Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review

Changes in brain functional connectivity and muscle strength independent of elbow flexor atrophy following upper limb immobilization in young females

Muscle disuse induces a decline in muscle strength that exceeds the rate and magnitude of muscle atrophy, suggesting that factors beyond the muscle contribute to strength loss. The purpose of this study was to characterize changes in the brain and neuromuscular system in addition to muscle size following upper limb immobilization in young females. Using a within-participant, unilateral design, 12 females (age: 20.6 ± 2.1 years) underwent 14 days of upper arm immobilization using an elbow brace and sling. Bilateral measures of muscle strength (isometric and isokinetic dynamometry), muscle size (magnetic resonance imaging), voluntary muscle activation capacity, corticospinal excitability, cortical thickness and resting-state functional connectivity were collected before and after immobilization. Immobilization induced a significant decline in isometric elbow flexion (-21.3 ± 19.2%, interaction: P = 0.0440) and extension (-19.9 ± 15.7%, interaction: P = 0.0317) strength in the immobilized arm only. There was no significant effect of immobilization on elbow flexor cross-sectional area (CSA) (-1.2 ± 2.4%, interaction: P = 0.466), whereas elbow extensor CSA decreased (-2.9 ± 2.9%, interaction: P = 0.0177) in the immobilized arm. Immobilization did not differentially alter voluntary activation capacity, corticospinal excitability, or cortical thickness (P > 0.05); however, there were significant changes in the functional connectivity of brain regions related to movement planning and error detection (P < 0.05). This study reveals that elbow flexor strength loss can occur in the absence of significant elbow flexor muscle atrophy, and that the brain represents a site of functional adaptation in response to upper limb immobilization in young females.

Targeting Crosstalk of Signaling Pathways among Muscles-Bone-Adipose Tissue: A Promising Therapeutic Approach for Sarcopenia

The aging process is associated with the development of a wide range of degenerative disorders in mammals. These diseases are characterized by a progressive decline in function at multiple levels, including the molecular, cellular, tissue, and organismal. Furthermore, it is responsible for various healthcare costs in developing and developed countries. Sarcopenia is the deterioration in the quality and functionality of muscles, which is extremely concerning as it manages many functions in the human body. This article reviews the molecular crosstalk involved in sarcopenia and the specific roles of many mediator molecules in establishing cross-talk between muscles, bone, and fatty tissues, eventually leading to sarcopenia. Besides, the involvement of various etiological factors, such as neurology, endocrinology, lifestyle, etc., makes it exceedingly difficult for clinicians to develop a coherent hypothesis that may lead to the well-organized management system required to battle this debilitating disease. The several hallmarks contributing to the progression of the disease is a vital question that needs to be addressed to ensure an efficient treatment for sarcopenia patients. Also, the intricate molecular mechanism involved in developing this disease requires more studies. The direct relationship of cellular senescence with aging is one of the pivotal issues contributing to disease pathophysiology. Some patented treatment strategies have been discussed, including drugs undergoing clinical trials and emerging options like miRNA and protein-enclosed extracellular vesicles. A clear understanding of the secretome, including the signaling pathways involved between muscles, bone, and fatty tissues, is extremely beneficial for developing novel therapeutics for curing sarcopenia.

Subscapularis atrophy and function after arthroscopic Trillat procedure

INTRODUCTION

Several studies have reported a strength deficit in internal rotation (IR) following a Latarjet procedure, which can persist for months or even years. The arthroscopic Trillat procedure does not require splitting the subscapularis muscle, potentially making it less damaging.

HYPOTHESIS

The arthroscopic Trillat procedure does not cause any atrophy or strength deficit in the subscapularis muscle.

METHODS

This was a single center, retrospective study of patients treated between 2013 and 2021. Included were patients who had chronic anterior shoulder instability with an indication for surgical stabilization using an arthroscopic Trillat procedure and who underwent a CT scan before surgery and a second one at 6 months postoperative. The following morphological parameters were measured on all the rotator cuff muscles: cross-sectional area (CSA), thickness and fatty infiltration using the mean muscle attenuation (MMA) measurement. Isokinetic tests were done 1 year post-surgery.

RESULTS

One hundred seventeen patients underwent arthroscopic Trillat surgery between 2013 and 2021; 58 were included, 30 were analyzed and 17 patients underwent isokinetic testing. The CSA of the subscapularis was significantly smaller by 5.3% (17.0 vs. 16.1; p = 0.03). None of the other rotator cuff muscles had a smaller CSA. The MMA of the subscapularis increased significantly while the MMA of the external rotators decreased postoperatively. No strength deficit was found at 1 year postoperative in the internal and external rotators.

DISCUSSION

The arthroscopic Trillat procedure produces minor atrophy of the subscapularis muscle at 6 months, with no strength deficit at 1 year postoperative. Several studies have reported a deficit in internal rotation strength after a Latarjet procedure, ranging from 6% to 19% depending on the study.

LEVEL OF EVIDENCE

IV.

Combined action observation and mental imagery versus neuromuscular electrical stimulation as novel therapeutics during short-term knee immobilization

Limb immobilization causes rapid declines in muscle strength and mass. Given the role of the nervous system in immobilization-induced weakness, targeted interventions may be able to preserve muscle strength, but not mass, and vice versa. The purpose of this study was to assess the effects of two distinct interventions during 1 week of knee joint immobilization on muscle strength (isometric and concentric isokinetic peak torque), mass (bioimpedance spectroscopy and ultrasonography), and neuromuscular function (transcranial magnetic stimulation and interpolated twitch technique). Thirty-nine healthy, college-aged adults (21 males, 18 females) were randomized into one of four groups: immobilization only (n = 9), immobilization + action observation/mental imagery (AOMI) (n = 10), immobilization + neuromuscular electrical stimulation (NMES) (n = 12), or control group (n = 8). The AOMI group performed daily video observation and mental imagery of knee extensions. The NMES group performed twice daily stimulation of the quadriceps femoris. Based on observed effect sizes, it appears that AOMI shows promise as a means of preserving voluntary strength, which may be modulated by neural adaptations. Strength increased from PRE to POST in the AOMI group, with +7.2% (Cohen's d = 1.018) increase in concentric isokinetic peak torque at 30°/s. However, NMES did not preserve muscle mass. Though preliminary, our findings highlight the specific nature of clinical interventions and suggest that muscle strength can be independently targeted during rehabilitation. This study was prospectively registered: ClinicalTrials.gov NCT05072652.

