Dorsiflexion Specific Ankle Robotics to Enhance Motor Learning After Stroke: A Preliminary Report.

Background Robotics has emerged as a promising avenue for gait retraining of persons with chronic hemiparetic gait and footdrop, yet there is a gap regarding the biomechanical adaptations that occur with locomotor learning. We developed an ankle exoskeleton (AMBLE) enabling dorsiflexion assist-as-needed across gait cycle sub-events to train and study the biomechanics of motor learning stroke. This single-armed, non-controlled study investigates effects of nine hours (9 weeks x 2 sessions/week) locomotor task-specific ankle robotics training on gait biomechanics and functional mobility in persons with chronic hemiparetic gait and foot drop. Subjects include N = 16 participants (8 male, 8 female) age 53 ± 12 years with mean 11 ± 8 years since stroke. All baseline and post-training outcomes including optical motion capture for 3-D gait biomechanics are conducted during unassisted (no robot) over-ground walking conditions. Findings: Robotics training with AMBLE produced significant kinematic improvements in ankle peak dorsiflexion angular velocity (°/s, + 44 [49%], p < 0.05), heel-first foot strikes (%steps, + 14 [15%], p < 0.01) toe-off angle (°, + 83[162%], p < 0.05), and paretic knee flexion (°, + 20 [30%], p < 0.05). Improvements in gait temporal-spatial parameters include increased paretic step length (cm, + 12 [20%], p < 0.05), reduced paretic swing duration (%GC, -3[6%], p < 0.05), and trend toward improved step length symmetry (-16 [11%], p = 0.08). Functional improvements include 10-meter comfortable (m/s, + 13 [16%], p < 0.01) and fastest (m/s, + 13 [15%], p < 0.01) walking velocities, 6-minute timed walk distance (m, + 16 [19%], p < 0.01) and Dynamic Gait Index scores (+ 15 [15%], p < 0.01). Subjects' perceived improvements surpassed the minimal clinically important difference on the Stroke Impact Scale (SIS) mobility subscale (+ 11 [19%], p < 0.05). Conclusions AMBLE training improves paretic ankle neuromotor control, paretic knee flexion, and gait temporal-distance parameters during unassisted over-ground walking in persons with chronic stroke and foot drop. This locomotor learning indexed by an increase in volitional autonomous (non-robotic) control of paretic ankle across training translated to improvements in functional mobility outcomes. Larger randomized clinical trials are needed to investigate the effectiveness of task-specific ankle robotics, and precise training characteristics to durably improve gait, balance, and home and community-based functional mobility for persons with hemiparetic gait and foot drop. Clinical trial identifier: NCT04594837.

Anodization as a scalable nanofabrication method to engineer mechanobactericidal nanostructures on complex geometries.

Bacterial contamination of implant surfaces is one of the primary causes of their failure, and this threat has been further exacerbated due to the emergence of drug-resistant bacteria. Nanostructured mechanobactericidal surfaces that neutralize bacteria via biophysical forces instead of traditional biochemical routes have emerged as a potential remedy against this issue. Here, we report on the bactericidal activity of titania nanotubes (TNTs) prepared by anodization, a well-established and scalable method. We investigate the differences in bacterial behavior between three different topographies and demonstrate the applicability of this technique on complex three-dimensional (3D) geometries. It was found that the metabolic activity of bacteria on such surfaces was lower, indicative of disturbed intracellular processes. The differences in deformations of the cell wall of Gram-negative and positive bacteria were investigated from electron micrographs Finally, nanoindentation experiments show that the nanotubular topography was durable enough against forces typically experienced in daily life and had minimal deformation under forces exerted by bacteria. Our observations highlight the potential of the anodization technique for fabricating mechanobactericidal surfaces for implants, devices, surgical instruments, and other surfaces in a healthcare setting in a cheap, scalable way.

Cellular Senescence Program is Sensitive to Physical Differences in Polymeric Tissue Scaffolds.

