Effect of exercise on the motor unit

Exercise interventions can improve muscle strength, endurance, and electrical activity of lumbar extensors in individuals with non-specific low back pain: a systematic review with meta-analysis

Exercise interventions have been recommended for people with non-specific low back pain. The literature is scarce regarding the effects of exercise on muscle strength, endurance, and electrical activity of lumbar extensor muscles. Electronic searches were carried out from May 2020 until August 2020 in the following databases: PUBMED, CENTRAL, EMBASE, PEDro, SPORTDiscus, Scielo, and LILACS. Only randomized controlled trials with passive and active control groups were included. The methodological quality of the included studies was performed using the Physiotherapy Evidence Database Scale. Eight studies, involving 508 participants, were included in metanalytical procedures. Exercise interventions demonstrated superior effects on muscle activity (Electromyography) when compared with active controls (p < 0.0001). Exercise interventions demonstrated superior effects on muscle endurance (Sorensen Test) when compared with passive (p = 0.0340) and active controls (p = 0.0276). Exercise interventions demonstrated superior effects on muscle strength (Machine) when compared with passive controls (p = 0.0092). Exercise interventions can improve muscle strength, endurance, and electrical activity in people with non-specific low back pain.

A randomised clinical trial comparing 35 Hz versus 50 Hz frequency stimulation effects on hand motor recovery in older adults after stroke

More solid data are needed regarding the application of neuromuscular electrical stimulation (NMES) in the paretic hand following a stroke. A randomised clinical trial was conducted to compare the effects of two NMES protocols with different stimulation frequencies on upper limb motor impairment and function in older adults with spastic hemiparesis after stroke. Sixty nine outpatients were randomly assigned to the control group or the experimental groups (NMES with 50 Hz or 35 Hz). Outcome measures included motor impairment tests and functional assessment. They were collected at baseline, after 4 and 8 weeks of treatment, and after a follow-up period. NMES groups showed significant changes (p < 0.05) with different effect sizes in range of motion, grip and pinch strength, the Modified Ashworth Scale, and the muscle electrical activity in the extensors of the wrist. The 35 Hz NMES intervention showed a significant effect on Barthel Index. Additionally, there were no significant differences between the groups in the Box and Block Test. Both NMES protocols proved evidence of improvements in measurements related to hand motor recovery in older adults following a stroke, nevertheless, these findings showed that the specific stimulation frequency had different effects depending on the clinical measures under study.

Muscle fibre activation and fatigue with low-load blood flow restricted resistance exercise-An integrative physiology review

Blood flow-restricted resistance exercise (BFRRE) has been shown to induce increases in muscle size and strength, and continues to generate interest from both clinical and basic research points of view. The low loads employed, typically 20%-50% of the one repetition maximum, make BFRRE an attractive training modality for individuals who may not tolerate high musculoskeletal forces (eg, selected clinical patient groups such as frail old adults and patients recovering from sports injury) and/or for highly trained athletes who have reached a plateau in muscle mass and strength. It has been proposed that achieving a high degree of muscle fibre recruitment is important for inducing muscle hypertrophy with BFRRE, and the available evidence suggest that fatiguing low-load exercise during ischemic conditions can recruit both slow (type I) and fast (type II) muscle fibres. Nevertheless, closer scrutiny reveals that type II fibre activation in BFRRE has to date largely been inferred using indirect methods such as electromyography and magnetic resonance spectroscopy, while only rarely addressed using more direct methods such as measurements of glycogen stores and phosphocreatine levels in muscle fibres. Hence, considerable uncertainity exists about the specific pattern of muscle fibre activation during BFRRE. Therefore, the purpose of this narrative review was (1) to summarize the evidence on muscle fibre recruitment during BFRRE as revealed by various methods employed for determining muscle fibre usage during exercise, and (2) to discuss reported findings in light of the specific advantages and limitations associated with these methods.

Inactivity and exercise training differentially regulate abundance of Na+-K+-ATPase in human skeletal muscle

Physical inactivity is a global health risk that can be addressed through application of exercise training suitable for an individual's health and age. People's willingness to participate in physical activity is often limited by an initially poor physical capability and early onset of fatigue. One factor associated with muscle fatigue during intense contractions is an inexcitability of skeletal muscle cells, reflecting impaired transmembrane Na⁺/K⁺ exchange and membrane depolarization, which are regulated via the transmembranous protein Na⁺-K⁺-ATPase (NKA). This short review focuses on the plasticity of NKA in skeletal muscle in humans after periods of altered usage, exploring NKA upregulation with exercise training and downregulation with physical inactivity. In human skeletal muscle, the NKA content quantified by [³H]ouabain binding site content shows robust, yet tightly constrained, upregulation of 8-22% with physical training, across a broad range of exercise training types. Muscle NKA content in humans undergoes extensive downregulation with injury that involves substantial muscular inactivity. Surprisingly, however, no reduction in NKA content was found in the single study that investigated short-term disuse. Despite clear findings that exercise training and injury modulate NKA content, the adaptability of the individual NKA isoforms in muscle (α_1-3 and β_1-3) and of the accessory and regulatory protein FXYD1 are surprisingly inconsistent across studies, for exercise training as well as for injury/disuse. Potential reasons for this are explored. Finally, we provide suggestions for future studies to provide greater understanding of NKA regulation during exercise training and inactivity in humans.

Exploring the Potential Effectiveness of Combining Optimal Nutrition With Electrical Stimulation to Maintain Muscle Health in Critical Illness: A Narrative Review

Muscle wasting occurs rapidly within days of an admission to the intensive care unit (ICU). Concomitant muscle weakness and impaired physical functioning can ensue, with lasting effects well after hospital discharge. Early physical rehabilitation is a promising intervention to minimize muscle weakness and physical dysfunction. However, there is an often a delay in commencing active functional exercises (such as sitting on the edge of bed, standing and mobilizing) due to sedation, patient alertness, and impaired ability to cooperate in the initial days of ICU admission. Therefore, there is high interest in being able to intervene early through nonvolitional exercise strategies such as electrical muscle stimulation (EMS). Muscle health characterized as the composite of muscle quantity, as well as functional and metabolic integrity, may be potentially maintained when optimal nutrition therapy is provided in complement with early physical rehabilitation in critically ill patients; however, the type, dosage, and timing of these interventions are unclear. This article explores the potential role of nutrition and EMS in maintaining muscle health in critical illness. Within this article, we will evaluate fundamental concepts of muscle wasting and evaluate the effects of EMS, as well as the effects of nutrition therapy on muscle health and the clinical and functional outcomes in critically ill patients. We will also highlight current research gaps in order to advance the field forward in this important area.

