Influence of electrical stimulation of the tibialis anterior muscle in paraplegic subjects. 1. Contractile properties

Approach to Small Animal Neurorehabilitation by Locomotor Training: An Update

Neurorehabilitation has a wide range of therapies to achieve neural regeneration, reorganization, and repair (e.g., axon regeneration, remyelination, and restoration of spinal circuits and networks) to achieve ambulation for dogs and cats, especially for grade 1 (modified Frankel scale) with signs of spinal shock or grade 0 (deep pain negative), similar to humans classified with ASIA A lesions. This review aims to explain what locomotor training is, its importance, its feasibility within a clinical setting, and some possible protocols for motor recovery, achieving ambulation with coordinated and modulated movements. In addition, it cites some of the primary key points that must be present in the daily lives of veterinarians or rehabilitation nurses. These can be the guidelines to improve this exciting exercise necessary to achieve ambulation with quality of life. However, more research is essential in the future years.

Insights on the Potential Mechanisms of Action of Functional Electrical Stimulation Therapy in Combination With Task-Specific Training: A Scoping Review

OBJECTIVES

This scoping review was undertaken to synthetize and appraise the literature on the potential mechanisms of action of functional electrical stimulation therapy in combination with task-specific training (FEST + TST) in the rehabilitation following stroke, spinal cord injury, traumatic brain injury, or multiple sclerosis.

MATERIALS AND METHODS

The literature search was performed using multiple databases (including APA, PsycInfo, Medline, PubMed, EMBASE, CCRCT, and Cochrane Database of Systematic Reviews) from 1946 to June 2020. The literature search used the following terms: (spinal cord injury, paraplegia, tetraplegia, quadriplegia, stroke, multiple sclerosis, traumatic brain injury, or acquired brain injury) AND (functional electrical stimulation or FES). The search included clinical and preclinical studies without limits to language.

RESULTS

Of the 8209 titles retrieved from the primary search, 57 publications fulfilled the inclusion and exclusion criteria for this scoping review. While most publications were clinical studies (n = 50), there were only seven preclinical studies using animal models. The results of this review suggest that FEST + TST can result in multiple effects on different elements from the muscle to the cerebral cortex. However, most studies were focused on the muscle changes after FEST + TST.

CONCLUSIONS

The results of this scoping review suggest that FEST + TST can result in multiple effects on different elements of the neuromuscular system, while most research studies were focused on the muscle changes after FEST + TST. Despite the efficacy of the FEST + TST in the neurorehabilitation after CNS injury or disease, the results of this review underline an important knowledge gap with regards to the actual mechanism of action of FEST + TST.

Neuromodulation of the cerebellum rescues movement in a mouse model of ataxia

Deep brain stimulation (DBS) relieves motor dysfunction in Parkinson's disease, and other movement disorders. Here, we demonstrate the potential benefits of DBS in a model of ataxia by targeting the cerebellum, a major motor center in the brain. We use the Car8 mouse model of hereditary ataxia to test the potential of using cerebellar nuclei DBS plus physical activity to restore movement. While low-frequency cerebellar DBS alone improves Car8 mobility and muscle function, adding skilled exercise to the treatment regimen additionally rescues limb coordination and stepping. Importantly, the gains persist in the absence of further stimulation. Because DBS promotes the most dramatic improvements in mice with early-stage ataxia, we postulated that cerebellar circuit function affects stimulation efficacy. Indeed, genetically eliminating Purkinje cell neurotransmission blocked the ability of DBS to reduce ataxia. These findings may be valuable in devising future DBS strategies.

Effects of Functional Electrical Stimulation Cycling of Different Duration on Viscoelastic and Electromyographic Properties of the Knee in Patients with Spinal Cord Injury

The benefits of functional electrical stimulation during cycling (FES-cycling) have been ascertained following spinal cord injury. The instrumented pendulum test was applied to chronic paraplegic patients to investigate the effects of FES-cycling of different duration (20-min vs. 40-min) on biomechanical and electromyographic characterization of knee mobility. Seven adults with post-traumatic paraplegia attended two FES-cycling sessions, a 20-min and a 40-min one, in a random order. Knee angular excursion, stiffness and viscosity were measured using the pendulum test before and after each session. Surface electromyographic activity was recorded from the rectus femoris (RF) and biceps femoris (BF) muscles. FES-cycling led to reduced excursion (p < 0.001) and increased stiffness (p = 0.005) of the knee, which was more evident after the 20-min than 40-min session. Noteworthy, biomechanical changes were associated with an increase of muscle activity and changes in latency of muscle activity only for 20-min, with anticipated response times for RF (p < 0.001) and delayed responses for BF (p = 0.033). These results indicate that significant functional changes in knee mobility can be achieved by FES-cycling for 20 min, as evaluated by the pendulum test in patients with chronic paraplegia. The observed muscle behaviour suggests modulatory effects of exercise on spinal network aimed to partially restore automatic neuronal processes.

Reduced specific force in patients with mild and severe facioscapulohumeral muscular dystrophy

BACKGROUND

Specific force, that is the amount of force generated per unit of muscle tissue, is reduced in patients with facioscapulohumeral muscular dystrophy (FSHD). The causes of reduced specific force and its relation with FSHD disease severity are unknown.

METHODS

Quantitative muscle magnetic resonance imaging (MRI), measurement of voluntary maximum force generation and quadriceps force-frequency relationship, and vastus lateralis muscle biopsies were performed in 12 genetically confirmed patients with FSHD and 12 controls.

RESULTS

Specific force was reduced by ~33% in all FSHD patients independent of disease severity. Quadriceps force-frequency relationship shifted to the right in severe FSHD compared to controls. Fiber type distribution in vastus lateralis muscle biopsies did not differ between groups.

