Effect of load during electrical stimulation training in spinal cord injury

Myofibrillar proteolysis in response to voluntary or electrically stimulated muscle contractions in humans

Knowledge about the effects of exercise on myofibrillar protein breakdown in human subjects is limited. Our purpose was to measure the changes in the degradation of myofibrillar proteins in response to different ways of eliciting muscle contractions using the local interstitial 3-methyl-histidine (3-MH) concentration as a marker for myofibrillar protein breakdown. Untrained males (n=8, 22-27 years, range) performed 210 maximal isokinetic eccentric contractions with each leg on an isokinetic dynamometer. One leg performed voluntary (VOL) and the other leg performed electrically induced contractions (ES). Microdialysis probes were placed in m. vastus lateralis in both the legs immediately after, and 1 and 3 days post-exercise. Interstitial 3-MH was higher in ES vs VOL immediately after exercise (P<0.05). One and 3 days post-exercise no difference between the two exercise types was observed. Only after ES did the histochemical stainings show significant disruption of cytoskeletal proteins. Furthermore, intracellular disruption and destroyed Z-lines were markedly more pronounced in ES vs VOL. In conclusion, the local level of interstitial 3-MH in the skeletal muscle was significantly enhanced after ES compared with VOL immediately after exercise, while the level of 3-MH did not change in the post-exercise period after VOL. These results indicate that the local myofibrillar breakdown is accelerated after ES associated with severe myofiber damage.

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

Cardiovascular Health and Exercise Rehabilitation in Spinal Cord Injury

There appears to be an increased prevalence and earlier onset of cardiovascular disease (CVD) in persons with SCI. Physical inactivity is thought to be a key factor in the increased risk for CVD. Physical inactivity is highly prevalent in persons with SCI and it appears that activities of daily living are not sufficient to maintain cardiovascular fitness and health. This systematic review examines the current literature regarding the risk for CVD and the effectiveness of varied exercise rehabilitation programs in attenuating the risk for CVD in SCI.

Myofibre damage in human skeletal muscle: effects of electrical stimulation versus voluntary contraction

Disruption to proteins within the myofibre after a single bout of unaccustomed eccentric exercise is hypothesized to induce delayed onset of muscle soreness and to be associated with an activation of satellite cells. This has been shown in animal models using electrical stimulation but not in humans using voluntary exercise. Untrained males (n=8, range 22-27 years) performed 210 maximal eccentric contractions with each leg on an isokinetic dynamometer, voluntarily (VOL) with one leg and electrically induced (ES) with the other leg. Assessments from the skeletal muscle were obtained prior to exercise and at 5, 24, 96 and 192 h postexercise. Muscle tenderness rose in VOL and ES after 24 h, and did not differ between groups. Maximal isometric contraction strength, rate of force development and impulse declined in the VOL leg from 4 h after exercise, but not in ES (except at 24 h). In contrast, a significant disruption of cytoskeletal proteins (desmin) and a rise of myogenic growth factors (myogenin) occurred only in ES. Intracellular disruption and destroyed Z-lines were markedly more pronounced in ES (40%) compared with VOL (10%). Likewise, the increase in satellite cell markers [neural cell adhesion molecule (N-CAM) and paired-box transcription factor (Pax-7)] was more pronounced in ES versus VOL. Finally, staining of the intramuscular connective tissue (tenascin C) was increased equally in ES and VOL after exercise. The present study demonstrates that in human muscle, the delayed onset of muscle soreness was not significantly different between the two treatments despite marked differences in intramuscular histological markers, in particular myofibre proteins and satellite cell markers. An increase in tenascin C expression in the midbelly of the skeletal muscle in both legs provides further evidence of a potential role for the extracellular matrix in the phenomenon of delayed onset of muscle soreness.

Musculoskeletal adaptations in chronic spinal cord injury: effects of long-term soleus electrical stimulation training

OBJECTIVE

The purpose of this study was to determine whether long-term electrical stimulation training of the paralyzed soleus could change this muscle's physiological properties (torque, fatigue index, potentiation index, torque-time integral) and increase tibia bone mineral density.

METHODS

Four men with chronic (>2 years) complete spinal cord injury (SCI; American Spinal Injury Association classification A) trained 1 soleus muscle using an isometric plantar flexion electrical stimulation protocol. The untrained limb served as a within-subject control. The protocol involved ~ 30 minutes of training each day, 5 days a week, for a period of 6 to 11 months. Mean compliance over 11 months of training was 91% for 3 subjects. A fourth subject achieved high compliance after only 5 months of training. Mean estimated compressive loads delivered to the tibia were approximately 110% of body weight. Over the 11 months of training, the muscle plantar flexion torque, fatigue index, potentiation index, and torque-time integral were evaluated periodically. Bone mineral density (dual-energy x-ray absorptiometry) was evaluated before and after the training program.