Sex differences in muscle-quality recovery following one week of knee joint immobilization and subsequent retraining

This manuscript represents the second phase of a clinical trial designed to examine the effects of knee joint immobilization and retraining on muscle strength and mass. In Phase 2, we examined sex differences in the recovery of multiple indices of muscle quality after a resistance training-based rehabilitation program. Following 1 week of immobilization, 27 participants (16 males, 11 females) exhibiting weakness underwent twice weekly resistance training sessions designed to re-strengthen their left knee. Unilateral retraining sessions utilizing leg press, extension, and curl exercises were conducted until participants could reproduce their pre-immobilization knee extension isometric maximal voluntary contraction (MVC) peak torque. Post-immobilization, both sexes demonstrated impaired MVC peak torque (males = -10.8%, females = -15.2%), specific torque (-9.8% vs. -13.1%), echo intensity of the vastus lateralis (+6.9% vs. +5.9%) and rectus femoris (+5.9% vs. +2.1), and extracellular water/intracellular water ratio (+7.8% vs. +9.0%). The number of retraining sessions for peak torque to return to baseline for males (median = 1, mean = 2.13) versus females (median = 2, mean = 2.91) was not significantly different, though the disparity in recovery times may be clinically relevant. Following retraining, specific torque was the only muscle-quality indicator that improved along with MVC peak torque (males = 20.1%, females = 22.4%). Our findings indicate that measures of muscle quality demonstrate divergent recovery rates following immobilization, with muscle mass lagging behind improvements in strength. Greater immobilization-induced strength loss among females suggests that sex-specific rehabilitation efforts may be justified.

Reorganization of Brain Resting-state Functional Connectivity Following 14 Days of Elbow Immobilization in Young Females

Limb immobilization is known to cause significant decreases in muscle strength and muscle mass as early as two days following the onset of immobilization. However, the decline in strength surpasses the decline in muscle mass, suggesting that factors in addition to muscle loss, such as neuroplasticity, contribute to the decrease in force production. However, little is known regarding immobilization-induced neural changes, although sensorimotor regions seem to be the most affected. The present study aimed to determine whether brain functional organization is altered following 14 days of unilateral elbow immobilization. Functional organization was quantified using resting-state functional connectivity, a measure of the synchronicity of the spontaneous discharge of different brain regions at rest. Data was obtained from twelve healthy young females before and after completing the immobilization period. A seed-to-voxel analysis was performed using seeds associated with cortical, subcortical, and cerebellar sensorimotor regions of the brain. The results showed changes predominantly involving cerebellar connectivity. For example, the immobilization period caused a decrease in connectivity between the motor cerebellar region of the immobilized arm and the left temporal lobe, and an increase between the same cerebellar region and the supplementary motor area. Overall, changes in connectivity occurred in regions typically associated with error detection and motor learning, suggesting a potential functional reorganization of the brain within 14 days of elbow immobilization.

Effect of Video-Assisted Isometric Strengthening Exercise Program on Pain and Muscle Strength Poststabilization of Lower Limb Fracture

The benefits of isometric strengthening exercises (ISEs) are compromised when patient teaching on ISEs is delayed and/or ineffectively delivered due to healthcare resources constraint, especially when health resources are stretched, as occurred during the COVID-19 pandemic. This study aims to examine the effect of a video-assisted ISE program on pain and muscle strength of patients following surgical stabilization of lower limb fracture. A quasi-experimental study with repeated measures was employed. Primary study outcomes were assessed using the Brief Pain Inventory and Manual Muscle Test. Effects over time were analyzed using generalized estimating equations. In comparison with usual care group (n = 32), the intervention group (n = 33) showed better pain reduction over time (p < .001, effect size [ES] = 0.39-1.77) and muscle strength preservation (p < .05; ES = 0.8-0.9). Patient acceptance of the intervention was favorable. Integration of video clips into patient teaching on ISEs is potentially beneficial in managing pain and muscle strength; it can be easily deployed to aid early ISE initiation.

Environmental enrichment through virtual reality as multisensory stimulation to mitigate the negative effects of prolonged bed rest

Prolonged bed rest causes a multitude of deleterious physiological changes in the human body that require interventions even during immobilization to prevent or minimize these negative effects. In addition to other interventions such as physical and nutritional therapy, non-physical interventions such as cognitive training, motor imagery, and action observation have demonstrated efficacy in mitigating or improving not only cognitive but also motor outcomes in bedridden patients. Recent technological advances have opened new opportunities to implement such non-physical interventions in semi- or fully-immersive environments to enable the development of bed rest countermeasures. Extended Reality (XR), which covers augmented reality (AR), mixed reality (MR), and virtual reality (VR), can enhance the training process by further engaging the kinesthetic, visual, and auditory senses. XR-based enriched environments offer a promising research avenue to investigate the effects of multisensory stimulation on motor rehabilitation and to counteract dysfunctional brain mechanisms that occur during prolonged bed rest. This review discussed the use of enriched environment applications in bedridden patients as a promising tool to improve patient rehabilitation outcomes and suggested their integration into existing treatment protocols to improve patient care. Finally, the neurobiological mechanisms associated with the positive cognitive and motor effects of an enriched environment are highlighted.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

Muscle damage induced by maximal eccentric exercise of the elbow flexors after 3-week immobilization

