Effects of lower limb unloading on skeletal muscle mass and function in humans

Previous short-term disuse dictates muscle gene expression and physiological adaptations to subsequent resistance exercise

Short-term unloading experienced following injury or hospitalisation induces muscle atrophy and weakness. The effects of exercise following unloading have been scarcely investigated. We investigated the functional and molecular adaptations to a resistance training (RT) programme following short-term unloading. Eleven males (22.09 ± 2.91 years) underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of knee extensor RT (three times/week). Data collection occurred at Baseline (LS0), after ULLS (LS10) and at active recovery (AR21). Knee extensor maximum voluntary contraction (MVC) was evaluated. Quadriceps volume was estimated by ultrasonography. Muscle fibre cross-sectional area, fibre type distribution, glycogen content and succinate dehydrogenase (SDH) activity were measured from vastus lateralis biopsies. Mitochondrial-related proteins were quantified by western blot and transcriptional responses were assessed by RNA sequencing. Following ULLS, quadriceps volume and MVC decreased significantly (3.7%, P < 0.05; 29.3%, P < 0.001). At AR21 (vs. LS10), MVC was fully restored (42%) and quadriceps volume increased markedly (18.6%, P < 0.001). Glycogen content and whole-body water increased at AR21 (14%, P < 0.001; 3.1%, P < 0.05). We observed a marked increase in fibre type I at AR21 (38%, P < 0.05). SDH immunoreactivity increased significantly after exercise (20%, P < 0.001). Mitochondrial fusion (MFN1, MFN2 and OPA1) and fission (DRP1) proteins were markedly increased by RT, and the most differentially expressed genes belonged to oxidative phosphorylation pathways. In contrast with what is usually observed after RT, oxidative metabolism, slow fibre type and mitochondrial dynamics were enhanced beyond expected. We propose that prior exposure to short-term muscle unloading may drive the nature of molecular adaptations to subsequent RT. KEY POINTS: Short-term unloading is often experienced during recovery from injuries and hospitalisation, leading to loss of muscle mass and strength. Although exercise can be beneficial in mitigating/reversing such alterations during disuse, only a few studies have focused on the effects of exercise following muscle unloading. With an integrative physiological approach, we aimed to elucidate the basic mechanisms of muscle function recovery in response to 21 days of resistance exercise that followed 10 days of unilateral lower limb suspension (ULLS), assessing whether the mechanisms underlying recovery are defined by a specific reversal of those that occurred during disuse. Resistance training was successful in recovering functional and structural muscle properties after 10 days of ULLS, but in contrast with what is usually observed in response to this training modality, oxidative metabolism and slow fibre type were mostly enhanced. We propose that prior exposure to short-term muscle unloading may drive the adaptations to subsequent exercise.

Changes to muscle and fascia tissue after eighteen days of ankle immobilization post-ankle sprain injury: an MRI case study

BACKGROUND

Ankle sprains often result in muscle atrophy and reduced range of motion, which can cause long-term ankle instabilities. Understanding the changes to muscle-such as atrophy-and concomitant changes to deep fascia-which may thicken alongside muscle loss-after ankle sprain injury is important to understanding structural changes about the joint and how they might contribute to longer-term impairments. Here, we employ advanced MRI to investigate skeletal muscle and fascial structural changes during the recovery period of one patient undergoing immobilization after ankle sprains.

MATERIAL AND METHODS

In this case study, a participant who suffered an ankle sprain underwent initial MRI scans and, after 21 days (18 of which included immobilization), a follow-up MRI. Techniques used included proton density, 3D stack of spirals, and diffusion tensor imaging to analyse muscle and fascia changes pre- and post-injury.

RESULTS

Results showed muscle atrophy in most shank muscles, with volume loss ranging from no change in the lateral gastrocnemius to 12.11% in the popliteus. Thigh muscles displayed hypertrophy of 6% in the hamstrings, while the quadriceps atrophied by 2.5%. Additionally, fascia thickness increased from 0.94 mm to 1.03 mm. Diffusion tensor imaging indicated that the biceps femoris experienced the most significant changes in physiological cross-sectional area, while the rectus femoris showed minimal change.

CONCLUSION

The findings highlight the variable responses of muscles and a notable thickening of deep fascia post-injury, underscoring its role in recovery from ankle sprains.

Neuromuscular mechanisms for the fast decline in rate of force development with muscle disuse - a narrative review

The removal of skeletal muscle tension (unloading or disuse) is followed by many changes in the neuromuscular system, including muscle atrophy and loss of isometric maximal strength (measured by maximal force, Fmax). Explosive strength, i.e. the ability to develop the highest force in the shortest possible time, to maximise rate of force development (RFD), is a fundamental neuromuscular capability, often more functionally relevant than maximal muscle strength. In the present review, we discuss data from studies that looked at the effect of muscle unloading on isometric maximal versus explosive strength. We present evidence that muscle unloading yields a greater decline in explosive relative to maximal strength. The longer the unloading duration, the smaller the difference between the decline in the two measures. Potential mechanisms that may explain the greater decline in measures of RFD relative to Fmax after unloading are higher recruitment thresholds and lower firing rates of motor units, slower twitch kinetics, impaired excitation-contraction coupling, and decreased tendon stiffness. Using a Hill-type force model, we showed that this ensemble of adaptations minimises the loss of force production at submaximal contraction intensities, at the expense of a disproportionately lower RFD. With regard to the high functional relevance of RFD on one hand, and the boosted detrimental effects of inactivity on RFD on the other hand, it seems crucial to implement specific exercises targeting explosive strength in populations that experience muscle disuse over a longer time.

Neuromodulatory Contribution to Muscle Force Production after Short-Term Unloading and Active Recovery

PURPOSE

Prior evidence has shown that neural factors contribute to the loss of muscle force after skeletal muscle disuse. However, little is known about the specific neural mechanisms altered by disuse. Persistent inward current (PIC) is an intrinsic property of motoneurons responsible for prolonging and amplifying the synaptic input, proportionally to the level of neuromodulation, thus influencing motoneuron discharge rate and force production. Here, we hypothesized that short-term unilateral lower limb suspension (ULLS) would reduce the neuromodulatory input associated with PIC, contributing to the reduction of force generation capacity. In addition, we tested whether physical exercise would restore the force generation capacity by reestablishing the initial level of neuromodulatory input.

METHODS

In 12 young adults, we assessed maximal voluntary contraction pre- and post-10 d of ULLS and after 21 d of active recovery (AR) based on resistance exercise. PIC was estimated from high-density surface electromyograms of the vastus lateralis muscle as the delta frequency (Δ F ) of paired motor units calculated during isometric ramped contractions.

