Skeletal muscle fibre type transformation following spinal cord injury

Intramuscular stimulation of tibialis anterior in human subjects: the effects of discharge variability on force production and fatigue

Continuous intramuscular stimulation of tibialis anterior (TA) was used to test the hypothesis that irregular trains of stimuli can increase force production and offset the magnitude of fatigue when compared with a continuous train of regular stimuli at an identical mean frequency (19 or 24 Hz). To achieve this, tungsten microelectrodes were inserted into the muscle belly into the motor point of the tibialis anterior muscle of able-bodied individuals (aged 19-50) and stimulated at current intensities ranging from 5 to 7 mA. The motor point was stimulated with a continuous train of regular stimulation at either 19 or 24 Hz (n = 11) or until the force declined below 25% of the peak force at the onset of stimulation. For the first seven subjects, no fatigue was exhibited, and thus, we simply compared the forces generated by the regular and irregular segments of the continuous train (120 sec for each segment). For four additional subjects, we delivered a higher frequency train (24 Hz) that elicited some fatigue. Once the force had declined below 25% of the initial peak force (which took between 140 and 210 sec), the continuous irregular train was integrated. Interestingly, for those subjects who exhibited muscular fatigue, force always began to rise again once the irregularity was incorporated into the continuous regular train of stimulation at the identical mean frequency (24 Hz). We conclude that incorporating irregularity into continuous trains of stimuli offers a significant advantage to the human neuromuscular system during both fatigued and nonfatigued states and could offer benefits to therapies such as functional electrical stimulation (FES).

The effects of electromyostimulation application timing on denervated skeletal muscle atrophy

INTRODUCTION

In this study we evaluated the effect of electromyostimulation (EMS) on myosin heavy chain (MHC) isoform expression in denervated rat muscles to determine the optimal timing for EMS application.

METHODS

EMS was initiated on post-injury day 1 for the group with denervation receiving immediate EMS (DIEMS) and on post-injury day 15 for the group with denervation receiving delayed EMS (DDEMS) in rat denervated muscles. Muscle wet weight and muscle fiber cross-sectional area (FCSA) were measured. MHC isoforms were analyzed in both protein homogenates and single muscle fibers.

RESULTS

The expression levels of IIx and IIb isoforms of MHC were significantly lower and higher, respectively, in the gastrocnemius muscles of the DIEMS group, but not the DDEMS group. The DIEMS group also showed larger FCSA and a lower proportion of hybrid single fibers compared with the DDEMS group.

DISCUSSION

These results indicate that immediate EMS is more effective than delayed EMS for aiding recovery of denervation-induced MHC changes. Muscle Nerve 56: E154-E161, 2017.

A lifestyle intervention program for successfully addressing major cardiometabolic risks in persons with SCI: a three-subject case series

INTRODUCTION

This study is a prospective case series analyzing the effects of a comprehensive lifestyle intervention program in three patients with chronic paraplegia having major risks for the cardiometabolic syndrome (CMS).

CASE PRESENTATION

Individuals underwent an intense 6-month program of circuit resistance exercise, nutrition using a Mediterranean diet and behavioral support, followed by a 6-month extension (maintenance) phase involving minimal support. The primary goal was a 7% reduction of body mass. Other outcomes analyzed insulin resistance using the HOMA-IR model, and plasma levels of fasting triglycerides and high-density lipoprotein cholesterol. All participants achieved the goal for 7% reduction of body mass and maintained the loss after the MP. Improvements were observed in 2/3 subjects for HOMA-IR and high-density lipoprotein cholesterol. All participants improved their risk for plasma triglycerides.

DISCUSSION

We conclude, in a three-person case series of persons with chronic paraplegia, a lifestyle intervention program involving circuit resistance training, a calorie-restrictive Mediterranean-style diet and behavioral support, results in clinically significant loss of body mass and effectively reduced component risks for CMS and diabetes. These results were for the most part maintained after a 6-month MP involving minimal supervision.

Microstimulation of single human motor axons in the toe extensors: force production during long-lasting trains of irregular and regular stimuli

Human motoneurones are known to discharge with a physiological variability of ~25% during voluntary contractions. Using microstimulation of single human motor axons, we have previously shown that delivering brief trains (10 pulses) of irregular stimuli, which incorporate discharge variability, generates greater contractile responses than trains of regular stimuli with identical mean frequency but zero variability. We tested the hypothesis that longer irregular (physiological) trains would produce greater contractile responses than regular (nonphysiological) trains of the same mean frequency (18 Hz) and duration (45 sec). Tungsten microelectrodes were inserted into the common peroneal nerve of human subjects, and single motor axons supplying the toe extensors (n = 14) were isolated. Irregular trains of stimuli showed greater contractile responses over identical mean frequencies in both fatigue-resistant and fatigable motor units, but because the forces were higher the rate of decline was higher. Nevertheless, forces produced by the irregular trains were significantly higher than those produced by the regular trains. We conclude that discharge irregularity augments force production during long as well as short trains of stimulation.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Two Experimental Strategies to Restore Muscle Mass in Adult Rats Following Spinal Cord Injury

Spinal cord injury decreases muscle mass and is associated with myofiber type trans formations in skeletal muscles. The present study evaluated the potential of motor- assisted cycling exercise or transplantation of fetal spinal cord tissue into the lesion cavity to inhibit or minimize these changes in skeletal muscles of 27 adult female Sprague-Dawley rats. Soleus (SO) and tibialis anterior (TA) muscles were studied 30 to 32 days after injury/intervention in the following groups: uninjured control ani mals (Con); spinal cord injured only (Tx); Tx with a 4-week exercise program con sisting of five weekly 60-minute sessions of cycling exercise initiated 5 days after in jury (TxEx); and Tx with fetal spinal cord tissue transplanted into the lesion cavity at the time of injury (TxTp). SO and TA muscle to body weight ratios were reduced significantly in the Tx group (24-30% decrease vs Con, p < 0.05) but were maintained with regular cycling exercise (6-8% decrease vs Con, no significant difference). The transplant had a beneficial effect on TA muscle mass (16% decrease vs Con, no sig nificant difference) but was not effective in limiting the effects of Tx on SO muscle mass. Immunohistochemistry and Northern analysis of TA and SO muscles revealed a Tx-induced reduction in myofiber cross sectional area (22% and 33% vs Con re spectively, p < 0.05) as well as a conversion in myosin heavy chain (MyHC) expres sion toward faster MyHC isoforms. Moreover, one month after injury, there was an increase in myofibers expressing more than one MyHC. mRNA encoding MyoD, a muscle-specific transcription factor, was increased in SO muscles suggesting that it may be involved in the long-term adaptations following spinal cord transection. Although cycling exercise was effective in preventing the decrease in myofiber area in both TA and SO, it did not inhibit the transformations of myofiber type. TA myofiber area was maintained in transplant recipients, however, this treatment was without conse quence on the size of SO myofibers. These results suggest that some of the normally observed spinal cord injury-induced skeletal muscle adaptations are minimized after one month of cycling exercise or fetal spinal cord tissue transplants. Key Words: Myosin heavy chain—Exercise—MyoD—Fetal tissue transplantation—Fiber types.