The present study investigated the effects of a 3-week immobilization (IM) on muscle damage induced by maximal eccentric exercise (MaxEC) to test the hypothesis that the IM would make muscles prone to muscle damage. Young healthy sedentary men were pseudo-randomly assigned to IM or control group (n = 12/group). Non-dominant arms of the IM group participants were immobilized at 90° elbow flexion by a cast for 21 days. All participants performed MaxEC consisting of five sets of six elbow flexor contractions by lowering a dumbbell set at 100% of pre-exercise maximal voluntary isometric contraction (MVC) strength of the non-dominant arm. This was performed at 2 days after the cast removal for the IM group. MVC torque, range of motion (ROM), muscle thickness (MT), muscle hardness, position sense (PS), and joint reaction angle (JRA) of the elbow flexors were measured at baseline, post-immobilization, and before, immediately after, and one to 5 days after MaxEC. The IM decreased MVC torque (-17 ± 2%), ROM (-2 ± 1%), MT (-7 ± 3%), and JRA (-12 ± 6%), and increased in muscle hardness (20 ± 6%) and PS (11 ± 2%) (p < 0.05). Changes in MVC (e.g., 2 days: -40 ± 5 vs. -30 ± 9%), ROM (2 days: -11 ± 2 vs. -9 ± 3%), muscle soreness (peak: 63 ± 22 vs. 48 ± 14 mm), plasma CK activity (peak: 7820 ± 4011 vs. 4980 ± 1363 IU/L), PS (maximal change: -23 ± 2 vs. -18 ± 3%), and JRA (maximal change: -37 ± 4 vs. -26 ± 3%) after MaxEC were greater (p < 0.05) for the IM than control group. These results supported the hypothesis and showed that the IM made the muscles more vulnerable to muscle damage induced by eccentric exercise.

Single-leg disuse decreases skeletal muscle strength, size, and power in uninjured adults: A systematic review and meta-analysis

We aimed to quantify declines from baseline in lower limb skeletal muscle size and strength of uninjured adults following single-leg disuse. We searched EMBASE, Medline, CINAHL, and CCRCT up to 30 January 2022. Studies were included in the systematic review if they (1) recruited uninjured participants; (2) were an original experimental study; (3) employed a single-leg disuse model; and (4) reported muscle strength, size, or power data following a period of single-leg disuse for at least one group without a countermeasure. Studies were excluded if they (1) did not meet all inclusion criteria; (2) were not in English; (3) reported previously published muscle strength, size, or power data; or (4) could not be sourced from two different libraries, repeated online searches, and the authors. We used the Cochrane Risk of Bias Assessment Tool to assess risk of bias. We then performed random-effects meta-analyses on studies reporting measures of leg extension strength and extensor size. Our search revealed 6548 studies, and 86 were included in our systematic review. Data from 35 and 20 studies were then included in the meta-analyses for measures of leg extensor strength and size, respectively (40 different studies). No meta-analysis for muscle power was performed due to insufficient homogenous data. Effect sizes (Hedges' g_av ) with 95% confidence intervals for leg extensor strength were all durations = -0.80 [-0.92, -0.68] (n = 429 participants; n = 68 aged 40 years or older; n ≥ 78 females); ≤7 days of disuse = -0.57 [-0.75, -0.40] (n = 151); >7 days and ≤14 days = -0.93 [-1.12, -0.74] (n = 206); and >14 days = -0.95 [-1.20, -0.70] (n = 72). Effect sizes for measures of leg extensor size were all durations = -0.41 [-0.51, -0.31] (n = 233; n = 32 aged 40 years or older; n ≥ 42 females); ≤7 days = -0.26 [-0.36, -0.16] (n = 84); >7 days and ≤14 days = -0.49 [-0.67, -0.30] (n = 102); and >14 days = -0.52 [-0.74, -0.30] (n = 47). Decreases in leg extensor strength (cast: -0.94 [-1.30, -0.59] (n = 73); brace: -0.90 [-1.18, -0.63] (n = 106)) and size (cast: -0.61[-0.87, -0.35] (n = 41); brace: (-0.48 [-1.04, 0.07] (n = 41)) following 14 days of disuse did not differ for cast and brace disuse models. Single-leg disuse in adults resulted in a decline in leg extensor strength and size that reached a nadir beyond 14 days. Bracing and casting led to similar declines in leg extensor strength and size following 14 days of disuse. Studies including females and males and adults over 40 years of age are lacking.

Survival in a Collapsed Stable for 37 Days After Avalanche Burial in 1755

In 1755 in Bergemoletto, Italy, an avalanche buried 4 people (2 women, a girl, and a boy) and several animals in a stable. After 37 d in a pitch-dark confined space, 3 of the 4 people were rescued alive. The 3 survivors had only goat milk, a few chestnuts, a few kg of raw kid meat, and meltwater for nutrition. We describe the longest-known survival in an avalanche burial and discuss the medical and psychological problems of the survivors. The boy died. When they were extricated, all 3 survivors were exhausted, cachectic, and unable to stand or walk. They were severely malnourished and were experiencing tingling, tremors, and weakness in the legs; constipation; changes in taste; and amenorrhea. One of the women had persistent eye problems and developed symptoms consistent with post-traumatic stress disorder. The survivors were given slow refeeding. It took from 1 to 6 wk before they could walk. We compare this case to other long-duration burials, especially mining accidents, and describe the rescue and patient care after long-duration burials. This case demonstrates that people can overcome extremely adverse conditions and survive.

Impact of different ground-based microgravity models on human sensorimotor system

This review includes current and updated information about various ground-based microgravity models and their impact on the human sensorimotor system. All known models of microgravity are imperfect in a simulation of the physiological effects of microgravity but have their advantages and disadvantages. This review points out that understanding the role of gravity in motion control requires consideration of data from different environments and in various contexts. The compiled information can be helpful to researchers to effectively plan experiments using ground-based models of the effects of space flight, depending on the problem posed.

Loss of neuromuscular junction integrity and muscle atrophy in skeletal muscle disuse

Physical inactivity (PI) is a major risk factor of chronic diseases. A major aspect of PI is loss of muscle mass and strength. The latter phenomenon significantly impacts daily life and represent a major issue for global health. Understandably, skeletal muscle itself has been the major focus of studies aimed at understanding the mechanisms underlying loss of mass and strength. Relatively lesser attention has been given to the contribution of alterations in somatomotor control, despite the fact that these changes can start very early and can occur at multiple levels, from the cortex down to the neuromuscular junction (NMJ). It is well known that exposure to chronic inactivity or immobilization causes a disproportionate loss of force compared to muscle mass, i.e. a loss of specific or intrinsic whole muscle force. The latter phenomenon may be partially explained by the loss of specific force of individual muscle fibres, but several other players are very likely to contribute to such detrimental phenomenon. Irrespective of the length of the disuse period, the loss of force is, in fact, more than two-fold greater than that of muscle size. It is very likely that somatomotor alterations may contribute to this loss in intrinsic muscle force. Here we review evidence that alterations of one component of somatomotor control, namely the neuromuscular junction, occur in disuse. We also discuss some of the novel players in NMJ stability (e.g., homer, bassoon, pannexin) and the importance of new established and emerging molecular markers of neurodegenerative processes in humans such as agrin, neural-cell adhesion molecule and light-chain neurofilaments.