RESULTS

The values of Δ F were reduced after 10 d of ULLS (-33%, P < 0.001), but were fully reestablished after the AR (+29.4%, P < 0.001). The changes in estimated PIC values were correlated ( r = 0.63, P = 0.004) with the reduction in maximal voluntary contraction after ULLS (-29%, P = 0.002) and its recovery after the AR (+28.5%, P = 0.003).

CONCLUSIONS

Our findings suggest that PIC estimates are reduced by muscle disuse and may contribute to the loss of force production and its recovery with exercise. Overall, this is the first study demonstrating that, in addition to peripheral neuromuscular changes, central neuromodulation is a major contributor to the loss of force generation capacity after disuse, and can be recovered after resistance exercise.

Lower limb suspension induces threshold-specific alterations of motor units properties that are reversed by active recovery

PURPOSE

This study aimed to non-invasively test the hypothesis that (a) short-term lower limb unloading would induce changes in the neural control of force production (based on motor units (MUs) properties) in the vastus lateralis muscle and (b) possible changes are reversed by active recovery (AR).

METHODS

Ten young males underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of AR. During ULLS, participants walked exclusively on crutches with the dominant leg suspended in a slightly flexed position (15°-20°) and with the contralateral foot raised by an elevated shoe. The AR was based on resistance exercise (leg press and leg extension) and executed at 70% of each participant's 1 repetition maximum, 3 times/week. Maximal voluntary isometric contraction (MVC) of knee extensors and MUs properties of the vastus lateralis muscle were measured at baseline, after ULLS, and after AR. MUs were identified using high-density electromyography during trapezoidal isometric contractions at 10%, 25%, and 50% of the current MVC, and individual MUs were tracked across the 3 data collection points.

RESULTS

We identified 1428 unique MUs, and 270 of them (18.9%) were accurately tracked. After ULLS, MVC decreased by 29.77%, MUs absolute recruitment/derecruitment thresholds were reduced at all contraction intensities (with changes between the 2 variables strongly correlated), while discharge rate was reduced at 10% and 25% but not at 50% MVC. Impaired MVC and MUs properties fully recovered to baseline levels after AR. Similar changes were observed in the pool of total as well as tracked MUs.

CONCLUSION

Our novel results demonstrate, non-invasively, that 10 days of ULLS affected neural control predominantly by altering the discharge rate of lower-threshold but not of higher-threshold MUs, suggesting a preferential impact of disuse on motoneurons with a lower depolarization threshold. However, after 21 days of AR, the impaired MUs properties were fully restored to baseline levels, highlighting the plasticity of the components involved in neural control.

Post-operative protein supplementation following orthopaedic surgery: A systematic review

Decreased mechanical loading after orthopaedic surgery predisposes patients to develop muscle atrophy. The purpose of this review was to assess whether the evidence supports oral protein supplementation can help decrease postoperative muscle atrophy and/or improve patient outcomes following orthopaedic surgery. A systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA). PubMed (MEDLINE), Embase, Scopus, and Web of Science were searched for randomized controlled trials that assessed protein or amino acid supplementation in patients undergoing orthopaedic surgery. Two investigators independently conducted the search using relevant Boolean operations. Primary outcomes included functional or physiologic measures of muscle atrophy or strength. Fourteen studies including 611 patients (224 males, 387 females) were analyzed. Three studies evaluated protein supplementation after ACL reconstruction (ACLR), 3 after total hip arthroplasty (THA), 5 after total knee arthroplasty (TKA), and 3 after surgical treatment of hip fracture. Protein supplementation showed beneficial effects across all types of surgery. The primary benefit was a decrease in muscle atrophy compared to placebo as measured by muscle cross sectional area. Multiple authors also demonstrated improved functional measures and quicker achievement of rehabilitation benchmarks. Protein supplementation has beneficial effects on mitigating muscle atrophy in the postoperative period following ACLR, THA, TKA, and surgical treatment of hip fracture. These effects often correlate with improved functional measures and quicker achievement of rehabilitation benchmarks. Further research is needed to evaluate long-term effects of protein supplementation and to establish standardized population-specific regimens that maximize treatment efficacy in the postoperative period.

Long-term human spaceflight and inflammaging: Does it promote aging?

Spaceflight and its associated stressors, such as microgravity, radiation exposure, confinement, circadian derailment and disruptive workloads represent an unprecedented type of exposome that is entirely novel from an evolutionary stand point. Within this perspective, we aimed to review the effects of prolonged spaceflight on immune-neuroendocrine systems, brain and brain-gut axis, cardiovascular system and musculoskeletal apparatus, highlighting in particular the similarities with an accelerated aging process. In particular, spaceflight-induced muscle atrophy/sarcopenia and bone loss, vascular and metabolic changes, hyper and hypo reaction of innate and adaptive immune system appear to be modifications shared with the aging process. Most of these modifications are mediated by molecular events that include oxidative and mitochondrial stress, autophagy, DNA damage repair and telomere length alteration, among others, which directly or indirectly converge on the activation of an inflammatory response. According to the inflammaging theory of aging, such an inflammatory response could be a driver of an acceleration of the normal, physiological rate of aging and it is likely that all the systemic modifications in turn lead to an increase of inflammaging in a sort of vicious cycle. The most updated countermeasures to fight these modifications will be also discussed in the light of their possible application not only for astronauts' benefit, but also for older adults on the ground.

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.

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.

Effects of short-term unloading and active recovery on human motor unit properties, neuromuscular junction transmission and transcriptomic profile

