Knowledge of the upper extremity (UE) effort exerted under real-world conditions is important for understanding how persons with motor or sensory disorders perform the postural shifts necessary to complete many activities of daily living while standing. To this end, a feedback controller, named the “Posture Follower Controller”, was developed to aid in task-dependent posture shifting by individuals with spinal cord injury standing with functional neuromuscular stimulation. In this experimental feasibility study, the controller modulated activation to the paralyzed lower extremity muscles as a function of the position of overall center of pressure (CoP), which was prescribed to move in a straight line in forward and diagonal directions. Posture-dependent control of stimulation enabled leaning movements that translated the CoP up to 48 mm away from the nominal position during quiet standing. The mean 95% prediction ellipse area, a measure of the CoP dispersion in the forward, forward-right, and forward-left directions, was
Spinal cord injury (SCI) often results in partial or total paralysis of the trunk and lower extremity (LE) muscles. Implanted neuroprostheses (NPs) utilizing functional neuromuscular stimulation (FNS) can restore basic standing function in individuals with SCI, providing them with the independence to accomplish several activities of daily living [
One benefit to the ability to adjust posture from the erect stance is the ability to prepare for a functional task, such as reaching and manipulating objects on shelves. This also gives users the ability to reach the full extent of their standing workspace, thereby providing them with greater independence and access to objects in the environment. Another benefit is adjusting posture laterally, to rest muscles on one side of the body as a means of mitigating fatigue and prolonging overall standing times. These benefits are further supported by the work of Abbas and Gillette [
To achieve these changes in posture, the location of the projection of the total body CoM on the base of support will have to change smoothly and continuously. Thus, control systems that can automatically maintain standing balance should include CoM position feedback and modulate stimulation to the LE muscles as posture is adjusted away from the erect stance. One such system (the Posture Follower Conftroller or PFC) was developed and tested in simulation [
The next step in the development and deployment of posture-dependent control systems is to implement the PFC in the laboratory environment with a standing NP user. The simulation results in [
The aims of this work are (1) to explore the effectiveness of a PFC to enable NP users to lean away from the erect posture and (2) to examine the contribution of UE effort to leaning postures in real subjects with SCI using a NP for standing. In this exploratory study, standing performance was determined by the following metrics: (1) maximum resultant UE effort contributed to leaning movements in the AP and ML directions as LE muscle activation is modulated and (2) CoP-tracking deviations from a prescribed straight-line path.
A 27-year-old male with motor incomplete C5 tetraplegia (AIS C) participated in the experiments. He was approximately 185.4 cm tall and weighed 58.5 kilograms when the experiments were conducted. He received a 16-channel implanted LE NP one year prior to data collection and was a regular user for reconditioning exercise and standing. At the time of the study, he could stand quietly in the neutral position for 25 minutes with 93% body weight (BW) supported by his legs, utilizing UE effort only for a light touch on the walker to maintain balance. Prior to participating in the experiments, the subject signed informed consent forms approved by the Institutional Review Board of the Louis Stokes Cleveland Veterans Affairs Medical Center.
