Elsevier

Medical Engineering & Physics

Volume 86, December 2020, Pages 47-56
Medical Engineering & Physics

A closed-loop self-righting controller for seated balance in the coronal and diagonal planes following spinal cord injury

https://doi.org/10.1016/j.medengphy.2020.10.010Get rights and content

Highlights

  • Self-righting controller using neuromuscular stimulation restores erect trunk posture.

  • Controller worked for individuals paralyzed by various levels of spinal cord injury.

  • External control unit used accelerometer-measured trunk tilt to regulate stimulation.

  • One chest-worn sensor corrected posture deviations in coronal and diagonal planes.

  • Controller assessed by response time, recovery time and percentage maximum deviation.

Abstract

Spinal cord injury (SCI) often results in loss of the ability to keep the trunk erect and stable while seated. Functional neuromuscular stimulation (FNS) can cause muscles paralyzed by SCI to contract and assist with trunk stability. We have extended the results of a previously reported threshold-based controller for restoring upright posture using FNS in the sagittal plane to more challenging displacements of the trunk in the coronal plane. The system was applied to five individuals with mid-thoracic or higher SCI, and in all cases the control system successfully restored upright sitting. The potential of the control system to maintain posture in forward-sideways (diagonal) directions was also tested in three of the subjects. In all cases, the controller successfully restored posture to erect. Clinically, these results imply that a simple, threshold based control scheme can restore upright sitting from forward, lateral or diagonal leaning without a chest strap; and that removal of barriers to upper extremity interaction with the surrounding environment could potentially allow objects to be more readily retrieved from around the wheelchair. Technical performance of the system was assessed in terms of three variables: response time, recovery time and percent maximum deviation from erect. Overall response and recovery times varied widely among subjects in the coronal plane (415±213 ms and 1381±883 ms, respectively) and in the diagonal planes (530±230 ms and 1800±820 ms, respectively). Average response time was significantly lower (p < 0.05) than the recovery time in all cases. The percent maximum deviation from erect was of the order of 40% or less for 9 out of 10 cases in the coronal plane and 5 out of 6 cases in diagonal directions.

Introduction

Spinal Cord Injury (SCI) often results in paralysis of the muscles of the lower limbs and trunk, which can lead to the loss of functional control of seated posture and balance. Trunk control is the ability for people to maintain and direct motion of the torso with the muscles of the hips and trunk, and it is an essential ability that allows them to effectively interact with their environment with their upper extremities bilaterally without holding on to the wheelchair. Loss of trunk control and stability due to SCI or other movement disorders, usually requires the use of devices, straps, or voluntary effort by the arms to maintain trunk posture in a stable position. This often limits the functional area that can be explored in the upper-limb workspace [1]. Trunk instability is a major concern for individuals with SCI [2], which often results in a decline in the ability to undertake activities of daily living (ADLs) including wheelchair propulsion [3] and extended reaching tasks.

Functional Neuromuscular Stimulus (FNS) is a technique in which implanted or surface electrodes deliver current to the motor nerve which cause the muscles paralyzed by SCI to contract and produce force. A carefully planned coordinated recruitment of such activated muscles using engineering control methods could mitigate some of the issues listed above, and stabilize the trunk to improve seated posture and enable more functional interactions with objects in the environment [1,4,5].

Earlier studies on trunk postural control had extensively explored the strategies used by the intact central nervous system to maintain stability in erect seated postures [6,7]. In these studies, perturbation forces were applied to the trunks of seated able-bodied individuals and the various static and dynamic characteristic responses were captured and analyzed. In particular, important factors that affect stability such as intrinsic stiffness and damping for the intact system were determined and their implications to trunk stability examined. In other studies, musculoskeletal models were used to perform in-silico experiments to explore the potential impact of recruiting a variety of paralyzed muscles with FNS to support and maintain trunk posture statically in different planes around the workspace of the seated operator [8,9,10]. Overall, these studies provided the main tools to develop more advanced dynamic control systems that could be deployed in individuals with SCI and other movement disorders to maintain trunk posture in response to either internally generated or externally applied perturbations.

Studies directly involving individuals with SCI have explored the use of FNS to help maintain a stable seated posture [11,12], and in cases of loss of stability to restore the trunk to a desired posture [13,14]. However, these developments had concentrated on stability exclusively in the sagittal plane and the restoration of posture from deviations from upright in the forward direction only [15]. Similar issues associated with stability during lateral bending in the coronal plane or intermediate directions still need to be addressed [16].

The purpose of the current study was to explore the design and deployment of a disturbance-rejection closed-loop controller that uses FNS in individuals with SCI to restore upright posture, not only in the sagittal plane as explored in previous studies [13,14], but also in the coronal plane or diagonally between lateral and anterior-posterior directions. The results have implications for maintaining upright seated posture and balance during ADLs such as driving and wheelchair propulsion, as well as for the development of more advanced control systems in the future.

Section snippets

Study participants

Five individuals paralyzed due to SCI at mid-thoracic or higher levels were recruited from a cohort of individuals with implanted neuroprostheses who were participants in other related studies in our laboratory [17,18,19,20]. The main inclusion criterion for all participants was absence of volitional trunk control; i.e. complete loss of voluntary function in the hip and trunk extensor and trunk lateral flexor muscles. Table 1 displays their clinical characteristics as well as the implanted

Muscle stimulation parameters

The baseline and high stimulation PWs for each subject and for each muscle are listed in Table 2. The baseline stimulation used during regular erect sitting varied between 0–80 µs with most values being closer to 10 µs for most muscles. On the other hand, the stimulation during the active section of the controller varied between 40 and 250 µs depending on the subject and muscle.

Tilt sensor calibration

The values (with 95% confidence bounds) of the slope and intercept in Eq. (1) were mS= -172 (-184.1, -160) and bS=

Discussion

Seated stability is an important function for individuals with various levels of SCI. Stability of seated postures ensures safety during the execution of various ADLs such as driving, wheelchair propulsion, manipulating objects, transfers, etc. The study presented here is the first to examine the potential of ensuring a stable erect posture in the coronal and diagonal planes using feedback control of neural stimulation. The system consistently returned all subjects to an upright sitting posture

Conclusions

We have designed and deployed a real-time self-righting feedback controller for restoration of trunk posture in the coronal and diagonal planes in individuals with various levels of SCI using FNS of their paralyzed muscles. The controller was tested in five individuals with cervical and thoracic level SCI who had been implanted with various FNS systems for seated trunk stability. The results showed that the self-righting feedback controller successfully restored posture to erect in all the

Declaration of Competing Interest

All authors declare no competing interests in this work.

Ethical approval

The research was approved by the Institutional Review Board of Louis Stokes Cleveland Veterans Affairs Medical Center - Approval Number IRB #07101-H36.

Acknowledgements

The authors would like to acknowledge the contributions of our study participants, the Motion Study Laboratory at the Louis Stokes Cleveland Veterans Affairs Medical Center, and the Cleveland APT Center.

Funding

This material was based on work supported in part by the National Institutes of Health (Grant 1R01NS101043-01) and the Department of Defense, SCIR Program (Grant W81XWH-17-1-0240).

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