Electrophysiological alterations of the neuromuscular junction (NMJ) and motor unit potential (MUP) with unloading are poorly studied. We aimed to investigate these aspects and the underlying molecular mechanisms with short-term unloading and active recovery (AR). Eleven healthy males underwent a 10-day unilateral lower limb suspension (ULLS) period, followed by 21-day AR based on resistance exercise. Quadriceps femoris (QF) cross-sectional area (CSA) and isometric maximum voluntary contraction (MVC) were evaluated. Intramuscular electromyographic recordings were obtained during 10% and 25% MVC isometric contractions from the vastus lateralis (VL). Biomarkers of NMJ molecular instability (serum c-terminal agrin fragment, CAF), axonal damage (neurofilament light chain) and denervation status were assessed from blood samples and VL biopsies. NMJ and ion channel transcriptomic profiles were investigated by RNA-sequencing. QF CSA and MVC decreased with ULLS. Increased CAF and altered NMJ transcriptome with unloading suggested the emergence of NMJ molecular instability, which was not associated with impaired NMJ transmission stability. Instead, increased MUP complexity and decreased motor unit firing rates were found after ULLS. Downregulation of ion channel gene expression was found together with increased neurofilament light chain concentration and partial denervation. The AR period restored most of these neuromuscular alterations. In conclusion, the human NMJ is destabilized at the molecular level but shows functional resilience to a 10-day unloading period at least at relatively low contraction intensities. However, MUP properties are altered by ULLS, possibly due to alterations in ion channel dynamics and initial axonal damage and denervation. These changes are fully reversed by 21 days of AR. KEY POINTS: We used integrative electrophysiological and molecular approaches to comprehensively investigate changes in neuromuscular integrity and function after a 10-day unilateral lower limb suspension (ULLS), followed by 21 days of active recovery in young healthy men, with a particular focus on neuromuscular junction (NMJ) and motor unit potential (MUP) properties alterations. After 10-day ULLS, we found significant NMJ molecular alterations in the absence of NMJ transmission stability impairment. These findings suggest that the human NMJ is functionally resilient against insults and stresses induced by short-term disuse at least at relatively low contraction intensities, at which low-threshold, slow-type motor units are recruited. Intramuscular electromyography analysis revealed that unloading caused increased MUP complexity and decreased motor unit firing rates, and these alterations could be related to the observed changes in skeletal muscle ion channel pool and initial and partial signs of fibre denervation and axonal damage. The active recovery period restored these neuromuscular changes.

Popliteal Blood Flow With Lower-Extremity Injury Mobility Devices

BACKGROUND

Ambulation devices may differ in their utility, muscle activation patterns, and how they affect regional blood flow. This study aimed to evaluate popliteal blood flow and vessel dimensions in response to ambulation with a hands-free crutch (HFC), axillary crutches (AC), a medical kneeling scooter (MKS), and regular walking in healthy adults.

METHODS

HFC, AC, MKS, and regular walking were completed in a random order by 40 adults aged 18-45 years. Participants ambulated at a comfortable pace for 10 minutes with each device. At baseline and immediately following each trial, a trained operator used diagnostic ultrasonography to capture popliteal vein and artery dimensional and flow characteristics.

RESULTS

Significant increases were observed from baseline (0.65 ± 0.23 cm) in venous diameter following walking (0.71 ± 0.21 cm, P = .012) and MKS (0.73 ± 0.21 cm, P = .003). Venous blood flow was also significantly different between conditions (P = .009) but was only greater following walking (124 ± 79 mL/min) compared to MKS (90 ± 64 mL/min, P = .021). No differences were observed in arterial dimensions between ambulation conditions. Significant increases were found in arterial blood flow from baseline (107 ± 69 mL/min) following walking (184 ± 97 mL/min, P < .001) and HFC (163 ± 86 mL/min, P < .001). Arterial blood flow following walking was greater than AC (132 ± 72 mL/min, P = .016) and MKS (128 ± 74 mL/min, P = .003).

CONCLUSION

We found an average decrease in venous time-averaged mean velocity between walking and use of the MKS, but no such decrease with either HFCs or use of ACs in this healthy experimental cohort.

LEVEL OF EVIDENCE

Level III, diagnostic comparative study.

Testing mechanisms for weight support distribution during inverted quadrupedalism in primates

A key characteristic of primate above-branch arboreal locomotion is hindlimb-biased weight support, subverting the typical mammalian condition in which the majority of the body weight is supported by the forelimb. This shift is thought to reflect an adaptation toward the arboreal niches exploited by early primates. However, above-branch quadrupedalism represents only one locomotor mode employed by primates in arboreal contexts. Inverted quadrupedal gaits, in which primates are suspended beneath branches by their hands and feet, have been documented in more than 50 primate taxa. This gait is characterized by a return to forelimb-biased weight distributions and a transition from peak vertical forces being greatest in the hindlimb to being greatest in the forelimb, which may occur to protect the hindlimb from high magnitudes of tensile loading when inverted. In this study, we compare kinetic and kinematic data during upright and inverted quadrupedalism in Lemur catta, Varecia variegata, Cebus capucinus, and Saimiri sciureus. These data are referenced against a classical inverted quadrupedal model: the two-toed sloth (Choloepus didactylus). Our findings show that inverted quadrupedalism in primates is differentiated from above-branch quadrupedalism by increases in forelimb weight support, forelimb contact times, and both forelimb and hindlimb joint excursions. Previously postulated biomechanical models outlining mechanisms relating to the control of weight support during upright walking do not translate well to inverted quadrupedal walking. We suggest that inverted primates may simply be adopting basal neuromuscular gait characteristics and applying them facultatively to this infrequent locomotor behavior.

Acute Effects of Sedentary Behavior on Ankle Torque Assessed with a Custom-Made Electronic Dynamometer

Inactivity negatively influences general health, and sedentary behaviour is known to impact the musculoskeletal system. The aim of the study was to assess the impact of time spent in active and sedentary behaviour on foot muscle strength. In this observational study, we compared the acute effects of one day of prolonged sitting and one day of low-to-moderate level of activity on ankle torque in one group of eight healthy participants. Peak ankle torque was measured using a portable custom-made electronic dynamometer. Three consecutive maximal voluntary isometric contractions for bilateral plantar flexor and dorsiflexor muscles were captured at different moments in time. The average peak torque significant statistically decreased at 6 h (p = 0.019) in both static and active behaviours, with a higher average peak torque in the active behaviour (p < 0.001). Age, gender, body mass index and average steps did not have any significant influence on the average value of maximal voluntary isometric contraction. The more time participants maintained either static or active behaviour, the less force was observed during ankle torque testation. The static behaviour represented by the sitting position was associated with a higher reduction in the average peak ankle torque during a maximal voluntary isometric contraction when compared to the active behaviour.

Anti-gravity treadmill rehabilitation improves gait and muscle atrophy in patients with surgically treated ankle and tibial plateau fractures after one year: A randomised clinical trial

OBJECTIVE

To compare the one-year postoperative outcomes of anti-gravity treadmill rehabilitation with those of standard rehabilitation in patients with ankle or tibial plateau fractures.

DESIGN

An open-label prospective randomised study.

SETTING

Three trauma centres.

SUBJECTS

Patients were randomised into the intervention (anti-gravity treadmill) or control (standard protocol) rehabilitation group.

MAIN MEASURES

The primary endpoint was changes in the Foot and Ankle Outcome Score for ankle fractures and Knee Injury and Osteoarthritis Outcome Score for tibial plateau fractures from baseline to 12 months after operation. Secondary endpoints were the subscores of these scores, muscle atrophy (leg circumference at 20 cm above and 10 cm below the knee joint) and the Dynamic Gait Index.