Low-Load Blood-Flow-Restricted Exercise to Prevent Muscle Atrophy and Decline in Functional Performance in a Patient Recovering From a Malleolus Fracture. A Case Report

This present case presentation offers supportive evidence that low-load blood-flow-restriction exercise (LL-BFRE) may be a feasible intervention to preserve skeletal muscle mass and, in part, lower-limb muscle function after a fracture of the lateral malleolus. A 28-year-old female patient sustained a radiographically verified stabile fracture of the lateral malleolus and was treated with a walker cast. She was allowed weight bearing on the limb within the limits of her pain threshold and to perform unloaded plantar and dorsiflexion movements of the ankle. The patient performed 12 weeks of home-based LL-BFRE 4 times per week to diminish declines in functional performance, muscle strength, and skeletal muscle atrophy. We observed that LL-BFRE was feasible with no exercise-related adverse events in the early stage of rehabilitation. The patient experienced no-or-low pain during exercise. Vastus lateralis muscle volume, and thigh and calf circumference was preserved.

Motor unit dysregulation following 15 days of unilateral lower limb immobilisation

Disuse atrophy, caused by situations of unloading such as limb immobilisation, causes a rapid yet diverging reduction in skeletal muscle function when compared to muscle mass. While mechanistic insight into the loss of mass is well studied, deterioration of muscle function with a focus towards the neural input to muscle remains underexplored. This study aimed to determine the role of motor unit adaptation in disuse-induced neuromuscular deficits. Ten young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation with intramuscular electromyography (iEMG) bilaterally recorded from the vastus lateralis (VL) during knee extensor contractions normalised to maximal voluntary contraction (MVC), pre and post disuse. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in motor unit (MU) discharge and MU potential (MUP) characteristics. VL CSA was reduced by approximately 15% which was exceeded by a two-fold decrease of 31% in muscle strength in the immobilised limb, with no change in either parameter in the non-immobilised limb. Parameters of MUP size were reduced by 11% to 24% with immobilisation, while neuromuscular junction (NMJ) transmission instability remained unchanged, and MU firing rate decreased by 8% to 11% at several contraction levels. All adaptations were observed in the immobilised limb only. These findings highlight impaired neural input following immobilisation reflected by suppressed MU firing rate which may underpin the disproportionate reductions of strength relative to muscle size. KEY POINTS: Muscle mass and function decline rapidly in situations of disuse such as bed rest and limb immobilisation. The reduction in muscle function commonly exceeds that of muscle mass, which may be associated with the dysregulation of neural input to muscle. We have used intramuscular electromyography to sample individual motor unit and near fibre potentials from the vastus lateralis following 15 days of unilateral limb immobilisation. Following disuse, the disproportionate loss of muscle strength when compared to size coincided with suppressed motor unit firing rate. These motor unit adaptations were observed at multiple contraction levels and in the immobilised limb only. Our findings demonstrate neural dysregulation as a key component of functional loss following muscle disuse in humans.

The impact of altering participant MRI scanning position on back muscle volume measurements

Objectives

Muscle volume may reflect both strength and functional capability and hence is a parameter often measured to assess the effect of various interventions. The aim of the current study was to determine the sensitivity of muscle volume calculations on participant postural position and hence gauge possible errors that may arise in longitudinal studies, especially those where an intervention leads to large muscle changes and potentially the degree of spinal curvature.

Methods

Twenty healthy participants (22-49 years, 10 male and 10 female), were recruited and MRI images acquired with them lying in four different positions; neutral spine (P1), decreased lordosis (P2), increased lordosis (P3) and neutral spine repeated (P4). Images were analysed in Simpleware ScanIP, and lumbar muscle volume and Cobb's angle, as an indicator of spine curvature, determined.

Results

After comparing volume determinations, no statistically significant differences were found for P1 - P2 and P1 - P4, whereas significant changes were determined for P2 - P3 and P1 - P3. P2 and P3 represent the two extremes of spinal curvature with a difference in Cobb's angle of 17°. However, the mean difference between volume determinations was only 29 cm³. These results suggest the differences in muscle volume determinations are generally greater with increasing differences in curvature between measurements, but that overall the effects are small.

Conclusions

Thus, generally, spinal muscle volume determinations are robust in terms of participant positioning.

Advances in knowledge

Differences in muscle volume calculations appear to become larger the greater the difference in spinal curvature between positions. Thus, spinal curvature should not have a major impact on the results of spinal muscle volume determinations following interventions in longitudinal studies.

Cardiovasomobility: an integrative understanding of how disuse impacts cardiovascular and skeletal muscle health

Cardiovasomobility is a novel concept that encompasses the integration of cardiovascular and skeletal muscle function in health and disease with critical modification by physical activity, or lack thereof. Compelling evidence indicates that physical activity improves health while a sedentary, or inactive, lifestyle accelerates cardiovascular and skeletal muscle dysfunction and hastens disease progression. Identifying causative factors for vascular and skeletal muscle dysfunction, especially in humans, has proven difficult due to the limitations associated with cross-sectional investigations. Therefore, experimental models of physical inactivity and disuse, which mimic hospitalization, injury, and illness, provide important insight into the mechanisms and consequences of vascular and skeletal muscle dysfunction. This review provides an overview of the experimental models of disuse and inactivity and focuses on the integrated responses of the vasculature and skeletal muscle in response to disuse/inactivity. The time course and magnitude of dysfunction evoked by various models of disuse/inactivity are discussed in detail, and evidence in support of the critical roles of mitochondrial function and oxidative stress are presented. Lastly, strategies aimed at preserving vascular and skeletal muscle dysfunction during disuse/inactivity are reviewed. Within the context of cardiovasomobility, experimental manipulation of physical activity provides valuable insight into the mechanisms responsible for vascular and skeletal muscle dysfunction that limit mobility, degrade quality of life, and hasten the onset of disease.

