor muscles for eight patients post-stroke after a 12-weeks FastFES gait retraining program.

Results

An median increase of 0.07 (Range [−0.01,0.22]) was seen in simulated activation averaged across all plantar flexors during the double support phase of gait from pre- to post-intervention. A concurrent increase in walking speed and plantar flexor induced forward center of mass acceleration by the plantar flexors was seen post-intervention for seven of the eight subject simulations. Additionally, post-training, the plantar flexors had an simulated increase in contribution to knee flexion acceleration during double support.

Conclusions

For the first time, muscle-actuated musculoskeletal models were used to simulate the effect of a gait retraining intervention on post-stroke muscle model predicted activation and function. The simulations showed a new pattern of simulated activation for the plantar flexor muscles after training, suggesting that the subjects activated these muscles with more appropriate timing following the intervention. Functionally, simulations calculated that the plantar flexors provided greater contribution to knee flexion acceleration after training, which is important for increasing swing phase knee flexion and foot clearance.

Validation of an adjustment equation for the burst superimposition technique in subjects post-stroke

INTRODUCTION

Deficits in voluntary force generation may be due to incomplete activation or decreased maximum force-generating ability (MFGA) of the targeted muscle. The validity of techniques used to measure MFGA in individuals post-stroke has not been assessed. The objective of this study was to determine the reliability of the MFGA predicted using an adjusted burst superimposition method within a post-stroke population.

METHODS

Differences in paretic-side plantar flexor muscle MFGA between 2 trials was calculated using the standard and adjusted burst superimposition technique for 17 individuals post-stroke (15 men, 58.7 ± 10 years of age, ≥6 months post-stroke) to assess reliability of the techniques.

RESULTS

The adjusted measurement was shown to be more reliable (P = 0.03), especially when volitional effort differed by >40 N.

CONCLUSIONS

Reliable measurement of the MFGA in individuals who have sustained a stroke is of clinical importance. These results suggest that the adjusted burst superimposition method may be useful when performing multiple measurements of muscle performance.

Hybrid models of the neuromusculoskeletal system improve subject-specificity

Muscle-actuated simulations of pathological gait have the capacity to identify muscle impairments and compensatory strategies, but the lack of subject-specific solutions prevents the prescription of personalized therapies. Conversely, electromyographic-driven models are limited to muscles for which data are available but can capture the true neural drive initiated by an individual subject. In order to improve subject-specificity and enforce physiological constraints on muscle activity, we propose a hybrid strategy for the optimization of subject-specific muscle patterns that involves forward dynamic simulation of whole body movement coupled with electromyographic-driven models of muscle subsets. In this paper we apply the hybrid approach to an example of post-stroke gait and demonstrate its unique ability to account for the unusual muscle activation patterns and muscle properties in patients with neuromuscular impairments.

Comparison of electromyography and joint moment as indicators of co-contraction

Antagonistic muscle activity can impair performance, increase metabolic cost, and increase joint stability. Excessive antagonist muscle activity may also cause an undesirable increase in joint contact forces in certain populations such as persons with knee osteoarthritis. Co-contraction of antagonistic muscles measured by electromyography (EMG) is a popular method used to infer muscle forces and subsequent joint forces. However, EMG alone cannot completely describe joint loads that are experienced. This study compares a co-contraction index from EMG to a co-contraction index calculated from simulated muscle moments during gait. Co-contraction indices were calculated from nine healthy, able-bodied subjects during treadmill walking at self-selected speed. Musculoskeletal simulations that tracked experimental kinematics and kinetics were generated for each subject. Experimentally measured EMG was used to constrain the model's muscle excitation for the vastus lateralis and semimembranosus muscles. Using the model's excitations as constrained by EMG, muscle activation and muscle moments were calculated. A common co-contraction index (CCI) based on EMG was compared with co-contraction based on normalized modeled muscle moments (MCCI). While the overall patterns were similar, the co-contraction predicted by MCCI was significantly lower than CCI. Because a simulation can account for passive muscle forces not detected with traditional EMG analysis, MCCI may better reflect physiological knee joint loads. Overall, the application of two co-contraction methods provides a more complete description of muscle co-contraction and joint loading than either method individually.