RESULTS

Initially, 73 patients (37 vs 36) underwent randomisation. After 12 months, 29 patients in the intervention group and 24 patients in the control group could be analysed. No significant difference was noted in the Foot and Ankle Outcome Score (80.8 ± 18.4 and 78.4 ± 21.1) and Knee Injury and Osteoarthritis Outcome Score (84.8 ± 15.2 and 81.7 ± 17.0). The change in the Dynamic Gait Index from 12 weeks to 12 months differed significantly between the groups (P = 0.04). Patients with tibial plateau fractures had a 3 cm wider thigh circumference in the intervention group than those in the control group (95% confidence interval: -0.2 to 6.3 cm, P = 0.08).

CONCLUSION

One year after surgery, patients who had undergone anti-gravity treadmill rehabilitation showed better gait than patients in the control group, and those with tibial plateau fractures had less muscle atrophy.

Early Weightbearing Versus Nonweightbearing After Operative Treatment of an Ankle Fracture: A Multicenter, Noninferiority, Randomized Controlled Trial

BACKGROUND

Acute ankle fractures can occur during sports activities, and unstable ankle fractures are commonly treated operatively. However, controversy exists about the optimal time to allow weightbearing.

HYPOTHESIS

Early weightbearing after the stable fixation of an ankle fracture is not inferior to nonweightbearing in terms of ankle function assessed at 12 months after injury.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

A total of 258 patients were assessed for eligibility. Of these patients, 194 were randomly allocated to either the early weightbearing group (95 patients who were allowed weightbearing at 2 weeks postoperatively) or the nonweightbearing group (99 patients who were not allowed weightbearing until 6 weeks postoperatively). The primary outcome measure was the mean difference in the Olerud-Molander ankle score (OMAS) between the groups, assessed at the 12-month follow-up examination. The secondary outcome measures were the time to return to preinjury activities and patients' subjective satisfaction. Complications such as hardware loosening or failure, fracture displacement, and nonunion were evaluated.

RESULTS

The mean difference in the OMAS for the early weightbearing group compared with the nonweightbearing group was 1.6 (95% CI, -1.9 to 5.0) in the intention-to-treat analysis. The lower limit of the 95% CI (-1.9) exceeded the noninferiority margin of -8, indicating that early weightbearing was not inferior to nonweightbearing. The difference in the proportion of patients who were satisfied or very satisfied with their treatment was not statistically significant (84.3% vs 76.2%; P = .19); however, the time taken to return to preinjury activities was shorter with early weightbearing than with nonweightbearing (9.1 ± 3.0 vs 11.0 ± 3.0 weeks; P < .001). No cases of nonunion were observed in either group.

CONCLUSION

Early weightbearing after the operative treatment of an unstable ankle fracture was not inferior to nonweightbearing in terms of OMAS assessed at 12 months after injury. The patients' subjective satisfaction was similar between the groups, although the time taken to return to preinjury activities was shorter in the early weightbearing group.

REGISTRATION

NCT02029170 (ClinicalTrials.gov identifier).

Age-related changes in muscle architecture and metabolism in humans: The likely contribution of physical inactivity to age-related functional decline

In the United Kingdom (UK), it is projected that by 2035 people aged >65 years will make up 23 % of the population, with those aged >85 years accounting for 5% of the total population. Ageing is associated with progressive changes in muscle metabolism and a decline in functional capacity, leading to a loss of independence. Muscle metabolic changes associated with ageing have been linked to alterations in muscle architecture and declines in muscle mass and insulin sensitivity. However, the biological features often attributed to muscle ageing are also seen in controlled studies of physical inactivity (e.g. reduced step-count and bed-rest), and it is currently unclear how many of these ageing features are due to ageing per se or sedentarism. This is particularly relevant at a time of home confinements reducing physical activity levels during the Covid-19 pandemic. Current knowledge gaps include the relative contribution that physical inactivity plays in the development of many of the negative features associated with muscle decline in older age. Similarly, data demonstrating positive effects of government recommended physical activity guidelines on muscle health are largely non-existent. It is imperative therefore that research examining interactions between ageing, physical activity and muscle mass and metabolic health is prioritised so that it can inform on the "normal" muscle ageing process and on strategies for improving health span and well-being. This review will focus on important changes in muscle architecture and metabolism that accompany ageing and highlight the likely contribution of physical inactivity to these changes.

The Application of Creatine Supplementation in Medical Rehabilitation

Numerous health conditions affecting the musculoskeletal, cardiopulmonary, and nervous systems can result in physical dysfunction, impaired performance, muscle weakness, and disuse-induced atrophy. Due to its well-documented anabolic potential, creatine monohydrate has been investigated as a supplemental agent to mitigate the loss of muscle mass and function in a variety of acute and chronic conditions. A review of the literature was conducted to assess the current state of knowledge regarding the effects of creatine supplementation on rehabilitation from immobilization and injury, neurodegenerative diseases, cardiopulmonary disease, and other muscular disorders. Several of the findings are encouraging, showcasing creatine's potential efficacy as a supplemental agent via preservation of muscle mass, strength, and physical function; however, the results are not consistent. For multiple diseases, only a few creatine studies with small sample sizes have been published, making it difficult to draw definitive conclusions. Rationale for discordant findings is further complicated by differences in disease pathologies, intervention protocols, creatine dosing and duration, and patient population. While creatine supplementation demonstrates promise as a therapeutic aid, more research is needed to fill gaps in knowledge within medical rehabilitation.

Nucleoprotein-enriched diet enhances protein synthesis pathway and satellite cell activation via ERK1/2 phosphorylation in unloaded rat muscles

NEW FINDINGS

What is the central question of this study? The purpose of this study was to determine whether the nucleotides in a nucleoprotein-enriched diet could ameliorate the unloading-associated decrease in soleus muscle mass and fibre size. What is the main finding and its importance? The results indicate that the nucleotides in the nucleoprotein-enriched diet could ameliorate the unloading-associated decrease in type I fibre size and muscle mass, most probably owing to the activation of protein synthesis pathways and satellite cell proliferation and differentiation via ERK1/2 phosphorylation. Thus, nucleotide supplementation appears to be an effective countermeasure for muscle atrophy.