Motor effects of movement representation techniques and cross-education: a systematic review and meta-analysis

INTRODUCTION

The objective was to assess the impact of movement representation techniques (MRT) through motor imagery (MI), action observation (AO) and visual mirror feedback (VMF) and cross-education training (CE) on strength, range of motion (ROM), speed, functional state and balance during experimental immobilization processes in healthy individuals, in patients with injuries that did not require surgery and in those with surgical processes that did or did not require immobilization.

EVIDENCE ACQUISITION

MEDLINE, EMBASE, CINAHL and Google Scholar were searched. Thirteen meta-analyses were conducted.

EVIDENCE SYNTHESIS

Regarding the immobilized participants, in the healthy individuals, MI showed significant results regarding maintenance of strength and ROM, with low-quality evidence. Regarding the process with no immobilization, VMF and MI techniques showed significant changes in maintaining ROM in patients with injury without surgery, with very low-quality evidence. Results had shown that MI demonstrated significantly higher maintenance of strength and speed in patients undergoing surgery, with low-quality evidence. No significant results were found in ROM. Low-quality evidence showed better results in AO plus usual care compared with usual treatment in isolation with respect to maintenance of functional state and balance. CE training demonstrated maintenance of strength in patients undergoing surgery, with moderate evidence; however, not in healthy experimentally immobilized individuals. VMF did not show significant results in maintaining ROM after surgery without immobilization, nor did MI in maintaining strength after surgery and immobilization.

CONCLUSIONS

MRT and CE training have been shown to have a significant impact on the improvement of various motor variables and on physical maintenance in general.

Effects of age on muscle power, postural control and functional capacity after short-term immobilization and retraining

OBJECTIVES

This study investigated the effect of lower limb immobilization and retraining on postural control and muscle power in healthy old and young men.

METHODS

Twenty men, nine old (OM:67.3±4.4 years) and eleven young (YM:24.4±1.6 years) underwent 2 weeks of unilateral whole-leg casting, followed by 4 weeks of retraining. Measures included center of pressure (CoP) sway length and area during single- and double-leg stance, maximal leg extensor muscle power, habitual and maximal 10-m gait speed, sit-to-stand performance, and 2-min step test.

RESULTS

After immobilization, leg extension muscle power decreased by 15% in OM (from 2.68±0.60 to 2.29±0.63 W/kg, p<0.05) and 17% in YM (4.37±0.76 to 3.63±0.69 W/kg, p<0.05). Double-leg CoP sway area increased by 45% in OM (218±82 to 317±145 mm²; p<0.05), with no change in YM (p=0.43). Physical function did not change after immobilization but sit-to-stand performance (+20%, p<0.05) and 2-min step test (+28%, p<0.05) increased in OM following retraining. In both groups, all parameters returned to baseline levels after retraining.

CONCLUSION

Two weeks of lower limb immobilization led to decreases in maximal muscle power in both young and old, whereas postural control was impaired selectively in old men. All parameters were restored in both groups after 4 weeks of resistance-based retraining.

Stimuli for Adaptations in Muscle Length and the Length Range of Active Force Exertion-A Narrative Review

Treatment strategies and training regimens, which induce longitudinal muscle growth and increase the muscles’ length range of active force exertion, are important to improve muscle function and to reduce muscle strain injuries in clinical populations and in athletes with limited muscle extensibility. Animal studies have shown several specific loading strategies resulting in longitudinal muscle fiber growth by addition of sarcomeres in series. Currently, such strategies are also applied to humans in order to induce similar adaptations. However, there is no clear scientific evidence that specific strategies result in longitudinal growth of human muscles. Therefore, the question remains what triggers longitudinal muscle growth in humans. The aim of this review was to identify strategies that induce longitudinal human muscle growth. For this purpose, literature was reviewed and summarized with regard to the following topics: (1) Key determinants of typical muscle length and the length range of active force exertion; (2) Information on typical muscle growth and the effects of mechanical loading on growth and adaptation of muscle and tendinous tissues in healthy animals and humans; (3) The current knowledge and research gaps on the regulation of longitudinal muscle growth; and (4) Potential strategies to induce longitudinal muscle growth. The following potential strategies and important aspects that may positively affect longitudinal muscle growth were deduced: (1) Muscle length at which the loading is performed seems to be decisive, i.e., greater elongations after active or passive mechanical loading at long muscle length are expected; (2) Concentric, isometric and eccentric exercises may induce longitudinal muscle growth by stimulating different muscular adaptations (i.e., increases in fiber cross-sectional area and/or fiber length). Mechanical loading intensity also plays an important role. All three training strategies may increase tendon stiffness, but whether and how these changes may influence muscle growth remains to be elucidated. (3) The approach to combine stretching with activation seems promising (e.g., static stretching and electrical stimulation, loaded inter-set stretching) and warrants further research. Finally, our work shows the need for detailed investigation of the mechanisms of growth of pennate muscles, as those may longitudinally grow by both trophy and addition of sarcomeres in series.

EMG Activity With Use of a Hands-Free Single Crutch vs a Knee Scooter

BACKGROUND

Foot and ankle injuries frequently require a period of nonweightbearing, resulting in muscle atrophy. Our previous study compared a hands-free single crutch (HFSC) to standard axillary crutches and found increased muscle recruitment and intensity while using the HFSC. Knee scooters are another commonly prescribed nonweightbearing device. The purpose of this study is to examine the electromyographic (EMG) differences between an HFSC and knee scooter, in conjunction with device preference and perceived exertion.

METHODS

A randomized crossover study was performed using 30 noninjured young adults. Wireless surface EMG electrodes were placed on the belly of the rectus femoris (RF), vastus lateralis (VL), lateral gastrocnemius (LG), and gluteus maximus (GM). Participants then ambulated along a 20-m walking area while 15 seconds of the gait cycle was recorded across 3 conditions: walking with a knee scooter, an HFSC, and with no assistive device. Mean muscle activity and peak EMG activity were recorded for each ambulatory modality. Immediately following testing, patient exertion and device preference was recorded.