The use of electromyography and magnetic resonance imaging to evaluate a core strengthening exercise programme

BACKGROUND

The question that was asked in the study was whether a training routine based on curl-up exercises with a load provided by body mass of the person increases local muscle strength or local muscle endurance.

OBJECTIVE

The aim of this study was to evaluate the effect of 4 weeks training based on a small load and low movement velocity on electrical activity (EMG), cross-sectional area (CSA) of core stabilisers.

METHODS

The EMG activity was measured in the rectus abdominis (RA), obliquus abdominis externus and erector spinae (ES) muscles. CSA of the muscles: RA, anterolateral abdominal, psoas major, quadratus lumborum, ES, and multifidus at the level of L3-L4 were measured too.

RESULTS

The training increased the CSA and thickness in most of the muscles studied. Statistically significant correlation was found only for the ES circumference (left side) and EMG activity for the right side (r= 0.627, p= 0.022) and left side (r= 0.624, p= 0.023).

CONCLUSIONS

The training programme resulted in a increase in the number of curl-up repetitions revealing an endurance increase in abdominal muscles. Furthermore, there was a increase in the EMG activity of the RA. An increase of the CSA of all tested muscles showed an increase of muscle active force.

Sustained maximal voluntary contraction produces independent changes in human motor axons and the muscle they innervate

The repetitive discharges required to produce a sustained muscle contraction results in activity-dependent hyperpolarization of the motor axons and a reduction in the force-generating capacity of the muscle. We investigated the relationship between these changes in the adductor pollicis muscle and the motor axons of its ulnar nerve supply, and the reproducibility of these changes. Ten subjects performed a 1-min maximal voluntary contraction. Activity-dependent changes in axonal excitability were measured using threshold tracking with electrical stimulation at the wrist; changes in the muscle were assessed as evoked and voluntary electromyography (EMG) and isometric force. Separate components of axonal excitability and muscle properties were tested at 5 min intervals after the sustained contraction in 5 separate sessions. The current threshold required to produce the target muscle action potential increased immediately after the contraction by 14.8% (p<0.05), reflecting decreased axonal excitability secondary to hyperpolarization. This was not correlated with the decline in amplitude of muscle force or evoked EMG. A late reversal in threshold current after the initial recovery from hyperpolarization peaked at −5.9% at ∼35 min (p<0.05). This pattern was mirrored by other indices of axonal excitability revealing a previously unreported depolarization of motor axons in the late recovery period. Measures of axonal excitability were relatively stable at rest but less so after sustained activity. The coefficient of variation (CoV) for threshold current increase was higher after activity (CoV 0.54, p<0.05) whereas changes in voluntary (CoV 0.12) and evoked twitch (CoV 0.15) force were relatively stable. These results demonstrate that activity-dependent changes in motor axon excitability are unlikely to contribute to concomitant changes in the muscle after sustained activity in healthy people. The variability in axonal excitability after sustained activity suggests that care is needed when using these measures if the integrity of either the muscle or nerve may be compromised.

Tongue muscle plasticity following hypoglossal nerve stimulation in aged rats

INTRODUCTION

Age-related decreases in tongue muscle mass and strength have been reported. It may be possible to prevent age-related tongue muscle changes using neuromuscular electrical stimulation (NMES). Our hypothesis was that alterations in muscle contractile properties and myosin heavy chain composition would be found after NMES.

METHODS

Fifty-four young, middle-aged, and old 344/Brown Norway rats were included in this study. Twenty-four rats underwent bilateral electrical stimulation of the hypoglossal nerves for 8 weeks and were compared with control or sham rats. Muscle contractile properties and myosin heavy chain (MHC) in the genioglossus (GG), styloglossus (SG), and hyoglossus (HG) muscles were examined.

RESULTS

Compared with unstimulated control rats, we found reduced muscle fatigue, increased contraction and half-decay times, and increased twitch and tetanic tension. Increased type I MHC was found, except for in GG in old and middle-aged rats.

CONCLUSION

Transitions in tongue muscle contractile properties and phenotype were found after NMES.

Myostatin facilitates slow and inhibits fast myosin heavy chain expression during myogenic differentiation

Skeletal muscles in the limb and body trunk are composed of heterogeneous myofibers expressing different isoforms of myosin heavy chain (Myh), including type I (slow, Myh7), IIA (intermediate, Myh2), IIX (fast, Myh1), and IIB (very fast, Myh4). While the contraction force and speed of a muscle are known to be determined by the relative abundance of myofibers expressing each Myh isoform, it is unclear how specific combinations of myofiber types are formed and regulated at the cellular and molecular level. We report here that myostatin (Mstn) positively regulates slow but negatively regulates fast Myh isoforms. Mstn was expressed at higher levels in the fast muscle myoblasts and myofibers than in the slow muscle counterparts. Interestingly, Mstn knockout led to a shift of Myh towards faster isoforms, suggesting an inhibitory role of Mstn in fast Myh expression. Consistently, when induced to differentiate, Mstn null myoblasts formed myotubes preferentially expressing fast Myh. Conversely, treatment of myoblasts with a recombinant Mstn protein upregulated Myh7 but downregulated Myh4 gene expression in newly formed myotubes. Importantly, both Mstn antibody and soluble activin type 2B receptor inhibited slow Myh7 and promoted fast Myh4 expression, indicating that myostatin acts through canonical activin receptor to regulate the expression of Myh genes. These results demonstrate a role of myostatin in the specification of myofiber types during myogenic differentiation.

A conditioning lesion provides selective protection in a rat model of Amyotrophic Lateral Sclerosis

Background

Amyotrophic Lateral Sclerosis (ALS) is neurodegenerative disease characterized by muscle weakness and atrophy due to progressive motoneuron loss. The death of motoneuron is preceded by the failure of neuromuscular junctions (NMJs) and axonal retraction. Thus, to develop an effective ALS therapy you must simultaneously preserve motoneuron somas, motor axons and NMJs. A conditioning lesion has the potential to accomplish this since it has been shown to enhance neuronal survival and recovery from trauma in a variety of contexts.

Methodology/Principal Findings

To test the effects of a conditioning lesion in a model of familial ALS we administered a tibial nerve crush injury to presymptomatic fALS^G93A rats. We examined its effects on motor function, motoneuron somas, motor axons, and NMJs. Our experiments revealed a novel paradigm for the conditioning lesion effect. Specifically we found that the motor functional decline in fALS^G93A rats that received a conditioning lesion was delayed and less severe. These improvements in motor function corresponded to greater motoneuron survival, reduced motor axonopathy, and enhanced NMJ maintenance at disease end-stage. Furthermore, the increased NMJ maintenance was selective for muscle compartments innervated by the most resilient (slow) motoneuron subtypes, but was absent in muscle compartments innervated by the most vulnerable (fast fatigable) motoneuron subtypes.

