Locomotion with various FES and non-FES systems including a novel stance-control brace.

Richard.B.Stein*, Frank. Hayday¹, SuLing Chong, Aiko Kido, Robert Rolf

Centre for Neuroscience, University of Alberta and Karl Hager Limb and Brace¹

Introduction

Many types of FES and non-FES systems have been developed for locomotion of paraplegic patients, but rarely are they compared in the same patient. This study presents a case study of a single patient who had a motor complete spinal cord injury at the T4/5 level in 2001. He is a 25 year old male, who has had a limited return of sensory function (ASIA C) and no major medical problems other than his spinal cord injury. He has tried a variety of systems for locomotion including: 1) a conventional wheelchair, 2) a leg-propelled wheelchair with FES of quadriceps and hamstring muscles (Stein et al., 2001), 3) conventional long-leg braces with forearm crutches or a walker, 4) a surface FES system using stimulation of quadriceps and common peroneal nerve (flexor reflex) and ankle-foot orthoses (AFOs) and 5) the FES system above with advanced long-leg braces incorporating knee joints that can be locked and unlocked automatically (Stance-control, Horton Prosthetics). He uses the systems in 4) and 5) with a walker having wheels in the front and back legs that lock (Lumex 2 wheeled folding walker). In addition, he has used a commercially available FES cycle ergometry system (Ergys, Therapeutic Technologies, Inc.) and an FES-rowing machine (Davoodi et al., 2002) to exercise and build up his legs. In this preliminary report we will compare objective measures of his performance (speed, endurance, changes in heart rate, physiological cost index) and his subjective reaction to the various systems.

Methods/Design

Physiological cost index (PCI). The PCI is a well accepted measure of the efficiency of producing a movement and is obtained by dividing the change in heart rate between rest and steady activity (beats/min.) by the velocity (m/min.) of the movement. PCI was measured as described previously (Stein et al., 2001). Essentially, subjects spent two minutes at rest, four minutes in the activity and at least four minutes for the heart rate to return to normal after the activity. Resting heart rate was measured by averaging the values for 2 min. before the activity and the final 2 min. after the activity. The active heart rate was measured over the last 2 min. of activity.

Leg-propelled wheelchair. Three 5x10 cm surface electrodes (Unipatch) were placed over the quadriceps muscles to stimulate rectus femoris, vastus lateralis and vastus medialis and 3 electrodes over the distal and proximal hamstrings. A rule base automatically switched stimulation to the quadriceps muscles when a flexion threshold was exceeded and stimulation of the hamstring muscles when an extension threshold was exceeded.

Surface FES. Electrodes were placed over the quadriceps muscles as described above and over the common peroneal nerve where it passes the head of the fibula in the conventional way (Kralj and Bajd, 1989). Hand switches were mounted on a rolling walker and the subject pressed the left or right switches to elicit a flexor reflex of the left or right legs. The quadriceps muscles were stimulated on each side except when the corresponding switch was depressed. Pressing both switches simultaneously activated quadriceps muscles on both sides for the sit to stand transition and this stimulation continued after releasing the switches to maintain the standing state. Pressing both switches again turned off the stimulation of the quadriceps after a delay for the subject to sit down.

Hybrid FES system with advanced braces. Two carbon fibre, long-leg braces were constructed using a commercially available system (Stance-Control, Horton Prosthetics). Each leg contains a switch on the medial and lateral aspects of the knee that can be in an unlocked, locked, or automatic position. In the automatic position the knee locks when the ankle extends and weight is supported by the leg, as occurs at heel strike. To unlock, the ankle must flex and the knee must be extended, as occurs when the weight of the body moves over the foot late in the stance phase of a normal gait cycle. The same electrodes were used as for the surface FES system above, but the rule base was modified. Pushing the hand switch first produces a brief activation of the quadriceps muscles to unlock the knee, followed by the flexor reflex stimulus to produce the swing of the leg. Releasing the button produces another brief activation of the quadriceps muscles to straighten the knee, so it will lock at the beginning of stance after heel strike. The stimulation is then turned off until the button is pushed again.

Results/Discussion

This subject, like the vast majority of paraplegics, used a conventional wheelchair for most activities, so the values for this mode of activity are used as a basis for comparison with the other methods of locomotion. Measurements were made on a 200 m indoor track. He can wheel long distances with little fatigue, but PCI was measured with the standard protocol described in Methods. His heart rate increased from 87 to 137 beats/min. on average during the exercise while he wheeled at 119 m/min. The value of PCI was 0.42 which is typical for healthy young SCI subjects (mean = 0.40) and comparable to the value for walking in a control population (mean = 0.33) (Stein et al., 2001).