CONCLUSIONS

Reduced quadriceps specific force is present in all FSHD patients regardless of disease severity or fatty infiltration. Early myopathic changes, including fibrosis, and non-muscle factors, such as physical fatigue and musculoskeletal pain, may contribute to reduced specific force.

Lim K, Jaiswal JK, Chen YW. Membrane Repair Deficit in Facioscapulohumeral Muscular Dystrophy

Deficits in plasma membrane repair have been identified in dysferlinopathy and Duchenne Muscular Dystrophy, and contribute to progressive myopathy. Although Facioscapulohumeral Muscular Dystrophy (FSHD) shares clinicopathological features with these muscular dystrophies, it is unknown if FSHD is characterized by plasma membrane repair deficits. Therefore, we exposed immortalized human FSHD myoblasts, immortalized myoblasts from unaffected siblings, and myofibers from a murine model of FSHD (FLExDUX4) to focal, pulsed laser ablation of the sarcolemma. Repair kinetics and success were determined from the accumulation of intracellular FM1-43 dye post-injury. We subsequently treated FSHD myoblasts with a DUX4-targeting antisense oligonucleotide (AON) to reduce DUX4 expression, and with the antioxidant Trolox to determine the role of DUX4 expression and oxidative stress in membrane repair. Compared to unaffected myoblasts, FSHD myoblasts demonstrate poor repair and a greater percentage of cells that failed to repair, which was mitigated by AON and Trolox treatments. Similar repair deficits were identified in FLExDUX4 myofibers. This is the first study to identify plasma membrane repair deficits in myoblasts from individuals with FSHD, and in myofibers from a murine model of FSHD. Our results suggest that DUX4 expression and oxidative stress may be important targets for future membrane-repair therapies.

A Comparison of FES and SCS for Neuroplastic Recovery After SCI: Historical Perspectives and Future Directions

There is increasing evidence that neuroplastic changes can occur even years after spinal cord injury, leading to reduced disability and better health which should reduce the cost of healthcare. In motor-incomplete spinal cord injury, recovery of leg function may occur if repetitive training causes afferent input to the lumbar spinal cord. The afferent input may be due to activity-based therapy without electrical stimulation but we present evidence that it is faster with electrical stimulation. This may be spinal cord stimulation or peripheral nerve stimulation. Recovery is faster if the stimulation is phasic and that the patient is trying to use their legs during the training. All the published studies are small, so all conclusions are provisional, but it appears that patients with more disability (AIS A and B) may need to continue using stimulation and for them, an implanted stimulator is likely to be convenient. Patients with less disability (AIS C and D) may make useful recovery and improve their quality of life from a course of therapy. This might be locomotion therapy but we argue that cycling with electrical stimulation, which uses biofeedback to encourage descending drive, causes rapid recovery and might be used with little supervision at home, making it much less expensive. Such an electrical therapy followed by conventional physiotherapy might be affordable for the many people living with chronic SCI. To put this in perspective, we present some information about what treatments are funded in the UK and the US.

Effects of ursolic acid on sub-lesional muscle pathology in a contusion model of spinal cord injury

Spinal Cord Injury (SCI) results in severe sub-lesional muscle atrophy and fiber type transformation from slow oxidative to fast glycolytic, both contributing to functional deficits and maladaptive metabolic profiles. Therapeutic countermeasures have had limited success and muscle-related pathology remains a clinical priority. mTOR signaling is known to play a critical role in skeletal muscle growth and metabolism, and signal integration of anabolic and catabolic pathways. Recent studies show that the natural compound ursolic acid (UA) enhances mTOR signaling intermediates, independently inhibiting atrophy and inducing hypertrophy. Here, we examine the effects of UA treatment on sub-lesional muscle mTOR signaling, catabolic genes, and functional deficits following severe SCI in mice. We observe that UA treatment significantly attenuates SCI induced decreases in activated forms of mTOR, and signaling intermediates PI3K, AKT, and S6K, and the upregulation of catabolic genes including FOXO1, MAFbx, MURF-1, and PSMD11. In addition, UA treatment improves SCI induced deficits in body and sub-lesional muscle mass, as well as functional outcomes related to muscle function, motor coordination, and strength. These findings provide evidence that UA treatment may be a potential therapeutic strategy to improve muscle-specific pathological consequences of SCI.

Potential of M-Wave Elicited by Double Pulse for Muscle Fatigue Evaluation in Intermittent Muscle Activation by Functional Electrical Stimulation for Motor Rehabilitation

Clinical studies on application of functional electrical stimulation (FES) to motor rehabilitation have been increasing. However, muscle fatigue appears early in the course of repetitive movement production training by FES. Although M-wave variables were suggested to be reliable indices of muscle fatigue in long lasting constant electrical stimulation under the isometric condition, the ability of M-wave needs more studies under intermittent stimulation condition, because the intervals between electrical stimulations help recovery of muscle activation level. In this paper, M-waves elicited by double pulses were examined in muscle fatigue evaluation during repetitive movements considering rehabilitation training with surface electrical stimulation. M-waves were measured under the two conditions of repetitive stimulation: knee extension force production under the isometric condition and the dynamic movement condition by knee joint angle control. Amplitude of M-wave elicited by the 2nd pulse of a double pulse decreased during muscle fatigue in both measurement conditions, while the change in M-waves elicited by single pulses in a stimulation burst was not relevant to muscle fatigue in repeated activation with stimulation interval of 1 s. Fatigue index obtained from M-waves elicited by 2nd pulses was suggested to provide good estimation of muscle fatigue during repetitive movements with FES.

Neuromuscular Electrical Stimulation-Induced Resistance Training After SCI: A Review of the Dudley Protocol

BACKGROUND

Neuromuscular electrical stimulation (NMES), often referred to as functional electrical stimulation (FES), has been used to activate paralyzed skeletal muscle in people with spinal cord injury (SCI). The goal of NMES has been to reverse some of the dramatic losses in skeletal muscle mass, to stimulate functional improvements in people with incomplete paralysis, and to produce some of the health benefits associated with exercise.

