Improving stand-to-sit maneuver for individuals with spinal cord injury

Abstract

Background: Users of neuroprostheses employing electrical stimulation (ES) generally complete the stand-to-sit (STS) maneuver with high knee angular velocities, increased upper limb support forces, and high peak impact forces at initial contact with the chair. Controlling the knee during STS descent is challenging in individuals with spinal cord injury (SCI) due to the decreasing joint moment available with increased knee angle in response to ES.

Methods: The goal of this study was to investigate the effects of incorporating either (1) a coupling mechanism that coordinates hip and knee flexion or (2) a mechanism that damps knee motion to keep the knee angular velocity constant during the STS transition. The coupling and damping were achieved by hydraulic orthotic mechanisms. Two subjects with SCI were enrolled and each served as their own controls when characterizing the performance of each mechanism during STS as compared to stimulation alone. Outcome measures such as hip-knee angle, knee angular velocity, upper limb support force, and impact force were analyzed to determine the effectiveness of the two mechanisms in providing controlled STS.

Results: The coordination between the hip and knee joints improved with each orthotic mechanism. The damping and hip-knee coupling mechanisms caused the hip and knee joint ratios of 1:1.1 and 1:0.99, respectively, which approached the 1:1 coordination ratio observed in nondisabled individuals during STS maneuver. The knee damping mechanism provided lower (p < 0.001) and a more constant knee angular velocity than the hip-knee coupling mechanism over the knee range of motion. Both the coupling and damping mechanisms were similarly effective at reducing upper limb support forces by 70 % (p < 0.001) and impact force by half (p ≤ 0.001) as compared to sitting down with stimulation alone.

Conclusions: Orthoses imposing simple kinematic constraints, such as 1:1 hip-knee coupling or knee damping, can normalize upper limb support forces, peak knee angular velocity, and peak impact force during the STS maneuvers.

Keywords: Biomechanics; Electrical stimulation; Hip-knee coupling; Hybrid neuroprosthesis; Knee damping; Orthotic knee mechanisms; Sitting impact force; Spinal cord injury; Stand-to-sit.

Fig. 1

Schematics for hydraulic circuits used to create (a) coupling and (b) damping

Fig. 2

Experimental setup for an individual with SCI using the (a) coupling and (b) damping mechanism to perform the STS maneuver

Fig. 3

Typical progression of the stand-to-sit maneuver from quiet standing (a) through initiation (b), early and late descent (c-d) and terminal impact (e-f). Coupling (top figure) required controlling the hips by means of upper limbs for knee flexion which resulted in greater forward trunk movement during STS than with the hydraulic damping mechanism (bottom figure) where knee flexion was controlled by the mechanism alone

Fig. 4

Representative average hip-knee angle and standard deviations demonstrating coordination of the hip and knee angles during the STS maneuver. Sitting with stimulation alone (blue) is characterized by exaggerated hip flexion at the beginning of the maneuver, followed by rapid knee flexion in the latter portion of the transition. The hip and knee angles approximate a 1:1 ratio when using the hip-knee coupling mechanism (purple) or the proportional valve damping mechanism (red) approximating that observed in nondisabled STS maneuvers (black) [27]

Fig. 5

Representative average knee angular velocity and standard deviations during STS. Knee flexion angular velocity was reduced and less variable for the coupling (purple) and damping (red) mechanisms compared to stimulation alone (blue). Nondisabled average angular velocity indicated by black line [27]

Fig. 6

Representative average upper limb support force with standard deviation indicating a reduction in upper limb reliance when using the coupling (purple) and damping (red) mechanisms as compared to stimulation only (blue). Nondisabled average upper limb support force indicated by black line [27]

Fig. 7

Representative average impact force and standard deviations for the different conditions indicating a reduction in the impact force when using the coupling (purple) or damping (red) mechanisms as compared to stimulation only (blue). Nondisabled average impact force indicated by black line [27]