ABSTRACT

Hindlimb unloading decreases both the protein synthesis pathway and satellite cell activation and results in muscle atrophy. Nucleotides are included in nucleoprotein and provide the benefits of increasing extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. ERK1/2 phosphorylation is also important in the activation of satellite cells, especially for myoblast proliferation and stimulating protein synthesis pathways. Therefore, we hypothesized that nucleotides in the nucleoproteins would ameliorate muscle atrophy by increasing the protein synthesis pathways and satellite cell activation during hindlimb unloading in rat soleus muscle. Twenty-four female Wistar rats were divided into four groups: control rats fed a basal diet without nucleoprotein (CON), control rats fed a nucleoprotein-enriched diet (CON+NP), hindlimb-unloaded rats fed a basal diet (HU) or hindlimb-unloaded rats fed a nucleoprotein-enriched diet (HU+NP). HU for 2 weeks resulted in reductions in phosphorylation of p70S6K and rpS6, the numbers of myoblast determination protein (MyoD)- and myogenin- positive nuclei, type I muscle fibre size and muscle mass. Both CON+NP and HU+NP rats showed an increase in ERK1/2, phosphorylation of p70S6K and rpS6, and the numbers of MyoD- and myogenin-positive nuclei compared with their basal diet groups. The NP diet also ameliorated the unloading-associated decrease in type I muscle fibre size and muscle mass. The results indicate that the nucleotides in the nucleoprotein-enriched diet could ameliorate the unloading-associated decrease in type I fibre size and muscle mass, most probably owing to the activation of protein synthesis pathways and satellite cell proliferation and differentiation via ERK1/2 phosphorylation. Thus, nucleotide supplementation appears to be an effective countermeasure for muscle atrophy.

Quantification of immobilization-induced changes in human calf muscle using speed-of-sound ultrasound: An observational pilot study

Short-term immobilization leads to fatty muscular degeneration, which is associated with various negative health effects. Based on literature showing very high correlations between MRI Dixon fat fraction and Speed-of-Sound (SoS), we hypothesized that we can detect short-term-immobilization-induced differences in SoS.Both calves of 10 patients with a calf cast on one side for a mean duration of 41 ± 26 days were examined in relaxed position using a standard ultrasound machine. Calf perimeters were measured for both sides. A flat Plexiglas-reflector, placed vertically on the opposite side of the probe with the calf in-between, was used as a timing reference for SoS. SoS was both manually annotated by two readers and assessed by an automatic annotation algorithm. The thickness values of the subcutaneous fat and muscle layers were manually read from the B-mode images. Differences between the cast and non-cast calves were calculated with a paired t test. Correlation analysis of SoS and calf perimeter was performed using Pearson's correlation coefficient.Paired t test showed significant differences between the cast and non-cast side for both SoS (P < .01) and leg perimeter (P < .001). SoS was reduced with the number of days after cast installment (r = -0.553, P = .097). No significant differences were found for muscle layer thickness, subcutaneous fat layer thickness, mean fat echo intensity, or mean muscle echo intensity.Short-term-immobilization led to a significant reduction in SoS in the cast calf compared to the healthy calf, indicating a potential role of SoS as a biomarker in detecting immobilization-induced fatty muscular degeneration not visible on B-mode ultrasound.

Quadriceps strength to body weight ratio is a significant indicator for initiating jogging after anterior cruciate ligament reconstruction

BACKGROUND

Quadriceps strength recovery after anterior cruciate ligament (ACL) reconstruction is an important criterion for progress in rehabilitation and return to sports. The purpose of this study was to determine whether quadriceps strength to body weight ratio (QS/BW) is a significant indicator for initiating jogging after ACL reconstruction.

METHODS

Isokinetic quadriceps strength at 60°/s was measured and a jogging trial was completed 3 months after ACL reconstruction with hamstring tendon autograft in 83 patients (36 male, 47 female; mean age, 26.6 ± 12.4 years). Based on the jogging trial results, patients were assigned to either a successful jogging group (mean velocity ≥ 9 km/h) or an unsuccessful jogging group (mean velocity < 9 km/h). The association between QS/BW and successful jogging after surgery was investigated by multivariate logistic regression analysis and the cut-off value was determined by receiver operating characteristic analysis.

RESULTS

Forty-four patients (53.0%) were assigned to the successful jogging group and 39 (47.0%) to the unsuccessful jogging group. QS/BW was independently associated with initiating jogging 3 months after surgery. The cut-off value of QS/BW for successful jogging was 1.45 Nm/kg (area under the curve = 0.94; sensitivity = 88.6%, specificity = 87.2%). All of the patients who initiated jogging with QS/BW of > 1.45 Nm/kg at 3 months returned to sports without recurrence or contralateral injury by 10 months after surgery.

CONCLUSIONS

QS/BW is a significant indicator for safely initiating jogging 3 months after ACL reconstruction. The cut-off value of QS/BW for initiating jogging was 1.45 Nm/kg.

One-leg inactivity induces a reduction in mitochondrial oxidative capacity, intramyocellular lipid accumulation and reduced insulin signalling upon lipid infusion: a human study with unilateral limb suspension

AIMS/HYPOTHESIS

Physical inactivity, low mitochondrial function, increased intramyocellular lipid (IMCL) deposition and reduced insulin sensitivity are common denominators of chronic metabolic disorders, like obesity and type 2 diabetes. Yet, whether low mitochondrial function predisposes to insulin resistance in humans is still unknown.

METHODS

Here we investigated, in an intervention study, whether muscle with low mitochondrial oxidative capacity, induced by one-legged physical inactivity, would feature stronger signs of lipid-induced insulin resistance. To this end, ten male participants (age 22.4 ± 4.2 years, BMI 21.3 ± 2.0 kg/m²) underwent a 12 day unilateral lower-limb suspension with the contralateral leg serving as an active internal control.

RESULTS

In vivo, mitochondrial oxidative capacity, assessed by phosphocreatine (PCr)-recovery half-time, was lower in the inactive vs active leg. Ex vivo, palmitate oxidation to ¹⁴CO₂ was lower in the suspended leg vs the active leg; however, this did not result in significantly higher [¹⁴C]palmitate incorporation into triacylglycerol. The reduced mitochondrial function in the suspended leg was, however, paralleled by augmented IMCL content in both musculus tibialis anterior and musculus vastus lateralis, and by increased membrane bound protein kinase C (PKC) θ. Finally, upon lipid infusion, insulin signalling was lower in the suspended vs active leg.

CONCLUSIONS/INTERPRETATION

Together, these results demonstrate, in a unique human in vivo model, that a low mitochondrial oxidative capacity due to physical inactivity directly impacts IMCL accumulation and PKCθ translocation, resulting in impaired insulin signalling upon lipid infusion. This demonstrates the importance of mitochondrial oxidative capacity and muscle fat accumulation in the development of insulin resistance in humans.

TRIAL REGISTRATION

ClinicalTrial.gov NCT01576250.

FUNDING

PS was supported by a 'VICI' Research Grant for innovative research from the Netherlands Organization for Scientific Research (Grant 918.96.618).

