An electrical knee lock system for functional electrical stimulation

Stance controlled knee flexion improves stimulation driven walking after spinal cord injury

Background

Functional neuromuscular stimulation (FNS) restores walking function after paralysis from spinal cord injury via electrical activation of muscles in a coordinated fashion. Combining FNS with a controllable orthosis to create a hybrid neuroprosthesis (HNP) has the potential to extend walking distance and time by mechanically locking the knee joint during stance to allow knee extensor muscle to rest with stimulation turned off. Recent efforts have focused on creating advanced HNPs which couple joint motion (e.g., hip and knee or knee and ankle) to improve joint coordination during swing phase while maintaining a stiff-leg during stance phase.

Methods

The goal of this study was to investigate the effects of incorporating stance controlled knee flexion during loading response and pre-swing phases on restored gait. Knee control in the HNP was achieved by a specially designed variable impedance knee mechanism (VIKM). One subject with a T7 level spinal cord injury was enrolled and served as his own control in examining two techniques to restore level over-ground walking: FNS-only (which retained a stiff knee during stance) and VIKM-HNP (which allowed controlled knee motion during stance). The stimulation pattern driving the walking motion remained the same for both techniques; the only difference was that knee extensor stimulation was constant during stance with FNS-only and modulated together with the VIKM to control knee motion during stance with VIKM-HNP.

Results

Stance phase knee angle was more natural during VIKM-HNP gait while knee hyperextension persisted during stiff-legged FNS-only walking. During loading response phase, vertical ground reaction force was less impulsive and instantaneous gait speed was increased with VIKM-HNP, suggesting that knee flexion assisted in weight transfer to the leading limb. Enhanced knee flexion during pre-swing phase also aided flexion during swing, especially when response to stimulation was compromised.

Conclusions

These results show the potential advantages of incorporating stance controlled knee flexion into a hybrid neuroprosthesis for walking. The addition of such control to FNS driven walking could also enable non-level walking tasks such as uneven terrain, slope navigation and stair descent where controlled knee flexion during weight bearing is critical.

Functional walking ability of paraplegic patients: comparison of functional electrical stimulation versus mechanical orthoses

BACKGROUND

Spinal cord injury (SCI) can cause partial or complete paralysis of the lower extremities, impairing the ability of individuals to stand and walk. Various types of mechanical orthoses, functional electrical stimulation (FES) systems and hybrid orthoses that incorporate FES have been designed to restore the ability of individuals with SCI to stand and walk. Standing and ambulation performance of SCI subjects using these different systems have been previously evaluated using energy consumption analysis, stability analysis and by quantitative gait analysis. Though FES-based systems are technologically more complex than passive mechanical systems, it is not apparent whether user performance is substantially improved with FES and hybrid orthoses compared to purely mechanical orthoses.

METHOD

An electronic search was performed via the Pubmed, Embase and ISI Web of Knowledge data base from 1960 to 2010. The abstracts, titles and full details of each individual study were assessed by the authors. The findings that were indicative of users' performance with the FES systems and hybrid systems were compared with that of mechanical orthoses. Moreover, the effects of using these different systems on physiological health were evaluated.

RESULTS

Twelve original articles and 5 review articles were selected by the author. However, most of the original articles were case studies. The results of previous investigations indicate that user performance with the mechanical orthoses was generally better than that of the hybrid and FES systems based on subject's stability and energy consumption while walking. Moreover, subject reportedly experienced a higher incidence of problems with the use of hybrid orthoses and FES systems compared with mechanical orthoses.

CONCLUSION

FES and hybrid orthoses offer considerable potential for restoring standing and walking abilities in persons with SCI. However, improvements in their designs and operation with subsequent objective evaluations are required to demonstrate that these systems enable users to improve their performance over that currently possible with passive, mechanical orthoses.