RESULTS

The trained limb fatigue index, potentiation index, and torque-time integral showed rapid and robust training effects (P<.05). Soleus electrical stimulation training yielded no changes to the proximal tibia bone mineral density, as measured by dual-energy x-ray absorptiometry. The subject with low compliance experienced fatigue index and torque-time integral improvements only when his compliance surpassed 80%. In contrast, his potentiation index showed adaptations even when compliance was low.

CONCLUSIONS

These findings highlight the persistent adaptive capabilities of chronically paralyzed muscle but suggest that preventing musculoskeletal adaptations after SCI may be more effective than reversing changes in the chronic condition.

Muscle oxygenation during prolonged electrical stimulation-evoked cycling in paraplegics

This study investigated cardiorespiratory responses and muscle oxygenation during prolonged electrical stimulation (ES)-evoked leg cycling in individuals with paraplegia (PARA). Four PARA and 6 able-bodied (AB) persons participated in this study. Subjects performed 10 min of passive cycling and 40 min of active cycling (PARA, ES cycling; AB, voluntary cycling) at workloads selected to elicit an equivalent oxygen uptake between groups. Cycling power output, cardiorespiratory responses, mechanical efficiency, and quadriceps muscle oxygenation (measured with near-infrared spectroscopy) were measured over the duration of the exercise. Oxygen uptake was similar in both groups during active cycling (PARA, 737 ± 177 mL·min–1; AB, 840 ± 90 mL·min–1). The cycling power output for PARA individuals commenced at 8.8 W, but varied considerably over 40 min. PARA individuals demonstrated markedly lower gross mechanical efficiency (~1.3%) during ES cycling compared with AB individuals performing voluntary exercise (~12.6%). During ES cycling, muscle oxygen saturation (SO2) decreased to approximately 72 ± 19%, whereas SO2 during volitional cycling was unaltered from resting levels. Muscle oxygenated haemoglobin initially decreased (–23%) during ES cycling, but returned to resting levels after 10 min. Deoxygenated haemoglobin initially rose during the first 5 min of ES cycling, and remained elevated by 28% thereafter. Upon cessation of ES cycling, lower-limb muscle oxygenation increased (+93%), suggesting reactive hyperaemia in PARA individuals after such exercise. During ES cycling, muscle oxygenation followed a different pattern to that observed in AB individuals performing voluntary cycling at an equivalent VO2. Equilibrium between oxygen demand and oxygen delivery was reached during prolonged ES cycling, despite the lack of neural adjustments of leg vasculature in the paralyzed lower limbs.

Spastic tail muscles recover from myofiber atrophy and myosin heavy chain transformations in chronic spinal rats

Without intervention after spinal cord injury (SCI), paralyzed skeletal muscles undergo myofiber atrophy and slow-to-fast myofiber type transformations. We hypothesized that chronic spasticity-associated neuromuscular activity after SCI would promote recovery from such deleterious changes. We examined segmental tail muscles of chronic spinal rats with long-standing tail spasticity (7 mo after sacral spinal cord transection; older chronic spinals), chronic spinal rats that experienced less spasticity early after injury (young chronic spinals), and rats without spasticity after transection and bilateral deafferentation (spinal isolated). These were compared with tail muscles of age-matched normal rats. Using immunohistochemistry, we observed myofiber distributions of 15.9 +/- 3.5% type I, 18.7 +/- 10.7% type IIA, 60.8 +/- 12.6% type IID(X), and 2.3 +/- 1.3% type IIB (means +/- SD) in young normals, which were not different in older normals. Young chronic spinals demonstrated transformations toward faster myofiber types with decreased type I and increased type IID(X) paralleled by atrophy of all myofiber types compared with young normals. Spinal isolated rats also demonstrated decreased type I myofiber proportions and increased type II myofiber proportions, and severe myofiber atrophy. After 4 mo of complete spasticity (older chronic spinals), myofiber type transformations were reversed, with no significant differences in type I, IIA, IID(X), or IIB proportions compared with age-matched normals. Moreover, after this prolonged spasticity, type I, IIA, and IIB myofibers recovered from atrophy, and type IID(X) myofibers partially recovered. Our results indicate that early after transection or after long-term spinal isolation, relatively inactive tail myofibers atrophy and transform toward faster myofiber types. However, long-term spasticity apparently produces neuromuscular activity that promotes recovery of myofiber types and myofiber sizes.

Rehabilitative Therapies after Spinal Cord Injury

We review some basic and highly relevant concepts in the effort to develop improved rehabilitative interventions for subjects with spinal cord injury (SCI). Interventions that are likely to contribute to improved sensorimotor function include (1) practice of the specific motor task that needs to be improved; and (2) combining the training with one or more interventions--such as pharmacological modulation of the excitability of spinal neural networks, implantation of selected cell types such as olfactory ensheathing glia (OEG), and/or modulation of the excitability of the spinal cord via epidural stimulation. Upon improvement of the neural control of the musculature following SCI, it will always be prudent to maximize the torque output from these activation patterns by assuring that muscle mass is maintained. Therefore, it seems quite feasible that considerable improvement in locomotor performance can be achieved by improved coordination of motor pools, as well as effective recovery of muscle mass, which will assist in the potential generation of normal forces among agonistic and antagonistic muscle groups.

