Estimates of persistent inward current in human motor neurons during postural sway

Persistent inward current (PIC) is a membrane property critical for increasing gain of motor neuron output. In humans, most estimates of PIC are made from plantarflexor or dorsiflexor motor units with the participant in a seated position with the knee flexed. This seated and static posture neglects the task-dependent nature of the monoaminergic drive that modulates PIC activation. Seated estimates may drastically underestimate the amount of PIC that occurs in human motor neurons during functional movement. The current study estimated PIC using the conventional paired motor unit technique which uses the difference between reference unit firing frequency at test unit recruitment and reference unit firing frequency at test unit de-recruitment (∆F) during triangular-shaped, isometric ramps in plantarflexion force as an estimate of PIC. Estimates of PIC were also made during standing anterior postural sway, a postural task that elicits a ramped increase and decrease in soleus motor unit activation similar to the conventional seated ramp contractions. For each motor unit pair, ∆F estimates of PIC made during conventional isometric ramps in the seated posture were compared to those made during standing postural sway. Baseline reciprocal inhibition (RI) was also measured in each posture using the post-stimulus time histogram (PSTH) technique. Hyperpolarizing input has been shown to have a reciprocal relationship with PIC in seated posture and RI was measured to examine if the same reciprocal relationship holds true during functional PIC estimation. It was hypothesized that an increase in ∆F would be seen during standing compared to sitting due to greater neuromodulatory input. We found that ∆F estimates during standing postural sway were equal (2.44 ± 1.17, p=0.44) to those in seated PIC estimates (2.73± 1.20) using the same motor unit pair. Reciprocal inhibition was significantly lower when measured in a standing posture (0.0031 ± 0.0251,

Reactive Stepping with Functional Neuromuscular Stimulation in Response to Forward-Directed Perturbations

Background: Implanted motor system neuroprostheses can be effective at increasing personal mobility of persons paralyzed by spinal cord injuries. However, currently available neural stimulation systems for standing employ patterns of constant activation and are unreactive to changing postural demands. Methods: In this work, we developed a closed-loop controller for detecting forward-directed body disturbances and initiating a stabilizing step in a person with spinal cord injury. Forward-directed pulls at the waist were detected with three body-mounted triaxial accelerometers. A finite state machine was designed and tested to trigger a postural response and apply stimulation to appropriate muscles so as to produce a protective step when the simplified jerk signal exceeded predetermined thresholds. Results: The controller effectively initiated steps for all perturbations with magnitude between 10 and 17.5 s body weight, and initiated a postural response with occasional steps at 5% body weight. For perturbations at 15 and 17.5% body weight, the dynamic responses of the subject exhibited very similar component time periods when compared with able-bodied subjects undergoing similar postural perturbations. Additionally, the reactive step occurred faster for stronger perturbations than for weaker ones (p \u3c .005, unequal varience t-test.) Conclusions: This research marks progress towards a controller which can improve the safety and independence of persons with spinal cord injury using implanted neuroprostheses for standing

Novel instrumented frame for standing exercising of users with complete spinal cord injuries

This paper describes a Functional Electrical Stimulation (FES) standing system for rehabilitation of bone mineral density (BMD) in people with Spinal Cord Injury (SCI). BMD recovery offers an increased quality of life for people with SCI by reducing their risk of fractures. The standing system developed comprises an instrumented frame equipped with force plates and load cells, a motion capture system, and a purpose built 16-channel FES unit. This system can simultaneously record and process a wide range of biomechanical data to produce muscle stimulation which enables users with SCI to safely stand and exercise. An exergame provides visual feedback to the user to assist with upper-body posture control during exercising. To validate the system an alternate weight-shift exercise was used; 3 participants with complete SCI exercised in the system for 1 hour twice-weekly for 6 months. We observed ground reaction forces over 70% of the full body-weight distributed to the supporting leg at each exercising cycle. Exercise performance improved for each participant by an increase of 13.88 percentage points of body-weight in the loading of the supporting leg during the six-month period. Importantly, the observed ground reaction forces are of higher magnitude than other studies which reported positive effects on BMD. This novel instrumentation aims to investigate weight bearing standing therapies aimed at determining the biomechanics of lower limb joint force actions and postural kinematics

A Robust Nonlinear Control Strategy for Unsupported Paraplegic Standing Using Functional Electrical Stimulation: Controller Synthesis and Simulation