Sarcopenic Obesity in Adults With Spinal Cord Injury: A Cross-Sectional Study

OBJECTIVES

To describe (1) the frequency and utility of clinically relevant spinal cord injury (SCI)-specific and general population thresholds for obesity and sarcopenic obesity; and (2) the fat and lean soft tissue distributions based on the neurologic level of injury and the American Spinal Injury Association Impairment Scale.

DESIGN

Cross-sectional.

SETTING

Tertiary SCI rehabilitation hospital.

PARTICIPANTS

Persons (N=136; men, n=100; women, n=36) with chronic (mean ± SD: 15.6±11.3y postinjury) tetraplegia (n=66) or paraplegia (n=70).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Body composition was assessed with anthropometrics and whole-body dual-energy x-ray absorptiometry. Muscle atrophy was quantified using a sarcopenia threshold of appendicular lean mass index (ALMI) (men, ≤7.26kg/m²; women, ≤5.5kg/m²). Obesity was defined by percentage body fat (men, ≥25%; women, ≥35%), visceral adipose tissue (≥130cm²), and SCI-specific obesity thresholds (body mass index [BMI] ≥22kg/m²; waist circumference ≥94cm). Sarcopenic obesity was defined as the presence of both sarcopenia and obesity. Groups were compared based on impairment characteristics using an analysis of covariance.

RESULTS

Sarcopenic obesity was prevalent in 41.9% of the sample. ALMI was lower among participants with motor-complete (6.2±1.3kg/m²) versus motor-incomplete (7.5±1.6kg/m²) injuries (P<.01). Whole-body fat was greater among participants with tetraplegia (28.8±11.2kg) versus paraplegia (24.1±8.7kg; P<.05). Compared with general population guidelines (20.6%), SCI-specific BMI thresholds identified all the participants with obesity (77.9%) based on percentage body fat (72.1%).

CONCLUSIONS

The observed frequency of sarcopenic obesity in this sample of individuals with chronic SCI is very high, and identification of obesity is dissimilar when using SCI-specific versus general population criteria.

Skeletal muscle fiber type: using insights from muscle developmental biology to dissect targets for susceptibility and resistance to muscle disease

Skeletal muscle fibers are classified into fiber types, in particular, slow twitch versus fast twitch. Muscle fiber types are generally defined by the particular myosin heavy chain isoforms that they express, but many other components contribute to a fiber's physiological characteristics. Skeletal muscle fiber type can have a profound impact on muscle diseases, including certain muscular dystrophies and sarcopenia, the aging-induced loss of muscle mass and strength. These findings suggest that some muscle diseases may be treated by shifting fiber type characteristics either from slow to fast, or fast to slow phenotypes, depending on the disease. Recent studies have begun to address which components of muscle fiber types mediate their susceptibility or resistance to muscle disease. However, for many diseases it remains largely unclear why certain fiber types are affected. A substantial body of work has revealed molecular pathways that regulate muscle fiber type plasticity and early developmental muscle fiber identity. For instance, recent studies have revealed many factors that regulate muscle fiber type through modulating the activity of the muscle regulatory transcription factor MYOD1. Future studies of muscle fiber type development in animal models will continue to enhance our understanding of factors and pathways that may provide therapeutic targets to treat muscle diseases. WIREs Dev Biol 2016, 5:518-534. doi: 10.1002/wdev.230 For further resources related to this article, please visit the WIREs website.

Strategies for Rapid Muscle Fatigue Reduction during FES Exercise in Individuals with Spinal Cord Injury: A Systematic Review

Background

Rapid muscle fatigue during functional electrical stimulation (FES)-evoked muscle contractions in individuals with spinal cord injury (SCI) is a significant limitation to attaining health benefits of FES-exercise. Delaying the onset of muscle fatigue is often cited as an important goal linked to FES clinical efficacy. Although the basic concept of fatigue-resistance has a long history, recent advances in biomedical engineering, physiotherapy and clinical exercise science have achieved improved clinical benefits, especially for reducing muscle fatigue during FES-exercise. This review evaluated the methodological quality of strategies underlying muscle fatigue-resistance that have been used to optimize FES therapeutic approaches. The review also sought to synthesize the effectiveness of these strategies for persons with SCI in order to establish their functional impacts and clinical relevance.

Methods

Published scientific literature pertaining to the reduction of FES-induced muscle fatigue was identified through searches of the following databases: Science Direct, Medline, IEEE Xplore, SpringerLink, PubMed and Nature, from the earliest returned record until June 2015. Titles and abstracts were screened to obtain 35 studies that met the inclusion criteria for this systematic review.

Results

Following the evaluation of methodological quality (mean (SD), 50 (6) %) of the reviewed studies using the Downs and Black scale, the largest treatment effects reported to reduce muscle fatigue mainly investigated isometric contractions of limited functional and clinical relevance (n = 28). Some investigations (n = 13) lacked randomisation, while others were characterised by small sample sizes with low statistical power. Nevertheless, the clinical significance of emerging trends to improve fatigue-resistance during FES included (i) optimizing electrode positioning, (ii) fine-tuning of stimulation patterns and other FES parameters, (iii) adjustments to the mode and frequency of exercise training, and (iv) biofeedback-assisted FES-exercise to promote selective recruitment of fatigue-resistant motor units.

Conclusion

Although the need for further in-depth clinical trials (especially RCTs) was clearly warranted to establish external validity of outcomes, current evidence was sufficient to support the validity of certain techniques for rapid fatigue-reduction in order to promote FES therapy as an integral part of SCI rehabilitation. It is anticipated that this information will be valuable to clinicians and other allied health professionals administering FES as a treatment option in rehabilitation and aid the development of effective rehabilitation interventions.