RESULTS

The RF, LG, and GM showed increased peak EMG activity percentage, and the LG showed increased mean muscle activity while using the HFSC compared with the knee scooter. When comparing the knee scooter and HFSC to walking, both showed increased muscle activity in the RF, VL, and LG but no difference in the GM. There was no statistical difference in participant preference, whereas the HFSC had a statistically significant higher perceived exertion than the knee scooter (P < .001).

CONCLUSION

In this group of young, healthy noninjured volunteers, the HFSC demonstrated increased peak EMG activity in most muscle groups tested compared with the knee scooter.

LEVEL OF EVIDENCE

Level II, prospective comparative study.

Short-Term Immobilization Promotes a Rapid Loss of Motor Evoked Potentials and Strength That Is Not Rescued by rTMS Treatment

Short-term limb immobilization results in skeletal muscle decline, but the underlying mechanisms are incompletely understood. This study aimed to determine the neurophysiologic basis of immobilization-induced skeletal muscle decline, and whether repetitive Transcranial Magnetic Stimulation (rTMS) could prevent any decline. Twenty-four healthy young males (20 ± 0.5 years) underwent unilateral limb immobilization for 72 h. Subjects were randomized between daily rTMS (n = 12) using six 20 Hz pulse trains of 1.5 s duration with a 60 s inter-train-interval delivered at 90% resting Motor Threshold (rMT), or Sham rTMS (n = 12) throughout immobilization. Maximal grip strength, EMG activity, arm volume, and composition were determined at 0 and 72 h. Motor Evoked Potentials (MEPs) were determined daily throughout immobilization to index motor excitability. Immobilization induced a significant reduction in motor excitability across time (−30% at 72 h; p < 0.05). The rTMS intervention increased motor excitability at 0 h (+13%, p < 0.05). Despite daily rTMS treatment, there was still a significant reduction in motor excitability (−33% at 72 h, p < 0.05), loss in EMG activity (−23.5% at 72 h; p < 0.05), and a loss of maximal grip strength (−22%, p < 0.001) after immobilization. Interestingly, the increase in biceps (Sham vs. rTMS) (+0.8 vs. +0.1 mm, p < 0.01) and posterior forearm (+0.3 vs. +0.0 mm, p < 0.05) skinfold thickness with immobilization in Sham treatment was not observed following rTMS treatment. Reduced MEPs drive the loss of strength with immobilization. Repetitive Transcranial Magnetic Stimulation cannot prevent this loss of strength but further investigation and optimization of neuroplasticity protocols may have therapeutic benefit.

Upper body strength endurance evaluation: A comparison between the handgrip strength and three body weight tests

BACKGROUND: The hand-grip strength test has been widely adopted to evaluate upper limb strength. Other field based tests as push-ups and pull-ups are commonly used for the same purpose. It is however unclear if these may be used interchangeably for upper body strength evaluation. OBJECTIVE: The purpose of this investigation was to evaluate strength endurance of the upper body and understand which test could be the most appropriate for upper body evaluation. METHODS: Thirty-eight healthy young male participants were tested with three tests comprised of: 1) push-ups (PS), 2) pull-ups (PL) and 3) parallel dips (PD) performed to exhaustion. Grip strength (GS), total number of repetitions, time-to-complete the test, repetition cadence and rate of perceived exertion (RPE) were also retrieved for investigation. RESULTS: Repetitions, time-to-complete the test and repetition cadence significantly differed across the three tests (p< 0.001). No difference in the RPE was present. No correlation was present between GS and the other tests. No correlation was present between RPE and performance values and time-to-complete the tests. BMI was positively correlated to RPE in all tests. All tests strongly correlate to each other (PS vs. PL r= 0.55; PS vs. PD r= 0.64; PL vs. PD r= 0.70) and to time-to-complete the test (PS r= 0.79; PL r= 0.69; PD r= 0.66). Only the results of the PD correlate to their respective repetition cadence (r= 0.66). CONCLUSIONS: GS is not suitable to evaluate strength endurance. PS, PL and PD are all suitable to evaluate strength endurance. However, PD may be preferred to evaluate the upper body, if velocity also needs to be taken into account.

Nonuniform loss of muscle strength and atrophy during bed rest: a systematic review

Muscle atrophy and decline in muscle strength appear very rapidly with prolonged disuse or mechanical unloading after acute hospitalization or experimental bed rest. The current study analyzed data from short-, medium-, and long-term bed rest (5-120 days) in a pooled sample of 318 healthy adults and modeled the mathematical relationship between muscle strength decline and atrophy. The results show a logarithmic disuse-induced loss of strength and muscle atrophy of the weight-bearing knee extensor muscles. The greatest rate of muscle strength decline and atrophy occurred in the earliest stages of bed rest, plateauing later, and likely contributed to the rapid neuromuscular loss of function in the early period. In addition, during the first 2 wk of bed rest, muscle strength decline is much faster than muscle atrophy: on day 5, the ratio of muscle atrophy to strength decline as a function of bed rest duration is 4.2, falls to 2.4 on day 14, and stabilizes to a value of 1.9 after ∼35 days of bed rest. Positive regression revealed that ∼79% of the muscle strength loss may be explained by muscle atrophy, while the remaining is most likely due to alterations in single fiber mechanical properties, excitation-contraction coupling, fiber architecture, tendon stiffness, muscle denervation, neuromuscular junction damage, and supraspinal changes. Future studies should focus on neural factors as well as muscular factors independent of atrophy (single fiber excitability and mechanical properties, architectural factors) and on the role of extracellular matrix changes. Bed rest results in nonuniform loss of isometric muscle strength and atrophy over time, where the magnitude of change was greater for muscle strength than for atrophy. Future research should focus on the loss of muscle function and the underlying mechanisms, which will aid in the development of countermeasures to mitigate or prevent the decline in neuromuscular efficiency.NEW & NOTEWORTHY Our study contributes to the characterization of muscle loss and weakness processes reflected by a logarithmic decline in muscle strength induced by chronic bed rest. Acute short-term hospitalization (≤5 days) associated with periods of disuse/immobilization/prolonged time in the supine position in the hospital bed is sufficient to significantly decrease muscle mass and size and induce functional changes related to weakness in maximal muscle strength. By bringing together integrated evaluation of muscle structure and function, this work identifies that 79% of the loss in muscle strength can be explained by muscle atrophy, leaving 21% of the functional loss unexplained. The outcomes of this study should be considered in the development of daily countermeasures for preserving neuromuscular integrity as well as preconditioning interventions to be implemented before clinical bed rest or chronic gravitational unloading (e.g., spaceflights).