Conclusions/Significance

These findings support the development of strategies aimed at mimicking the conditioning lesion effect to treat ALS as well as underlined the importance of considering the heterogeneity of motoneuron sub-types when evaluating prospective ALS therapeutics.

Comparison of two lower-body modes of endurance training on lower-body strength development while concurrently training

The most recent American College of Sports Medicine (1998) recommendations for quantity and quality of exercise includes both resistance and endurance exercise components. Skeletal muscle adaptations to resistance-only and endurance-only programs may be different and possibly antagonistic when both types of training are imposed concurrently. The present study examined the effect of two different modes of lower-body endurance exercise (i.e., cycle ergometry and incline treadmill walking) on lower-body strength development with concurrent resistance training designed to improve lower-body strength (i.e., bilateral leg press 1 repetition maximum [RM]). Thirty untrained participants (22 men and 8 women, ages 18-23) were randomly assigned to one of 3 training groups (resistance only [R], N = 10; resistance + cycle ergometry [RC], N = 10; and resistance + incline treadmill [RT], N = 10). The 3 training groups exercised twice per week for 9 weeks. The reduced frequency of exercise treatments were selected specifically to avoid overtraining for in-season athletes attempting to maintain offseason conditioning. Body mass and body composition measurements were taken pre- and post-training. Before training began, 3 weeks of training, 6 weeks of training, and after training, the participants also performed a 1RM test for lower-body strength. Analysis of variance comparisons with repeated measures revealed the following statistically significant changes (alpha = 0.05) in the 3 training groups over time: (a) when men and women were combined, body mass of R was significantly greater than RC and RT post-training; (b) body mass of men only was significantly greater than RC and RT post-training; (c) body composition of men only was significantly smaller for RC and RT compared with R; (d) when men and women were combined, percent change in strength revealed significantly greater gains in R compared with RT at 6 weeks; (e) when men and women were combined, percent change in strength revealed significantly greater gains in R compared with RC and RT post-training; (f) percent change in strength for men only was significantly greater for R compared with RT at 3 weeks; (g) percent change in strength for men only was significantly greater for R compared with RC and RT at 6 weeks, and RC was significantly greater than RT at 6 weeks; (h) percent change in strength in men only was significantly greater for R compared with RC and RT post-training, and RC was significantly greater than RT post-training; and (i) percent change in strength in women was significantly greater in R compared with RT post-training. The findings confirm previous studies that reported attenuated strength development with concurrent resistance and endurance training compared with resistance-only training. More importantly, this study indicates that the mode of endurance exercise in concurrent training regimens may play a role in the development of strength. Specifically, it seems that cycling is superior to treadmill endurance training for an individual with the goal of developing strength in a multijoint movement (i.e., leg press or squat) in the lower-body because it more closely mimics the biomechanical movement of these exercises.

Lower thoracic spinal cord stimulation to restore cough in patients with spinal cord injury: results of a National Institutes of Health-Sponsored clinical trial. Part II: clinical outcomes

OBJECTIVE

To evaluate the clinical effects of spinal cord stimulation (SCS) to restore cough in subjects with cervical spinal cord injury.

DESIGN

Clinical trial assessing the clinical outcomes and side effects associated with the cough system.

SETTING

Outpatient hospital or residence.

PARTICIPANTS

Subjects (N=9; 8 men, 1 woman) with cervical spinal cord injury.

INTERVENTIONS

SCS was performed at home by either the subjects themselves or caregivers on a chronic basis and as needed for secretion management.

MAIN OUTCOME MEASURES

Ease in raising secretions, requirement for trained caregiver support related to secretion management, and incidence of acute respiratory tract infections.

RESULTS

The degree of difficulty in raising secretions improved markedly, and the need for alternative methods of secretion removal was virtually eliminated. Subject life quality related to respiratory care improved, with subjects reporting greater control of breathing problems and enhanced mobility. The incidence of acute respiratory tract infections fell from 2.0+/-0.5 to 0.7+/-0.4 events/subject year (P<.01), and mean level of trained caregiver support related to secretion management measured over a 2-week period decreased from 16.9+/-7.9 to 2.1+/-1.6 and 0.4+/-0.3 times/wk (P<.01) at 28 and 40 weeks after implantation of the device, respectively. Three subjects developed mild hemodynamic effects that abated completely with continued SCS. Subjects experienced mild leg jerks during SCS, which were well tolerated. There were no instances of bowel or bladder leakage.

CONCLUSIONS

Restoration of cough via SCS is safe and efficacious. This method improves life quality and has the potential to reduce the morbidity and mortality associated with recurrent respiratory tract infections in this patient population.

Submental sEMG and hyoid movement during Mendelsohn maneuver, effortful swallow, and expiratory muscle strength training

PURPOSE

This study investigated the concurrent biomechanical and electromyographic properties of 2 swallow-specific tasks (effortful swallow and Mendelsohn maneuver) and 1 swallow-nonspecific (expiratory muscle strength training [EMST]) swallow therapy task in order to examine the differential effects of each on hyoid motion and associated submental activation in healthy adults, with the overall goal of characterizing task-specific and overload properties of each task.

METHOD

Twenty-five healthy male and female adults (M = 25 years of age) participated in this prospective, experimental study with 1 participant group. Each participant completed all study tasks (including normal swallow, Mendelsohn maneuver swallow, effortful swallow, and EMST task) in random order during concurrent videofluoroscopy and surface electromyography recording.

RESULTS

Results revealed significant differences in the trajectory of hyoid motion as measured by overall displacement and angle of elevation of the hyoid bone. As well, timing of hyoid movement and amplitude differences existed between tasks with regard to the activation of the submental musculature.

CONCLUSIONS

Study results demonstrated differential effects of the 3 experimental tasks on the principles of task specificity and overload. These principles are important in the development of effective rehabilitative programs. Subsequent direction for future research is suggested.

Rehabilitation of children and adults who have neuromuscular diseases

Neuromuscular diseases are a broad group of disorders that affect the motor unit. Recent advances in genetics and molecular biology have greatly furthered understanding of these diseases. Unfortunately, this has not greatly modified treatment strategies. This article addresses some common features of these diseases, and some less commonly addressed issues.

Postmenopausal HRT and tibolone in relation to muscle strength and body composition

OBJECTIVES

Sarcopenia, the loss of muscle mass with age, has a great impact on physical function, and especially in postmenopausal women, who experience a greater decline in muscle strength than do men of similar age. Conventional hormone replacement therapy (HRT) may diminish this loss of muscle strength and may even increase muscle strength. However, HRT is not currently promoted for this indication because of its negative side effects, which is why tibolone, a synthetic steroid with oestrogenic, progestogenic, and androgenic activity, may be an alternative option. The aim of this article was to review data on the effect of HRT and tibolone on muscle strength and body composition in postmenopausal women.