Effects of Use and Disuse on Non-paralyzed and Paralyzed Skeletal Muscles

Skeletal muscle is an integral part of the somatic nervous system and plays a primary role in the performance of physical activities. Because physical activity is vital to countering the effects of aging and age related diseases and is a key component in the maintenance of healthy body composition it is important to understand the effects of use and disuse on skeletal muscle. While voluntary muscle activity provides optimal benefits to muscle and the maintenance of healthy body composition, neuromuscular electrical stimulation may be a viable alternative activity for individuals with paralysis. Body composition with a healthy muscle to fat ratio has been associated with healthy blood lipid and glucose profiles that may decrease the risk of cardiovascular and metabolic diseases.

PPARδ preserves a high resistance to fatigue in the mouse medial gastrocnemius after spinal cord transection

INTRODUCTION

Skeletal muscle oxidative capacity decreases and fatigability increases after spinal cord injury. Transcription factor peroxisome proliferator-activated receptor δ (PPARδ) promotes a more oxidative phenotype.

METHODS

We asked whether PPARδ overexpression could ameliorate these deficits in the medial gastrocnemius of spinal cord transected (ST) adult mice.

RESULTS

Time-to-peak tension and half-relaxation times were longer in PPARδ-Con and PPARδ-ST compared with littermate wild-type (WT) controls. Fatigue index was 50% higher in PPARδ-Con than WT-Con and 70% higher in the PPARδ-ST than WT-ST. There was an overall higher percent of darkly stained fibers for succinate dehydrogenase in both PPARδ groups.

CONCLUSIONS

The results indicate a conversion toward slower, more oxidative, and less fatigable muscle properties with overexpression of PPARδ. Importantly, the elevated fatigue resistance was maintained after ST, suggesting that enhanced PPARδ expression, and possibly small molecule agonists, could ameliorate the increased fatigability routinely observed in chronically paralyzed muscles.

Effect of adjusting pulse durations of functional electrical stimulation cycling on energy expenditure and fatigue after spinal cord injury

The purpose of the current study was to determine the effects of three different pulse durations (200, 350, and 500 microseconds [P200, P350, and P500, respectively]) on oxygen uptake (VO2), cycling performance, and energy expenditure (EE) percentage of fatigue of the knee extensor muscle group immediately and 48 to 72 h after cycling in persons with spinal cord injury (SCI). A convenience sample of 10 individuals with motor complete SCI participated in a repeated-measures design using a functional electrical stimulation (FES) cycle ergometer over a 3 wk period. There was no difference among the three FES protocols on relative VO2 or cycling EE. Delta EE between exercise and rest was 42% greater in both P500 and P350 compared with P200 (p = 0.07), whereas recovery VO2 was 23% greater in P350 compared with P200 (p = 0.03). There was no difference in the outcomes of the three pulse durations on muscle fatigue. Knee extensor torque significantly decreased immediately after (p < 0.001) and 48 to 72 h after (p < 0.001) FES leg cycling. Lengthening pulse duration did not affect submaximal or relative VO2 or EE, total EE, and time to fatigue. Greater recovery VO2 and delta EE were noted in P350 and P500 compared with P200. An acute bout of FES leg cycling resulted in torque reduction that did not fully recover 48 to 72 h after cycling.

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.

Monitoring hemodynamic changes in stroke-affected muscles using near-infrared spectroscopy

The oxygenation level of a tissue is an important marker of the health of the tissue and has a direct effect on performance. It has been shown that the blood flow to the paretic muscles of hemiparetic post-stroke patients is significantly reduced compared to non-paretic muscles. It is hypothesized that hemodynamic activity in paretic muscles is suppressed as compared to non-paretic muscles, and that oximetry can be used to measure this disparity in real-time. In order to test this hypothesis, a custom-made oximetry device was used to measure hemodynamic activity in the forearm extensor muscles in post-stroke patients’ paretic and non-paretic sides and in a control population during three exercise levels calibrated to the subject’s maximum effort. The change in oxygenation (ΔOxy) and blood volume (ΔBV) were calculated and displayed in real-time. Results show no apparent difference in either ΔOxy or ΔBV between control subjects’ dominant and non-dominant muscles. However, the results show a significant difference in ΔOxy between paretic and non-paretic muscles, as well as a significant difference between normalized post-stroke and control data. Further work will be necessary to determine if the observed difference between the paretic and non-paretic muscles changes over the course of physical therapy and can be correlated with functional improvements.

Asynchronous recruitment of low-threshold motor units during repetitive, low-current stimulation of the human tibial nerve

Motoneurons receive a barrage of inputs from descending and reflex pathways. Much of our understanding about how these inputs are transformed into motor output in humans has come from recordings of single motor units during voluntary contractions. This approach, however, is limited because the input is ill-defined. Herein, we quantify the discharge of soleus motor units in response to well-defined trains of afferent input delivered at physiologically-relevant frequencies. Constant frequency stimulation of the tibial nerve (10–100 Hz for 30 s), below threshold for eliciting M-waves or H-reflexes with a single pulse, recruited motor units in 7/9 subjects. All 25 motor units recruited during stimulation were also recruited during weak (<10% MVC) voluntary contractions. Higher frequencies recruited more units (n = 3/25 at 10 Hz; n = 25/25 at 100 Hz) at shorter latencies (19.4 ± 9.4 s at 10 Hz; 4.1 ± 4.0 s at 100 Hz) than lower frequencies. When a second unit was recruited, the discharge of the already active unit did not change, suggesting that recruitment was not due to increased synaptic drive. After recruitment, mean discharge rate during stimulation at 20 Hz (7.8 Hz) was lower than during 30 Hz (8.6 Hz) and 40 Hz (8.4 Hz) stimulation. Discharge was largely asynchronous from the stimulus pulses with “time-locked” discharge occurring at an H-reflex latency with only a 24% probability. Motor units continued to discharge after cessation of the stimulation in 89% of trials, although at a lower rate (5.8 Hz) than during the stimulation (7.9 Hz). This work supports the idea that the afferent volley evoked by repetitive stimulation recruits motor units through the integration of synaptic drive and intrinsic properties of motoneurons, resulting in “physiological” recruitment which adheres to Henneman’s size principle and results in relatively low discharge rates and asynchronous firing.