Kinematic comparison of split-belt and single-belt treadmill walking and the effects of accommodation

INTRODUCTION

Instrumented treadmills are becoming increasingly more common in gait laboratories. Instrumented side-split treadmills allow the collection of forces under each foot during walking. However, there may be a tendency to increase the base of support when walking on these treadmills, influencing other frontal plane mechanics as well. Therefore, the purpose of this study was to examine the effect of walking on a side-split instrumented treadmill on base of gait and frontal plane kinematics of the lower extremity.

METHODS

Twenty subjects walked on both a split and a single-belt treadmill. Base of gait and frontal plane kinematic angles and variability data were recorded. A one-way ANOVA was used to determine differences between the single and split-belt conditions at baseline and following a 10 min accommodation on the split-belt. The relationships between the change in base of gait and change in each kinematic variable were also determined.

RESULTS

On average, the base of gait was 3.7 cm wider on the split-belt treadmill with a 4mm gap between belts. No significant differences were observed in the mean values of lower extremity kinematics or kinematic variability at baseline or following the 10 min accommodation. However, the increase in base of gait was significantly related to a decrease in peak knee and hip adduction angles.

CONCLUSION

The 4mm gap between the treadmill belts significantly increased the mean base of gait in all subjects. This did not alter mean frontal plane kinematics. However, as base of gait increased, the tendency towards hip and knee abduction also increased.

Stabilisation of walking by intrinsic muscle properties revealed in a three-dimensional muscle-driven simulation

A fundamental question in movement science is how humans perform stable movements in the presence of disturbances such as contact with objects. It remains unclear how the nervous system, with delayed responses to disturbances, maintains the stability of complex movements. We hypothesised that intrinsic muscle properties (i.e. the force-length-velocity properties of muscle fibres and tendon elasticity) may help stabilise human walking by responding instantaneously to a disturbance and providing forces that help maintain the movement trajectory. To investigate this issue, we generated a 3D muscle-driven simulation of walking and analysed the changes in the simulation's motion when a disturbance was applied to models with and without intrinsic muscle properties. Removing the intrinsic properties reduced the stability; this was true when the disturbing force was applied at a variety of times and in different directions. Thus, intrinsic muscle properties play a unique role in stabilising walking, complementing the delayed response of the central nervous system.

Strategies used during a challenging weighted walking task in healthy adults and individuals with knee osteoarthritis

Knee osteoarthritis (OA) is a disease that affects millions of people. While numerous gait differences have been identified between healthy adults and adults with knee OA under normal and challenging conditions, adults with knee OA have not been studied during a challenging weighted walking task. Investigation of the effect of weighted walking on the initial contact and loading response phases of gait was undertaken in 20 healthy and 20 knee OA subjects ages 40-85 years old walking at 1.0m/s while unweighted and weighted with 1/6th of their body weight in a weight vest. Subjects were grouped according to their Kellgren and Lawrence radiographic score and healthy subjects were age-matched to those with knee OA. ANOVA revealed significant effects for hip flexion angle at initial contact, step length, initial double support percent, and load rate. Post hoc t-tests revealed that subjects with knee OA had a larger initial double support percent and hip flexion angle at initial contact and a decreased load rate compared to unweighted, healthy adults. Also, both groups increased their initial double support percent in response to the challenging weighted walking task, but only the healthy adults increased their hip flexion angle at initial contact and decreased their load rate. During the weighted condition, the knee OA group had a shorter step length compared to the healthy group. Because the knee OA group only made minor compensations to their gait strategy, it appears that they may be unable or prefer not to adjust their gait mechanics due to underlying issues.