A typical cellular senescence program involves exposing cells to DNA-damaging agents such as ionization radiation or chemotherapeutic drugs, which cause multipronged changes, including increased cell size and volume, the onset of enhanced oxidative stress, and inflammation. In the present study, we examined if the senescence onset decision is sensitive to the design, porosity, and architecture of the substrate. To address this, we generated a library of polymeric scaffolds widely used in tissue engineering of varied stiffness, architecture, and porosity. Using irradiated A549 lung cancer cells, we examined the differences between cellular responses in these 3D scaffold systems and observed that senescence onset is equally diminished. When compared to the two-dimensional (2D) culture formats, there were profound changes in cell size and senescence induction in three-dimensional (3D) scaffolds. We further establish that these observed differences in the senescence state can be attributed to the altered cell spreading and cellular interactions on these substrates. This study elucidates the role of scaffold architecture in the cellular senescence program.

Cooperative stiffening of flexible high aspect ratio nanostructures impart mechanobactericidal activity to soft substrates.

Understanding how bacteria interact with surfaces with micrometer and/or sub-micrometer roughness is critical for developing antibiofouling and bactericidal topographies. A primary research focus in this field has been replicating and emulating bioinspired nanostructures on various substrates to investigate their mechanobactericidal potential. Yet, reports on polymer substrates, especially with very high aspect ratios, have been rare, despite their widespread use in our daily lives. Specifically, the role of a decrease in stiffness with an increase in the aspect ratio of nanostructures may be consequential for the mechanobactericidal mechanism, which is biophysical in nature. Therefore, this work reports on generating bioinspired high aspect ratio nanostructures on poly(ethylene terephthalate) (PET) surfaces to study and elucidate their antibacterial and antibiofouling properties. Biomimetic nanotopographies with variable aspect ratios were generated via maskless dry etching of PET in oxygen plasma. It was found that both high and low-aspect ratio structures effectively neutralized Gram-negative bacterial contamination by imparting damage to their membranes but were unable to inactivate Gram-positive cells. Notably, the clustering of the soft, flexible tall nanopillars resulted in cooperative stiffening, as revealed by the nanomechanical behavior of the nanostructures and validated with the help of finite element simulations. Moreover, external capillary forces augmented the killing efficiency by enhancing the strain on the bacterial cell wall. Finally, experimental and computational investigation of the durability of the nanostructured surfaces showed that the structures were robust enough to withstand forces encountered in daily life. Our results demonstrate the potential of the single-step dry etching method for the fabrication of mechanobactericidal topographies and their potential in a wide variety of applications to minimize bacterial colonization of soft substrates like polymers.

Theoretical and computational investigations into mechanobactericidal activity of nanostructures at the bacteria-biomaterial interface: a critical review.

Mechanobactericidal surfaces kill bacteria upon contact by posing landscapes hostile to them and have rapidly gained popularity amongst researchers over the past decade. But several fundamental aspects of the physical interactions between bacteria and nanostructures and the underlying killing mechanisms are still poorly understood. This is partly attributable to the difficulties associated with the characterization of the bacteria-nanostructure interface in a biological environment during the killing process and to the stochastic and non-linear behaviors generally associated with biological systems. However, several analytical and computational models have presented and analyzed possible killing routes and have proven useful in understanding different aspects of the phenomena. Analytical models formulate equations, often based on energy considerations, and aim to predict optimal nanostructure dimensions. They are more widely used than computational models that try to simulate the killing process and the stress or strain fields in the cell membrane through numerical methods. These models provide insights into the forces responsible for the spontaneous penetration of the cell into the nanostructures, which are still highly debated in the field. They have also helped to correlate the nanostructure dimensions with their bactericidal activity to optimize such values and facilitate the translation of this technology to physiological conditions. This review focuses on the rupture of the bacterial cell wall by nanopillars or similar high aspect ratio structures and applying these principles to the deformation of the cell membrane. Many recent interesting experimental results that either refute our current understanding or have the potential to challenge the current consensus are also discussed. Lastly, the limitations of the current strategies and opportunities to address the unresolved gaps in the field are also presented.