Recently, we developed a new type of wheelchair that can be propelled by flexion and extension of the knee (Stein et al., 2001). One or both knee movements can be coupled to the rotation of the wheels to produce forward movement of the wheelchair. A model has been developed that allows the coupling to be optimized, based on the relative strength of the knee flexor (hamstring) and knee extensor (quadriceps) muscle groups for each subject (Stein et al., 2003). With these optimal settings the subject was able to wheel for several km around the indoor track, as shown in Fig. 1. The velocity declined slowly from 85 to 40 m/min. as he wheeled more than 2.5 km over a period of 52 min. Note that the heart rate increased only from 93 beats/min. at rest (distance = 0) to about 110 beats/min. over this whole period. The PCI was initially about 0.1 for the first 500 m and gradually increased, but remained more efficient than arm wheeling. In a 4 min. trial on another day, his PCI was 0.07. The average value for motor complete SCI subjects is 0.18 which is more than twice as efficient as for arm wheeling. The subject liked the leg-propelled wheelchair, but his strong desire was to be able to walk.

He had been fitted with conventional long-leg braces and he used these from time to time for exercise. He could use the braces with forearm crutches or a walker. He achieved an average speed of 8.8 m/min. using a “swing-to” gait in which he moves the walker forward and then supports his weight with both arms. The legs are then brought up to the new walker position. The heart rate increased from 88 to 119 beats/min. on average for a PCI of 3.5. Thus, the speed and efficiency was an order of magnitude less than for wheeling with comparable effort. His endurance for walking was less than 100 m with the long leg braces. These limitations explain why he and most other motor-complete paraplegics do not frequently use long-leg braces, except for exercise over short distances.

Fig. 1. Heart rate and velocity (top) while a subject uses the leg-propelled wheelchair with FES over a distance of 2.5 km. The heart rate increased very little from rest (distance=0) throughout the period of test (52 min.) while the velocity was quite well maintained. The physiological cost index (PCI) is initially very low, but gradually increases (bottom).

We also tested the subject with a surface FES system (see Methods). Ankle-foot orthoses were used to provide additional stability at the ankle joints, together with a rolling walker. With this system he was only able to walk at a speed of 3.4 m/min. and had to sit down after less than 3 min. of use. The active heart rate increased to about 140 beats/min. and the PCI was 16. He had not been extensively trained with this FES system and the distance and efficiency would presumably increase, if training were provided over a period of time. Nonetheless, the effort was so high and the speed and efficiency so low that we felt he would not use such a system on a regular basis, except perhaps for exercise. This has been the usual experience with other surface FES systems for walking after complete SCI.

Recently, advanced long-leg braces have become available commercially that have a knee joint that can be locked and unlocked automatically during walking. Once the knee is locked no further stimulation is needed, which reduces the fatigue and effort required. We only have preliminary results from the subject using this system over the past month. His walking speed is very slow (2.5 m/min.), and the PCI is high (12). Nonetheless, he can already walk up to 40 m at a time and he does not fatigue to the same extent as with the FES system alone, since he can stand without stimulation to “catch his breath” (Mayogoitia & Andrews, 1990). He reports that his arms and palms get tired from the weight they support. This is now the limiting factor, rather than the legs fatiguing. He likes the fact that he is walking with an alternating gait in a “normal” fashion, rather than using a “swing-to” gait or a wheelchair and is enthusiastic about working to improve his performance.

In summary, the subject can locomote most efficiently with a leg-propelled wheelchair using an FES system to flex and extend his knees. He can travel several km with less change in heart rate than when propelling the wheelchair conventionally with his arms. He has also tried several walking systems and is most enthused about a hybrid system that combines stance-control, long-leg braces and an FES system to move the legs and lock and unlock the braces. His performance in preliminary trials has been rather limited, although it is improving steadily with use. We are also working to optimize the system for subjects with motor-complete SCI.

Fig. 2. Comparison of PCI (beats/m), velocity of locomotion (m/min.) and change in heart rate (beats/min.) for five different methods of locomotion. The values are on a logarithmic scale because of the wide range. Note that the effort (PCI) is least for the leg-propelled wheelchair using FES. The velocity is highest with the conventional wheelchair and the change in heart rate is greatest using FES alone.

Acknowledgment. This research was supported by the Canadian Institutes for Health Research. We thank K. James and R. Vandenberg for excellent technical assistance in design and construction of the leg-propelled wheelchair and the advanced braces respectively.

References

[1]Davoodi, R., Andrews, B.J., Wheeler, G.D., Lederer, R. (2002). Development of an indoor rowing machine with manual FES controller for total body exercise in paraplegia. IEEE Trans. Neural Sys. Rehab. Eng. 10: 197-203.

[2]Kralj, A., Bajd, T. (1989) Functional Electrical Stimulation, Standing and Walking after Spinal Cord Injury. CRC Press, Boca Raton FL.

[3]Mayagoitia, R.E.,Andrews, B.J. (1990). Stability during standing for 30 minutes in a hybrid FRO. Advances in External Control of Human Extremities X (Popovic, D., ed.). Nauka, Belgrade, Yugoslavia.

[4]Stein, R.B, Chong, S.L., James, KB., Bell, G.J. (2001). Improved efficiency with a wheelchair propelled by the legs using voluntary activity or electrical stimulation. Arch. Phys. Med. Rehab. 82: 1198-1203.

[5]Stein, R.B., Roetenberg, D., Chong, S.L., James, KB. (2003). Model-based customization and optimization of a wheelchair modified for leg control. Med. Eng. Phys.25:11-19.