PHYSIOLOGICAL BENEFITS OF ELECTRICALLY-ASSISTED AMBULATION

 IN PERSONS WITH SPINAL CORD INJURIES

 

Patrick L. Jacobs

 

Department of Neurological Surgery, and the Miami Project to Cure Paralysis

University of Miami School of Medicine

 

1600 Northwest 10th Avenue, R-48; Miami, Florida, USA 33136

 

ABSTRACT

Clinical investigation of Functional Electrical Stimulation (FES) in persons with spinal cord injury (SCI) has generally concentrated on hardware and control system issues in the attempt to increase functional outcome.  Previous studies have reported deficits in FES ambulation pace and endurance with extremely low gait efficiency. Such characteristics may limit functional daily use of these systems and restrict their use to research laboratory settings. Within the engineering model, FES systems are often considered solely a means to produce movements judged functional in the performance of daily tasks, such as feeding or locomotion. Thus, the practicality of FES ambulation, has been often been deemed questionable.

Survivors of SCI are known to experience significant negative physiological outcomes and higher rates of secondary disability than non-disabled peers. Exercise opportunities following SCI are often limited to activities using intact musculature above the lesion. Electrically stimulated cycle ergometry is an example of FES used for exercise purposes.  However, the use of FES ambulation systems has not generally been regarded as appropriate for exercise purposes. This presentation describes the experiences of this laboratory in which the physiological adaptations of over thirty persons with SCI were examined following a period of exercise training using a commercial FES ambulation system. Quantitative performance data will be presented on the significant effects of FES ambulation training on lower extremity musculature mass, lower extremity local circulation, lower extremity venous characteristics and pooling, upper extremity work capacity, as well as central cardiovascular and hemodynamic functioning.

 

INTRODUCTION

Survivors of spinal cord injury (SCI) are faced with challenges in virtually every daily activity. Limitations in voluntary motor control and bowel/bladder control are the most obvious. However, essentially every physical system displays a level of dysfunction following interruption of normal neurological innervation. Muscle atrophy is apparent in the paralyzed limbs within weeks following traumatic SCI. Similar atrophy develops within the vascular system and results in diminished circulation throughout the affected limbs. Persons with SCI exhibit significantly slower rates of lower extremity peripheral healing and dramatically higher rates of skin ulcers. Osteoporosis is a consequence of complete SCI and results in relatively higher rates of fractures.1,2 The lifestyle of persons with SCI tends to be sedentary with a number of increased health risk factors including decreased cardiovascular fitness,3 unfavorable blood lipid profiles,4,5 increased rate of diabetes,6 and increased incidence of hypertension.7,8

It has been well established within the general population that exercise activity provides physiological benefits in deconditioned persons. Beneficial exercise activities in persons with SCI are generally limited to activities utilizing the muscles spared above the point of injury. Upper extremity exercise capacity is limited by relatively smaller size, greater peripheral resistance, and less mechanical efficiency (in comparison to leg exercise). Decreased venous return from the paralyzed limbs has been associated with decreased stroke volume and compensatory increases of heart rate at subpeak levels of arm exercise.9,10 Persons with SCI paraplegia also reach lower levels of peak power output and peak oxygen uptake (VO2peak) than matched cohorts without disability. A notable incidence of shoulder and elbow injuries (60-70%) has been reported recently in the general SCI population with even higher injury rates in the wheelchair athletic population.11,12 While regular wheelchair and arm ergometry exercise training has been demonstrated to provide training adaptations, these benefits are generally limited to the peripheral systems above the point of injury. Thus, neither the central cardiovascular, nor the peripheral systems below the injury site, are significantly affected by this type of training.

Systems of functional electrical stimulation (FES) have been demonstrated to allow functional patterns of electrically-assisted movement within the paralyzed lower extremities.13 To date, FES walking systems have not been accepted as a reasonable means of functional locomotion due to low pace of walking and extremely high energy costs. However, relatively few controlled studies have examined the use of this technology as a means to address the various systemic complications associated with SCI. In fact, the very nature of high energy requirements and slow walking pace suggest that FES walking may provide a productive exercise modality in this population. It is the purpose of this paper to present the findings of a series of studies which examined the effect of FNS ambulation on numerous physiological outcome measures.

 

METHODS AND RESULTS

Thirty persons with complete SCI paraplegia, injury levels ranging from T4 to T11, participated in a series of research studies over a five year period. Testing and training procedures were explained to the subjects and written informed consents were obtained in accordance with The University of Miami Human Subjects Medical Sciences Subcommittee. Electrically-assisted knee extension strengthening exercises were performed until each subject was able to stand for three minutes. The FES walking program was 12 weeks in length and consisted of three weekly sessions, typically three ambulation bouts per day. A commercially available walking system, the Parastep-1, was used for both the strengthening and ambulation training. Physiological testing was performed prior to and following the FES ambulation program.  The following is a summary of the methods and findings of the study series:

  

Exercise Performance:  The distance and time of each walking bout was recorded and walking pace was calculated.14 Daily and weekly values of average and peak ambulation distance, time, and pace were also calculated.  All subjects displayed a regular increase of time, distance, and pace of FES ambulation over the 12 weeks period. The mean of each variable tended to increased linearly despite profound variability between subjects. Subjects tended to plateau in walking time at weeks eight through twelve with the mean time per walking bout greater than 20 minutes.

