Ankle, knee, and hip moments during standing with and without joint contractures: simulation study for functional electrical stimulation

Joint contractures have been one of the contraindications for use of functional electrical stimulation for standing in paraplegic patients. A simulation study using a three-segment link mechanical model of the human body was performed to calculate the muscle moments at the ankles, knees, and hips during standing with and without having joint contractures. The knee and hip angles were varied in 5 degrees increments, whereas the ankle angles were varied in 1 degree increments. It was assumed that energy efficient posture was obtained with the least sum of the squared moments of the ankles, knees, and hips joints by the muscles. Ankles at 5 degrees of dorsiflexion, knees at 0 degrees, and hips at 15 degrees of extension resulted in the most energy efficient posture without joint contractures. The muscle moments increased with the increase in angle of contractures. The joint contractures at ankle angles > or = 6 degrees of plantar flexion, knee angles > or = 20 degrees of flexion, and/or hip angles > or = 20 degrees of flexion produce a potentially unstable posture. These findings suggest that some degree of joint contractures can be tolerated in paraplegic patients using functional electrical stimulation for standing.     

The use of selective electrical stimulation of the quadriceps to improve standing function in paraplegia.

Persons with spinal cord injury (SCI) can benefit significantly from functional neuromuscular stimulation (FNS) systems for standing if manual tasks can be performed while upright. Using FNS to sufficiently activate the knee extensors to rise from a sitting position often results in inadvertent activation of the rectus femoris and/or sartorius, which flex the hip. In this study, intramuscular electrodes implanted in the vastus lateralis and medialis of four subjects with SCI were used to activate these muscles individually and simultaneously to measure knee extension moment. Support forces applied to the arms and feet were measured while upright to quantify the effects of recruiting rectus femoris and/or sartorius. In three of the four subjects, vastus lateralis, by itself, generated adequate knee extension moment for rising from a chair and to maintain static standing. Simultaneous activation of the vastus lateralis and medialis using a bifurcated electrode generated adequate knee extension moment in one subject, and was within 10% of the required moment in another. While upright, activation of the rectus femoris resulted in arm support force increases of 4-11% body weight, while deactivation resulted in arm support force decreases of 6-9% body weight. The results indicate that selective activation of the vastus lateralis, individually or in combination with vastus medialis, can improve current FNS standing systems by reducing the arm support forces required to remain upright.     

Tetanic responses of electrically stimulated paralyzed muscle atvarying interpulse intervals

The influence of stimulus interpulse interval (IPI) on torque output during electrically-evoked contractions was investigated for the knee extensor muscles of paralyzed subjects. The parameters measured were the rise time, magnitude, and relaxation time of the contraction at stimulus IPI's ranging from 62 to 7 ms. Torque output increased as IPI's were decreased from 62 to 15 ms. Peak torques were recorded at IPI's of 12-15 ms; IPI's less than these resulted in an insignificant loss of torque. Rise times decreased as IPI's were decreased. Relaxation time generally increased as IPI's were decreased with the longest relaxation times occurring with stimulation at an IPI of 12 ms. To demonstrate the influence of IPI on muscle fatigue, the effect of prolonged stimulation at short (12 ms) and long (50 ms) IPI's was also compared. After 30 s of stimulation with an IPI of 12 ms, mean torque had declined to 5 +/- 3 percent and after 30 s of stimulation with an IPI of 50 ms, mean torque had declined to 82 +/- 4 percent of the initial value. Knowledge of how stimulus IPI influences the response of paralyzed muscle to electrical stimulation may assist in the development of rehabilitation devices which utilize these technologies.     

Simulation of a functional neuromuscular stimulation powered mechanical gait orthosis with coordinated joint locking.

The purpose of this study was to examine a hybrid orthosis system (HOS) for walking after spinal-cord injury (SCI) that coordinates the mechanical locking and unlocking of knee and ankle joints of a reciprocating gait orthosis (RGO), while propulsive forces are injected and unlocked joints controlled with functional neuromuscular stimulation (FNS). The likely effectiveness of the HOS in terms of forward progression, stability, and posture of paraplegic gait was determined in this simulation study. A three-dimensional computer model of a HOS combining FNS with an RGO incorporating feedback control of muscle activation and joint locking was developed. An anthropomorphic human model included passive joint moments and a foot-ground contact model adapted from other studies. A model of the RGO reciprocally coupled the hips and locked and unlocked the knee and ankle joints during stance and swing respectively. The actions of muscles under FNS activation were modeled via closed-loop control of joint torque inputs. A walking aid that mimicked canes and voluntary upper extremity actions maintained lateral stability by providing the necessary shoulder forces and moments. The simulated HOS achieved gait speeds of 0.51 +/- 0.03 m/s, stride lengths of 0.85 +/- 0.04 m, and cadences of 72 +/- 4 steps/min, exceeding the reported performance of other assistive gait systems. Although minimal forward trunk tilt was found to be necessary during specific phases of gait, posture, and stability were significantly improved over FNS-only systems.     