Gait and Balance Biomechanics in Older Adults With and Without Human Immunodeficiency Virus.

Balance deficits impose limitations and can impede safe walking contributing to falls and falls-related complications. The objective of this study was to perform an in-depth balance assessment and compare domains of limitations in older men with and without HIV infection. Fifteen sedentary African American men either with HIV (n = 6) or without HIV (n = 9 controls) participated. Standing balance was assessed under quiet stance on dual synchronized force plates during three 30 s trials with eyes open. Participants also completed standardized clinical instruments of balance, including the Berg Balance Scale (BBS) and Dynamic Gait Index (DGI). Older participants with HIV have lower BBS and DGI scores than controls (both p < .05). Adults with HIV have nearly twice the magnitude greater center of pressure (COP) sway variability than controls (1.42 ± 1.20 cm2 vs. 0.71 ± 0.1 cm2, p < .05). These data demonstrating differences in COP sway area between groups may further support evidence of potential fall risk and contribute to frailty in older adults with HIV.

Mimicking Insect Wings: The Roadmap to Bioinspiration.

Insect wings possess unique, multifaceted properties that have drawn increasing attention in recent times. They serve as an inspiration for engineering of materials with exquisite properties. The structure-function relationships of insect wings are yet to be documented in detail. In this review, we present a detailed understanding of the multifunctional properties of insect wings, including micro- and nanoscale architecture, material properties, aerodynamics, sensory perception, wettability, optics, and antibacterial activity, as investigated by biologists, physicists, and engineers. Several established modeling strategies and fabrication methods are reviewed to engender novel ideas for biomimetics in diverse areas.

Recognizing Hemiparetic Ankle Deficits Using Wearable Pressure Sensors.

OBJECTIVE: To provide proof-of-concept for a novel method to recognize impaired push-off and foot-drop deficits in hemiparetic gait using analog pressure sensors. These data may enhance feedback from a modular ankle exoskeleton (such as Anklebot) for stroke rehabilitation, which now employs on/off foot switches under the foot. METHODS: A pressure sensor was positioned on the posterior side of the calcaneus. Experiments were conducted on two healthy subjects with normal walking and with hip circumduction and foot drop, the latter to mimic hemiparetic gait post-stroke. RESULTS: Unlike the foot switches, the pressure sensor yielded data during swing. The initial swing and terminal stance readings followed local foot-shoe dynamics and were thus able to detect foot drop swing deficits while also providing push-off information during stance. DISCUSSION: The analog pressure sensors provided more information than foot switches, even during stance. This system may provide clinicians with a tool to monitor foot drop and push-off.

Task-specific ankle robotics gait training after stroke: a randomized pilot study.

BACKGROUND: An unsettled question in the use of robotics for post-stroke gait rehabilitation is whether task-specific locomotor training is more effective than targeting individual joint impairments to improve walking function. The paretic ankle is implicated in gait instability and fall risk, but is difficult to therapeutically isolate and refractory to recovery. We hypothesize that in chronic stroke, treadmill-integrated ankle robotics training is more effective to improve gait function than robotics focused on paretic ankle impairments. FINDINGS: Participants with chronic hemiparetic gait were randomized to either six weeks of treadmill-integrated ankle robotics (n = 14) or dose-matched seated ankle robotics (n = 12) videogame training. Selected gait measures were collected at baseline, post-training, and six-week retention. Friedman, and Wilcoxon Sign Rank and Fisher's exact tests evaluated within and between group differences across time, respectively. Six weeks post-training, treadmill robotics proved more effective than seated robotics to increase walking velocity, paretic single support, paretic push-off impulse, and active dorsiflexion range of motion. Treadmill robotics durably improved gait dorsiflexion swing angle leading 6/7 initially requiring ankle braces to self-discarded them, while their unassisted paretic heel-first contacts increased from 44 % to 99.6 %, versus no change in assistive device usage (0/9) following seated robotics. CONCLUSIONS: Treadmill-integrated, but not seated ankle robotics training, durably improves gait biomechanics, reversing foot drop, restoring walking propulsion, and establishing safer foot landing in chronic stroke that may reduce reliance on assistive devices. These findings support a task-specific approach integrating adaptive ankle robotics with locomotor training to optimize mobility recovery. CLINICAL TRIAL IDENTIFIER: NCT01337960. https://clinicaltrials.gov/ct2/show/NCT01337960?term=NCT01337960&rank=1.