OBJECTIVE

The purpose of this brief review is to describe a quantifiable resistance training form of NMES developed by Gary A. Dudley.

METHODS

People with motor complete SCI were first tested to confirm that an NMES-induced muscle contraction of the quadriceps muscle could be achieved. The contraction stimulus consisted of biphasic pulses at 35 Hz performed with increasing current up to what was needed to produce full knee extension. Four sets of 10 knee extensions were elicited, if possible. Training occurred biweekly for 3 to 6 months, with ankle weights being increased up to an added weight of 9.1 kg if the 40 repetitions could be performed successfully for 2 sessions.

RESULTS

Many participants have performed this protocol without adverse events, and all participants showed progression in the number of repetitions and/or the amount of weight lifted. Large increases in muscle mass occur, averaging 30% to 40%. Additional physiological adaptations to stimulated muscle have also been reported.

CONCLUSIONS

These results demonstrate that the affected skeletal muscle after SCI responds robustly to progressive resistance training many years after injury. Future work with NMES should determine whether gains in lean mass translate to improved health, function, and quality of life.

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Changes of the force-frequency relationship in the rat medial gastrocnemius muscle after total transection and hemisection of the spinal cord

The relationships between the stimulation frequency and the force developed by motor units (MUs) of the medial gastrocnemius muscle were compared between intact rats and animals after total transection or hemisection of the spinal cord at the low thoracic level. The experiments on functionally isolated MUs were carried out 14, 30, 90, and 180 days after the spinal cord injury. Axons of investigated MUs were stimulated with trains of pulses at 10 progressively increased frequencies (from 1 to 150 Hz), and the force-frequency curves were plotted. Spinal cord hemisection resulted in a considerable leftward shift of force-frequency curves in all types of MUs. After the total transection, a leftward shift of the curve was observed in fast MUs, whereas there was a rightward shift in slow MUs. These changes coincided with a decrease of stimulation frequencies necessary to evoke 60% of maximal force. Moreover, the linear correlation between these stimulation frequencies and the twitch contraction time observed in intact rats was disrupted in all groups of animals with spinal cord injury. The majority of the observed changes reached the maximum 1 mo after injury, whereas the effects evoked by spinal cord hemisection were significantly smaller and nearly constant in the studied period. The results of this study can be important for the prediction of changes in force regulation in human muscles after various extends of spinal cord injury and in evaluation of the frequency of functional electrical stimulation used for training of impaired muscles.

Evoked EMG and muscle fatigue during isokinetic FES-cycling in individuals with SCI

PURPOSE

This study investigated whether muscle fatigue during functional electrical stimulation (FES)-induced cycling was associated with changes occurring in evoked electromyographic signals (eEMG, M-waves) in individuals with spinal cord injury. We also explored the effects of recovery intervals between exercise sessions on the relationship between eEMG and muscle torque.

METHODS

Eight individuals with spinal cord injury performed three FES-cycling sessions of 15-min duration, with 5 min of recovery between them. The quadriceps muscles were electrically stimulated as the prime agonist to produce cycling. Pedal torques and surface eEMG signals were synchronously processed and recorded for offline analysis.

RESULTS

Large Torque decreases (20-44%) were observed in the first 5 min of cycling during the three exercise bouts, while changes of similar magnitude did not occur on any of the M-wave time-series (less than 19%). Between 5 and 15 min of cycling, muscle fatigue lowered the plateau baselines of Torque (ranging from 41% to 62%), M-wave peak-to-peak amplitude (PtpA) and Area (ranging from 60% to 98%) time-series, yet the magnitudes of these reductions were not consistent between them.

CONCLUSION

We concluded that muscle fatigue during FES-cycling was not associated with, nor could be predicted by, eEMG signals. Nonetheless, the consistency between M-waves and Torque time-curves in their direction of change clearly warrants further investigation.

Effects of endurance and strength-directed electrical stimulation training on the performance and histological properties of paralyzed human muscle: a pilot study

Electrical stimulation (ES) improves muscle properties after spinal cord injury (SCI), but cycling power output (PO) remains low. We investigated the effect of endurance and strength ES training on these parameters. Assessments of quadriceps strength and fatigue resistance, cycling PO, and muscle biopsies were made in four well-trained SCI subjects (three cyclists and one rower) before and after additional weight training in the cyclists and once in the rower. Weight training improved muscle strength, but cycling PO was low in all subjects. There was no effect of training type on biopsy data. Biopsies showed non-specific signs of pathology, predominance of type IIa fibers, and uniform metabolic activity. Oxidative activity was low, as were capillary:fiber ratios in the cyclists. Cycling PO is limited by factors other than muscle strength. Future ES training studies should attempt to improve muscle oxidative capacity to optimize the potential benefits of ES exercise.

Muscle after spinal cord injury

The morphological and contractile changes of muscles below the level of the lesion after spinal cord injury (SCI) are dramatic. In humans with SCI, a fiber-type transformation away from type I begins 4-7 months post-SCI and reaches a new steady state with predominantly fast glycolytic IIX fibers years after the injury. There is a progressive drop in the proportion of slow myosin heavy chain (MHC) isoform fibers and a rise in the proportion of fibers that coexpress both the fast and slow MHC isoforms. The oxidative enzymatic activity starts to decline after the first few months post-SCI. Muscles from individuals with chronic SCI show less resistance to fatigue, and the speed-related contractile properties change, becoming faster. These findings are also present in animals. Future studies should longitudinally examine changes in muscles from early SCI until steady state is reached in order to determine optimal training protocols for maintaining skeletal muscle after paralysis.