Background: Functional electrical stimulation (FES) is known as a promising technique for movement generation in the paralyzed limbs through electrical stimulation of the muscle nerves. This paper focuses on the FES based control of upright standing in paraplegic patients. In this study a new approach for controlling the upright posture has been proposed. The posture control strategies proposed in the previous works were based on controlling the angular joint position, and none of them were focused on controlling the CoP dynamics directly. Since the CoP is representative of posture balance dynamics, in this study the adopted FES based control strategy was designed to control the CoP dynamics directly.Method: In the proposed strategy, the controller has determined the stimulation intensity of ankle muscles in a manner to restrict the center of pressure (CoP) in a specific zone to guarantee the posture balance during unsupported standing. The proposed approach is based on a new cooperative based combination between two different controllers. Utilizing this strategy, until the CoP is confined within the stable zone, an adaptive controller is active and tries to preserve the posture stability. When the CoP goes out the stable zone, sliding mode control, as a nonlinear control technique presenting remarkable properties of robustness, is activated and tries to back the CoP within the preference zone. In this manner, not only the posture balance can be guaranteed but also the balance dynamics can be similar to the elicited dynamic postural behavior in the normal subjects.Results: Extended evaluations carried out through the simulation studies on a musculoskeletal model. According to the achieved results, the proposed control strategy is not only robust against the external disturbances but also insensitive to the initial postural conditions.Conclusion: The achieved results prove the acceptable performance of the proposed control strategy

Control systems for function restoration, exercise, fitness and health in spinal cord injury

We describe original research contributions to the engineering development of systems which aim to restore function and enable effective exercise for people with spinal cord injury (SCI). Our work utilises functional electrical stimulation (FES) of paralysed muscle. Improving function and general health through participation in exercise is vital to the enhancement of quality of life, well-being and promotion of longevity. Crucial to the development of this research has been judicious use of advanced methods of feedback control engineering; this has been a key enabling factor in many of our original contributions. The consequences of a spinal cord injury can be severe. The primary effects may include; paralysis and loss of sensation in the legs, arms and trunk; disruption of bladder and bowel function; and disruption of the autonomic regulation of blood pressure, heart rate and lung function. If the abdominal and chest muscles are paralysed, breathing will be compromised, and patients with a high-level cervical injury may require mechanical ventilation. These primary effects of a spinal cord injury may, over time, lead to a range of debilitating secondary medical complications. These include reduced cardiovascular fitness, urinary tract infection and an associated risk of kidney disease, reduced bone mineral density, the possible development of pressure sores, and muscle spasticity. People with paralysed chest and abdominal muscles are at increased risk of respiratory infection. Consideration of these factors has led us to focus our research programme in this field on novel engineering solutions which have relevance to the secondary consequences of spinal cord injury, and which may help to alleviate some of their effects. In this thesis we describe our contributions in the following areas: 1. Control of Paraplegic Standing; This work concerns upright stance, and aims to provide; (i) automatic feedback control of balance during stance, with the arms free for functional tasks; (ii) methods and apparatus for dynamic standing therapy, which may help to enhance the individual's retained balance skills. This area of work has successfully demonstrated the automatic control of balance during quiet standing in paraplegic subjects. Further, we have established the feasibility of ankle stiffness control in paraplegic subjects using FES, and we have shown that this can be combined with volitional upper-body inputs to achieve stable, arm-free balance. 2. Lower-limb Cycling: Lower-limb cycling, achieved through electrical stimulation of paralysed leg-actuating muscles, is an effective exercise intervention. We have described refinements to the engineering design of an FES-cycling system, based upon the adaptation of commercially-available recumbent tricycles (of various designs), some of which are equipped with an auxiliary electric motor. We have contributed new methods of feedback control of key variables including cycle cadence and exercise workrate. These contributions have facilitated further detailed study of the effect of the exercise on cardiopulmonary fitness, bone integrity, spasticity, muscle condition, and factors relating to the likelihood of skin breakdown (i.e. the development of pressure sores). 3. Upper-limb Exercise in Tetraplegia; We have developed a new exercise modality for patients with a cervical-level injury and significant loss of arm function. The system allows effective arm ergometry by combining volitional motion with electrical stimulation of the paralysed upper-arm muscles. This work has developed new apparatus and exercise testing protocols, and has examined the effect of the exercise on cardiopulmonary fitness and muscle strength in experiments with tetraplegic subjects. 4. Modelling and Control of Stimulated Muscle; This fundamental area of research has investigated dynamic modelling and feedback control design approaches for electrically-stimulated muscle. This work has been applied in the three areas mentioned above. We identify promising areas for future research. These include extension of work on lower- limb cycling to patients with incomplete injuries, to those with cervical-level injuries, and to children with SCI. We wish to participate in a multi-centre clinical study of implanted nerve- root stimulation technology for restoration of bladder and bowel control, and for lower-limb exercise (including cycling). We have initiated a study of treadmill-based gait therapy for incomplete-lesion patients. The goals of this study are to develop test protocols for accurate characterisation of cardiopulmonary status, and to determine whether this form of cyclical lower-limb exercise has a positive impact on retained voluntary leg function. It is often the case that it is those people most severely affected by neurological impairment who stand to gain the most from these approaches (e.g. high-level tetraplegia, paediatric spinal cord injury, etc.). We must therefore continue to seek ways in which the work can be developed for the maximum benefit of these patients. In conclusion, this thesis has described original research contributions to the engineering development of systems which aim to restore important function and to enable effective exercise for people with spinal cord injury. An important facet of our work has been the application of feedback control methods; this has been an enabling factor in several areas of study. We have focused on areas which promise improved fitness and general health, and which may alleviate some of the secondary consequences of spinal cord injury. This work encompasses fundamental research, clinical studies, and the pursuit of technology transfer into clinical practice. Finally, we recognise the growing awareness of and interest in central nervous system plasticity, and in the broad field of central neural regeneration and repair. It is therefore timely to ask whether cyclical exercise interventions can lead to improvement of volitional function in patients with incomplete or discomplete lesions. Such improvements may, we speculate, result from the strengthening of muscles which retain at least partial volitional control, or from neural plasticity and re-organisation, or from regeneration effects (neurogenesis and functional connectivity). A key requirement in this line of investigation, and a major challenge, will be to develop or to utilise methods which can detect changes in a patient's volitional function and neurological status, and which can isolate the source of such changes. Should reliable methods become available, the way to the study of recovery of function through cyclical exercise would be opened. These considerations will remain, we propose, an indispensable complement to cell-based surgical interventions which may become available in the future