The time course of neuromuscular impairment during short-term disuse in young women

Skeletal muscle disuse results in rapid functional declines. Previous studies have typically been at least 1 week in duration and focused on the responsiveness of men. Herein, we report the timeline of initial impairments in strength, voluntary activation (VA), and motor unit control during 2 weeks of knee joint immobilization. Thirteen women (mean age =21 years) underwent 2 weeks of left knee joint immobilization via ambulation on crutches and use of a brace. Participants visited the laboratory for testing on seven occasions (two familiarization visits, pretest, 48 and 72 h, 1 and 2 weeks). Knee extensor isometric and concentric isokinetic strength at two velocities (180 and 360 degrees⋅s^-1 ), VA, and submaximal vastus lateralis motor unit activity were evaluated. Moderate-to-large decreases in isometric and concentric strength at 180 degrees⋅s^-1 and VA were observed within 48 hours. Isometric strength continued to decline beyond 72 h, whereas other variables plateaued. The B-term of the motor unit mean firing rate versus action potential amplitude relationship demonstrated a moderate increase 1 week into immobilization, suggesting that greater firing rates were necessary to maintain pretest torque levels. Concentric strength at a velocity of 360 degrees s^-1 was not affected. Decreases in knee extensor strength occur within a matter of days after immobilization, although the time course and magnitude vary among assessment methods. These changes are mediated by the nervous system's capacity to activate skeletal muscle. Clinically appropriate interventions which target nervous system plasticity should be implemented early to minimize the rapid functional impairments associated with disuse.

Post-COVID-19 Fatigue: Potential Contributing Factors

Much of the spotlight for coronavirus disease 2019 (COVID-19) is on the acute symptoms and recovery. However, many recovered patients face persistent physical, cognitive, and psychological symptoms well past the acute phase. Of these symptoms, fatigue is one of the most persistent and debilitating. In this "perspective article," we define fatigue as the decrease in physical and/or mental performance that results from changes in central, psychological, and/or peripheral factors due to the COVID-19 disease and propose a model to explain potential factors contributing to post-COVID-19 fatigue. According to our model, fatigue is dependent on conditional and physiological factors. Conditional dependency comprises the task, environment, and physical and mental capacity of individuals, while physiological factors include central, psychological, and peripheral aspects. This model provides a framework for clinicians and researchers. However, future research is needed to validate our proposed model and elucidate all mechanisms of fatigue due to COVID-19.

Impact of anti-gravity treadmill rehabilitation therapy on the clinical outcomes after fixation of lower limb fractures: A randomized clinical trial

OBJECTIVE

To compare the effects of anti-gravity treadmill rehabilitation with those of standard rehabilitation on surgically treated ankle and tibial plateau fractures.

DESIGN

Open-label prospective randomized multicenter study.

SETTING

Three level 1 trauma centers.

SUBJECTS

Patients with tibial plateau or ankle fractures who underwent postoperative partial weight-bearing were randomized into the intervention (anti-gravity treadmill use) or control (standard rehabilitation protocol) groups.

MAIN MEASURES

The primary endpoint was the change in the Foot and Ankle Outcome Score for ankle fractures and total Knee injury and Osteoarthritis Outcome Score for tibial plateau fractures (0-100 points) from baseline (T1) to six weeks after operation (T4) in both groups. Leg circumference of both legs was measured to assess thigh muscle atrophy in the operated leg.

RESULTS

Thirty-seven patients constituted the intervention and 36 the control group, respectively; 14 patients dropped out during the follow-up period. Among the 59 remaining patients (mean age 42 [range, 19-65] years), no difference was noted in the Foot and Ankle Outcome Score (54.2 ± 16.1 vs. 56.0 ± 16.6) or Knee injury and Osteoarthritis Outcome Score (52.8 ± 18.3 vs 47.6 ± 17.7) between the intervention and control groups 6 weeks after operation. The change in the leg circumference from T1 to T4 was greater by 4.6 cm in the intervention group (95% confidence interval: 1.2-8.0, P = 0.005). No adverse event associated with anti-gravity treadmill rehabilitation was observed.

CONCLUSION

No significant difference was noted in patient-reported outcomes between the two groups. Significant differences in muscular atrophy of the thigh were observed six weeks after operation.

Contralateral effects of eccentric resistance training on immobilized arm

This study compared the effects of contralateral eccentric-only (ECC) and concentric-/eccentric-coupled resistance training (CON-ECC) of the elbow flexors on immobilized arm. Thirty healthy participants (18-34 y) were randomly allocated to immobilization only (CTRL; n = 10), immobilization and ECC (n = 10), or immobilization and CON-ECC group (n = 10). The non-dominant arms of all participants were immobilized (8 h·day^-1 ) for 4 weeks, during which ECC and CON-ECC were performed by the dominant (non-immobilized) arm 3 times a week (3-6 sets of 10 repetitions per session) with an 80%-120% and 60%-90% of one concentric repetition maximum (1-RM) load, respectively, matching the total training volume. Arm circumference, 1-RM and maximal voluntary isometric contraction (MVIC) strength, biceps brachii surface electromyogram amplitude (sEMG_RMS ), rate of force development (RFD), and joint position sense (JPS) were measured for both arms before and after immobilization. CTRL showed decreases (P < .05) in MVIC (-21.7%), sEMG_RMS (-35.2%), RFD (-26.0%), 1-RM (-14.4%), JPS (-87.4%), and arm circumference (-5.1%) of the immobilized arm. These deficits were attenuated or eliminated by ECC and CON-ECC, with greater effect sizes for ECC than CON-ECC in MVIC (0.29: +12.1%, vs -0.18: -0.1%) and sEMG_RMS (0.31:17.5% vs -0.15: -5.9%). For the trained arm, ECC showed greater effect size for MVIC than CON-ECC (0.47 vs 0.29), and increased arm circumference (+2.9%), sEMG_RMS (+77.9%), and RDF (+31.8%) greater (P < .05) than CON-ECC (+0.6%, +15.1%, and + 15.8%, respectively). The eccentric-only resistance training of the contralateral arm was more effective to counteract the negative immobilization effects than the concentric-eccentric training.