METHODS

Medline, Pubmed, Embase, and Sumsearch were searched for articles on the effect of HRT and tibolone on muscle strength and body composition, using the Mesh terms hormone replacement therapy and clinical trial combined with muscle strength or body composition. Tibolone was added as search term with clinical trial and muscle strength or body composition.

RESULTS

Three of five randomized controlled trials reported a significant positive effect of HRT on muscle strength but not on body composition. Tibolone significantly increased handgrip strength and isometric knee extension strength in one randomized placebo-controlled, double-blind trial and increased mean knee extensor strength in one cross-sectional study. Tibolone also increased the lean body mass and decreased the accumulation of body fat.

CONCLUSIONS

HRT and tibolone increase muscle strength. Unlike HRT, tibolone also increases lean body mass and significantly reduces the total body fat content. Further research is recommended to determine whether tibolone is a safe treatment for sarcopenia.

Surface electromyographic activity of the submental muscles during swallow and expiratory pressure threshold training tasks

The use of expiratory muscle strength trainers improves parameters related to pulmonary function, speech, and cough in both healthy and patient populations. Recently, it has been speculated that expiratory strength training may alter the force generation of muscles used during the swallow process. Specifically, the use of the trainer may result in increased activation of the submental muscle complex. Support for this hypothesis was tested by examining the timing and amplitude of submental muscle activity obtained using surface EMG. These muscles are known to be important for normal swallow function. Twenty participants (10 males, 10 females; mean age = 29 years) were recruited to participate in a one-session study. Participants were asked to perform two swallows (saliva swallow and water swallow) and develop an expiratory pressure set at 25% and 75% of their maximum expiratory pressure (MEP) using an expiratory muscle strength trainer. These tasks allowed comparison of muscle activity during both the swallow and expiratory tasks completed with the trainer. Results indicated that the patterns of activation in the submental muscle group while training on the expiratory device had longer duration of activation with higher amplitude of EMG activity when compared with the swallowing condition. These findings indicate that expiratory muscle strength training (EMST) increases motor unit recruitment of the submental muscle complex. Discussion centers on the potential benefit of EMST as a treatment modality for dysphagia characterized by decreased amplitude of hyoid movement during swallowing.

Neuromuscular electrical stimulation in neurorehabilitation

This review provides a comprehensive overview of the clinical uses of neuromuscular electrical stimulation (NMES) for functional and therapeutic applications in subjects with spinal cord injury or stroke. Functional applications refer to the use of NMES to activate paralyzed muscles in precise sequence and magnitude to directly accomplish functional tasks. In therapeutic applications, NMES may lead to a specific effect that enhances function, but does not directly provide function. The specific neuroprosthetic or "functional" applications reviewed in this article include upper- and lower-limb motor movement for self-care tasks and mobility, respectively, bladder function, and respiratory control. Specific therapeutic applications include motor relearning, reduction of hemiplegic shoulder pain, muscle strengthening, prevention of muscle atrophy, prophylaxis of deep venous thrombosis, improvement of tissue oxygenation and peripheral hemodynamic functioning, and cardiopulmonary conditioning. Perspectives on future developments and clinical applications of NMES are presented.

Gene transfer demonstrates that muscle is not a primary target for non-cell-autonomous toxicity in familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive paralysis arising from the premature death of motor neurons. An inherited form is caused by a dominant mutation in the ubiquitously expressed superoxide dismutase (SOD1). SOD1 mutant expression within motor neurons is a determinant of onset and early disease, and mutant accumulation within microglia accelerates disease progression. Muscle also is a likely primary source for toxicity, because retraction of motor axons from synaptic connections to muscle is among the earliest presymptomatic events. To test involvement of muscle in ALS, viral delivery of transcription-mediated siRNA is shown to suppress mutant SOD1 accumulation within muscle alone but to be insufficient to maintain grip strength, whereas delivery to both motor neurons and muscle is sufficient. Use of a deletable mutant gene to diminish mutant SOD1 from muscle did not affect onset or survival. Finally, follistatin expression encoded by adeno-associated virus chronically inhibited myostatin and produced sustained increases in muscle mass, myofiber number, and fiber diameter, but these increases did not affect survival. Thus, SOD1-mutant-mediated damage within muscles is not a significant contributor to non-cell-autonomous pathogenesis in ALS, and enhancing muscle mass and strength provides no benefit in slowing disease onset or progression.

Effects of regular resistance training on muscle histopathology and morphometry in elderly patients with chronic kidney disease

OBJECTIVE

To evaluate muscle histopathological outcomes, adaptation in muscle fiber area, and fiber type proportion after a resistance training program for patients with chronic kidney disease.

DESIGN

Vastus lateralis muscle biopsies before and after the study period in seven patients and six healthy subjects performing the exercise program and in five patients in a nonexercising comparison group.

RESULTS

The change in degree of histopathological abnormality did not differ between the groups after the exercise program. Muscle fiber type proportion and muscle fiber area was also the same after the observation period.

CONCLUSIONS

Patients with chronic kidney disease have muscle histopathological abnormalities already in the predialysis phase. There was no indication that the exercise program had disadvantageous effects on muscle histopathology in these patients. There were no differences in muscle fiber area or in fiber type proportion after the exercise program within or between the groups. Thus, a workload of 60% of one repetition maximum was sufficient to increase muscular strength and endurance in patients with chronic kidney disease but not to increase muscle fiber area or to change muscle fiber type proportion.

Synergy of insulin-like growth factor-1 and exercise in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of the neuromuscular system resulting in paralysis and ultimately death. Currently, no effective therapy is prescribed for patients; however, several therapeutic strategies are showing promise. Either exercise or treatment with adeno-associated virus/insulin-like growth factor-1 alone has therapeutic benefits in an amyotrophic lateral sclerosis transgenic mouse model. We show here that activity duration affects the therapeutic benefit associated with exercise, with 6- and 12-hour exposure to a running wheel providing significant motor function benefits and increased survival. Remarkably, a combination of insulin-like growth factor-1 gene delivery and exercise has profound effects on survival and function, indicative of synergistic effects with exercise and insulin-like growth factor-1. Our results indicate that a drug treatment in combination with appropriate exercise may provide the most promising therapy for amyotrophic lateral sclerosis to date.