An Engineering Model of Human Balance Control-Part I: Biomechanical Model.

We developed a balance measurement tool (the balanced reach test (BRT)) to assess standing balance while reaching and pointing to a target moving in three-dimensional space according to a sum-of-sines function. We also developed a three-dimensional, 13-segment biomechanical model to analyze performance in this task. Using kinematic and ground reaction force (GRF) data from the BRT, we performed an inverse dynamics analysis to compute the forces and torques applied at each of the joints during the course of a 90 s test. We also performed spectral analyses of each joint's force activations. We found that the joints act in a different but highly coordinated manner to accomplish the tracking task-with individual joints responding congruently to different portions of the target disk's frequency spectrum. The test and the model also identified clear differences between a young healthy subject (YHS), an older high fall risk (HFR) subject before participating in a balance training intervention; and in the older subject's performance after training (which improved to the point that his performance approached that of the young subject). This is the first phase of an effort to model the balance control system with sufficient physiological detail and complexity to accurately simulate the multisegmental control of balance during functional reach across the spectra of aging, medical, and neurological conditions that affect performance. Such a model would provide insight into the function and interaction of the biomechanical and neurophysiological elements making up this system; and system adaptations to changes in these elements' performance and capabilities.

Increased reward in ankle robotics training enhances motor control and cortical efficiency in stroke.

Robotics is rapidly emerging as a viable approach to enhance motor recovery after disabling stroke. Current principles of cognitive motor learning recognize a positive relationship between reward and motor learning. Yet no prior studies have established explicitly whether reward improves the rate or efficacy of robotics-assisted rehabilitation or produces neurophysiologic adaptations associated with motor learning. We conducted a 3 wk, 9-session clinical pilot with 10 people with chronic hemiparetic stroke, randomly assigned to train with an impedance-controlled ankle robot (anklebot) under either high reward (HR) or low reward conditions. The 1 h training sessions entailed playing a seated video game by moving the paretic ankle to hit moving onscreen targets with the anklebot only providing assistance as needed. Assessments included paretic ankle motor control, learning curves, electroencephalograpy (EEG) coherence and spectral power during unassisted trials, and gait function. While both groups exhibited changes in EEG, the HR group had faster learning curves (p = 0.05), smoother movements (p

Modular ankle robotics training in early subacute stroke: a randomized controlled pilot study.

UNLABELLED: BACKGROUND. Modular lower extremity robotics may offer a valuable avenue for restoring neuromotor control after hemiparetic stroke. Prior studies show that visually guided and visually evoked practice with an ankle robot (anklebot) improves paretic ankle motor control that translates into improved overground walking. OBJECTIVE: To assess the feasibility and efficacy of daily anklebot training during early subacute hospitalization poststroke. METHODS: Thirty-four inpatients from a stroke unit were randomly assigned to anklebot (n = 18) or passive manual stretching (n = 16) treatments. All suffered a first stroke with residual hemiparesis (ankle manual muscle test grade 1/5 to 4/5), and at least trace muscle activation in plantar- or dorsiflexion. Anklebot training employed an "assist-as-needed" approach during >200 volitional targeted paretic ankle movements, with difficulty adjusted to active range of motion and success rate. Stretching included >200 daily mobilizations in these same ranges. All sessions lasted 1 hour and assessments were not blinded. RESULTS: Both groups walked faster at discharge; however, the robot group improved more in percentage change of temporal symmetry (P = .032) and also of step length symmetry (P = .038), with longer nonparetic step lengths in the robot (133%) versus stretching (31%) groups. Paretic ankle control improved in the robot group, with increased peak (P ≤ .001) and mean (P ≤ .01) angular speeds, and increased movement smoothness (P ≤ .01). There were no adverse events. CONCLUSION: Though limited by small sample size and restricted entry criteria, our findings suggest that modular lower extremity robotics during early subacute hospitalization is well tolerated and improves ankle motor control and gait patterning.