Satellite cells in ageing: use it or lose it

Individuals that maintain healthy skeletal tissue tend to live healthier, happier lives as proper muscle function enables maintenance of independence and actuation of autonomy. The onset of skeletal muscle decline begins around the age of 30, and muscle atrophy is associated with a number of serious morbidities and mortalities. Satellite cells are responsible for regeneration of skeletal muscle and enter a reversible non-dividing state of quiescence under homeostatic conditions. In response to injury, satellite cells are able to activate and re-enter the cell cycle, creating new cells to repair and create nascent muscle fibres while preserving a small population that can return to quiescence for future regenerative demands. However, in aged muscle, satellite cells that experience prolonged quiescence will undergo programmed cellular senescence, an irreversible non-dividing state that handicaps the regenerative capabilities of muscle. This review examines how periodic activation and cycling of satellite cells through exercise can mitigate senescence acquisition and myogenic decline.

Redox modulation of muscle mass and function

Muscle mass and strength are very important for exercise performance. Training-induced musculoskeletal injuries usually require periods of complete immobilization to prevent any muscle contraction of the affected muscle groups. Disuse muscle wasting will likely affect every sport practitioner in his or her lifetime. Even short periods of disuse results in significant declines in muscle size, fiber cross sectional area, and strength. To understand the molecular signaling pathways involved in disuse muscle atrophy is of the utmost importance to develop more effective countermeasures in sport science research.

We have divided our review in four different sections. In the first one we discuss the molecular mechanisms involved in muscle atrophy including the main protein synthesis and protein breakdown signaling pathways. In the second section of the review we deal with the main cellular, animal, and human atrophy models. The sources of reactive oxygen species in disuse muscle atrophy and the mechanism through which they regulate protein synthesis and proteolysis are reviewed in the third section of this review. The last section is devoted to the potential interventions to prevent muscle disuse atrophy with especial consideration to studies on which the levels of endogenous antioxidants enzymes or dietary antioxidants have been tested.

Exercise Countermeasures to Neuromuscular Deconditioning in Spaceflight

The mechanical unloading of spaceflight elicits a host of physiological adaptations including reductions in muscle mass, muscle strength, and muscle function and alterations in central interpretation of visual, vestibular, and proprioceptive information. Upon return to a terrestrial, gravitational environment, these result in reduced function and performance, the potential consequences of which will be exacerbated during exploration missions to austere and distant destinations such as the moon and Mars. Exercise is a potent countermeasure to unloading-induced physiological maladaptations and has been employed since the early days of spaceflight. In-flight exercise hardware has evolved from rudimentary and largely ineffective devices to the current suite onboard the International Space Station (ISS) comprised of a cycle ergometer, treadmill, and resistance exercise device; these contemporary devices have either fully protected or significantly attenuated neuromuscular degradation in spaceflight. However, unlike current microgravity operations on the ISS, future exploration missions will include surface operations in partial gravity environments, which will require greater physiological capacity and work output of their crews. For these flights, it is critical to identify physiological thresholds below which task performance will be impaired and to develop exercise countermeasures-both pre- and in-flight-to ensure that crewmembers are able to safely and effectively complete physically demanding mission objectives. © 2020 American Physiological Society. Compr Physiol 10:171-196, 2020.

Negative Effects of Long-duration Spaceflight on Paraspinal Muscle Morphology

STUDY DESIGN

Prospective case series.

OBJECTIVE

Determine the extent of paraspinal muscle cross-sectional area (CSA) and attenuation change after long-duration spaceflight and recovery on Earth. Determine association between in-flight exercise and muscle atrophy.

SUMMARY OF BACKGROUND DATA

Long-duration spaceflight leads to marked muscle atrophy. However, another negative consequence of disuse is intramuscular fatty infiltration. Notably, few studies have investigated the effects of spaceflight on intramuscular fatty infiltration, or how muscle atrophy is associated with in-flight exercise.

METHODS

We analyzed computed tomography scans of the lumbar spine (L1/L2) from 17 long-duration astronauts and cosmonauts to determine paraspinal muscle CSA and attenuation. Computed tomography scans were collected preflight, postflight, 1-year postflight, and, in a subset, 2 to 4 years postflight. We measured CSA (mm) and attenuation (Hounsfield Units) of the erector spinae (ES), multifidus (MF), psoas (PS), and quadratus lumborum (QL) muscles. We used paired t tests to compare muscle morphology at each postflight time point to preflight values and Pearson correlation coefficients to determine the association between muscle changes and in-flight exercise.

RESULTS

ES, MF, and QL CSA and attenuation were significantly decreased postflight compared with preflight (-4.6% to -8.4% and -5.9% to -8.8%, respectively, p < 0.05 for all). CSA of these muscles equaled or exceeded preflight values upon Earth recovery, however QL and PS attenuation remained below preflight values at 2 to 4 years postflight. More resistance exercise was associated with less decline in ES and MF CSA, but greater decline in PS CSA. Increased cycle ergometer exercise was associated with less decline of QL CSA. There were no associations between in-flight exercise and muscle attenuation.

CONCLUSION

Both CSA and attenuation of paraspinal muscles decline after long-duration spaceflight, but while CSA returns to preflight values within 1 year of recovery, PS and QL muscle attenuation remain reduced even 2 to 4 years postflight. Spaceflight-induced changes in paraspinal muscle morphology may contribute to back pain commonly reported in astronauts.

LEVEL OF EVIDENCE

4.

Sarcopenia: Aging-Related Loss of Muscle Mass and Function

Sarcopenia is a loss of muscle mass and function in the elderly that reduces mobility, diminishes quality of life, and can lead to fall-related injuries, which require costly hospitalization and extended rehabilitation. This review focuses on the aging-related structural changes and mechanisms at cellular and subcellular levels underlying changes in the individual motor unit: specifically, the perikaryon of the α-motoneuron, its neuromuscular junction(s), and the muscle fibers that it innervates. Loss of muscle mass with aging, which is largely due to the progressive loss of motoneurons, is associated with reduced muscle fiber number and size. Muscle function progressively declines because motoneuron loss is not adequately compensated by reinnervation of muscle fibers by the remaining motoneurons. At the intracellular level, key factors are qualitative changes in posttranslational modifications of muscle proteins and the loss of coordinated control between contractile, mitochondrial, and sarcoplasmic reticulum protein expression. Quantitative and qualitative changes in skeletal muscle during the process of aging also have been implicated in the pathogenesis of acquired and hereditary neuromuscular disorders. In experimental models, specific intervention strategies have shown encouraging results on limiting deterioration of motor unit structure and function under conditions of impaired innervation. Translated to the clinic, if these or similar interventions, by saving muscle and improving mobility, could help alleviate sarcopenia in the elderly, there would be both great humanitarian benefits and large cost savings for health care systems.