Health maintenance and management in childhood disability

Health and health-related needs of children with disabilities are very broad in scope, and it is impossible to adequately cover all aspects in a single article. This article has tried, however, to highlight the issues common to disability, and lay the groundwork for the development of health maintenance guidelines for this population. The prevalence of childhood disability is on the rise, yet life expectancies are improving, and it is not uncommon for children even with severe disabilities to live well into adulthood. The ICIDH2 paradigm shift to focus on health and function rather than impairment and disability fits well with the national initiative to promote health for all. There is a paucity of information on the impact of childhood interventions on health in adulthood, yet it would be prudent to surmise that early interventions will be effective. Directed attention to the basic health needs and preventive measures for the management of children with disabilities is required. This can be difficult because the management of a child with disability is dynamic and challenging, and health care providers may find their time and energies consumed with just managing the more obvious and striking disability-related problems. The widespread ramifications of disability on both the individual and society, call for teamwork between family, health care providers, and the community. Although the specific needs of such a diverse group vary widely; as a group, children with disabilities have many common needs. These include the need for basic health maintenance and health promotion measures such as nutrition, immunizations and physical fitness, as well as coordination of services, psychologic and family support, technical assistance, funding resources, communication between health and education fields, and access to a "medical home," the site for primary care of the child. As a tree is bent, so it will grow. Efforts to promote growth and maximize opportunities for development, to inculcate healthy eating habits, to encourage exercise and socialization behaviors, and to strengthen the bond between the child with disability, family, and community will help minimize disability-related problems in adulthood. As children with disabilities are drawn into the mainstream of the concept of health, we should start to see the much awaited change in societal attitudes toward disability. The conditions resulting in childhood disability are many varied. Rather than developing condition specific health guidelines, it would be more advantageous to use a general health framework of anticipatory guidance, growth, development, medical management, psychologic and vocational counseling, and resource planning. Working off this framework, health maintenance and promotion measures can then be further individualized to suit the child and family's specific needs.

Neural control of motor output: can training change it?

Henneman's size principle of motor unit recruitment and rate coding reduces fatigue, minimizes error in transfer of information from the nervous system, and produces smooth force output. Plasticity present at various sites of the motor system may change endurance, force, speed, or precision with training, but not the recruitment order.

The role of the inferior head of the human lateral pterygoid muscle in the generation and control of horizontal mandibular force

The aim was to test the hypothesis that the inferior head (IH) of the human lateral pterygoid muscle (LP) is involved in the generation and fine control of horizontal isometric mandibular force. Although previous studies provided some evidence for this, they had limitations that necessitate a re-examination. In eight participants, electromyographic (EMG) activity was recorded from the IHLP unilaterally, as well as bilateral surface recordings from the masseter (M) and anterior temporalis (AT), and the submandibular group of muscles (SUBM), during the generation of horizontal isometric mandibular force in a direction contralateral to the side of the IHLP recording. Isometric force at 5-8 mm open from the intercuspal position was exerted on a transducer (attached by a bar to the upper teeth) by a rod attached to the lower teeth. Participants tracked a target on a video screen that required 5-s holding periods at each 100 gwt (0.98 N) between 400 gwt (3.92 N) and 800 gwt (7.84 N). The mean of multi-unit EMG activity from all muscles during the most stable 2-s force-holding periods increased significantly with each force increment (GLM repeated measures: P<0.0001). When normalized, the multi-unit data from the IHLP exhibited the steepest rate of increase. The mean firing rates of 21 IHLP single motor units (SMUs) significantly increased with force (GLM repeated measures: P<0.0001). Two SMUs fired in advance of force onset, which suggests a role in force initiation. There were close associations between fluctuations in force and in IHLP SMU firing rates and multi-unit activity, but a similar correspondence was not as clear for the other recorded jaw muscles. These findings suggest that the IHLP is important in the generation and fine control of contralaterally directed, horizontal jaw forces.

Resistance training and bone mineral density in adolescent females

OBJECTIVE

To examine the effects of 15 months of resistance training on bone mineral density (BMD) in female adolescents (aged 14 to 17 years).

STUDY DESIGN

Participants were randomly assigned to either a training (n = 46) or control group (n = 21). BMD and body composition were measured by using dual-energy x-ray absorptiometry. Strength was assessed by means of one-repetition maximums for the leg press and bench press. The exercise group trained 30 to 45 minutes a day, 3 days per week, using 15 different resistance exercises. Control participants remained sedentary (<2 hours of exercise per week).

RESULTS

Leg strength increased significantly (40%) in the exercise group, but there were no changes in the control group. Femoral neck BMD increased significantly in the training group (1.035 to 1.073 g/cm(2), P <.01) but not in the control group (1.034 to 1.048 g/cm(2)). No significant changes were seen in either group in lumbar spine BMD (1.113 to 1.142 g/cm(2) and 1.158 to 1.190 g/cm(2), respectively) or total body BMD (1.103 to 1.134 g/cm(2) and 1.111 to 1.129 g/cm(2), respectively).

CONCLUSION

Resistance training is a potential method for increasing bone density in adolescents, although such a program would be best done as part of the school curriculum.

Using the exercise test to create the exercise prescription

Exercise testing can provide valuable information to aid the primary care physician in developing a safe and effective exercise program for his or her patients. This review presents the most recent recommendations for the components of an exercise program as well as methods to accomplish appropriate prescription writing for the various subsets of individuals from the healthy patient to the patient with chronic disease. In addition, a plea is made for physicians to encourage all patients to engage in at least some kind of regular exercise activity in an attempt to counteract the increasingly sedentary lifestyles found in our culture.

Downhill running preferentially increases CGRP in fast glycolytic muscle fibers

Calcitonin gene-related peptide (CGRP) is present in some spinal cord motoneurons and at neuromuscular junctions in skeletal muscle. We previously reported increased numbers of CGRP-positive (CGRP+) motoneurons supplying hindlimb extensors after downhill exercise (Homonko DA and Theriault E, Inter J Sport Med 18: 1-7, 1997). The present study identifies the responding population with respect to muscle and motoneuron pool and correlates changes in CGRP with muscle fiber type-identified end plates. Twenty seven rats were divided into the following groups: control and 72 h and 2 wk postexercise. FluoroGold was injected into the soleus, lateral gastrocnemius, and the proximal (mixed fiber type) or distal (fast-twitch glycolytic) regions of the medial gastrocnemius (MG). Untrained animals ran downhill on a treadmill for 30 min. The number of FluoroGold/CGRP+ motoneurons within proximal and distal MG increased by 72 h postexercise (P<0.05). No significant changes were observed in soleus or lateral gastrocnemius motoneurons postexercise. The number of alpha-bungarotoxin/CGRP+ motor end plates in the MG increased exclusively at fast-twitch glycolytic muscle fibers 72 h and 2 wk postexercise (P<0.05). One interpretation of these results is that unaccustomed exercise preferentially activates fast-twitch glycolytic muscle fibers in the MG.