The standing NP consisted of one surgically implanted 16-channel stimulator telemeter [
To target the right GMED for postural control in the ML direction and bilateral tibialis anterior (TA) and gastrocnemius (GS) for postural control in the AP direction, the implanted stimulation system was supplemented with self-adhesive surface electrodes. These muscles were recruited because they were not available in the subject’s implanted system. Surface stimulation was delivered at a constant frequency of 20 Hz, variable pulse width up to 250
Real-time control of stimulation was implemented with a custom software developed in MATLAB/Simulink R7.9 and the xPC Target toolbox (MathWorks Inc., Natick, MA). A Windows (Microsoft Inc., Redmond, WA) host computer was utilized to build customized applications, while a dedicated (target) computer with the Pentium Dual-Core 3 GHz microprocessor (Intel Inc., Santa Clara, CA) with 2 GB of RAM was responsible for running the applications in real time. The host and target computers communicated via the TCP/IP protocol. Data were acquired using a NI PCI-6071E board (National Instruments Inc., Austin, TX). For the experiments described, all real-time controller and stimulation parameters were sampled at 40 Hz. The stimulus values for erect standing were determined by clinical observation whereby the subject exhibited ample knee, hip, and trunk extension to achieve an erect posture without discomfort. Baseline standing stimulation values are listed in Table
Muscle pulse amplitudes and pulse widths for baseline standing. Muscles that were always recruited for baseline standing are indicated with a “P”, while those recruited by the controller are indicated with a “C.” Muscles that were supplemented with surface electrodes are indicated with a “
Muscle | Function | Pulse amplitude (mA) | Baseline standing PW ( |
Threshold PW ( |
Saturation PW ( |
---|---|---|---|---|---|
Right gluteus maximus (right GMX) | P | 20.0 | 248 | 2 | 250 |
Right hamstring (right HM) | P | 20.0 | 250 | 64 | 250 |
Left gluteus maximus (left GMX) | P | 20.0 | 145 | 5 | 150 |
Left gluteus medius (left GMED) | C | 20.0 | 61.5 | 13 | 110 |
Right quadratus lumborum (right QL) | — | — | — | — | — |
Right erector spinae (right ES) | P | 2.1 | 90 | 10 | 125 |
Left quadratus lumborum (left QL) | — | 18.0 | 0 | 10 | 50 |
Left erector spinae (left ES) | — | 18.0 | 0 | 35 | 70 |
Right quadriceps 1 (right QD 1) | P | 0.8 | 90 | 30 | 90 |
Right quadriceps 2 (right QD 2) | P | 0.8 | 90 | 24 | 90 |
Left quadriceps 1 (left QD 1) | P | 0.8 | 250 | 64 | 250 |
Left quadriceps 2 (left QD 2) | P | 0.8 | 250 | 48 | 250 |
Right quadriceps 3 (right QD 3) | P | 0.8 | 100 | 32 | 100 |
Right posterior adductor (right PA) | C | 20.0 | 86 | 2 | 170 |
Left quadriceps 3 (left QD 3) | P | 0.8 | 250 | 72 | 250 |
Left posterior adductor (left PA) | C | 20.0 | 128.5 | 7 | 250 |
Right gluteus medius (right GMED) | C |
100.0 | 0 | 80 | 250 |
Right tibialis anterior (right TA) | C |
30.0 | 0 | 80 | 100 |
Right gastrocnemius (right GS) | C |
100.0 | 0 | 20 | 65 |
Left tibialis anterior (left TA) | C |
30.0 | 0 | 70 | 90 |
Left gastrocnemius (left GS) | C |
100.0 | 0 | 30 | 70 |
In this study, the overall center of pressure (CoP) position (a function of the location of the vertical ground reaction force vector) was used as the feedback signal for the PFC. A more suitable feedback signal would be the orthogonal projection of the whole-body CoM or center of gravity (CoG). However, there are challenges in implementing the CoM position (a function of the location of the total body mass, as the feedback signal). Currently, there is no means for the quantity to be computed or estimated from body-mounted sensors in real time rapidly and accurately enough to use as a stimulus control signal with a paralyzed user. The CoM, CoG, and CoP are equivalent during static conditions. Thus, the overall CoP position was used as a surrogate because it can be readily obtained from two force plates (AMTI, Watertown, MA) in the laboratory, making it a more practical control signal for this exploratory study. The laboratory-based PFC took the form of a proportional feedback controller, so it tracks voluntary changes in posture by mapping changes in CoP to changes in LE muscle activations (Figure
Control setup. The user stands erect on force plates, which measure the center of pressure (CoP) position in the anterior-posterior (AP) and medial-lateral (ML) directions. The user leans away from the erect stance, adjusting the overall CoP position towards the ends of the paths in the forward and diagonal directions. The force plates continuously track the resulting changes in the CoP position and the posture follower controller converts the changes in the CoP position to muscle activation, which is applied to the lower extremities.
