Robotic upright stand trainer (RobUST) and postural control in individuals with spinal cord injury

Abstract

Context/objective: Assessed feasibility and potential effectiveness of using a novel robotic upright stand trainer (RobUST) to deliver postural perturbations or provide assistance-as-needed at the trunk while individuals with spinal cord injury (SCI) performed stable standing and self-initiated trunk movements. These tasks were assessed with research participants' hands on handlebars for self-balance assistance (hands on) and with hands off (free hands).

Design: Proof of concept study.

Participants: Four individuals with motor complete (n = 3) or incomplete (n = 1) SCI who were not able to achieve independent standing and presented a neurological lesion level ranging from cervical 4 to thoracic 2.

Outcome measures: Ground reaction forces, trunk displacement, and electromyography activity of trunk and lower limb muscles.

Results: Research participants received continuous pelvic assistance via RobUST, and manual trainer assistance at the knees to maintain standing. Participants were able to attempt all tasks. Free hands trunk perturbations resulted in greater load bearing-related sensory information (73% ipsilateral vertical loading), trunk displacement (57%), and muscle activation compared to hands on. Similarly, free hands stable standing with RobUST assistance-as-needed resulted in 8.5% larger bodyweight bearing, 112% larger trunk movement velocity, and higher trunk muscles activation compared to standing with hands on. Self-initiated trunk movements controlled by hands on showed 116% greater trunk displacement, 10% greater vertical ground reaction force, and greater ankle muscle activation compared to free hands.

Conclusion: RobUST established a safe and challenging standing environment for individuals with SCI and has the potential to improve training paradigms and assessments of standing postural control.

Keywords: Force field; Perturbation; Postural control; Robotics; Spinal cord injury; Standing.

Figure 1

Left: Robotic upright stand trainer (RobUST) setup and components (left): (a) aluminum frame, (b) motion capture cameras, (c) visual feedback, (d) electrical box, (e) computer and controller setup, (f) force platforms, (g) motors/encoders. Right: representative SCI individual standing with RobUST: knees extension is manually assisted by a trainer, while RobUST applies constant force to assist hips extension, and provides a force field at the trunk to assist-as-needed.

Figure 2

Representative trunk perturbations with free hands or hands on handlebars applied on participants A121 and A124. Pert: perturbation resultant force; HB Fz: vertical force applied to the handlebar; antero-posterior (AP) or medio-lateral (ML) displacement of the trunk geometric center; R: right; UT: upper trapezius; TA: tibialis anterior; MG: medial gastrocnemius; ES: erector spinae; MH: medial hamstrings; vertical (z), medio-lateral (x) and antero-posterior (y) force (F) of the right force plate. Gray, shaded area reflects a period of standing with hands on the handlebars to reposition the trunk after the perturbation.

Figure 3

Effects of trunk perturbations with free hands or hands on fixed handlebars. Fz (vertical ground reaction force) and Fh (horizontal ground reaction force), expressed as percent change from baseline (quiet standing immediately prior to perturbation), are reported for the side ipsilateral (Ipsi) to the perturbation, contralateral (Contra) to the perturbation, and for the anterior and posterior (AP) perturbations pooled together. Peak EMG normalized by baseline value is reported for each perturbation direction (L: left; R: right; B: back; F: front) for the following muscles: upper trapezius, UT; erector spinae, ES; rectus abdominis, RA; external oblique, OB; adductor, AD; vastus lateralis, VL; tibialis anterior, TA; medial gastrocnemius, MG. Data are shown as mean ± standard deviation. Medium (0.50-0.79, *) or large (≥ 0.80, **) effect size is reported.

Figure 4

Kinetic, kinematic, and electromyography (EMG) time series data for two participants (A121 and A124) during stable standing with (a) RobUST force field and free hands (FF-free hands), or (b) hands on handlebars without RobUST FF (hands on-No FF). RobUST FF was activated when A121 trunk moved beyond the force field boundary. HB Fz: vertical force applied to the handlebar; antero-posterior (AP) or medio-lateral (ML) displacement of the trunk geometric center; R: right; UT: upper trapezius; Fz: force plate vertical force.

Figure 5

Effects of RobUST force field and free hands (FF-free hands), or hands on handlebars without RobUST force field (hands on-No FF) on stable standing characteristics. Fz: combined force plate mean vertical force, expressed as percent body weight (%BW). Mean Vel.: mean velocity of the trunk geometric center. iEMG: integrated EMG normalized by baseline (quiet sitting) values is reported for right UT: upper trapezius; OB: external oblique; ES: erector spinae; VL: vastus lateralis; MG: medial gastrocnemius; TA: tibialis anterior. Data are shown as mean ± standard deviation. Medium (0.50-0.79, *) or large (≥ 0.80, **) effect size is reported.

Figure 6

Kinetic, kinematic and EMG variables assessed during self-initiated antero-posterior (AP), medio-lateral (ML) and circular (Circle) trunk movements performed with RobUST force field and free hands (FF-free hands), or hands on handlebars without RobUST force field (hands on-No FF). R: right force plate; L: left force plate. Fz (vertical ground reaction force) and Fh (horizontal ground reaction force) are expressed as percent body weight (%BW). iEMG: integrated EMG normalized by baseline (quiet sitting) values is reported for right UT: upper trapezius; ES: erector spinae; OB: external oblique; TA: tibialis anterior; MG: medial gastrocnemius; VL: vastus lateralis. Data are shown as mean ± standard deviation. Medium (0.50-0.79, *) or large (≥ 0.80, **) effect size is reported.