Trunk Rehabilitation Using Cable-Driven Robotic Systems

Upper body control is required to complete many daily tasks. One needs to stabilize the head and trunk over the pelvis, as one shifts the center of mass to interact with the world. While healthy individuals can perform activities that require leaning, reaching, and grasping readily, those with neurological and musculoskeletal disorders present with control deficits. These deficits can lead to difficulty in shifting the body center of mass away from the stable midline, leading to functional limitations and a decline in the quality of activity. Often these patient groups use canes, walkers, and wheelchairs for support, leading to occasional strapping or joint locking of the body for trunk stabilization. Current rehabilitation strategies focus on isolated components of stability. This includes strengthening, isometric exercises, hand-eye coordination tasks, isolated movement, and proprioceptive training. Although all these components are evidence based and directly correlate to better stability, motor learning theories such as those by Nikolai Bernstein, suggest that task and context specific training can lead to better outcomes. In specific, based on our experimentation, we believe functional postural exploration, while encompassing aspects of strengthening, hand-eye coordination, and proprioceptive feedback can provide better results. In this work, we present two novel cable robotic platforms for seated and standing posture training. The Trunk Support Trainer (TruST) is a platform for seated posture rehabilitation that provides controlled external wrench on the human trunk in any direction in real-time. The Stand Trainer is a platform for standing posture rehabilitation that can control the trunk, pelvis, and knees, simultaneously. The system works through the use of novel force-field algorithms that are modular and user-specific. The control uses an assist-as-needed strategy to apply forces on the user during regions of postural instability. The device also allows perturbations for postural reactive training. We have conducted several studies using healthy adult populations and pilot studies on patient groups including cerebral palsy, cerebellar ataxia, and spinal cord injury. We propose new training methods that incorporate motor learning theory and objective interventions for improving posture control. We identify novel methods to characterize posture in form of the “8-point star test”. This is to assess the postural workspace. We also demonstrate novel methods for functional training of posture and balance. Our results show that training with our robotic platforms can change the trunk kinematics. Specifically, healthy adults are able to translate the trunk further and rotate the trunk more anteriorly in the seated position. In the standing position, they can alter their reach strategy to maintain the upper trunk more vertically while reaching. Similarly, Cerebral Palsy patients improve their trunk translations, reaching workspace, and maintain a more vertical posture after training, in the seated position. Our results also showed that an Ataxia patient was able to improve their reaching workspace and trunk translations in the standing position. Finally, our results show that the robotic platforms can successfully reduce trunk and pelvis sway in spinal cord injury patients. The results of the pilot studies suggest that training with our robotic platforms and methods is beneficial in improving trunk control

The influence of the reciprocal hip joint link in the advanced reciprocating gait orthosis on standing performance in paraplegia

The effect of reciprocally linking the hip hinges of a hip-knee-ankle-foot orthosis on standing performance was studied in a comparative trial of the Advanced Reciprocating Gait Orthosis (ARGO) and an ARGO in which the Bowden cable was removed (A_GO). Six male subjects with spinal cord injury (SCI) at T4 to T12 level participated in the study, which was conducted using a single case experimental design. Standing balance, the ability to handle balance disturbances (standing stability), and the performance of a functional hand task during standing were assessed in both orthosis configurations in the order A_GO-ARGO-A_GO-ARGO. No significant differences with respect to standing performance were found for the two orthosis configurations. However, the results indicate that the crutch force needed for maintaining balance during various tasks, especially for quiet standing with two crutches, may be much higher in the orthosis without Bowden cable. Therefore, it is very likely that the reciprocal hip joint link in the ARGO provides a substantial and clinically relevant reduction of upper body effort required for standing under functional conditions