Force-frequency and force-length properties in skeletal muscle following unilateral focal ischaemic insult in a rat model

AIM

Our purpose was to quantify skeletal muscle properties following unilateral focal ischaemic insult (stroke) in a rat model.

METHODS

Male rats were divided into two groups: stroke and 2 weeks recovery (n = 8) and control group (n = 7). Stroke was induced in the area of the motor neocortex containing hind limb corticospinal neurones. Contractile properties of the medial gastrocnemius muscle were measured in situ in both limbs. Force-length and force-frequency properties were measured before and 35 min after 5 min fatiguing stimulation.

RESULTS

Stroke resulted in bilateral tetanic fade during 200 Hz stimulation. When normalized to 100 Hz contractions, force at 200 Hz was 95.4 +/- 0.9% for the paretic muscles, 96.7 +/- 1.7% for non-paretic muscles and 102.2 +/- 1.0% for muscles of control rats (P = 0.006). Prior to fatiguing contractions, there was no difference in the length dependence of force. During repetitive contractions, active force fell significantly to 19 +/- 4 and 25 +/- 5% of initial force in paretic and non-paretic muscles of animals with a stroke respectively. In control animals active force fell to 37 +/- 5%. During repetitive contractions, fusion index increased in muscles of stroke animals to 1.0 +/- 0 but in control animals it was 0.95 +/- 0.02. There was selective force depression at short lengths for fatigued paretic muscle (significant difference at muscle lengths less than reference length -2 mm).

CONCLUSION

The tetanic fade at high stimulation frequencies indicates that there may be activation failure following focal ischaemic insult. The greater magnitude of fatigue and selective depression at short lengths following repetitive contractions should be investigated further.

Effects of chronic electrical stimulation on paralyzed expiratory muscles

Following spinal cord injury, the expiratory muscles develop significant disuse atrophy characterized by reductions in their weight, fiber cross-sectional area, and force-generating capacity. We determined the extent to which these physiological alterations can be prevented with electrical stimulation. Because a critical function of the expiratory muscles is cough generation, an important goal was the maintenance of maximal force production. In a cat model of spinal cord injury, short periods of high-frequency lower thoracic electrical spinal cord stimulation (SCS) at the T(10) level (50 Hz, 15 min, twice/day, 5 days/wk) were initiated 2 wk following spinalization and continued for a 6-mo period. Airway pressure (P)-generating capacity was determined by SCS. Five acute, spinalized animals served as controls. Compared with controls, initial P fell from 43.9 +/- 1.0 to 41.8 +/- 0.7 cmH(2)O (not significant) in the chronic animals. There were small reductions in the weight of the external oblique, internal oblique, transverses abdominis, internal intercostal, and rectus abdominis muscles (not significant for each). There were no significant changes in the population of fast muscle fibers. Because prior studies (Kowalski KE, Romaniuk JR, DiMarco AF. J Appl Physiol 102: 1422-1428, 2007) have demonstrated significant atrophy following spinalization in this model, these results indicate that expiratory muscle atrophy can be prevented by the application of short periods of daily high-frequency stimulation. Because the frequency of stimulation is similar to the expected pattern of clinical use for cough generation, the daily application of electrical stimulation could potentially serve the dual purpose of maintenance of expiratory muscle function and airway clearance.

Nutrient supplementation post ambulation in persons with incomplete spinal cord injuries: a randomized, double-blinded, placebo-controlled case series

OBJECTIVE

To examine effects of protein-carbohydrate intake on ambulation performance in persons with incomplete spinal cord injury (SCI).

DESIGN

Double-blinded treatment with washout and placebo crossover.

SETTING

Academic medical center.

PARTICIPANTS

Three subjects aged 34 to 43 years with incomplete SCI at C5-T4.

INTERVENTIONS

Subjects walked to fatigue on 5 consecutive days. On fatigue, participants consumed 48g of vanilla-flavored whey and 1g/kg of body weight of carbohydrate (CH(2)O). Weekend rest followed, and the process was repeated. A 2-week washout was interposed and the process repeated using 48g of vanilla-flavored soy.

MAIN OUTCOME MEASURES

Oxygen consumed (Vo(2); in L/min), carbon dioxide evolved (Vco(2)), respiratory exchange ratio (RER: Vco(2)/Vo(2)), time (in minutes), and distance walked (in meters) were recorded. Caloric expenditure was computed as Vo(2) by time by 21kJ/L (5kcal/L) of oxygen consumed. Data were averaged across the final 2 ambulation sessions for each testing condition.

RESULTS

Despite slow ambulation velocities (range, .11-.34m/s), RERs near or above unity reflected reliance on CH(2)O fuel substrates. Average ambulation time to fatigue was 17.8% longer; distance walked 37.9% longer, and energy expenditure 12.2% greater with the whey and CH(2)O supplement than with the soy drink.

CONCLUSIONS

Whey and CH(2)O ingestion after fatiguing ambulation enhanced ensuing ambulation by increasing ambulation distance, time, and caloric expenditure in persons with incomplete SCI.

Low-frequency fatigue in individuals with spinal cord injury

BACKGROUND/OBJECTIVE

This study examined magnitude and recovery of low-frequency fatigue (LFF) in the quadriceps after electrically stimulated contractions in spinal cord-injured (SCI) and able-bodied subjects.

SUBJECTS

Nine SCI (ASIA A-C, levels C5-T9, injured 13.6 +/- 12.2 years) and 9 sedentary able-bodied subjects completed this study.

METHODS

Fatigue was evoked in 1 thigh, and the nonfatigued leg served as a control. The fatigue test for able-bodied subjects lasted 15 minutes. For SCI, stimulation was adjusted so that the relative drop in force was matched to the able-bodied group. Force was assessed at 20 (P20) and 100 Hz (P100), and the ratio of P20/P100 was used to evaluate LFF in thighs immediately after, at 10, 20, and 60 minutes, and at 2, 4, 6, and 24 hours after a fatigue test.