Evaluation of the applicability of HRT as a preservative of muscle strength in women

OBJECTIVES

To review the studies that have been undertaken on the effects of postmenopausal hormone replacement therapy (HRT); especially oestrogen (+progestin) regimens on the preservation of muscle strength. Current knowledge of the mechanisms and actions of steroid- and sex hormones on skeletal muscle tissue will be used in an attempt to clarify the mechanism of action of a possible effect. The objective is to arrive at an agreement on whether or not postmenopausal oestrogen administration has a positive influence on skeletal muscle tissue.

METHODS

Peer-reviewed publications were assessed.

RESULTS

An age-related decrement in muscle strength can be found in both men and women. However, in women, an extra decline can be observed around the time of menopause. A possible relationship between the additional diminution in muscle strength and altered hormone concentrations after the onset of menopause has been suggested. Since women nowadays spend one-third of their life postmenopausal, it is extremely important to keep the decline in muscle mass as small as possible. Besides the continuation of a physically active lifestyle, HRT was suggested to serve as a protective mechanism. Although, the usefulness of HRT as a preservative of muscle strength appeared controversial.

CONCLUSIONS

Skeletal muscle strength is sensitive to training up to a high age, though continuation of physical activity does not appear to protect skeletal muscles completely from age-related decrements. Therefore, the development of another preventive method would be useful. Considering the present knowledge it has all the hallmarks that HRT can be a useful tool in the maintenance of muscle strength in postmenopausal women. None the less, further research is necessary to endorse this theory.

Neuromuscular and psychomotor abnormalities in patients with schizophrenia and their first-degree relatives

In previous studies of schizophrenic patients, neuromuscular (histopathological and electrophysiological) and psychomotor (finger tapping) abnormalities were found. The present study was designed to investigate relationships between these abnormalities and a family history of psychosis in 14 schizophrenic patients and 25 unaffected first-degree relatives compared to 14 healthy controls. Muscle biopsies were performed in either m. tibialis anterior or m. lateralis. Macro EMG recordings were made from m. tibialis anterior. A finger tapping test was used to investigate psychomotor performance. Neuromuscular abnormalities (muscle biopsies and/or macro EMG) and/or aberrant psychomotor performance (finger tapping test) were found in 13 (93%) patients, 14 (56%) first-degree relatives and in three (21%) controls. A statistically significant relationship for the psychomotor, but not neuromuscular changes to a family history of psychosis was found using a logistic regression method. The percentage of patients, relatives and healthy controls exhibiting were 36/40/7% in the muscle biopsy, 50/20/0% in the macro EMG, and 71/82/14% in the finger tapping investigations. A higher frequency of neuromuscular and psychomotor abnormalities was found in patients with schizophrenia and their first-degree relatives compared to healthy controls. The relationship between psychomotor findings and a family history of psychosis indicate that central aspects of motor aberrations are associated with a hereditary disposition of psychosis. The neuromuscular as well as psychomotor changes indicate that schizophrenia may be a systemic disease involving the central nervous system as well as peripheral organs. An altered cell membrane is suggested to be an underlying factor based on the type of neuromuscular findings.

Muscle biopsy, macro EMG, and clinical characteristics in patients with schizophrenia

BACKGROUND

In a previous study of motor unit properties in patients with schizophrenia, muscle fiber histologic and electrophysiologic abnormalities were observed. The present study was designed to compare patients with schizophrenia with healthy control subjects with regard to muscle fiber histology and motor unit function. A second objective was to relate these variables to clinical characteristics.

METHODS

Twelve patients with first-episode schizophrenia and fifteen patients with chronic schizophrenia (DSM-III-R) and 27 matched control subjects were included in the study. Muscle biopsies were performed either in m. tibialis anterior or m. vastus lateralis. Electromyographic recordings (macro EMG) were made from the m. tibialis anterior motor units. Psychiatric ratings included the PANSS and extrapyramidal side effects.

RESULTS

Seven of the muscle biopsy specimens from the patients and one from the control subjects were classified as abnormal (p =.049). The most frequent abnormality was atrophic muscle fibers. Eight patients and no control subjects exhibited pathological macro EMG (p =.032). The findings were present in chronic as well as in first-episode patients with schizophrenia.

CONCLUSIONS

In approximately 50% of the patients, neuromuscular abnormalities were found either in the muscle biopsy or the macro EMG investigations. The results indicate that either a common pathologic process or different pathological processes are at hand in the neuromuscular system in patients with schizophrenia. The findings are compatible with a disturbed cell membrane function.

EMG power spectrum analysis of first dorsal interosseous muscle in pianists

PURPOSE

Long-term piano training may induce potential biochemical and structural adaptations in hand intrinsic muscles or alter the motor strategy in the nervous systems.

METHODS

This study investigated, by electromyography (EMG) and strain gauge, whether this pedagogical training is aerobic or anaerobic by nature. Changes in EMG power spectrum during incremental isometric muscle contractions before and after a sustained, fatiguing muscular performance were studied in the first dorsal interosseous muscle (FDI) of 13 female pianists and 15 sedentary controls. The maximal voluntary contraction (MVC) force of the FDI and the time needed to induce fatigue were also measured.

RESULTS

During fatigue, the median frequency (MF) shifted toward the low frequencies in both groups. The MF of 50% and 25%MVC subsequent to fatigue were significantly lower than those before fatigue in the control group (104.0 +/- 45.5 Hz vs 116.7 +/- 41.4 Hz at 50%, P < 0.05; 114.7 +/- 43.4 Hz vs 123.3 +/- 46.7 Hz at 25%, P < 0.05). There were no significant MF changes within the pianists before and after fatigue. The root mean squares (RMS) of 50%, 25%, and 10%MVC of control subjects after fatigue were significantly higher than those before fatigue, but not in pianists. There was no difference in MF and RMS between the two groups at any level of %MVC before or after fatigue, but the pianists tended to show higher MF at each level of %MVC. Although the MVC was similar in both groups, the pianists, however, needed a much longer time to induce fatigue than controls (14.3 +/- 5.8 min vs 5.8 +/- 3.3 min, P < 0.005).

CONCLUSIONS

These observations are discussed in terms of the training-induced metabolic adaptations, the changes in the strategy of motor unit recruitment, and the possibility of differences in muscle fiber composition. This study indicates that piano training should not be classified as power training, rather as endurance training. Such findings may also provide information about fine motor training for athletes and other professionals.