A Systematic Review and Meta-Analysis on Neural Adaptations Following Blood Flow Restriction Training: What We Know and What We Don't Know

Objective: To summarize the existing evidence on the long-term effects of low-load (LL) blood flow restricted (BFR) exercise on neural markers including both central and peripheral adaptations. Methods: A systematic review and meta-analysis was conducted according to the PRISMA guidelines. The literature search was performed independently by two reviewers in the following electronic databases: PubMed, Web of Science, Scopus and CENTRAL. The systematic review included long-term trials investigating the effects of LL-BFR training in healthy subjects and compared theses effects to either LL or high-load (HL) training without blood flow restriction. Results: From a total of N = 4499 studies, N = 10 studies were included in the qualitative synthesis and N = 4 studies in a meta-analysis. The findings indicated that LL-BFR resulted in enhanced levels of muscle excitation compared to LL training with pooled effect sizes of 0.87 (95% CI: 0.38-1.36). Compared to HL training, muscle excitation following LL-BFR was reported as either similar or slightly lower. Differences between central activation between LL-BFR and LL or HL are less clear. Conclusion: The summarized effects in this systematic review and meta-analysis highlight that BFR training facilitates neural adaptations following LL training, although differences to conventional HL training are less evident. Future research is urgently needed to identify neural alterations following long-term blood flow restricted exercise.

Quadriceps muscle electromyography activity during physical activities and resistance exercise modes in younger and older adults

BACKGROUND

Understanding the root cause of the age-related impairment in muscle adaptive remodelling with resistance exercise training (RET) and developing pragmatic and accessible resistance exercise for older adults, are essential research directives.

METHODS

We sought to determine whether indices of quadriceps muscle EMG activity in response to different modes of RET and activities of daily living (ADL), differed between 15 healthy younger (25 ± 3 years) and 15 older (70 ± 5 years) adults. On four separate days, participants completed a maximal voluntary contraction (MVC) of the knee extensors, followed by a 15 m walking task, stair climbing task (i.e. ADL) and lower-limb RET through body-weight squats (BW-RET) and seated knee extensions on a machine (MN-RET) or via elastic bands (EB-RET). Surface quadriceps electromyography (EMG) was measured throughout all tasks to provide indirect estimates of changes in muscle activity.

RESULTS

MVC was significantly greater in young vs. older adults (Young: 256 ± 72 vs. Old: 137 ± 48 N·m, P < 0.001). EMG activity during all exercise tasks was significantly higher in older vs. younger adults when expressed relative to maximal EMG achieved during MVC (P < 0.01, for all). In addition, relative quadriceps muscle EMG activity was significantly greater in EB-RET (Young: 20.3 ± 8.7 vs. Old: 37.0 ± 10.7%) and MN-RET (Young: 22.9 ± 10.3, vs. Old: 37.8 ± 10.8%) compared with BW-RET (Young: 8.6 ± 2.9 vs. Old: 27.0 ± 9.3%), in young and older adults (P < 0.001). However, there was no significant difference in quadriceps EMG between EB-RET and MN-RET (P > 0.05).

CONCLUSIONS

In conclusion, relative quadriceps muscle EMG activity was higher across a range of activities/exercise modes in older vs. younger adults. The similar quadriceps muscle EMG activity between EB-RET and MN-RET provides a platform for detailed investigation of the neuromuscular and muscle metabolic responses to such pragmatic forms of RET to strengthen the evidence-base for this mode of RET as a potential countermeasure to sarcopenia.

Over 25 Years of Pediatric Botulinum Toxin Treatments: What Have We Learned from Injection Techniques, Doses, Dilutions, and Recovery of Repeated Injections?

Botulinum toxin type A (BTXA) has been used for over 25 years in the management of pediatric lower and upper limb hypertonia, with the first reports in 1993. The most common indication is the injection of the triceps surae muscle for the correction of spastic equinus gait in children with cerebral palsy. The upper limb injection goals include improvements in function, better positioning of the arm, and facilitating the ease of care. Neurotoxin type A is the most widely used serotype in the pediatric population. After being injected into muscle, the release of acetylcholine at cholinergic nerve endings is blocked, and a temporary denervation and atrophy ensues. Targeting the correct muscle close to the neuromuscular junctions is considered essential and localization techniques have developed over time. However, each technique has its own limitations. The role of BTXA is flexible, but limited by the temporary mode of action as a focal spasticity treatment and the restrictions on the total dose deliverable per visit. As a mode of treatment, repeated BTXA injections are needed. This literature reviewed BTXA injection techniques, doses and dilutions, the recovery of muscles and the impact of repeated injections, with a focus on the pediatric population. Suggestions for future studies are also discussed.

Declines in skeletal muscle quality vs. size following two weeks of knee joint immobilization

Background

Disuse of a muscle group, which occurs during bedrest, spaceflight, and limb immobilization, results in atrophy. It is unclear, however, if the magnitude of decline in skeletal muscle quality is similar to that for muscle size. The purpose of this study was to examine the effects of two weeks of knee joint immobilization on vastus lateralis and rectus femoris echo intensity and cross-sectional area.

Methods

Thirteen females (mean ± SD age = 21 ± 2 years) underwent two weeks of left knee joint immobilization via ambulating on crutches and use of a brace. B-mode ultrasonography was utilized to obtain transverse plane images of the immobilized and control vastus lateralis and rectus femoris at pretest and following immobilization. Effect size statistics and two-way repeated measures analyses of variance were used to interpret the data.

Results

No meaningful changes were demonstrated for the control limb and the rectus femoris of the immobilized limb. Analyses showed a large increase in vastus lateralis echo intensity (i.e., decreased muscle quality) for the immobilized limb (p = .006, Cohen’s d = .918). For vastus lateralis cross-sectional area, no time × limb interaction was observed (p = .103), but the effect size was moderate (d = .570). There was a significant association between the increase in vastus lateralis echo intensity and the decrease in cross-sectional area (r =  − .649, p = .016).

Conclusion

In female participants, two weeks of knee joint immobilization resulted in greater deterioration of muscle quality than muscle size. Echo intensity appears to be an attractive clinical tool for monitoring muscle quality during disuse.