Age at spinal cord injury determines muscle strength

As individuals with spinal cord injury (SCI) age they report noticeable deficits in muscle strength, endurance and functional capacity when performing everyday tasks. These changes begin at ~45 years. Here we present a cross-sectional analysis of paralyzed thenar muscle and motor unit contractile properties in two datasets obtained from different subjects who sustained a cervical SCI at different ages (≤46 years) in relation to data from uninjured age-matched individuals. First, completely paralyzed thenar muscles were weaker when C6 SCI occurred at an older age. Muscles were also significantly weaker if the injury was closer to the thenar motor pools (C6 vs. C4). More muscles were strong (>50% uninjured) in those injured at a younger (≤25 years) vs. young age (>25 years), irrespective of SCI level. There was a reduction in motor unit numbers in all muscles tested. In each C6 SCI, only ~30 units survived vs. 144 units in uninjured subjects. Since intact axons only sprout 4-6 fold, the limits for muscle reinnervation have largely been met in these young individuals. Thus, any further reduction in motor unit numbers with time after these injuries will likely result in chronic denervation, and may explain the late-onset muscle weakness routinely described by people with SCI. In a second dataset, paralyzed thenar motor units were more fatigable than uninjured units. This gap widened with age and will reduce functional reserve. Force declines were not due to electromyographic decrements in either group so the site of failure was beyond excitation of the muscle membrane. Together, these results suggest that age at SCI is an important determinant of long-term muscle strength, and fatigability, both of which influence functional capacity.

Altered content of AMP-activated protein kinase isoforms in skeletal muscle from spinal cord injured subjects

AMP-activated protein kinase (AMPK) is a pivotal regulator of energy homeostasis. Although downstream targets of AMPK are widely characterized, the physiological factors governing isoform expression of this protein kinase are largely unknown. Nerve/contractile activity has a major impact on the metabolic phenotype of skeletal muscle, therefore likely to influence AMPK isoform expression. Spinal cord injury represents an extreme form of physical inactivity, with concomitant changes in skeletal muscle metabolism. We assessed the influence of longstanding and recent spinal cord injury on protein abundance of AMPK isoforms in human skeletal muscle. We also determined muscle fiber type as a marker of glycolytic or oxidative metabolism. In subjects with longstanding complete injury, protein abundance of the AMPKγ3 subunit, as well as myosin heavy chain (MHC) IIa and IIx, were increased, whereas abundance of the AMPKγ1 subunit and MHC I were decreased. Similarly, abundance of AMPKγ3 and MHC IIa proteins were increased, whereas AMPKα2, -β1, and -γ1 subunits and MHC I abundance was decreased during the first year following injury, reflecting a more glycolytic phenotype of the skeletal muscle. However, in incomplete cervical lesions, partial recovery of muscle function attenuated the changes in the isoform profile of AMPK and MHC. Furthermore, exercise training (electrically stimulated leg cycling) partly normalized mRNA expression of AMPK isoforms. Thus, physical activity affects the relative expression of AMPK isoforms. In conclusion, skeletal muscle abundance of AMPK isoforms is related to physical activity and/or muscle fiber type. Thus, physical/neuromuscular activity is an important determinant of isoform abundance of AMPK and MCH. This further underscores the need for physical activity as part of a treatment regimen after spinal cord injury to maintain skeletal muscle metabolism.

The relationship between exercise-induced muscle fatigue, arterial blood flow and muscle perfusion after 56 days local muscle unloading

In the light of the dynamic nature of habitual plantar flexor activity, we utilized an incremental isokinetic exercise test (IIET) to assess the work-related power deficit (WoRPD) as a measure for exercise-induced muscle fatigue before and after prolonged calf muscle unloading and in relation to arterial blood flow and muscle perfusion. Eleven male subjects (31 ± 6 years) wore the HEPHAISTOS unloading orthosis unilaterally for 56 days. It allows habitual ambulation while greatly reducing plantar flexor activity and torque production. Endpoint measurements encompassed arterial blood flow, measured in the femoral artery using Doppler ultrasound, oxygenation of the soleus muscle assessed by near-infrared spectroscopy, lactate concentrations determined in capillary blood and muscle activity using soleus muscle surface electromyography. Furthermore, soleus muscle biopsies were taken to investigate morphological muscle changes. After the intervention, maximal isokinetic torque was reduced by 23·4 ± 8·2% (P<0·001) and soleus fibre size was reduced by 8·5 ± 13% (P = 0·016). However, WoRPD remained unaffected as indicated by an unchanged loss of relative plantar flexor power between pre- and postexperiments (P = 0·88). Blood flow, tissue oxygenation, lactate concentrations and EMG median frequency kinematics during the exercise test were comparable before and after the intervention, whereas the increase of RMS in response to IIET was less following the intervention (P = 0·03). In conclusion, following submaximal isokinetic muscle work exercise-induced muscle fatigue is unaffected after prolonged local muscle unloading. The observation that arterial blood flow was maintained may underlie the unchanged fatigability.

Potential role of oxidative stress on the prescription of rehabilitation interventions in spinal cord injury

STUDY DESIGN

Review article.

OBJECTIVES

To provide an overview of free radical biology, particularly with respect to muscle physiology, as well as the potential effects of muscle morphological changes, physical capacity and nutritional status on oxidative stress in people with chronic spinal cord injury (SCI). The potential implications of these factors for determining the optimal dosage of rehabilitation training interventions in people with chronic SCI will also be presented.

SETTING

Vancouver, BC, Canada.

METHODS

Literature review.

RESULTS

Not applicable.

CONCLUSION

There has been a great deal of focus on rehabilitation exercise interventions providing intensive practice of movements to enhance functional recovery and physical capacity following SCI. However, there is still much to be understood about the appropriate dosage of training parameters (e.g. frequency, duration). It has been known for several decades that exercise increases free radical production, leading to oxidative stress. To date, there has been little consideration of the potential interaction of oxidative stress with training parameters on functional outcomes in chronic SCI. Furthermore, individuals with chronic SCI face many secondary consequences of their injury, such as muscle atrophy, change in muscle fiber type, general deconditioning and nutritional status, that are known to influence free radical production and antioxidant capacity. Better understanding of the potential confounding effects of oxidative stress associated with exercise will improve our ability to determine the optimal 'dose' of rehabilitation training to maximize functional recovery following SCI.