  

Energy Cost/Efficiency:  Following the training period, subjects participated in a peak electrically assisted ambulation test. Subjects walked along a 10 m walkway with incrementally increased pace from minimal to peak velocity. Metabolic activity was continually assessed via open circuit spirometry and heart rate was measured with direct palpation. Energy cost and energy efficiency of FES walking was calculated and compared with data from literature of nondisabled ambulation.  The energy cost of FES ambulation was found to be quite high at stationary standing, slow controlled pace, and throughout increasing velocity. Results indicated that the primary source of the energy cost was related to the increased energy uptake due to coming to an upright stance. FES walking was approximately 8 times less energy efficient than that of non-disabled walking.


      

Arm Ergometry Stress Testing:  A peak incremental arm ergometry test was performed by all subjects.15 Power output was increased 10 watts per three minute work stage to the point of volitional exhaustion. Metabolic activity was continually assessed via open circuit spirometry and heart rate was measured with a 12 lead EKG. Pre- and post-training cardiopulmonary responses were compared a peak exertion and at matched subpeak levels of power output.  Following the 12 week FES walking program, subjects displayed significant increased in peak power output and VO2peak (25% and 14.9%, respectively). At matched levels of subpeak arm effort, HR was significantly lower (12 - 15 beats/min). Subsequent studies confirmed that the decreased subpeak HR values were accompanied with significantly increased values of stroke volume.

   

Lower Extremity Muscular Mass:  Anthropometric measure were taken at seven sites including midthigh and calf as well as respective skinfold measures. Thigh cross sectional area (CSA) was calculated.14  Total volumes of each measure was calculated mathematically.  Anthropometric testing revealed significant increases in thigh girth (mean=2.6cm) and calf girth (mean=1.4cm).  The increased girth was calculated (with skinfolds) to relate increases in thigh CSA ranging from 20 to 25% in most subjects. A subsequent study used sequential MR images to assess thigh total CSA, muscle mass CSA of each compartment, and adipose CSA. These studies confirmed these estimates and documented increases up to 48% in the anterior thigh compartments.

  

Lower Extremity Circulation:  Doppler ultrasound technology was used to the end diastolic CSA of the common femoral artery.16 These CSAs were taken at rest and at regular intervals following five minutes of thigh occlusion.   Significant increases in CSA of the common femoral artery (25%) and arterial inflow volume (56%) were noted following training. Similar increases were documented in the pulse volume and arterial inflow volume responses to occlusion at matched time intervals.

   

Bone Density:   Bone mineral density at the femoral head, neck, and Ward=s triangle were measured using dual-photon densitometry prior to and following the FES training.17  There were no significant effects of training noted in bone mineral density following 12 weeks of FES ambulation training.     

Psychological:  Two measures were used to assess possible psychological effects of participation in a FES walking program.18 The Physical Self subscale (PSS) of the Tennessee Self-Concept Scale and the Beck Depression Inventory (BDI) were utilized prior to and after the 12 week training program. Following training, significant increases were reported in the PSS and the scores on the BDI were reported to indicate significant improvements in that domain.

 

CONCLUSION

Generally, the use of FES has been regarded as a possible means to provide >functional= movement, such as self feeding or gait, in persons in whom such actions have been limited by disease or injury. Unfortunately, the current level of technology is unable to provide reasonable levels of function when compared with the non-diseased state. The use of FES systems exercise modalities may offer a means to restore diminished physiological function. This series of research studies has demonstrated that regular use of a commercial FES >walking= system can enhance the function of a number of physical systems in persons with SCI. However, controlled studies are needed to assess the possible beneficial effects of such training programs on the various health risk factors evident in the SCI population, including unfavorable blood lipid profiles, increased rates of diabetes and hypertension, as well as high rates of pressure sores and urinary tract infections.

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10)  VanLoan M, McCluer S, Loftin JM, Boileau RA. Comparisons of physiological responses to maximal arm exercise among able-bodied, paraplegics, and quadriplegics. Paraplegia. 1987;25:397-405.

11)  Curtis KA, Drysdale GA, Lanza RD, et al. Shoulder pain in wheelchair users with tetraplegia and paraplegia. Arch Phys Med Rehabil. 1999;80:453-457.

12)  Goldstein B, Young J, Escobedo EM. Rotator cuff repairs in individuals with paraplegia. Am J Phys Med rehabil. 1997;76:316-322.

13)  Graupe D, Kahn KH. Functional electrical stimulation for ambulation for paraplegics. Malabar, FL. Krieger; 1994.

14)  Klose KJ, Jacobs PL, Broton JG, et al. Evaluation of a training program for persons with SCI paraplegia using the Parastep-1 ambulation system: Ambulation performance and anthropometric measures. Arch Phys Med Rehabil. 1997;78:789-793.

15)  Jacobs PL, Nash, MS, Klose KJ, et al. Evaluation of a training program for persons with SCI paraplegia using the Parastep-1 ambulation system: Effects on physiological responses to peak arm ergometry. Arch Phys Med Rehabil. 1997;78:794-798.

16)  Nash MS, Jacobs PL, Montalvo BM, et al. Evaluation of a training program for persons with SCI paraplegia using the Parastep-1 ambulation system: Lower extremity blood flow and hyperremic responses to occlusion are augted by ambulation training. Arch Phys Med Rehabil. 1997;78:808-14.

17)  Needham-Shropshire BM, Broton JG, Klose KJ, et al. Evaluation of a training program for persons with SCI paraplegia using the Parastep-1 ambulation system: Lack of effect on bone density. Arch Phys Med Rehabil. 1997;78:799-803.

18)  Guest, RS, Klose KJ,  Needham-Shropshire BM, Jacobs PL. Evaluation of a training program for persons with SCI paraplegia using the Parastep-1 ambulation system: Effect on physical self-concept and depression. Arch Phys Med Rehabil. 1997;78:804-807.