Effects of fetal spinal cord tissue transplants and cycling exercise on the soleus muscle in spinalized rats

Studies were carried out to determine if an intraspinal transplant (Trpl) of fetal spinal cord tissue or hind limb exercise (Ex) affected the changes in myosin heavy chain (MyHC) composition or myofiber size that occur following a complete transection (Tx) of the lower thoracic spinal cord of the adult rat. In one group of animals, transplants were made acutely, whereas in a second group, daily cycling exercise was initiated 5 days after injury, with animals in both groups being sacrificed 90 days after injury. The soleus muscle is normally composed of myofibers expressing either type I (90%) or type IIa (10%) MyHC. Following a spinal transection, expression of type I MyHC isoform decreased (18% of myofibers), type IIa MyHC expression increased (65% of myofibers), and the majority of myofibers (80%) expressed type IIx MyHC. Most myofibers coexpressed multiple MyHC isoforms. Compared with Tx only, with Ex or with Trpl, there was a decrease in the number of myofibers expressing type I or IIa isoforms but little change in expression of IIx MyHC. Myofibers expressing the IIb isoform appeared in several transplant recipients but not after exercise. Transection resulted in atrophy of type I myofibers to approximately 50% of normal size, whereas myofibers were significantly larger after exercise (74% of control) and in Trpl recipients (77% of control). Type IIa myofibers also were significantly larger in Trpl recipients compared with the Tx only group. Overall, the mean myofiber size was significantly greater after exercise and in Trpl recipients compared with myofibers in Tx only animals. Thus, although neither strategy shifted the MyHC profile towards the control, both interventions influenced the extent of atrophy observed after spinalization. These data suggest that palliative strategies can be developed to modulate some of the changes in hind limb muscles that occur following a spinal cord injury.

The effects of training through high-frequency electrical stimulation on the physiological properties of single human thenar motor units

The relative impact of training on motor units (MUs) with differing physiological characteristics remains controversial. To examine this issue, we longitudinally tracked the contractile and electrical characteristics of six human thenar MUs in 2 young healthy subjects before, during, and following an intermittent, high-frequency electrical stimulation program. Responses of MUs with differing baseline physiological characteristics varied widely. While the twitch and maximal tetanic tensions of the slower and fatigue-resistant MUs increased, tensions of the faster and more fatigable MUs declined. The fatigue resistance of the faster and more fatigable MUs, on the other hand, increased while that of the slower MUs remained unchanged. Although electrical stimulation of individual MUs allowed their training to be precisely controlled, it will be of practical importance to determine whether similar divergent MU contractile changes also occur with voluntary training.

Nonlinear tension summation of different combinations of motor units in the anesthetized cat peroneus longus muscle

The purpose of this study was to examine the linearity of summation of the forces produced by the stimulation of different combinations of type identified motor units (MUs) in the cat peroneus longus muscle (PL) under isometric conditions. The muscle was fixed at its twitch optimal length, and the tension produced by the single MU was recorded during 24- and 72-Hz stimulation. The summation analysis was first carried out for MUs belonging to the same functional group, and then different combinations of fast fatigable (FF) MUs were added to the nonfatigable slow (S) and fatigue resistant (FR) group. The tension resulting from the combined stimulation of increasing numbers of MUs (measured tension) was evaluated and compared with the linearly predicted value, calculated by adding algebraically the tension produced by the individual MUs assembled in the combination (calculated tension). Tension summation displayed deviations from linearity. S and FR MUs mainly showed marked more than linear summation; FF MUs yielded either more or less than linear summation; and, when the FF units were recruited after the S and FR MUs, less than linear summation always occurred. The magnitude of the nonlinear summation appeared stimulus frequency dependent for the fatigable FF and FI group. The relationship between measured tension and calculated tension for each MU combination was examined, and linear regression lines were fitted to each set of data. The high correlation coefficients and the different slope values for the different MU-type combinations suggested that the nonlinear summation was MU-type specific. The mechanisms of nonlinear summations are discussed by considering the consequences of internal shortening and thus the mechanical interactions among MUs and shifts in muscle fiber length to a more or less advantageous portion of single MU length-tension curves.

Hand dominance and motor unit firing behavior

Daily preferential use was shown to alter physiological and mechanical properties of skeletal muscle. This study was aimed at revealing differences in the control strategy of muscle pairs in humans who show a clear preference for one hand. We compared the motor unit (MU) recruitment and firing behavior in the first dorsal interosseous (FDI) muscle of both hands in eight male volunteers whose hand preference was evaluated with the use of a standard questionnaire. Myoelectric signals were recorded while subjects isometrically abducted the index finger at 30% of the maximal voluntary contraction (MVC) force. A myoelectric signal decomposition technique was used to accurately identify MU firing times from the myoelectric signal. In MUs of the dominant hand, mean values for recruitment threshold, initial firing rate, average firing rate at target force, and discharge variability were lower when compared with the nondominant hand. Analysis of the cross-correlation between mean firing rate and muscle force revealed cross-correlation peaks of longer latency in the dominant hand than in the nondominant side. This lag of the force output with respect to fluctuations in the firing behavior of MUs is indicative of a greater mechanical delay in the dominant FDI muscle. MVC force was not significantly different across muscle pairs, but the variability of force at the submaximal target level was higher in the nondominant side. The presence of lower average firing rates, lower recruitment thresholds, and greater firing rate/force delay in the dominant hand is consistent with the notion of an increased percentage of slow twitch fibers in the preferentially used muscle, allowing twitch fusion and force buildup to occur at lower firing rates. It is suggested that a lifetime of preferred use may cause adaptations in the fiber composition of the dominant muscle such that the mechanical effectiveness of its MUs increased.

American College of Sports Medicine Position Stand. The recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness, and flexibility in healthy adults

ACSM Position Stand on The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness, and Flexibility in Adults. Med. Sci. Sports Exerc., Vol. 30, No. 6, pp. 975-991, 1998. The combination of frequency, intensity, and duration of chronic exercise has been found to be effective for producing a training effect. The interaction of these factors provide the overload stimulus. In general, the lower the stimulus the lower the training effect, and the greater the stimulus the greater the effect. As a result of specificity of training and the need for maintaining muscular strength and endurance, and flexibility of the major muscle groups, a well-rounded training program including aerobic and resistance training, and flexibility exercises is recommended. Although age in itself is not a limiting factor to exercise training, a more gradual approach in applying the prescription at older ages seems prudent. It has also been shown that aerobic endurance training of fewer than 2 d.wk-1, at less than 40-50% of VO2R, and for less than 10 min-1 is generally not a sufficient stimulus for developing and maintaining fitness in healthy adults. Even so, many health benefits from physical activity can be achieved at lower intensities of exercise if frequency and duration of training are increased appropriately. In this regard, physical activity can be accumulated through the day in shorter bouts of 10-min durations. In the interpretation of this position stand, it must be recognized that the recommendations should be used in the context of participant's needs, goals, and initial abilities. In this regard, a sliding scale as to the amount of time allotted and intensity of effort should be carefully gauged for the cardiorespiratory, muscular strength and endurance, and flexibility components of the program. An appropriate warm-up and cool-down period, which would include flexibility exercises, is also recommended. The important factor is to design a program for the individual to provide the proper amount of physical activity to attain maximal benefit at the lowest risk. Emphasis should be placed on factors that result in permanent lifestyle change and encourage a lifetime of physical activity.