Stance control knee mechanism for lower-limb support in hybrid neuroprosthesis

A hydraulic stance control knee mechanism (SCKM) was developed to fully support the knee against flexion during stance and allow uninhibited motion during swing for individuals with paraplegia using functional neuromuscular stimulation (FNS) for gait assistance. The SCKM was optimized for maximum locking torque for body-weight support and minimum resistance when allowing for free knee motion. Ipsilateral and contralateral position and force feedback were used to control the SCKM. Through bench and nondisabled testing, the SCKM was shown to be capable of supporting up to 70 N-m, require no more than 13% of the torque achievable with FNS to facilitate free motion, and responsively and repeatedly unlock under an applied flexion knee torque of up to 49 N-m. Preliminary tests of the SCKM with an individual with paraplegia demonstrated that it could support the body and maintain knee extension during stance without the stimulation of the knee extensor muscles. This was achieved without adversely affecting gait, and knee stability was comparable to gait assisted by knee extensor stimulation during stance.

Hybrid functional electrical stimulation with medial linkage knee-ankle-foot orthoses in complete paraplegics

We have previously restored ambulation in paraplegics by performing hybrid functional electrical stimulation (FES) with medial linkage knee-ankle-foot orthosis (MLKAFO). The most common MLKAFO (hinge-type MLKAFO) has the hypothetical axis that is lower than the physiological hip joint position, resulting in slow velocity and short step length. A new MLKAFO (sliding-type MLKAFO), which uses sliding medial linkages, has been developed to correct the axial discrepancy of the hinge-type MLKAFO that causes limited hip joint excursion. There have been reports of instability associated with sliding medial linkages, but the mechanism of this instability is unclear. The purpose of the present study was to evaluate the effects of FES with MLKAFOs on ambulation in paraplegics. Two complete paraplegic patients (levels T8 and T12, respectively) participated in this study. Kinematics data during ambulation were obtained using a motion analysis system. We measured gait velocity and hip progression during the standing phase. The sliding-type MLKAFO produced faster gait velocity than did the hinge-type MLKAFO, but it caused pelvis instability without FES. Pelvis instability was controlled by hybrid FES using the sliding-type MLKAFO. With hybrid FES, the sliding-type MLKAFO provides better gait performance than the hinge-type MLKAFO, but the hinge-type MLKAFO provides greater pelvis stability during walking. Moreover, FES provides sufficient propulsion to allow the complete paraplegics to walk.

Muscle fatigue from intermittent stimulation with low and high frequency electrical pulses

OBJECTIVE

To evaluate muscle fatigue resulting from intermittent low frequency and high frequency stimulation for the application of closed-loop control in functional electrical stimulation (FES).

DESIGN

Nonrandomized trial.

SETTING

General community, a referral center, institutional practice, and ambulatory care.

PATIENTS

Twenty healthy nondisabled men volunteered for the normal muscle group. Four paraplegic men with implanted percutaneous intramuscular electrodes for FES volunteered for the paralyzed muscle group.

INTERVENTION

The stimulation frequency was set at low (20 Hz) or high (100 Hz). Stimulation was administered in 4-second bursts at the start of 60-second, 120-second, and 240-second periods (duty cycles of 1/15, 1/30, and 1/60, respectively).

MAIN OUTCOME MEASUREMENTS

Knee extensor torques were measured during intermittent electrical stimulation. A strength decrement index (SDI) was used to assess muscle fatigue. Actual knee extensor torques in the paraplegic men were also measured with an isokinetic dynamometer.

RESULTS

Muscle fatigue was significantly greater at 20 Hz than at 100 Hz for both the nondisabled and the paraplegic subjects (p < .0001). Muscle fatigue at the 1/15 cycle was significantly reduced (p < .01).

CONCLUSIONS

Muscle fatigue was greater at the lower frequency (20 Hz) than at the higher frequency (100 Hz) during intermittent electrical stimulation, suggesting that intermittent high frequency stimulation may be valuable in the development of closed-loop control strategies for FES.