RESULTS

The magnitude of LFF (up to 1 hour after fatigue) was not different between able-bodied and patients with SCI. However, recovery of LFF over 24 hours was greater in able-bodied compared with patients with SCI in both the experimental (P < 0.001) and control legs (P < 0.001). The able-bodied group showed a gradual recovery of LFF over time in the experimental leg, whereas the SCI group did not.

CONCLUSIONS

These results show that individuals with SCI are more susceptible to LFF than able-bodied subjects. In SCI, simply assessing LFF produced considerable LFF and accounted for a substantial portion of the response. We propose that muscle injury is causing the dramatic LFF in SCI, and future studies are needed to test whether "fatigue" in SCI is actually confounded by the effects of muscle injury.

Twitch and Tetanic Properties of Human Thenar Motor Units Paralyzed by Chronic Spinal Cord Injury

Little is known about how human motor units respond to chronic paralysis. Our aim was to record surface electromyographic (EMG) signals, twitch forces, and tetanic forces from paralyzed motor units in the thenar muscles of individuals ( n = 12) with chronic (1.5–19 yr) cervical spinal cord injury (SCI). Each motor unit was activated by intraneural stimulation of its motor axon using single pulses and trains of pulses at frequencies between 5 and 100 Hz. Paralyzed motor units ( n = 48) had small EMGs and weak tetanic forces ( n = 32 units) but strong twitch forces, resulting in half-maximal force being achieved at a median of only 8 Hz. The distributions for cumulative twitch and tetanic forces also separated less for paralyzed units than for control units, indicating that increases in stimulation frequency made a smaller relative contribution to the total force output in paralyzed muscles. Paralysis also induced slowing of conduction velocities, twitch contraction times and EMG durations. However, the elevated ratios between the twitch and the tetanic forces, but not contractile speed, correlated significantly with the extent to which unit force summated in response to different frequencies of stimulation. Despite changes in the absolute values of many electrical and mechanical properties of paralyzed motor units, most of the distributions shifted uniformly relative to those of thenar units obtained from control subjects. Thus human thenar muscles paralyzed by SCI retain a population of motor units with heterogeneous contractile properties because chronic paralysis influenced all of the motor units similarly.

Fatigue of muscles weakened by death of motoneurons

Weakness is a characteristic of muscles influenced by the postpolio syndrome (PPS), amyotrophic lateral sclerosis (ALS), and spinal cord injury (SCI). The strength deficits relate to changes in muscle use and to the chronic denervation that can follow the spinal motoneuron death common to these disorders. PPS, ALS, and SCI also involve variable amounts of supraspinal neuron death, the effects of which on muscle weakness remains unclear. Nevertheless, weakness of muscle itself defines the functional consequences of these disorders. A weaker muscle requires an individual to work that muscle at higher than usual intensities relative to its maximal capacity, inducing progressive fatigue and an increased sense of effort. Little evidence is available to suggest that the fatigue commonly experienced by individuals with these disorders relates to an increase in the intrinsic fatigability of the muscle fibers. The only exception is when SCI induces chronic muscle paralysis. To reduce long-term functional deficits in these disorders, studies must identify the signaling pathways that influence neuron survival and determine the factors that encourage and limit sprouting of motor axons. This may ensure that a greater proportion of the fibers in each muscle remain innervated and available for use.

Fatigue properties of human thenar motor units paralysed by chronic spinal cord injury

Human muscles paralysed chronically by spinal cord injury (SCI) fatigue excessively. Whether these reductions in force reflect a decrease in the fatigue resistance of the motor units is unknown. Our aim was to determine the fatigability of thenar motor units paralysed chronically (10 +/- 2 years) by cervical SCI. Surface electromyographic activity (EMG) and force were recorded from 17 paralysed motor units (n = 7 subjects) in response to intraneural motor axon stimulation (13 pulses at 40 Hz, 1 s(-1) for 2 min). Unit force decreased progressively, reaching 8-60% of initial after 2 min, whereas both the amplitude and area of the first EMG potentials in the trains increased significantly (both P < 0.05). Thus, transmission of neural signals to the sarcolemma was effective and the reduction in force must reflect impaired processes in the muscle fibres. The median fatigue index for paralysed units (0.31), the ratio of the force at 2 min compared to the initial force, was significantly lower than that for units from control subjects (0.85, P < 0.05), but the distribution of fatigue indices for each population had a similar shape (ranges: 0.08-0.60 and 0.41-0.95, respectively). Hence, chronic paralysis did not limit the range of fatigability typically found for thenar units, only its magnitude. These findings suggest that all paralysed units underwent similar reductions in fatigue resistance. After fatigue, paralysed unit forces were reduced at all frequencies (1-100 Hz, P < 0.05). Twitch contraction and half-relaxation times were increased, as was the frequency needed to produce half maximal force (P < 0.05). Thus, stimulation protocols used to produce functional movements in paralysed muscles need to accommodate the significant and rapid fatigue of the motor units.