Clinical application of a modular ankle robot for stroke rehabilitation.

BACKGROUND: Advances in our understanding of neuroplasticity and motor learning post-stroke are now being leveraged with the use of robotics technology to enhance physical rehabilitation strategies. Major advances have been made with upper extremity robotics, which have been tested for efficacy in multi-site trials across the subacute and chronic phases of stroke. In contrast, use of lower extremity robotics to promote locomotor re-learning has been more recent and presents unique challenges by virtue of the complex multi-segmental mechanics of gait. OBJECTIVES: Here we review a programmatic effort to develop and apply the concept of joint-specific modular robotics to the paretic ankle as a means to improve underlying impairments in distal motor control that may have a significant impact on gait biomechanics and balance. METHODS: An impedance controlled ankle robot module (anklebot) is described as a platform to test the idea that a modular approach can be used to modify training and measure the time profile of treatment response. RESULTS: Pilot studies using seated visuomotor anklebot training with chronic patients are reviewed, along with results from initial efforts to evaluate the anklebot's utility as a clinical tool for assessing intrinsic ankle stiffness. The review includes a brief discussion of future directions for using the seated anklebot training in the earliest phases of sub-acute therapy, and to incorporate neurophysiological measures of cerebro-cortical activity as a means to reveal underlying mechanistic processes of motor learning and brain plasticity associated with robotic training. CONCLUSIONS: Finally we conclude with an initial control systems strategy for utilizing the anklebot as a gait training tool that includes integrating an Internal Model-based adaptive controller to both accommodate individual deficit severities and adapt to changes in patient performance.

Changes in passive ankle stiffness and its effects on gait function in people with chronic stroke.

Mechanical impedance of the ankle is known to influence key aspects of ankle function. We investigated the effects of robot-assisted ankle training in people with chronic stroke on the paretic ankle's passive stiffness and its relationship to overground gait function. Over 6 wk, eight participants with residual hemiparetic deficits engaged in a visuomotor task while seated that required dorsiflexion (DF) or plantar flexion (PF) of their paretic ankle with an ankle robot ("anklebot") assisting as needed. Passive ankle stiffness (PAS) was measured in both the trained sagittal and untrained frontal planes. After 6 wk, the PAS decreased in both DF and PF and reverted into the variability of age-matched controls in DF. Changes in PF PAS correlated strongly with gains in paretic step lengths (Spearman rho = -0.88, p = 0.03) and paretic stride lengths (Spearman rho = -0.82, p = 0.05) during independent floor walking. Moreover, baseline PF PAS were correlated with gains in paretic step lengths (Spearman rho = 0.94, p = 0.01), paretic stride lengths (Spearman rho = 0.83, p = 0.05), and single-support stance duration (Spearman rho = 0.94, p = 0.01); and baseline eversion PAS were correlated with gains in cadence (Spearman rho = -0.88, p = 0.03). These findings suggest that ankle robot-assisted, visuomotor-based, isolated ankle training has a positive effect on paretic ankle PAS that strongly influences key measures of gait function.

Short-term ankle motor performance with ankle robotics training in chronic hemiparetic stroke.