Cellular mechanisms of tissue fibrosis. 4. Structural and functional consequences of skeletal muscle fibrosis

Skeletal muscle fibrosis can be a devastating clinical problem that arises from many causes, including primary skeletal muscle tissue diseases, as seen in the muscular dystrophies, or it can be secondary to events that include trauma to muscle or brain injury. The cellular source of activated fibroblasts (myofibroblasts) may include resident fibroblasts, adult muscle stem cells, or inflammatory or perivascular cells, depending on the model studied. Even though it is likely that there is no single source for all myofibroblasts, a common mechanism for the production of fibrosis is via the transforming growth factor-β/phosphorylated Smad3 pathway. This pathway and its downstream targets thus provide loci for antifibrotic therapies, as do methods for blocking the transdifferentiation of progenitors into activated fibroblasts. A structural model for the extracellular collagen network of skeletal muscle is needed so that measurements of collagen content, morphology, and gene expression can be related to mechanical properties. Approaches used to study fibrosis in tissues, such as lung, kidney, and liver, need to be applied to studies of skeletal muscle to identify ways to prevent or even cure the devastating maladies of skeletal muscle.

Reduced satellite cell numbers with spinal cord injury and aging in humans

INTRODUCTION

Both sarcopenia and spinal cord injury (SCI) are characterized by the loss of skeletal muscle mass and function. Despite obvious similarities in atrophy between both models, differences in muscle fiber size and satellite cell content may exist on a muscle fiber type-specific level.

METHODS

In the present study, we compared skeletal muscle fiber characteristics between wheelchair-dependent young males with SCI (n = 8, 32 ± 4 yr), healthy elderly males (n = 8, 75 ± 2 yr), and young controls (n = 8, 31 ± 3 yr). Muscle biopsies were collected to determine skeletal muscle fiber type composition, fiber size, and satellite cell content.

RESULTS

Severe atrophy and a shift toward approximately 90% Type II muscle fibers were observed in muscle obtained from males with SCI. Muscle fiber size was substantially smaller in both the SCI (Types I and II fibers) and elderly subjects (Type II fibers) when compared with the controls. Satellite cell content was substantially lower in the wheelchair-dependent SCI subjects in both the Types I and II muscle fibers (0.049 ± 0.019 and 0.050 ± 0.005 satellite cells per fiber, respectively) when compared with the young controls (0.104 ± 0.011 and 0.117 ± 0.009 satellite cells per fiber, respectively). In the elderly, the number of satellite cells was lower in the Type II muscle fibers only (0.042 ± 0.005 vs 0.117 ± 0.009 satellite cells per fiber in the elderly vs young controls, respectively).

CONCLUSION

This is the first study to show that muscle fiber atrophy as observed with SCI (Types I and II fibers) and aging (Type II fibers) is accompanied by a muscle fiber type-specific reduction in satellite cell content in humans.

Expression of genes involved in fatty acid transport and insulin signaling is altered by physical inactivity and exercise training in human skeletal muscle

Physical deconditioning is associated with the development of chronic diseases, including type 2 diabetes and cardiovascular disease. Exercise training effectively counteracts these developments, but the underlying mechanisms are largely unknown. To gain more insight into these mechanisms, muscular gene expression levels were assessed after physical deconditioning and after exercise training of the lower limbs in humans by use of gene expression microarrays. To exclude systemic effects, we used human models for local physical inactivity (3 wk of unilateral limb suspension) and for local exercise training (6 wk of functional electrical stimulation exercise of the extremely deconditioned legs of individuals with a spinal cord injury). The most interesting subset of genes, those downregulated after deconditioning as well as upregulated after exercise training, contained 18 genes related to both the "insulin action" and "adipocytokine signaling" pathway. Of these genes, the three with strongest up/downregulation were the muscular fatty acid-binding protein-3 (FABP3), the fatty acid oxidizing enzyme hydroxyacyl-CoA dehydrogenase (HADH), and the mitochondrial fatty acid transporter solute carrier 25 family member A20 (SLC25A20). The expression levels of these genes were confirmed using RT-qPCR. The results of the present study indicate an important role for a decreased transport and metabolism of fatty acids, which provides a link between physical activity levels and insulin signaling.

Role of alpha-actinin-3 in contractile properties of human single muscle fibers: a case series study in paraplegics

A common nonsense polymorphism in the ACTN3 gene results in the absence of α-actinin-3 in XX individuals. The wild type allele has been associated with power athlete status and an increased force output in numeral studies, though the mechanisms by which these effects occur are unclear. Recent findings in the Actn3(-/-) (KO) mouse suggest a shift towards 'slow' metabolic and contractile characteristics of fast muscle fibers lacking α-actinin-3. Skinned single fibers from the quadriceps muscle of three men with spinal cord injury (SCI) were tested regarding peak force, unloaded shortening velocity, force-velocity relationship, passive tension and calcium sensitivity. The SCI condition induces an 'equal environment condition' what makes these subjects ideal to study the role of α-actinin-3 on fiber type expression and single muscle fiber contractile properties. Genotyping for ACTN3 revealed that the three subjects were XX, RX and RR carriers, respectively. The XX carrier's biopsy was the only one that presented type I fibers with a complete lack of type II(x) fibers. Properties of hybrid type II(a)/II(x) fibers were compared between the three subjects. Absence of α-actinin-3 resulted in less stiff type II(a)/II(x) fibers. The heterozygote (RX) exhibited the highest fiber diameter (0.121±0.005 mm) and CSA (0.012±0.001 mm(2)) and, as a consequence, the highest peak force (2.11±0.14 mN). Normalized peak force was similar in all three subjects (P = 0.75). Unloaded shortening velocity was highest in R-allele carriers (P<0.001). No difference was found in calcium sensitivity. The preservation of type I fibers and the absence of type II(x) fibers in the XX individual indicate a restricted transformation of the muscle fiber composition to type II fibers in response to long-term muscle disuse. Lack of α-actinin-3 may decrease unloaded shortening velocity and increase fiber elasticity.

Depression and recovery of reflex amplitude during electrical stimulation after spinal cord injury

OBJECTIVE

The aim of this study was to quantify, for the first time, H-reflexes evoked during prolonged trains of wide-pulse neuromuscular electrical stimulation (WP-NMES) in individuals with chronic spinal cord injury (SCI). We hypothesised that after the first H-reflex, reflex amplitudes would be depressed (due to post-activation depression), but would recover and this recovery would be enhanced after a "burst" of 100 Hz WP-NMES.