Paretic muscle atrophy and non-contractile tissue content in individual muscles of the post-stroke lower extremity

Muscle atrophy is one of many factors contributing to post-stroke hemiparetic weakness. Since muscle force is a function of muscle size, the amount of muscle atrophy an individual muscle undergoes has implications for its overall force-generating capability post-stroke. In this study, post-stroke atrophy was determined bilaterally in fifteen leg muscles with volumes quantified using magnetic resonance imaging (MRI). All muscle volumes were adjusted to exclude non-contractile tissue content, and muscle atrophy was quantified by comparing the volumes between paretic and non-paretic sides. Non-contractile tissue or intramuscular fat was calculated by determining the amount of tissue excluded from the muscle volume measurement. With the exception of the gracilis, all individual paretic muscles examined had smaller volumes in the non-paretic side. The average decrease in volume for these paretic muscles was 23%. The gracilis volume, on the other hand, was approximately 11% larger on the paretic side. The amount of non-contractile tissue was higher in all paretic muscles except the gracilis, where no difference was observed between sides. To compensate for paretic plantar flexor weakness, one idea might be that use of the paretic gracilis actually causes the muscle to increase in size and not develop intramuscular fat. By eliminating non-contractile tissue from our volume calculations, we have presented volume data that more appropriately represents force-generating muscle tissue. Non-uniform muscle atrophy was observed across muscles and may provide important clues when assessing the effect of muscle atrophy on post-stroke gait.

Effects of fast functional electrical stimulation gait training on mechanical recovery in poststroke gait

Stroke leads to gait impairments that can negatively influence quality of life. Functional electrical stimulation (FES) applied during fast walking (FastFES) is an effective gait rehabilitation strategy that can lead to improvements in gait performance, walking speed and endurance, balance, activity, and participation poststroke. The effect of FastFES gait training on mechanical energy utilization is not well understood. The objective of this study was to test the effects of 12 weeks of FastFES gait training on mechanical recovery indices of poststroke gait. Kinematic data were collected from 11 stroke survivors before and after 12 weeks of FastFES training. Mechanical recovery was calculated from the positive changes in vertical, anterior-posterior, and medial-lateral components of center of mass energy. The average mechanical recovery increased from 34.5% before training to 40.0% after training. The increase was statistically significant (P = 0.014). The average self-selected walking speed increased from 0.4 m/s to 0.7 m/s after the 12-week FastFES training. The results indicate that the subjects were better able to generate and utilize the external mechanical energy of walking after FastFES gait training. FastFES gait training has the capacity to increase the gait speed, improve the mechanical recovery, and reduce the mechanical energy expenditure of stroke survivors when they walk.

Knee osteoarthritis affects the distribution of joint moments during gait

Alterations in lower extremity kinetics have been shown to exist in persons with knee osteoarthritis (OA), however few investigations have examined how the intersegmental coordination of the lower extremity kinetic chain varies in the presence of knee joint pathology. The objective of this study was to evaluate how knee OA and walking speed affect total support moment and individual joint contributions to the total support moment. Fifteen healthy subjects and 30 persons with knee OA participated in 3D walking analysis at constrained (1.0 m/s), self-selected and fastest tolerable walking speeds. Individual joint contributions to total support moment were analyzed using separate ANOVAs with one repeated measure (walking speed). Linear regression analysis was used to evaluate the relationship between walking speed and joint contribution. Persons with knee OA reduced the contribution of the knee joint when walking at constrained (p = 0.04) and self-selected walking speeds (p = 0.009). There was a significant increase in the ankle contribution and a significant decrease in the hip contribution when walking speed was increased (p < 0.004), however individual walking speeds were not significantly related to joint contributions. This suggests that the relationship between walking speed and joint contribution is dependent on the individual's control strategy and we cannot estimate the joint contribution solely based on walking speed. The slower gait speed observed in persons with knee OA is not responsible for the reduction in knee joint moments, rather this change is likely due to alterations in the neuromuscular strategy of the lower extremity kinetic chain in response to joint pain or muscle weakness.