Cerebrovascular accident (stroke) often results in impaired motor control and persistent weakness that may lead to chronic disability, including deficits in gait and balance function. Finding ways to restore motor control may help reduce these deficits; however, little is known regarding the capacity or temporal profile of short-term motor adaptations and learning at the hemiparetic ankle. Our objective was to determine the short-term effects of a single session of impedance-controlled ankle robot ("anklebot") training on paretic ankle motor control in chronic stroke. This was a double-arm pilot study on a convenience sample of participants with chronic stroke (n = 7) who had residual hemiparetic deficits and an equal number of age- and sex-matched nondisabled control subjects. Training consisted of participants in each group playing a target-based video game with the anklebot for an hour, for a total of 560 movement repetitions in dorsiflexion/plantar flexion ranges followed by retest 48 hours later. Task difficulty was adjusted to ankle range of motion, with robotic assistance decreased incrementally across training. Assessments included robotic measures of ankle motor control on unassisted trials before and after training and at 48 hours after training. Following exposure to the task, subjects with stroke improved paretic ankle motor control across a single training session as indexed by increased targeting accuracy (21.6 +/- 8.0 to 31.4 +/- 4.8, p = 0.05), higher angular speeds (mean: 4.7 +/- 1.5 degrees/s to 6.5 +/- 2.6 degrees/s, p < 0.01, peak: 42.8 +/- 9.0 degrees/s to 45.6 +/- 9.4 degrees/s, p = 0.03), and smoother movements (normalized jerk: 654.1 +/- 103.3 s(-2) to 537.6 +/- 86.7 s(-2), p < 0.005, number of speed peaks: 27.1 +/- 5.8 to 23.7 +/- 4.1, p < 0.01). In contrast, nondisabled subjects did not make statistically significant gains in any metric after training except in the number of successful passages (32.3 +/- 7.5 to 36.5 +/- 6.4, p = 0.006). Gains in all five motor control metrics were retained (p > 0.05) at 48 hours in both groups. Robust maintenance of motor adaptation in the robot-trained paretic ankle over 48 hours may be indicative of short-term motor learning. Our initial results suggest that the anklebot may be a flexible motor learning platform with the potential to detect rapid changes in ankle motor performance poststroke.

Measurement of passive ankle stiffness in subjects with chronic hemiparesis using a novel ankle robot.

Our objective in this study was to assess passive mechanical stiffness in the ankle of chronic hemiparetic stroke survivors and to compare it with those of healthy young and older (age-matched) individuals. Given the importance of the ankle during locomotion, an accurate estimate of passive ankle stiffness would be valuable for locomotor rehabilitation, potentially providing a measure of recovery and a quantitative basis to design treatment protocols. Using a novel ankle robot, we characterized passive ankle stiffness both in sagittal and in frontal planes by applying perturbations to the ankle joint over the entire range of motion with subjects in a relaxed state. We found that passive stiffness of the affected ankle joint was significantly higher in chronic stroke survivors than in healthy adults of a similar cohort, both in the sagittal as well as frontal plane of movement, in three out of four directions tested with indistinguishable stiffness values in plantarflexion direction. Our findings are comparable to the literature, thus indicating its plausibility, and, to our knowledge, report for the first time passive stiffness in the frontal plane for persons with chronic stroke and older healthy adults.

Ankle training with a robotic device improves hemiparetic gait after a stroke.

BACKGROUND: Task-oriented therapies such as treadmill exercise can improve gait velocity after stroke, but slow velocities and abnormal gait patterns often persist, suggesting a need for additional strategies to improve walking. OBJECTIVES: To determine the effects of a 6-week visually guided, impedance controlled, ankle robotics intervention on paretic ankle motor control and gait function in chronic stroke. METHODS: This was a single-arm pilot study with a convenience sample of 8 stroke survivors with chronic hemiparetic gait, trained and tested in a laboratory. Subjects trained in dorsiflexion-plantarflexion by playing video games with the robot during three 1-hour training sessions weekly, totaling 560 repetitions per session. Assessments included paretic ankle ranges of motion, strength, motor control, and overground gait function. RESULTS: Improved paretic ankle motor control was seen as increased target success, along with faster and smoother movements. Walking velocity also increased significantly, whereas durations of paretic single support increased and double support decreased. CONCLUSIONS: Robotic feedback training improved paretic ankle motor control with improvements in floor walking. Increased walking speeds were comparable with reports from other task-oriented, locomotor training approaches used in stroke, suggesting that a focus on ankle motor control may provide a valuable adjunct to locomotor therapies.