METHODS

Soleus M-waves and H-reflexes evoked during WP-NMES (1 ms pulse width) of the tibial nerve were quantified in nine individuals with SCI. WP-NMES was delivered in two patterns: "constant-frequency" (15 or 20 Hz for 12 s) and "burst-like" (15-100-15 Hz or 20-100-20 Hz; 4 s each phase) at an intensity that evoked an M-wave between 10% and 15% of the maximal M-wave (M(max)).

RESULTS

During constant frequency stimulation, after the initial depression from the first to the second H-reflex (1st: 57% M(max); 2nd: 25% M(max)), H-reflexes did not recover significantly and were 37% M(max) at the end of the stimulus train. During the burst-like pattern, after the initial depression (1st: 62% M(max); 2nd: 30%), reflexes recovered completely by the end of the stimulation (to 55% M(max)) as they were not significantly different from the first H-reflex. M-waves were initially depressed (1st: 12% M(max); 2nd: 7% M(max)) then did not change throughout the stimulation and were not significantly different between stimulation patterns. An analysis of covariance indicated that the depression in M-wave amplitude did not account for the depression in H-reflex amplitude.

CONCLUSIONS

Relatively large H-reflexes were recorded during both patterns of NMES. The brief burst of 100 Hz stimulation restored H-reflexes to their initial amplitudes, effectively reversing the effects of post-activation depression.

SIGNIFICANCE

For individuals with chronic SCI, generating contractions through central pathways may help reduce muscle atrophy and produce contractions that are more fatigue-resistant for rehabilitation, exercise programs, or to perform activities of daily living.

Motor unit recruitment when neuromuscular electrical stimulation is applied over a nerve trunk compared with a muscle belly: quadriceps femoris

Neuromuscular electrical stimulation (NMES) can be delivered over a nerve trunk or muscle belly and both can generate contractions through peripheral and central pathways. Generating contractions through peripheral pathways is associated with a nonphysiological motor unit recruitment order, which may limit the efficacy of NMES rehabilitation. Presently, we compared recruitment through peripheral and central pathways for contractions of the knee extensors evoked by NMES applied over the femoral nerve vs. the quadriceps muscle. NMES was delivered to evoke 10 and 20% of maximum voluntary isometric contraction torque 2–3 s into the NMES (time1) in two patterns: 1) constant frequency (15 Hz for 8 s); and 2) step frequency (15–100-15 Hz and 25–100-25 Hz for 3–2-3 s, respectively). Torque and electromyographic activity recorded from vastus lateralis and medialis were quantified at the beginning (time1) and end (time2; 6–7 s into the NMES) of each pattern. M-waves (peripheral pathway), H-reflexes, and asynchronous activity (central pathways) during NMES were quantified. Torque did not differ regardless of NMES location, pattern, or time. For both muscles, M-waves were ∼7–10 times smaller and H-reflexes ∼8–9 times larger during NMES over the nerve compared with over the muscle. However, unlike muscles studied previously, neither torque nor activity through central pathways were augmented following 100 Hz NMES, nor was any asynchronous activity evoked during NMES at either location. The coefficient of variation was also quantified at time2to determine the consistency of each dependent measure between three consecutive contractions. Torque, M-waves, and H-reflexes were most variable during NMES over the nerve. In summary, NMES over the nerve produced contractions with the greatest recruitment through central pathways; however, considering some of the limitations of NMES over the femoral nerve, it may be considered a good complement to, as opposed to a replacement for, NMES over the quadriceps muscle for maintaining muscle quality and reducing contraction fatigue during NMES rehabilitation.

A biomechanical analysis of exercise in standing, supine, and seated positions: Implications for individuals with spinal cord injury

CONTEXT/OBJECTIVE

The distal femur is the primary fracture site in patients with osteoporosis after spinal cord injury (SCI).

OBJECTIVE

To mathematically compare the compression and shear forces at the distal femur during quadriceps stimulation in the standing, supine, and seated positions. A force analysis across these positions may be a consideration for people with SCI during neuromuscular electrical stimulation of the quadriceps.

DESIGN

A biomechanical model.

SETTING

Research laboratory.

OUTCOME MEASURES

Compression and shear forces from the standing, supine, and seated biomechanical models at the distal femur during constant loads generated by the quadriceps muscles.

RESULTS

The standing model estimated the highest compressive force at 240% body weight and the lowest shear force of 24% body weight at the distal femur compared with the supine and seated models. The supine model yielded a compressive force of 191% body weight with a shear force of 62% body weight at the distal femur. The seated model yielded the lowest compressive force of 139% body weight and the highest shear force of 215% body weight.

CONCLUSIONS

When inducing a range of forces in the quadriceps muscles, the seated position yields the highest shear forces and lowest compressive forces when compared with the supine and standing positions. Standing with isometric contractions generates the highest compressive loads and lowest shear forces. Early active resistive standing may provide the most effective means to prevent bone loss after SCI.

Respiratory muscle plasticity

Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it. As such, respiratory muscle is in a constant state of remodeling, and the basis of muscle's plasticity is its ability to change protein expression and resultant protein balance in response to varying environmental conditions. Here, we will describe the changes of respiratory muscle imposed by extrinsic changes in mechanical load, activity, and innervation. Although there is a large body of literature on the structural and functional plasticity of respiratory muscles, we are only beginning to understand the molecular-scale protein changes that contribute to protein balance. We will give an overview of key mechanisms regulating protein synthesis and protein degradation, as well as the complex interactions between them. We suggest future application of a systems biology approach that would develop a mathematical model of protein balance and greatly improve treatments in a variety of clinical settings related to maintaining both muscle mass and optimal contractile function of respiratory muscles.