Effects of post-fracture non-weight-bearing immobilization on muscle atrophy, intramuscular and intermuscular adipose tissues in the thigh and calf

OBJECTIVE

Disuse and/or a non-weight-bearing condition changes muscle composition, with decreased skeletal muscle tissue and increased fat within (intramuscular adipose tissue, IntraMAT) and between (intermuscular adipose tissue, InterMAT) given muscles. Excessive adipose tissue contributes to dysfunctional and metabolically impaired muscle. How these adipose tissues change during orthopedic treatment (e.g., cast immobilization, daily use of crutches) is not well documented. This study aimed to quantify changes in IntraMAT, InterMAT, and thigh and calf muscle tissue during orthopedic treatment.

MATERIALS AND METHODS

We studied 8 patients with fifth metatarsal bone or fibular fractures. The ankle joint involved underwent plaster casting for approximately 4 weeks, with crutches used during that time. Axial T1-weighted MRI at the mid-thigh and a 30% proximal site at the calf were obtained to measure IntraMAT and InterMAT cross-sectional areas (CSAs) and skeletal muscle tissue CSA before treatment and 4 weeks afterward.

RESULTS

Thigh and calf muscle tissue CSAs were significantly decreased from before to after treatment: thigh, 85.8 ± 7.6 to 77.1 ± 7.3 cm²; calf, 53.3 ± 5.5 to 48.9 ± 5.0 cm² (p < 0.05). None of the IntraMAT or InterMAT changes was statistically significant. There was a relation between the percentage change of thigh IntraMAT CSA and muscle tissue CSA (r_s = -0.86, p < 0.01).

CONCLUSIONS

The 4 weeks of treatment primarily induced skeletal muscle atrophy with less of an effect on IntraMAT or InterMAT. There is a risk of increasing IntraMAT relatively by decreasing skeletal muscle tissue size during orthopedic treatment.

Exercise recovery increases skeletal muscle H2O2 emission and mitochondrial respiratory capacity following two-weeks of limb immobilization

Extended periods of skeletal muscle disuse result in muscle atrophy. Following limb immobilization, increased mitochondrial reactive oxygen species (ROS) production may contribute to atrophy through increases in skeletal muscle protein degradation. However, the effect of skeletal muscle disuse on mitochondrial ROS production remains unclear. This study investigated the effect of immobilization, followed by two subsequent periods of restored physical activity, on mitochondrial H₂O₂ emissions in adult male skeletal muscle. Middle-aged men (n = 30, 49.7 ± 3.84 y) completed two weeks of unilateral lower-limb immobilization, followed by two weeks of baseline-matched activity, consisting of 10,000 steps a day, then completed two weeks of three times weekly supervised resistance training. Vastus lateralis biopsies were taken at baseline, post-immobilization, post-ambulatory recovery, and post-resistance-training. High-resolution respirometry was used simultaneously with fluorometry to determine mitochondrial respiration and hydrogen peroxide (H₂O₂) production in permeabilized muscle fibres. Mitochondrial H₂O₂ emission with complex I and II substrates, in the absence of ADP, was greater following immobilization, however, there was no effect on mitochondrial respiration. Both ambulatory recovery and resistance training, following the period of immobilization, increased in mitochondrial H₂O₂ emissions. These data demonstrated that 2 weeks of immobilization increases mitochondrial H₂O₂ emissions, but subsequent retraining periods of ambulatory recovery and resistance training also led to in robust increases in mitochondrial H₂O₂ emissions in skeletal muscle.

Recent advances in musculoskeletal physiotherapy for haemophilia

Physiotherapy is directed towards the movement needs and potential of individuals, providing treatment and rehabilitation to develop, maintain and restore maximum movement and functional ability throughout the lifespan. Recent systematic reviews and randomized controlled trials have extended evidence for the clinical efficacy of physiotherapy interventions and rehabilitation for people with haemophilia. This narrative review synthesizes recent evidence to discuss; differentiating musculoskeletal bleeding and haemophilic arthropathy, efficacy of physiotherapy and rehabilitation for acute musculoskeletal bleeding and arthropathy, as well as monitoring musculoskeletal health. Whilst robust evidence is emerging, there is a need for more well designed randomized clinical studies with larger numbers and homogeneity of participants and collaboration of all researchers and clinicians to identify a core set of outcome measures that can be used to monitor musculoskeletal health.

Hypoxia Aggravates Inactivity-Related Muscle Wasting

Poor musculoskeletal state is commonly observed in numerous clinical populations such as chronic obstructive pulmonary disease (COPD) and heart failure patients. It, however, remains unresolved whether systemic hypoxemia, typically associated with such clinical conditions, directly contributes to muscle deterioration. We aimed to experimentally elucidate the effects of systemic environmental hypoxia upon inactivity-related muscle wasting. For this purpose, fourteen healthy, male participants underwent three 21-day long interventions in a randomized, cross-over designed manner: (i) bed rest in normoxia (NBR; P_iO₂ = 133.1 ± 0.3 mmHg), (ii) bed rest in normobaric hypoxia (HBR; P_iO₂ = 90.0 ± 0.4 mmHg) and ambulatory confinement in normobaric hypoxia (HAmb; P_iO₂ = 90.0 ± 0.4 mmHg). Peripheral quantitative computed tomography and vastus lateralis muscle biopsies were performed before and after the interventions to obtain thigh and calf muscle cross-sectional areas and muscle fiber phenotype changes, respectively. A significant reduction of thigh muscle size following NBR (-6.9%, SE 0.8%; P < 0.001) was further aggravated following HBR (-9.7%, SE 1.2%; P = 0.027). Bed rest-induced muscle wasting in the calf was, by contrast, not exacerbated by hypoxic conditions (P = 0.47). Reductions in both thigh (-2.7%, SE 1.1%, P = 0.017) and calf (-3.3%, SE 0.7%, P < 0.001) muscle size were noted following HAmb. A significant and comparable increase in type 2× fiber percentage of the vastus lateralis muscle was noted following both bed rest interventions (NBR = +3.1%, SE 2.6%, HBR = +3.9%, SE 2.7%, P < 0.05). Collectively, these data indicate that hypoxia can exacerbate inactivity-related muscle wasting in healthy active participants and moreover suggest that the combination of both, hypoxemia and lack of activity, as seen in COPD patients, might be particularly harmful for muscle tissue.