Tail muscles become slow but fatigable in chronic sacral spinal rats with spasticity

Paralyzed skeletal muscle sometimes becomes faster and more fatigable after spinal cord injury (SCI) because of reduced activity. However, in some cases, pronounced muscle activity in the form of spasticity (hyperreflexia and hypertonus) occurs after long-term SCI. We hypothesized that this spastic activity may be associated with a reversal back to a slower, less fatigable muscle. In adult rats, a sacral (S2) spinal cord transection was performed, affecting only tail musculature and resulting in chronic tail spasticity beginning 2 wk later and lasting indefinitely. At 8 mo after injury, we examined the contractile properties of the segmental tail muscle in anesthetized spastic rats and in age-matched normal rats. The segmental tail muscle has only a few motor units (<12), which were easily detected with graded nerve stimulation, revealing two clear motor unit twitch durations. The dominant faster unit twitches peaked at 15 ms and ended within 50 ms, whereas the slower unit twitches only peaked at 30-50 ms. With chronic injury, this slow twitch component increased, resulting in a large overall increase (>150%) in the fraction of the peak muscle twitch force remaining at 50 ms. With injury, the peak muscle twitch (evoked with supramaximal stimulation) also increased in its time to peak (+48.9%) and half-rise time (+150.0%), and decreased in its maximum rise (-35.0%) and decay rates (-40.1%). Likewise, after a tetanic stimulation, the tetanus half-fall time increased by 53.8%. Therefore the slow portion of the muscle was enhanced in spastic muscles. Consistent with slowing, posttetanic potentiation was 9.2% lower and the stimulation frequency required to produce half-maximal tetanus decreased 39.0% in chronic spinals. Interestingly, in spastic muscles compared with normal, whole muscle twitch force was 81.1% higher, whereas tetanic force production was 38.1% lower. Hence the twitch-to-tetanus ratio increased 104.0%. Inconsistent with overall slowing, whole spastic muscles were 61.5% more fatigable than normal muscles. Thus contrary to the classical slow-to-fast conversion that is seen after SCI without spasticity, SCI with spasticity is associated with a mixed effect, including a preservation/enhancement of slow properties, but a loss of fatigue resistance.

Adaptive response of human tendon to paralysis

To gain insight into the adaptive response of human tendon to paralysis, we compared the mechanical properties of the in vivo patellar tendon in six men who were spinal cord-injured (SCI) and eight age-matched, able-bodied men. Measurements were taken by combining dynamometry, electrical stimulation, and ultrasonography. Tendon stiffness and Young's modulus, calculated from force-elongation and stress-strain curves, respectively, were lower by 77% (P < 0.01) and 59% (P < 0.05) in the SCI than able-bodied subjects. The cross-sectional area (CSA) of the tendon was 17% smaller (P < 0.05) in the SCI subjects, but there was no difference in tendon length between the two groups. Our results indicate that paralysis causes substantial deterioration of the structural and material properties of tendon. This needs to be taken into consideration in the design of electrical stimulation protocols for rehabilitation and experimental purposes, and when interpreting changes in the contractile speed of paralyzed muscle.

Influence of knee joint angle on muscle properties of paralyzed and nonparalyzed human knee extensors

Muscles of individuals with a spinal cord injury (SCI) exhibit an unexpected leftward shift in the force (torque)-frequency relationship. We investigated whether differences in torque-angle relationships between SCI and able-bodied control muscles could explain this shift. Electrically stimulated knee-extensor contractions were obtained at knee flexion angles of between 30 degrees and 90 degrees. Torque-frequency relationships were obtained at 30 degrees, 90 degrees, and optimum angle. Optimum angle was not different between groups but SCI-normalized torques were lower at the extreme angles. At all angles, SCI muscles produced higher relative torques at low stimulation frequencies. Thus, there was no evidence of a consistent change in the length of paralyzed SCI muscles, and the anomalous leftward shift in the torque-frequency relationship was not the result of testing the muscle at a relatively long length. The results provide valuable information about muscle changes occurring in various neurological disorders.

Increasing blood flow before exercise in spinal cord-injured individuals does not alter muscle fatigue

Previous studies have shown increased fatigue in paralyzed muscle of spinal cord-injured (SCI) patients (Castro M, Apple D Jr, Hillegass E, and Dudley GA. Eur J Appl Physiol 80: 373-378, 1999; Gerrits H, Hopman MTE, Sargeant A, and de Haan A. Clin Physiol 21: 105-113, 2001). Our purpose was to determine whether the increased muscle fatigue could be due to a delayed rise in blood flow at the onset of exercise in SCI individuals. Isometric electrical stimulation was used to induce fatigue in the quadriceps femoris muscle of seven male, chronic (>1 yr postinjury), complete (American Spinal Injury Association, category A) SCI subjects. Cuff occlusion was used to elevate blood flow before electrical stimulation, and the magnitude of fatigue was compared with a control condition of electrical stimulation without prior cuff occlusion. Blood flow was measured in the femoral artery by Doppler ultrasound. Prior cuff occlusion increased blood flow in the first 30 s of stimulation compared with the No-Cuff condition (1,350 vs. 680 ml/min, respectively; P < 0.001), although blood flow at the end of stimulation was the same between conditions (1,260 ± 140 vs. 1,160 ± 370 ml/min, Cuff and No-Cuff condition, respectively; P = 0.511). Muscle fatigue was not significantly different between prior cuff occlusion and the control condition (32 ± 13 vs. 35 ± 10%; P = 0.670). In conclusion, increased muscle fatigue in SCI individuals is not associated with the prolonged time for blood flow to increase at the onset of exercise.