Effects of unilateral robotic limb loading on gait characteristics in subjects with chronic stroke.

BACKGROUND: Hemiparesis after stroke often leads to impaired ankle motor control that impacts gait function. In recent studies, robotic devices have been developed to address this impairment. While capable of imparting forces to assist during training and gait, these devices add mass to the paretic leg which might encumber patients' gait pattern. The purpose of this study was to assess the effects of the added mass of one of these robots, the MIT's Anklebot, while unpowered, on gait of chronic stroke survivors during overground and treadmill walking. METHODS: Nine chronic stroke survivors walked overground and on a treadmill with and without the anklebot mounted on the paretic leg. Gait parameters, interlimb symmetry, and joint kinematics were collected for the four conditions. Repeated-measures analysis of variance (ANOVA) tests were conducted to examine for possible differences across four conditions for the paretic and nonparetic leg. RESULTS: The added inertia and friction of the unpowered anklebot had no statistically significant effect on spatio-temporal parameters of gait, including paretic and nonparetic step time and stance percentage, in both overground and treadmill conditions. Noteworthy, interlimb symmetry as characterized by relative stance duration was greater on the treadmill than overground regardless of loading conditions. The presence of the unpowered robot loading reduced the nonparetic knee peak flexion on the treadmill and paretic peak dorsiflexion overground (p < 0.05). CONCLUSIONS: Our results suggest that for these subjects the added inertia and friction of this backdriveable robot did not significantly alter their gait pattern.

A novel theoretical framework for the dynamic stability analysis, movement control, and trajectory generation in a multisegment biomechanical model.

We consider a simplified characterization of the postural control system that embraces two broad components: one representing the musculoskeletal dynamics in the sagittal plane and the other representing proprioceptive feedback and the central nervous system (CNS). Specifically, a planar four-segment neuromusculoskeletal model consisting of the ankle, knee, and hip degrees-of-freedom (DOFs) is described in this paper. The model includes important physiological constructs such as Hill-type muscle model, active and passive muscle stiffnesses, force feedback from the Golgi tendon organ, muscle length and rate feedback from the muscle spindle, and transmission latencies in the neural pathways. A proportional-integral-derivative (PID) controller for each individual DOF is assumed to represent the CNS analog in the modeling paradigm. Our main hypothesis states that all stabilizing PID controllers for such multisegment biomechanical models can be parametrized and analytically synthesized. Our analytical and simulation results show that the proposed representation adequately shapes a postural control that (a) possesses good disturbance rejection and trajectory tracking, (b) is robust against feedback latencies and torque perturbations, and (c) is flexible to embrace changes in the musculoskeletal parameters. We additionally present detailed sensitivity analysis to show that control under conditions of limited or no proprioceptive feedback results in (a) significant reduction in the stability margins, (b) substantial decrease in the available stabilizing parameter set, and (c) oscillatory movement trajectories. Overall, these results suggest that anatomical arrangement, active muscle stiffness, force feedback, and physiological latencies play a major role in shaping motor control processes in humans.

PID controller tuning for the first-order-plus-dead-time process model via Hermite-Biehler theorem.

This paper discusses PID stabilization of a first-order-plus-dead-time (FOPDT) process model using the stability framework of the Hermite-Biehler theorem. The FOPDT model approximates many processes in the chemical and petroleum industries. Using a PID controller and first-order Padé approximation for the transport delay, the Hermite-Biehler theorem allows one to analytically study the stability of the closed-loop system. We derive necessary and sufficient conditions for stability and develop an algorithm for selection of stabilizing feedback gains. The results are given in terms of stability bounds that are functions of plant parameters. Sensitivity and disturbance rejection characteristics of the proposed PID controller are studied. The results are compared with established tuning methods such as Ziegler-Nichols, Cohen-Coon, and internal model control.