The user stood at an erect, nominal stance with baseline (open-loop) stimulation. In this stance, the user stood upright with the feet approximately under the shoulders and each on a separate force plate. The erect, nominal stance is biomechanically defined as the standing posture in which the head, trunk, pelvis, and LEs are aligned as close to vertical as possible in sagittal and coronal planes with minimal to no axial rotation in the coronal plane. The components of the overall CoP position in the AP and ML direction were computed using (equation (
As the subject leans away from the erect stance and the CoP moves away from the nominal (erect) position, the resulting changes in AP and ML components are tracked by the PFC via a simple proportional control law. Assuming a linear relationship between changes in the CoP and muscle activation, the changes in activation to be applied to the LE muscles are computed according to (equation
Assuming that posture is adjusted in a slow and quasi-static manner, the PFC targets muscles to provide support, supplying stimulation that is optimal, as determined in [
To assess the standing performance with respect to maintaining and tracking posture according to a prescribed path, visual feedback of the overall CoP and specified paths were presented on a computer monitor in real time (Figure
Diagram of the visual feedback display. The subject stood erect at the nominal (NO) starting position and adjusted the overall center of pressure (CoP) position to track the moving circle to the end of the paths defined by the yellow circles. The subject tracked the moving circle along the same path to return to the NO position. Prior to conducting the experiments, the speed of the moving circle and the locations of the endpoints of the paths were tuned to ensure that the subject adjusted posture at a comfortable rate and within reasonable limits of his standing balance. During the experiments, the subject adjusted the overall CoP position (green) to track the moving circle (blue) in the forward (FO), forward-right (FR), and forward-left (FL) directions. As an additional visual cue, the currently specified path is defined by changing the color of its endpoint from yellow to red. In this image, the currently specified path is the one from NO to FR.
The proportional gain settings in (equation
The setup for testing the effects of the PFC is depicted in Figure
Set-up for experimental evaluation of the controller. The subject stands erect on force plates, while holding onto an instrumented walker and adjusting the overall center of pressure (CoP) position towards the end of paths in the forward and diagonal directions. The subject was provided with visual feedback while adjusting the overall CoP position. Reflective markers were mounted on the subject to track his joint positions as he adjusted posture.
A static trial was collected to obtain the UE forces exerted on the walker at the nominal erect standing posture. After instruction and sufficient practice to obviate learning effects, the subject adjusted posture by exerting volitional UE effort on the instrumented walker to ensure his overall CoP tracked the moving circle as it moved in the forward (FO) and diagonal directions (forward-right, FR; forward-left, FL). Five trials were collected, with two repetitions for tracking the circle to the ends of each of the three paths and returning to the nominal erect position completed per trial. The sequence of directions was randomized to avoid systematic error.
A repetition is distinguished by leaning movement onset and offset (Figure
The sample CoP profiles of the subject (blue) and the moving circle (red) of the two consecutive leaning movements during the tracking task. The top panel displays the CoP profiles in the AP direction, while the bottom panel displays the CoP profiles in the ML direction. Each movement is considered a separate repetition, which consists of a movement onset, dwell period, and movement offset. Movement onsets are indicated as the time point in which the moving circle initiates movement from the nominal starting position to the end of the specified path. Upon reaching the end of the specified path, the moving circle dwells there for 3 seconds. When the dwell period ends, the moving circle returns to the nominal position and remains there until it initiates travel along the next path. The first time point at which the moving circle acquires the nominal starting position on the return is the movement offset. Based on the orientation of the laboratory coordinate system, postural adjustments in the forward direction are indicated as CoPAP increasing from the nominal starting position. The postural shifts towards the left are indicated as CoPML increasing from the nominal. Thus, in both repetitions, the subject was tracking the moving circle in the forward-left direction.