Exercise Recommendations for Individuals with Spinal Cord Injury

Persons with spinal cord injury (SCI) exhibit deficits in volitional motor control and sensation that limit not only the performance of daily tasks but also the overall activity level of these persons. This population has been characterised as extremely sedentary with an increased incidence of secondary complications including diabetes mellitus, hypertension and atherogenic lipid profiles. As the daily lifestyle of the average person with SCI is without adequate stress for conditioning purposes, structured exercise activities must be added to the regular schedule if the individual is to reduce the likelihood of secondary complications and/or to enhance their physical capacity. The acute exercise responses and the capacity for exercise conditioning are directly related to the level and completeness of the spinal lesion. Appropriate exercise testing and training of persons with SCI should be based on the individual's exercise capacity as determined by accurate assessment of the spinal lesion. The standard means of classification of SCI is by application of the International Standards for Classification of Spinal Cord Injury, written by the Neurological Standards Committee of the American Spinal Injury Association. Individuals with complete spinal injuries at or above the fourth thoracic level generally exhibit dramatically diminished cardiac acceleration with maximal heart rates less than 130 beats/min. The work capacity of these persons will be limited by reductions in cardiac output and circulation to the exercising musculature. Persons with complete spinal lesions below the T(10) level will generally display injuries to the lower motor neurons within the lower extremities and, therefore, will not retain the capacity for neuromuscular activation by means of electrical stimulation. Persons with paraplegia also exhibit reduced exercise capacity and increased heart rate responses (compared with the non-disabled), which have been associated with circulatory limitations within the paralysed tissues. The recommendations for endurance and strength training in persons with SCI do not vary dramatically from the advice offered to the general population. Systems of functional electrical stimulation activate muscular contractions within the paralysed muscles of some persons with SCI. Coordinated patterns of stimulation allows purposeful exercise movements including recumbent cycling, rowing and upright ambulation. Exercise activity in persons with SCI is not without risks, with increased risks related to systemic dysfunction following the spinal injury. These individuals may exhibit an autonomic dysreflexia, significantly reduced bone density below the spinal lesion, joint contractures and/or thermal dysregulation. Persons with SCI can benefit greatly by participation in exercise activities, but those benefits can be enhanced and the relative risks may be reduced with accurate classification of the spinal injury.

Long-term spinal cord injury increases susceptibility to isometric contraction-induced muscle injury

Complete spinal cord injury (SCI) results in inactivation and unloading of affected skeletal muscles. Unloading causes an increased susceptibility of muscle to contraction-induced injury. This study used magnetic resonance imaging (MRI) to test the hypothesis that isometric contractions would evoke greater muscle damage to the quadriceps femoris muscle (mQF) of SCI subjects than that of able-bodied (AB) controls. MR images were taken of the mQF prior to, immediately post, and 3 days post electromyostimulation (EMS). EMS consisted of five sets of ten isometric contractions (2 s on/6 s off, 1 min between sets) followed by another three sets of ten isometric contractions (1 s on/1 s off, 30 s between sets). Average muscle cross-sectional area (CSA) and the relative areas of stimulated and injured muscle were obtained from MR images by quantifying the number of pixels with an elevated T2 signal. SCI subjects had significantly greater relative area [90 (2)% versus 66 (4)%, P<0.05; mean (SE)] but a lesser absolute area [16 (3) cm(2) versus 44 (6) cm(2), P<0.05] of mQF stimulated than AB controls. During EMS, peak torque was reduced by 66% and 37% for SCI and control subjects, respectively. Three days post EMS, there was a greater relative area of stimulated mQF injured for the SCI subjects [25 (6)% versus 2 (1)%, P<0.05]. Peak torque remained decreased by 22% on day 3 in the SCI group only. These results indicate that affected muscle years after SCI is more susceptible to contraction-induced muscle damage, as determined by MRI, compared to AB controls. They also support the contention that electrically elicited isometric contractions are sufficient to cause muscle damage after a prolonged period of inactivity.

Variability in fibre properties in paralysed human quadriceps muscles and effects of training

A spinal cord injury usually leads to an increase in contractile speed and fatigability of the paralysed quadriceps muscles, which is probably due to an increased expression of fast myosin heavy chain (MHC) isoforms and reduced oxidative capacity. Sometimes, however, fatigue resistance is maintained in these muscles and also contractile speed is slower than expected. To obtain a better understanding of the diversity of these quadriceps muscles and to determine the effects of training on characteristics of paralysed muscles, fibre characteristics and whole muscle function were assessed in six subjects with spinal cord lesions before and after a 12-week period of daily low-frequency electrical stimulation. Relatively high levels of MHC type I were found in three subjects and this corresponded with a high degree of fusion in 10-Hz force responses (r=0.88). Fatigability was related to the activity of succinate dehydrogenase (SDH) (r=0.79). Furthermore, some differentiation between fibre types in terms of metabolic properties were present, with type I fibres expressing the highest levels of SDH and lowest levels of alpha-glycerophosphate dehydrogenase. After training, SDH activity increased by 76+/-26% but fibre diameter and MHC expression remained unchanged. The results indicate that expression of contractile proteins and metabolic properties seem to underlie the relatively normal functional muscle characteristics observed in some paralysed muscles. Furthermore, training-induced changes in fatigue resistance seem to arise, in part, from an improved oxidative capacity.

Training by low-frequency stimulation of tibialis anterior in spinal cord-injured men

The tibialis anterior muscle of nine paraplegic men was chronically stimulated (2-6 h per day; at 10 Hz, 5 s on, 5 s off) under isometric loading conditions for 5 days per week for 4 weeks. After 4 weeks of training, muscle fatigue resistance in an electrically evoked test had increased by an average of 75% (P <.01, n = 9), but there were no changes in the relative composition of the three myosin heavy chain (MHC) isoforms. Five of the subjects continued training for an additional 5 weeks (2 h per day, 3 days per week). Although there was a tendency for twitch time to peak torque to increase after this additional period, no change occurred in relative MHC isoform content. However, in situ hybridization analysis revealed that even after 2 weeks of stimulation, there was evidence of upregulation of the mRNA for the MHC-I isoform and downregulation of the MHC-IIX isoform, a development that continued in weeks 4 and 9. This study provides evidence, at the level of gene transcription, that a fast-to-slow change in MHC isoform composition may be possible in human muscle when its usage is significantly increased.

