Understanding stand-to-sit maneuver: implications for motor system neuroprostheses after paralysis

Abstract

Standing up, standing, and walking functions can be restored to people with spinal cord injury by contracting the paralyzed hip, knee, and ankle muscles with electrical stimulation. Restoring these functions using electrical stimulation requires controlled activation to provide coordinated movements. However, the stand-to-sit (STS) maneuver involves eccentric contractions of the quadriceps to control lowering of the body to the seated position, which is difficult to achieve with stimulation alone and presents unique challenges to lower-limb neuroprostheses. In this study, we examined the biomechanics of the STS maneuver in five nondisabled individuals and five users of an implanted neuroprosthesis. Neuroprosthesis users relied heavily on their upper limbs during STS, with peak supporting forces approximately 25% body weight, and exhibited an average vertical acceleration at the impact six times higher than that of the nondisabled subjects (p < 0.001). Sitting with stimulation resulted in impact forces at initial contact with the seating surface averaging 1.4 times body weight and representing an average of twice the impact forces of the nondisabled subjects (p < 0.001). These results indicate a need for additional interventions to better control descent, minimize impact, and gently transition from standing to sitting to achieve a more natural movement and reduce the risk of injury.

Keywords: SCI; biomechanics; functional neuromuscular stimulation; impact force; neuroprosthesis; paralysis; spinal cord injury; stand-to-sit; upper-limb force; vertical acceleration.

Figure 1

Experimental setup for nondisabled subject performing stand-to-sit maneuver.

Figure 2

Typical progression of stand-to-sit (STS) maneuver demonstrated by subject I with spinal cord injury using functional neuromuscular stimulation (FNS) (top row) and nondisabled subject D (bottom row). In example shown, ramping down of stimulation for subject I was initiated 1.8 s before frame (a), which illustrates only last 10% of stimulation pattern. Stimulation ended after frame (a) and no further stimulation was applied for frames (b)–(f). Subjects using FNS typically initiated STS maneuver by leaning forward while stimulation was decreasing (a) and exaggerated flexion of hips (b), which led to passive locking of knees even after cessation of stimulation. Once their stimulated muscles were relaxed and posture changed to unlock knees, knees would flex (c) to allow descent toward chair (d)–(f).

Figure 3

Representative hip-knee angle plot with standard deviations for nondisabled control (Able-bodied) and subject with spinal cord injury (SCI) using functional neuromuscular stimulation (FNS). Hip and knee angles of nondisabled stand-to-sit (STS) maneuvers approximately follow a 1:1 ratio (dotted line). Subjects using FNS began STS maneuver with large flexion at hips in relation to knees for ~2:1 hip-knee angle ratio (thick solid line) but then shifted to rapid change in knee flexion with relatively little change in hip flexion for hip-knee ratio closer to 1:4 in latter part of maneuver.

Figure 4

Representative knee angular velocity with standard deviations for typical nondisabled control (Able-bodied) and subject with spinal cord injury (SCI) using functional neuromuscular stimulation (FNS) during stand-to-sit (STS) maneuver. Knee angular velocity for nondisabled participant was relatively constant throughout STS maneuver (dotted line). Knee angular velocity for subject using FNS increased to maximum in later stages of maneuver (thick solid line) and consistently peaked at values far exceeding those of nondisabled control.

Figure 5

Mean ± standard deviation peak upper-limb forces during stand-to-sit maneuver for each subject. Peak values were averaged within nondisabled (Able) and spinal cord injury (SCI) groups to compare between the two populations. ^*p < 0.05.

Figure 6

Mean ± standard deviation peak vertical accelerations at impact during stand-to-sit maneuver for each subject. Peak values were averaged within nondisabled (Able) and spinal cord injury (SCI) groups to compare between the two populations. ^*p < 0.05.

Figure 7

Mean ± standard deviation peak impact forces during stand-to-sit maneuver for each subject. Peak values were averaged within nondisabled (Able) and spinal cord injury (SCI) groups to compare between the two populations. ^*p < 0.05.