There were twelve repetitions in which the subject consistently maintained the starting position before movement onset, tracked the moving circle the entire distance to the end, and maintained the same nominal starting position after movement offset. Those repetitions were selected for analysis, and the overall CoP profiles were computed. To obtain the changes in the CoP relative to the value at the nominal position, the starting CoP position was subtracted from the resulting trajectories. UE effort, defined as the maximum resultant UE forces exerted with the PFC, was compared to the values exerted during the erect stance (equation
When movements were elicited in the forward or diagonal directions at gain settings larger than 0.4 of the changes in CoPAP, the modulated activation to the bilateral GS resulted in raising the heels off the ground so the subject stood on his toes. This heel-raising effect was diminished when the gain was set to values below 0.35 and the SPWs of the right and left GS were reduced from 100
The mean changes in CoP trajectories and stimulation pulse widths for leaning postures in the FO direction are represented in Figure
Mean changes across the five trials in (a) the overall CoP position and (b) muscle stimulation pulse widths as posture was shifted in the forward direction. In (a), the mean CoP profiles are presented for the anterior-posterior (AP) direction and the medial-lateral (ML) direction. In (b), the changes in stimulation PWs are presented for the following muscles: LGS (left gastrocnemius), RGS (right gastrocnemius), LTA (left tibialis anterior), RTA (right tibialis anterior), LGMED (left gluteus medius), RGMED (right gluteus medius), LPA (left posterior adductor), and RPA (right posterior adductor). In all plots of the repetitions in the forward direction, the mean profiles are indicated with bold solid lines and (±1) standard deviation is indicated with dashed lines.
Standing duration was an average of 1 minute and 55 seconds (±6 seconds) per trial. The mean maximum resultant UE forces exerted while changing posture were computed for each leaning direction and normalized as percentage of BW (Figure
Mean maximum resultant UE force during leaning movements in the forward (FO), forward-right (FR), and forward-left (FL) directions. As a reference, the maximum resultant UE force exerted while the subject stood in the nominal (NO) starting position during a static trial is also displayed. Error bars are included to indicate ±1 standard deviation of measurements across twelve repetitions.
CoP excursions in the AP and ML directions during the CoP-tracking task are displayed in Figure
(a) Typical posturogram and (b) 95% prediction ellipses for CoP-tracking tasks in the forward and diagonal directions. The 95% PEA for the leaning movements in the forward direction was 1276.5 mm2, 1141.8 mm2 in the forward-right direction, and 1645.2 mm2 in the forward-left direction. Based on the orientation of the laboratory coordinate system, postural adjustments in the forward direction are indicated as CoPAP increasing from the nominal starting position. Postural shifts towards the left are indicated as CoPML increasing from the nominal.
The mean width of the prediction ellipses (Figure
Mean widths of 95% prediction ellipses for CoP-tracking tasks in the forward (FO), forward-right (FR), and forward-left (FL) directions. The error bars are included to indicate ±1 standard deviation of measurements across twelve repetitions.
The aim of this study was to implement the PFC in the laboratory setting and conduct an experimental feasibility test with a standing NP user. This is the first study to our knowledge to investigate the modulation of LE stimulation in a standing NP user as posture is adjusted away from erect stance via a feedback controller. In this study, the feedback signal was the CoP position, which was readily obtained from force plates. As the subject leaned away from an erect stance, the PFC modulated stimulation proportionally according to the desire to effect postural change during the tracking tasks.