Noninvasive measurement of phosphocreatine recovery kinetics in single human muscles

The rate at which phosphocreatine (PCr) is resynthesized after exercise is related to muscle oxidative capacity (Vmax). With the use of a one-dimensional image-guided, localized nuclear magnetic resonance spectroscopy technique, PCr kinetics were monitored in the medial gastrocnemius of eight healthy subjects after voluntary, short duration, maximal rate exercise. Localized spectra were obtained every 6 s with <5% contamination from nonselected regions. Maximal rate exercise elicited near-maximal to maximal muscle activation, as indicated by the high-PCr hydrolysis rate (2.26 +/- 0.07 mM/s) and extensive PCr depletion. At the end of 9 s of maximal rate exercise, PCr was depleted by 61.4 +/- 2.4% and intracellular pH was 7.04 +/- 0.03. After 9 s of maximal rate exercise, PCr recovered with a rate constant (kPCr) of 1.87 +/- 0.15 min(-1) and a Vmax of 67.2 +/- 6.0 mM/min. Independent of prior activity, aerobic ATP synthesis rates reached 48.6 +/- 4.9 mM/min within 9 s. Extending maximal rate exercise to 30 s resulted in 92.0 +/- 1.2% PCr depletion and an intracellular pH of 6.45 +/- 0.07. The intracellular acidosis separated the direct relationship between kPCr and muscle Vmax but did not affect the initial PCr resynthesis rate.

Enzyme-histochemical and morphological characteristics of fast- and slow-twitch skeletal muscle after brain infarction in the rat

The right middle cerebral artery was permanently occluded in 12-week-old male spontaneously hypertensive rats. After the surgery the rats were subjected to repeated behavioural tests during the observation period. Fourteen weeks after surgery the fast-twitch extensor digitorum longus (EDL) and the slow-twitch soleus muscle of both sides were removed and examined with regard to muscle fibre characteristics obtained by histochemical and morphometrical methods. Comparisons were made with age-matched controls. Limb placement and the ability to traverse a beam or a rotating pole were repeatedly tested 2-13 weeks after the operation. In spite of permanent sensorimotor deficits in limb placement and when traversing a rotating pole or beam, no increase in pathological changes was noted in either EDL or soleus. The number and proportion of fibre types remained unchanged in both muscles. There was no difference in muscle fibre size in either EDL or soleus. It is concluded that brain infarction in the rat, although causing marked impairment of contralateral motor function, does not have a major influence on the muscle-fibre morphology or fibre-type composition, irrespective of muscle type.

Fast-to-slow muscle conversion by chronic electrostimulation: effects on mitochondrial respiratory chain function with possible implications for the gracilis neosphincter procedure

The effects of chronic, around the clock, low-frequency electrostimulation on the respiratory chain activity and cytochrome content of freshly isolated mitochondria were evaluated in rabbit skeletal muscle before and after 30 days of continuous or cyclical electrostimulation using a totally implantable system and a training programme now used in humans. The respiratory activity measured in state III increased strongly after electrostimulation. The efficiency of the respiratory chain increased significantly after electrostimulation but the activity of complex [(reduced nicotinamide adenine dinucleotide dehydrogenase) did not increase. The amount of cytochromes a and a3, b562, and c and c1 increased clearly after electrostimulation. The respiratory activity rate of mitochondria obtained after continuous electrostimulation was apparently higher than after cyclical electrostimulation. Chronic uninterrupted low-frequency electrostimulation, using a clinical training programme, induces an increase in mitochondrial respiratory chain activity in purified mitochondria of skeletal muscle. These changes are the basis of induced resistance to fatigue in fast-to-slow muscle conversion by chronic electrostimulation.

Motor unit recruitment strategy changes with skill acquisition

The modifications of motor unit recruitment strategy due to skill acquisition was determined in the elbow flexor-extensor muscles of normal human subjects. The median frequency of the power density spectra of the electromyograms recorded from the biceps and triceps muscles during a 3-s linear increase in flexion force in the range of 0-100% maximal voluntary contraction (MVC) was calculated for each subject, every 2 weeks over a total 6-week period during which subjects practiced linear flexion force increase three times a week. Electromyograms were recorded with two pairs of electrodes of different size and electrode spacing. It was shown that skill acquisition due to the 360 practice trials over the 6-week period caused an increase in the initial motor unit recruitment phase of the agonist's force generation cycle from about 0-65% MVC to about 0-85% MVC. The increase in the recruitment range was gradual and statistically significant for the measurements made every 2 weeks. The recruitment range of the antagonist triceps demonstrated a minor, but statistically insignificant, decrease over the same training period. There was a minor, but statistically insignificant, advantage of using small electrodes and inter-electrode spacing. It was concluded that skill acquisition, due to repeated functional use of a muscle in the same contraction mode, results in a slower, prolonged recruitment of motor units in the initial segment of the force generation cycle, thereby allowing a more precise and accurate control of the increments of force increase. Such conclusions reinforce the concept advocating the plasticity of motor unit control according to the functional demands imposed on the muscle. The results have significant implications in the design of various athletic, occupational and rehabilitation training modalities for optimal performance of various movement functions.

Motor unit estimates obtained using the new "MUESA" method

Motor unit number estimates were obtained from the extensor digitorum brevis and thenar muscles using a new method called MUESA. MUESA is distinguished from other estimation methods in the manner in which it deals with probabilistic motor unit activation, which is more commonly referred to as "alternation." Because of "alternation," incremental increases in the observed muscle potentials often cannot be interpreted in terms of the successive activation of single motor units. In the MUESA method, the nerve is subjected to a number of constant-intensity stimulus trains, and the resultant muscle response sequences are decomposed into their constituent motor unit action potentials. In general, if a stimulus train results in the probabilistic activation of n motor units, we can expect to see up to 2n different potentials, with each potential representing a unique combination of active and/or inactive motor units. If all 2n potentials are indeed observed, the decomposition of the observed potential sequence into its constituent motor unit action potentials is very straightforward. For the majority of the cases in which the number of observed potentials is not an integer power of 2, we have developed a novel decomposition method based on the analysis of the relative firing rates of the motor units.