Myofibrillar distribution of succinate dehydrogenase activity and lipid stores differs in skeletal muscle tissue of paraplegic subjects

Lack of physical activity has been related to an increased risk of developing insulin resistance. This study aimed to assess the impact of chronic muscle deconditioning on whole body insulin sensitivity, muscle oxidative capacity, and intramyocellular lipid (IMCL) content in subjects with paraplegia. Nine subjects with paraplegia and nine able-bodied, lean controls were recruited. An oral glucose tolerance test was performed to assess whole body insulin sensitivity. IMCL content was determined both in vivo and in vitro using (1)H-magnetic resonance spectroscopy and fluorescence microscopy, respectively. Muscle biopsy samples were stained for succinate dehydrogenase (SDH) activity to measure muscle fiber oxidative capacity. Subcellular distributions of IMCL and SDH activity were determined by defining subsarcolemmal and intermyofibrillar areas on histological samples. SDH activity was 57 ± 14% lower in muscle fibers derived from subjects with paraplegia when compared with controls (P < 0.05), but IMCL content and whole body insulin sensitivity did not differ between groups. In muscle fibers taken from controls, both SDH activity and IMCL content were higher in the subsarcolemmal region than in the intermyofibrillar area. This typical subcellular SDH and IMCL distribution pattern was lost in muscle fibers collected from subjects with paraplegia and had changed toward a more uniform distribution. In conclusion, the lower metabolic demand in deconditioned muscle of subjects with paraplegia results in a significant decline in muscle fiber oxidative capacity and is accompanied by changes in the subcellular distribution patterns of SDH activity and IMCL. However, loss of muscle activity due to paraplegia is not associated with substantial lipid accumulation in skeletal muscle tissue.

Clinical neurophysiology in the prognosis and monitoring of traumatic spinal cord injury

Preclinical studies for the repair of spinal cord injury (SCI) and potential therapies for accessing the inherent plasticity of the central nervous system (CNS) to promote recovery of function are currently moving into the translational stage. These emerging clinical trials of therapeutic interventions for the repair of SCI require improved assessment techniques and quantitative outcome measures to supplement the American Spinal Injuries Association (ASIA) Impairment Scales. This chapter attempts to identify those electrophysiological techniques that show the most promise for provision of objective and quantitative measures of sensory, motor, and autonomic function in SCI. Reviewed are: (1) somatosensory evoked potentials, including dermatomal somatosensory evoked potentials, and the electrical perceptual threshold as tests of the dorsal (posterior) column pathway; (2) laser evoked potentials and contact heat evoked potentials as tests of the anterior spinothalamic tract; (3) motor evoked potentials in limb muscles, in response to transcranial magnetic stimulation of the motor cortex as tests of the corticospinal tract, and the application of the technique to assessment of trunk and sphincter muscles; and (4) the sympathetic skin response as a test of spinal cord access to the sympathetic chain.

Integrating rehabilitation engineering technology with biologics

Rehabilitation engineers apply engineering principles to improve function or to solve challenges faced by persons with disabilities. It is critical to integrate the knowledge of biologics into the process of rehabilitation engineering to advance the field and maximize potential benefits to patients. Some applications in particular demonstrate the value of a symbiotic relationship between biologics and rehabilitation engineering. In this review we illustrate how researchers working with neural interfaces and integrated prosthetics, assistive technology, and biologics data collection are currently integrating these 2 fields. We also discuss the potential for further integration of biologics and rehabilitation engineering to deliver the best technologies and treatments to patients. Engineers and clinicians must work together to develop technologies that meet clinical needs and are accessible to the intended patient population.

Differential expression of metabolic genes essential for glucose and lipid metabolism in skeletal muscle from spinal cord injured subjects

Skeletal muscle plays an important role in the regulation of energy homeostasis; therefore, the ability of skeletal muscle to adapt and alter metabolic gene expression in response to changes in physiological demands is critical for energy balance. Individuals with cervical spinal cord lesions are characterized by tetraplegia, impaired thermoregulation, and altered skeletal muscle morphology. We characterized skeletal muscle metabolic gene expression patterns, as well as protein content, in these individuals to assess the impact of spinal cord injury on critical determinants of skeletal muscle metabolism. Our results demonstrate that mRNA levels and protein expression of skeletal muscle genes essential for glucose storage are reduced, whereas expression of glycolytic genes is reciprocally increased in individuals with spinal cord injury. Furthermore, expression of genes essential for lipid oxidation is coordinately reduced in spinal cord injured subjects, consistent with a marked reduction of mitochondrial proteins. Thus spinal cord injury resulted in a profound and tightly coordinated change in skeletal muscle metabolic gene expression program that is associated with the aberrant metabolic features of the tissue.

Predicting fatigue during electrically stimulated non-isometric contractions

Mathematical prediction of power loss during electrically stimulated contractions is of value to those trying to minimize fatigue and to those trying to decipher the relative contributions of force and velocity. Our objectives were to: (1) develop a model of non-isometric fatigue for electrical stimulation-induced, open-chain, repeated extensions of the leg at the knee; and (2) experimentally validate the model. A computer-controlled stimulator sent electrical pulses to surface electrodes on the thighs of 17 able-bodied subjects. Isometric and non-isometric non-fatiguing and fatiguing leg extension torque and/or angle at the knee were measured. Two existing mathematical models, one of non-isometric force and the other of isometric fatigue, were combined to develop the non-isometric force-fatigue model. Angular velocity and 3 new parameters were added to the isometric fatigue model. The new parameters are functions of parameters within the force model, and therefore additional measurements from the subject are not needed. More than 60% of the variability in the measurements was explained by the new force-fatigue model. This model can help scientists investigate the etiology of non-isometric fatigue and help engineers to improve the task performance of functional electrical stimulation systems.

Adaptive change in electrically stimulated muscle: a framework for the design of clinical protocols

Adult mammalian skeletal muscles have a remarkable capacity for adapting to increased use. Although this behavior is familiar from the changes brought about by endurance exercise, it is seen to a much greater extent in the response to long-term neuromuscular stimulation. The associated phenomena include a markedly increased resistance to fatigue, and this is the key to several clinical applications. However, a more rational basis is needed for designing regimes of stimulation that are conducive to an optimal outcome. In this review I examine relevant factors, such as the amount, frequency, and duty cycle of stimulation, the influence of force generation, and the animal model. From these considerations a framework emerges for the design of protocols that yield an overall functional profile appropriate to the application. Three contrasting examples illustrate the issues that need to be addressed clinically.

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.

The length-tension relationship of human dorsiflexor and plantarflexor muscles after spinal cord injury

STUDY DESIGN

A cross-sectional design.

OBJECTIVES

To examine the length-tension relationship of dorsiflexion (DF) and plantarflexion (PF) muscle groups in seven individuals with chronic spinal cord injury (SCI; C2-T7; age 43+/-10.1 years) and compare it with a group of age and sex-matched able-bodied (AB) controls.

SETTING

McMaster University, Hamilton, ON, Canada.

METHODS

Isometric single twitch properties, 0.5-s tetanic contractions (SCI) and maximal voluntary contractions (AB) were measured at nine joint angles from 20 degrees DF to 20 degrees PF.