Compared to the maximum resultant UE force exerted while the subject stood in the NO position during the static trial (6.75% BW), large percent differences in mean maximum resultant UE effort exerted were observed for all the leaning directions (
PEA and ellipse width were computed to determine CoP-tracking deviations. Across all leaning directions, the PEA increased as deviations in CoP tracking occurred (Figure
Lemay et al. [
The CoP position feedback, as measured with force plates, was a practical signal for laboratory-based exploratory experiments with the PFC. However, the long-term goal is to deploy the controller for home use. Force plates limit controller deployment to the laboratory setting, but advances in sensor technology enable the accurate capture of body motion outside of a controlled laboratory environment. Insole-pressure measurement devices are an appealing option for the measurement of CoP, given that the position of the feet on the floor relative to each other are specified. Each time the user stands, it is likely that the location of the feet will differ slightly. This is not a major issue in the laboratory, where the feet can be moved to fixed targets before each experiment. However, for implementation in the uncontrolled environments of the home and community, additional sensors would need to be added to determine the distances between the feet and their orientation before computing the CoP position. The CoM position is a global variable that can be implemented to detect the position of the body each time the user stands as well as to track the dynamic changes in posture as the user prepares for a functional task. Furthermore, the CoM position more accurately reflects the system dynamics and can change without commensurate displacements of the CoP. The CoM position is therefore an ideal parameter for controlling the entire system, particularly for faster movements or to recover from perturbations. Methods to estimate the CoM position from a network of body-mounted inertial measurement units are underdeveloped, and future work will verify such techniques and incorporate them into home-going systems employing the whole-body CoM position as the feedback signal.
A limitation of this study is the length of time the subject could stand during the experiments. Although the subject could stand quietly for 25 minutes at the time of testing, these experiments were more demanding because they entailed multiple repetitions of standing and adjusting posture in the different directions. To minimize fatigue induced by continuous activation of the muscles, the number of repetitions collected was limited, so that the subject’s total standing time did not exceed 10 minutes. This is consistent with elapsed standing times with conventional FNS systems [
Another limitation to this study is the availability of muscles for control as well as the directions in which the recruited muscles acted. The PFC, as implemented, assumed that the muscles acted independently and exclusively in the sagittal or coronal planes. Future work should explore and exploit the coupling between muscle actions and include cross terms to represent the effects of the GS and TA on ML movement and PA and GMED on AP movement. This involves extending the PFC to act in the generalized coronal plane and modulating all muscles simultaneously irrespective of assumed movement direction (including the postural muscles for hip extension/flexion or trunk extension/lateral bending not adjusted in the current study) to generate the globally optimal patterns of stimulation to realize a movement.
This study sought to determine the experimental feasibility of the PFC, a muscle activation controller that modulated LE activation according to changes in the CoP position, in a recipient of an implanted standing NP. The PFC enabled the subject to assume leaning postures in the FO, FR, and FL directions, by modulating LE muscle activation according to changes in the overall CoP position. More than twice the UE effort as a percentage of quiet standing were required to effect changes in CoP experimentally in this study as predicted from the simulations presented in [
We have explored the experimental feasibility of the PFC, a CoP-position tracking muscle activation controller with a recipient of an implanted standing NP. This is the first study to our knowledge that investigates feedback control of standing posture to enable user-selected leaning movements away from erect stance in an individual with SCI. As the CoP position was adjusted to track the moving circle along the various paths, the PFC continually updated activation to the user’s paralyzed LE musculature. Ellipse areas of the CoP traces indicate that the PFC provided the user with greater access to the standing workspace. Future work will evaluate the controller with the whole-body CoM position as the feedback signal and account for cross-coupling resulting from the anatomical actions of the contracting muscles. This will require the development and evaluation of a model that outputs CoM from data captured from body-mounted sensors and more advanced multidimensional control algorithms.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there is no conflict of interest regarding the publication of this paper.
This material is also the result of work supported with resources and the use of facilities at the Louis Stokes Cleveland Veterans Affairs Medical Center in Cleveland, OH. Funding for this work was provided by the National Institute of Neurological Disorders and Stroke (Grant No: R01NS040547) and the Craig H. Neilsen Foundation (Grant No: 459308).