Musculoskeletal plasticity after acute spinal cord injury: effects of long-term neuromuscular electrical stimulation training

Maintaining the physiologic integrity of paralyzed limbs may be critical for those with spinal cord injury (SCI) to be viable candidates for a future cure. No long-term intervention has been tested to attempt to prevent the severe musculoskeletal deterioration that occurs after SCI. The purposes of this study were to determine whether a long-term neuromuscular electrical stimulation training program can preserve the physiological properties of the plantar flexor muscles (peak torque, fatigue index, torque-time integral, and contractile speed) as well as influence distal tibia trabecular bone mineral density (BMD). Subjects began unilateral plantar flexion electrical stimulation training within 6 wk after SCI while the untrained leg served as a control. Mean compliance for the 2-yr training program was 83%. Mean estimated compressive loads delivered to the tibia were approximately 1-1.5 times body weight. The training protocol yielded significant trained versus untrained limb differences for torque (+24%), torque-time integral (+27%), fatigue index (+50%), torque rise time (+45%), and between-twitch fusion (+15%). These between-limb differences were even greater when measured at the end of a repetitive stimulation protocol (125 contractions). Peripheral quantitative computed tomography revealed 31% higher distal tibia trabecular BMD in trained limbs than in untrained limbs. The intervention used in this study was sufficient to limit many of the deleterious muscular and skeletal adaptations that normally occur after SCI. Importantly, this method of load delivery was feasible and may serve as the basis for an intervention to preserve the musculoskeletal properties of individuals with SCI.

Improvement of Metabolic and Cardiorespiratory Responses Through Treadmill Gait Training With Neuromuscular Electrical Stimulation in Quadriplegic Subjects

This work assessed the influence of treadmill gait training with neuromuscular electrical stimulation (NMES) on the metabolic and cardiorespiratory responses in quadriplegic subjects. The gait group (GG) (n=11) performed 6 months of treadmill training with 30-50% body weight support and with the help of physiotherapists, twice a week, allotting 20 min for each session. The control group (CG) (n=10), during the 6 months of training, did not perform any activity using NMES, performing instead conventional physiotherapy. Metabolic and cardiorespiratory responses (O(2) uptake [VO(2)], CO(2) production [VCO(2)], pulmonary ventilation (V(E)), heart rate [HR], and blood pressure [BP]) were measured on inclusion and after 6 months. For the GG, differences were found in all parameters after training (P<0.05), except for HR and diastolic BP. During gait, VO(2) (L/min) increased by 36%, VCO(2) (L/min) increased by 42.97%, V(E) (L/min) increased by 30.48%, and systolic BP (mm Hg) increased by 4.8%. For the CG, only VO(2) and VCO(2) (L/min) significantly increased at rest (30.82 and 16.39%, respectively) and during knee-extension exercise (26.29 and 17.37%, respectively). Treadmill gait with NMES was, therefore, more efficient toward increasing the aerobic capacity due to yielding higher metabolic and cardiovascular stresses.

Strategies That Improve Paralyzed Human Quadriceps Femoris Muscle Performance During Repetitive, Nonisometric Contractions

OBJECTIVE

To determine the effect of combining different stimulation frequencies on the ability of paralyzed human quadriceps muscle to produce a 50 degrees knee excursion repetitively when starting at 90 degrees of flexion.

DESIGN

Repeated-measures design.

SETTING

Clinical research laboratory.

PARTICIPANTS

Complete data were collected from 9 subjects aged 11 to 25 years (mean +/- standard deviation, 17.1+/-4.5y) with spinal cord injury (SCI).

INTERVENTION

Three protocols were each tested during separate sessions: 20-Hz trains of pulses followed by 66-Hz trains (C20+66), 33-Hz trains followed by 66-Hz trains (C33+66), and 66-Hz trains alone (C66). For each frequency, stimulation was repeated until the knee failed to produce a 50 degrees excursion. This approach allowed us to evaluate the response to stimulation with 20-, 33-, and 66-Hz and combinations of 20- and 66-Hz and 33- and 66-Hz trains.

MAIN OUTCOME MEASURE

Number of successful contractions.

RESULTS

The C20 and C33 did not differ (mean, 41.0+/-12.6 excursions and 42.0+/-12.3 excursions, respectively), and each produced more excursions than the C66 protocol. The C20+66 and C33+66 protocols produced 51.4+/-15.0 and 44.9+/-13.6 excursions, respectively, and the C20+66 was the best protocol overall (all P<or=.05).

CONCLUSIONS

This study showed that stimulation strategies that start with low frequencies and switch to higher frequencies as the muscle fatigues could improve the ability of functional electric stimulation applications to perform repetitive, nonisometric contractions in subjects with SCI.