Effects of training on contractile properties of paralyzed quadriceps muscle

Effects of two different training regimens on the contractile properties of the quadriceps muscle were studied in six individuals with spinal cord injury. Each subject had both limbs trained with the two regimens, consisting of stimulation with low frequencies (LF) at 10 HZ or high frequencies (HF) at 50 HZ; one limb of each subject was stimulated with the LF protocol and the other with the HF regimen. Twelve weeks of daily training increased tetanic tension by approximately 20%, which was not significantly different between training regimens. Interestingly, after HF but not LF training, the unusual high forces at the low frequency range of the force-frequency relationship decreased, possibly due to a reduced activation per impulse. After LF but not HF training, force oscillation amplitudes declined (by 33%) as relaxation tended to slow, which may have opposed possible effects of reduced activation as seen after HF training. Finally, fatigue resistance also increased rapidly after LF training (by 43%) but not after HF training. These results indicate that different types of training may selectively change different aspects of function in disused muscles.

Contractile properties and fatigue of quadriceps muscles in multiple sclerosis

Functional characteristics of electrically stimulated quadriceps muscles of patients with multiple sclerosis (MS) were determined to investigate whether adaptations in muscle properties contribute to the higher fatigability of these patients. The estimated maximal isometric force generating capacity of MS patients was only 11.2% (P < 0.05) lower than control subjects. However, the patients were only able to voluntarily exert 75 +/- 22% (n = 12) of their maximal capacity, against 94 +/- 6% (n = 7) for the control subjects. There were no differences in muscle speed, suggesting that muscle fiber distribution was not different in the MS patients due to reduced muscle usage. During a series of repeated contractions, greater decrements occurred in isometric force and in maximal rate of force rise in the MS patients (by 31.3 +/- 10.3% and 50.1 +/- 10.0%, respectively; n = 13) than control subjects (23.8 +/- 6.6% and 39.0 +/- 8.1%, n = 15), suggesting a lower oxidative capacity. The results indicate that increasing the mass of their muscles by training may help to reduce the excessive muscle fatigue of MS patients.

Contractile properties of the quadriceps muscle in individuals with spinal cord injury

Selected contractile properties and fatigability of the quadriceps muscle were studied in seven spinal cord-injured (SCI) and 13 able-bodied control (control) individuals. The SCI muscles demonstrated faster rates of contraction and relaxation than did control muscles and extremely large force oscillation amplitudes in the 10-Hz signal (65 +/- 22% in SCI versus 23 +/- 8% in controls). In addition, force loss and slowing of relaxation following repeated fatiguing contractions were greater in SCI compared with controls. The faster contractile properties and greater fatigability of the SCI muscles are in agreement with a characteristic predominance of fast glycolytic muscle fibers. Unexpectedly, the SCI muscles exhibited a force-frequency relationship shifted to the left, most likely as the result of relatively large twitch amplitudes. The results indicate that the contractile properties of large human locomotory muscles can be characterized using the approach described and that the transformation to faster properties consequent upon changes in contractile protein expression following SCI can be assessed. These measurements may be useful to optimize stimulation characteristics for functional electrical stimulation and to monitor training effects induced by electrical stimulation during rehabilitation of paralyzed muscles.

Influence of complete spinal cord injury on skeletal muscle within 6 mo of injury

This study examined the influence of spinal cord injury (SCI) on affected skeletal muscle. The right vastus lateralis muscle was biopsied in 12 patients as soon as they were clinically stable (average 6 wk after SCI), and 11 and 24 wk after injury. Samples were also taken from nine able-bodied controls at two time points 18 wk apart. Surface electrical stimulation (ES) was applied to the left quadriceps femoris muscle to assess fatigue at these same time intervals. Biopsies were analyzed for fiber type percent and cross-sectional area (CSA), fiber type-specific succinic dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPDH) activities, and myosin heavy chain percent. Controls showed no change in any variable over time. Patients showed 27-56% atrophy (P = 0.000) of type I, IIa, and IIax+IIx fibers from 6 to 24 wk after injury, resulting in fiber CSA approximately one-third that of controls. Their fiber type specific SDH and GPDH activities increased (P </= 0.001) from 32 to 90% over the 18 wk, thereby approaching or surpassing control values. The relative CSA of type I fibers and percentage of myosin heavy chain type I did not change. There was apparent conversion among type II fiber subtypes; type IIa decreased and type IIax+IIx increased (P </= 0.012). Force loss during ES did not change over time for either group but was greater (P = 0.000) for SCI patients than for controls overall (27 vs. 9%). The results indicate that vastus lateralis muscle shows marked fiber atrophy, no change in the proportion of type I fibers, and a relative independence of metabolic enzyme levels from activation during the first 24 wk after clinically complete SCI. Over this time, quadriceps femoris muscle showed moderately greater force loss during ES in patients than in controls. It is suggested that the predominant response of mixed human skeletal muscle within 6 mo of SCI is loss of contractile protein. Therapeutic interventions could take advantage of this to increase muscle mass.

Regression-based indices of fatigue in paraplegics' electrically stimulated quadriceps

We studied muscle fatigue in 19 paraplegic patients during an isometric sustained contraction. Both quadriceps (n = 38) were maximally stimulated for 126 s. We defined a nonlinear equation with four coefficients to fit the data and extract fatigue indices carrying information on the amount (asymptotic values, torque coordinate at the inflection point, total decrement from initial value) and the rate of torque decrease (maximum slope, time constant, time coordinate at the inflection point, time at which the torque reached 50% of initial value). Most of the indices reported in the literature provide information limited to the amount of torque decrease. Three of the coefficients we defined could be graphically estimated. The goodness of fit was very high (r2 = 0.993 +/- 0.001) but none of the indices provided information on both the amount and the rate of decrease. Major differences were encountered among the patients. Therefore, some of the computed indices varied to a great extent. We studied the curves exhibiting the same fatigue indices and defined a simple general fatigue index able to distinguish from muscles having different resistance to fatigue (asymptotic value x the time constant). This index is carrying both types of information and is expressed in relative torque x second (range from 1.81 to 22.82).