Development and operation of portable and laboratory electricalstimulation systems for walking in paraplegic subjects

Two new stimulation systems have been designed for use in functional neuromuscular stimulation of paralyzed people; one is portable and one is a nonportable laboratory system. Compared to previous systems, these have greatly enhanced capabilities, especially in terms of memory capacity, expandability, and user interface. They are extensively operator programmable. The laboratory stimulation system was designed to provide quick turnaround time for stimulation pattern or program changes while maintaining complete compatibility with the portable system. The lab system will also accomodate external closed-loop control.     

Control of functional neuromuscular stimulation systems forstanding and locomotion in paraplegics

The authors adopt a control-systems perspective in reviewing past applications of functional neuromuscular stimulation for providing lower extremity motor function in paralyzed individuals. Specifically, their approach emphasizes direct computer-controlled electrical stimulation of paralyzed muscle rather than triggering reflexes. In experimental settings it provides paralyzed individuals with the ability to do functional tasks while standing, to walk short distances on varying surfaces, to negotiate obstacles, and to climb and descend stairs     

Implanted functional electrical stimulation system for mobility in paraplegia: a follow-up case report.

A 16-channel functional electrical stimulation (FES) system has been implanted in a person with T10 paraplegia for over a year. The system consists of two eight-channel radio frequency controlled receiver-stimulators delivering stimuli through a network of 14 epimysial and two intramuscular electrodes. Using this system and a walker for support, the subject was able to stand up for 8 min and walk regularly for 20 m. The standing duration was limited by arm fatigue since upper extremities supported an average of 25% of body weight. This was due to suboptimal hip extension and some undesired recruitment of rectus femoris and sartorius with stimulation of quadriceps electrodes. The left quadriceps exhibited rapid fatigue that limited walking distance and duration. The metabolic energy requirements were well within the aerobic limits of the sedentary paraplegic population. At one-year follow-up evaluation all electrodes are functional except one intramuscular electrode. The implant caused no adverse physiological effects and the individual reported health benefits such as increased energy and overall fitness as a result of the FES system use. With further improvements in muscle response through innovative surgical techniques, the 16-channel implanted FES system can be a viable addition to exercise and mobility function in persons with paraplegia.     

Advancing step by step

Restoring movement to paralyzed limbs by means of electrical stimulation has been a research goal for over 30 years. Recently, those efforts have borne commercial fruit in a system that gives some people who are paralyzed limited use of their legs. Still, before a large number of paralyzed people can achieve full dexterity and mobility, much work has yet to be done. Formidable multidisciplinary problems remain, challenging the diverse teams of engineers, physicians, and therapists who are at work on them     

Design of a Variable Constraint Hip Mechanism for a Hybrid Neuroprosthesis to Restore Gait After Spinal Cord Injury

A variable constraint hip mechanism (VCHM) has been developed for a hybrid neuroprosthesis system (HNP) to provide postural stability and uninhibited sagittal hip rotation throughout the gait of individuals with paraplegia. This paper describes the design concepts used in the development of the VCHM. The VCHM utilizes a hydraulic system to reciprocally couple the hips or individually lock and/or free a hip to rotate in one or both sagittal directions. Bench testing results show the feasibility of utilizing a portable hydraulic system in controlling hip joint kinematics. The passive resistive torques of the VCHM against user hip rotation at hip angular velocities typical of gait does not exceed 10% of the achievable hip torque generated by functional neuromuscular stimulation of paralyzed muscle. With the state of the VCHM configured to reciprocally couple the hips, the normalized mechanical efficiency of the VCHM was determined to be 0.7. Since each hip will be independently driven by the FNS of muscle, high torque transfer efficiency between the hips is not essential for successful operation of the VCHM. Future work will focus on the development of a sensor-based feedback controller to modulate the hip constraints of the VCHM and validation of the VCHM as part of a HNP for paraplegic individuals implanted with FNS systems.