Comparison of MRI-based estimates of articular cartilage contact area in the tibiofemoral joint

Knee osteoarthritis (OA) detrimentally impacts the lives of millions of older Americans through pain and decreased functional ability. Unfortunately, the pathomechanics and associated deviations from joint homeostasis that OA patients experience are not well understood. Alterations in mechanical stress in the knee joint may play an essential role in OA; however, existing literature in this area is limited. The purpose of this study was to evaluate the ability of an existing magnetic resonance imaging (MRI)-based modeling method to estimate articular cartilage contact area in vivo. Imaging data of both knees were collected on a single subject with no history of knee pathology at three knee flexion angles. Intra-observer reliability and sensitivity studies were also performed to determine the role of operator-influenced elements of the data processing on the results. The method's articular cartilage contact area estimates were compared with existing contact area estimates in the literature. The method demonstrated an intra-observer reliability of 0.95 when assessed using Pearson's correlation coefficient and was found to be most sensitive to changes in the cartilage tracings on the peripheries of the compartment. The articular cartilage contact area estimates at full extension were similar to those reported in the literature. The relationships between tibiofemoral articular cartilage contact area and knee flexion were also qualitatively and quantitatively similar to those previously reported. The MRI-based knee modeling method was found to have high intra-observer reliability, sensitivity to peripheral articular cartilage tracings, and agreeability with previous investigations when using data from a single healthy adult. Future studies will implement this modeling method to investigate the role that mechanical stress may play in progression of knee OA through estimation of articular cartilage contact area.

Combined effects of fast treadmill walking and functional electrical stimulation on post-stroke gait

Gait dysfunctions are highly prevalent in individuals post-stroke and affect multiple lower extremity joints. Recent evidence suggests that treadmill walking at faster than self-selected speeds can help improve post-stroke gait impairments. Also, the combination of functional electrical stimulation (FES) and treadmill training has emerged as a promising post-stroke gait rehabilitation intervention. However, the differential effects of combining FES with treadmill walking at the fast versus a slower, self-selected speed have not been compared previously. In this study, we compared the immediate effects on gait while post-stroke individuals walked on a treadmill at their self-selected speed without FES (SS), at the SS speed with FES (SS-FES), at the fastest speed they are capable of attaining (FAST), and at the FAST speed with FES (FAST-FES). During SS-FES and FAST-FES, FES was delivered to paretic ankle plantarflexors during terminal stance and to paretic dorsiflexors during swing phase. Our results showed improvements in peak anterior ground reaction force (AGRF) and trailing limb angle during walking at FAST versus SS. FAST-FES versus SS-FES resulted in greater peak AGRF, trailing limb angle, and swing phase knee flexion. FAST-FES resulted in further increase in peak AGRF compared to FAST. We posit that the enhancement of multiple aspects of post-stroke gait during FAST-FES suggest that FAST-FES may have potential as a post-stroke gait rehabilitation intervention.

Knee contact force in subjects with symmetrical OA grades: differences between OA severities

In using musculoskeletal models, researchers can calculate muscle forces, and subsequently joint contact forces, providing insight into joint loading and the progression of such diseases as osteoarthritis (OA). The purpose of this study was to estimate the knee contact force (KCF) in patients with varying degrees of OA severity using muscle forces and joint reaction forces derived from OpenSim. Walking data was obtained from healthy individuals (n=14) and those with moderate (n=10) and severe knee OA (n=2). For each subject, we generated 3D, muscle-actuated, forward dynamic simulations of the walking trials. Muscle forces that reproduced each subject's gait were calculated. KCFs were then calculated using the vector sum of the muscle forces and joint reaction forces along the longitudinal axis of the femur. Moderate OA subjects exhibited a similar KCF pattern to healthy subjects, with lower second peaks (p=0.021). Although subjects with severe OA had similar initial peak KCF to healthy and moderate OA subjects (more than 4 times BW), the pattern of the KCF was very different between groups. After an initial peak, subjects with severe OA continually unloaded the joint, whereas healthy and moderate OA subjects reloaded the knee during late stance. In subjects with symmetric OA grades, there appears to be differences in loading between OA severities. Similar initial peaks of KCF imply that reduction of peak KCF may not be a compensatory strategy for OA patients; however, reducing duration of high magnitude loads may be employed.