RESULTS

In the DF muscles, peak-evoked twitch (PT) torque occurred at 20 degrees PF for SCI (3.4+/-1.1 N m) and AB (3.8+/-1.4 N m) with no difference in peak torque between groups, whereas peak summated force occurred at 10 degrees PF in AB and 20 degrees PF in SCI (P<0.01). In the PF muscles, PT torque occurred at 15 degrees DF in AB (18.6+/-2.6 N m) and at 5 degrees DF (6.8+/-3.3 N m; P<0.01) in SCI, and peak-summated force occurred at 15 degrees DF in AB. The SCI group did not show any change in PF peak-summated force with varying joint angles. Rates of contraction and relaxation were not different between the two groups.

CONCLUSIONS

The results suggest a significant change in the length-tension relationship of the PF muscles after SCI, but no change in the DF muscle group. Rehabilitation programs should focus on maintaining PF muscle length in order to optimize muscle strength and function after SCI.

Hemiparetic stroke alters vastus lateralis myosin heavy chain profiles between the paretic and nonparetic muscles

Skeletal muscle phenotype alterations following hemiparetic stroke contribute to disabilities associated with stroke. The phenotypic response following stroke is undefined. This investigation examined the myosin heavy chain (MHC) composition of the vastus lateralis (VL) of stroke survivors in paretic (P) and nonparetic (NP) muscle. Protein obtained from VL of 10 stroke survivors was isolated and purified, and MHC gel electrophoresis was performed. The MHC bands were quantified, and a paired sample two-tailed T test with significance set at p < or = 0.05 was performed. MHC I expression was significantly less in P versus NP VL (.93 vs. 1.00 arbitrary units [AU]). Significantly more IIx MHC was found in the P versus NP VL (1.33 vs. 1.0). No significant differences in type IIa MHC (1.07 P vs. 1.00 NP) were found. These changes in MHC composition suggest an alteration in muscle function due to stroke or the altered activity patterns of muscle following stroke.

Effect of a peripheral nerve block on torque produced by repetitive electrical stimulation

Neuromuscular electrical stimulation (NMES) generates contractions by activation of motor axons (peripheral mechanism), but the afferent volley also contributes by recruiting spinal motoneurons synaptically (central mechanism), which recruits motoneurons according to Henneman's size principle. Thus, we hypothesized that contractions that develop due to a combination of peripheral and central mechanisms will fatigue less rapidly than when electrically evoked contractions are generated by the activation of motor axons alone. Plantar-flexion torque evoked by NMES over the triceps surae was compared in five able-bodied subjects before (Intact) and during (Blocked) a complete anesthetic block of the tibial and common peroneal nerves. In the Blocked condition, plantar-flexion torque could only develop from the direct activation of motor axons beneath the stimulating electrodes. NMES was delivered using three protocols: protocol A, constant 100 Hz for 30 s; protocol B, four 2-s bursts of 100 Hz alternating with 20-Hz stimulation; and protocol C, alternating 100 Hz bursts (1 s on, 1 s off) for 30 s. The percent change in evoked plantar flexion torque from the beginning to the end of the stimulation differed ( P < 0.05) between Intact and Blocked conditions for all protocols (Intact: protocol A = +125%, B = +230%, C = +78%; Blocked: protocol A = −79%, B = −15%, C = −35%). These results corroborate previous evidence that NMES can evoke contractions via the recruitment of spinal motoneurons in addition to the direct recruitment of motor axons. We now show that NMES delivered for periods of up to 30 s generates plantar-flexion torque which decreases when only motor axons are recruited and increases when the central nervous system can contribute.

Assessment of gluteus maximus muscle area with different image analysis programs

OBJECTIVE

To determine the effectiveness of a percutaneous gluteal stimulation system (GSTIM) by comparing assessments of axial computed tomography (CT) scans for the pelvic area.

DESIGN

Comparing the measurements of the cross-sectional area (CSA) of the gluteus maximus muscle between raters and 2 image analysis programs.

SETTING

Retrospective axial CT scans of the pelvic area.

PARTICIPANTS

Men (N=9) with complete (below T6) spinal cord injury (SCI) and at least 2 years postinjury participated in the study (range, 29-75y; mean age, 51.8y).

INTERVENTION

Comparing gluteus maximus CSA before and after a period of GSTIM.

MAIN OUTCOME MEASURE

Measurements made by 2 expert and 2 nonexpert raters were used to compare the repeatability and reliability of measuring muscle CSA. The longitudinal study presented is from repeated CT scans obtained over a 2-year period for 1 representative participant who received a GSTIM system.

RESULTS

For repeatability, nonexpert raters measured a mean CSA of 35.2 cm2 (range, 20-45 cm2), while experts measured 21 cm2 (range, 10-35 cm2). A composite of all raters using the same program had SDs of 2.5 to 2.6 cm2 for a program available through the National Institutes of Health and 2.5 to 4.4 cm2 for a commercially available program. For reliability, differences between the 2 programs had mean differences in SD between 2.2 and 3.7 cm2.

CONCLUSIONS

The same rater and program (preferably the more reliable ImageJ) is recommended for the course of a longitudinal study. Otherwise, significant error would be introduced. Furthermore, significant increases in the CSA of gluteal muscle compared with preintervention (baseline) measurements were observed for the participant receiving GSTIM.

Contractile properties of the denervated external anal sphincter

OBJECTIVE

The purpose of this study was to evaluate the effect of denervation on contractile properties of the external anal sphincter (EAS) of the female rat.

STUDY DESIGN

Sham operation, pudendal nerve transection, pelvic neurectomy, or combined pudendal nerve transection/pelvic neurectomy was performed in young female rats. Contractile function of the EAS was determined after 2 weeks.

RESULTS

Maximal force-generating capacity of the EAS was not impaired by bilateral pudendal denervation or pelvic neurectomy. Twitch tension, however, was decreased, and fatigability increased after pelvic neurectomy. Combined bilateral pudendal nerve-transection plus pelvic neurectomy resulted in compromised force-generating capacity, decreased twitch tension, and increased fatigability of the anal sphincter.

CONCLUSION

Subtle changes in EAS function are detectable after pelvic neurectomy, but not pudendal denervation. In contrast, combination pudendal and pelvic neurectomy resulted in severe compromise of EAS function. These data suggest that EAS function is relatively preserved unless injury occurs to > 1 source of innervation.