The Influence of Ambulatory Aid on Lower-Extremity Muscle Activation During Gait

Context: Foot and ankle injuries are common and often require a nonweight-bearing period of immobilization for the involved leg. This nonweight-bearing period usually results in muscle atrophy for the involved leg. There is a dearth of objective data describing muscle activation for different ambulatory aids that are used during the aforementioned nonweight-bearing period. Objective: To compare activation amplitudes for 4 leg muscles during (1) able-bodied gait and (2) ambulation involving 3 different ambulatory aids that can be used during the acute phase of foot and ankle injury care. Design: Within-subject, repeated measures. Setting: University biomechanics laboratory. Participants: Sixteen able-bodied individuals (7 females and 9 males). Intervention: Each participant performed able-bodied gait and ambulation using 3 different ambulatory aids (traditional axillary crutches, knee scooter, and a novel lower-leg prosthesis). Main Outcome Measure: Muscle activation amplitude quantified via mean surface electromyography amplitude throughout the stance phase of ambulation. Results: Numerous statistical differences (P < .05) existed for muscle activation amplitude between the 4 observed muscles, 3 ambulatory aids, and able-bodied gait. For the involved leg, comparing the 3 ambulatory aids: (1) knee scooter ambulation resulted in the greatest vastus lateralis activation, (2) ambulation using the novel prosthesis and traditional crutches resulted in greater biceps femoris activation than knee scooter ambulation, and (3) ambulation using the novel prosthesis resulted in the greatest gastrocnemius activation (P < .05). Generally speaking, muscle activation amplitudes were most similar to able-bodied gait when subjects were ambulating using the knee scooter or novel prosthesis. Conclusions: Type of ambulatory aid influences muscle activation amplitude. Traditional axillary crutches appear to be less likely to mitigate muscle atrophy during the nonweighting, immobilization period that often follows foot or ankle injuries. Researchers and clinicians should consider these results when recommending ambulatory aids for foot or ankle injuries.

Effect of bimagrumab on thigh muscle volume and composition in men with casting-induced atrophy

BACKGROUND

Patients experiencing disuse atrophy report acute loss of skeletal muscle mass which subsequently leads to loss of strength and physical capacity. In such patients, especially the elderly, complete recovery remains a challenge even with improved nutrition and resistance exercise. This study aimed to explore the clinical potential of bimagrumab, a human monoclonal antibody targeting the activin type II receptor, for the recovery of skeletal muscle volume from disuse atrophy using an experimental model of lower extremity immobilization.

METHODS

In this double-blind, placebo-controlled trial, healthy young men (n = 24; mean age, 24.1 years) were placed in a full-length cast of one of the lower extremities for 2 weeks to induce disuse atrophy. After cast removal, subjects were randomized to receive a single intravenous (i.v.) dose of either bimagrumab 30 mg/kg (n = 15) or placebo (n = 9) and were followed for 12 weeks. Changes in thigh muscle volume (TMV) and inter-muscular adipose tissue (IMAT) and subcutaneous adipose tissue (SCAT) of the thigh, maximum voluntary knee extension strength, and safety were assessed throughout the 12 week study.

RESULTS

Casting resulted in an average TMV loss of -4.8% and comparable increases in IMAT and SCAT volumes. Bimagrumab 30 mg/kg i.v. resulted in a rapid increase in TMV at 2 weeks following cast removal and a +5.1% increase above pre-cast levels at 12 weeks. In comparison, TMV returned to pre-cast level at 12 weeks (-0.1%) in the placebo group. The increased adiposity of the casted leg was sustained in the placebo group and decreased substantially in the bimagrumab group at Week 12 (IMAT: -6.6%, SCAT: -3.5%). Knee extension strength decreased by ~25% in the casted leg for all subjects and returned to pre-cast levels within 6 weeks after cast removal in both treatment arms. Bimagrumab was well tolerated with no serious or severe adverse events reported during the study.

CONCLUSIONS

A single dose of bimagrumab 30 mg/kg i.v. safely accelerated the recovery of TMV and reversal of accumulated IMAT following 2 weeks in a joint-immobilizing cast.

Interventional strategies to combat muscle disuse atrophy in humans: focus on neuromuscular electrical stimulation and dietary protein

Numerous situations, such as the recovery from illness or rehabilitation after injury, necessitate a period of muscle disuse in otherwise healthy individuals. Even a few days of immobilization or bed rest can lead to substantial loss of skeletal muscle tissue and compromise metabolic health. The decline in muscle mass is attributed largely to a decline in postabsorptive and postprandial muscle protein synthesis rates. Reintroduction of some level of muscle contraction by the application of neuromuscular electrical stimulation (NMES) can augment both postabsorptive and postprandial muscle protein synthesis rates and, as such, prevent or attenuate muscle loss during short-term disuse in various clinical populations. Whereas maintenance of habitual dietary protein consumption is a prerequisite for muscle mass maintenance, supplementing dietary protein above habitual intake levels does not prevent muscle loss during disuse in otherwise healthy humans. Combining the anabolic properties of physical activity (or surrogates) with appropriate nutritional support likely further increases the capacity to preserve skeletal muscle mass during a period of disuse. Therefore, effective interventional strategies to prevent or alleviate muscle disuse atrophy should include both exercise (mimetics) and appropriate nutritional support.

The effect of thumb splinting on thenar muscles atrophy, pain, and function in subjects with thumb carpometacarpal joint osteoarthritis

BACKGROUND

When the first carpometacarpal joint of the wrist is immobilized using an orthosis to combat the effects of osteoarthritis, atrophy of the thenar muscles may occur.

OBJECTIVES

The aim of this study was to evaluate the thenar muscle diameter and cross-sectional area, joint function, and pain, before and after being supplied with an orthosis in patients with grades 1 and 2 carpometacarpal osteoarthritis compared to a control group.

STUDY DESIGN

Randomized clinical trial.

METHODS

A total of 25 volunteer patients were randomized into two groups (an orthosis group and a control group) using a randomization table. A visual analog scale, the Michigan Hand Questionnaire, and ultrasound were used to measure pain, function, and specific muscle cross-sectional areas at baseline and after 4 weeks in both groups.

RESULTS

Mean visual analog scale pain scores decreased by 20% after 4 weeks of splinting, while those in the control group decreased by 3%. Changes in scores were significantly different between both groups ( p = 0.001). There was no significant difference between the groups in either the Michigan Hand Questionnaire score or the muscle cross-sectional area.

CONCLUSION

A large and significant effect on perceived pain in patients with first carpometacarpal joint osteoarthritis was observed after 4 weeks of splint use. Differences in treatment effects were found with regard to muscle cross-sectional areas, but these were not significant. Clinical relevance Custom-made splints may be recommended for the treatment of first carpometacarpal joint osteoarthritis. Moderate to large but non-significant treatment effects were found with regard to muscle cross-sectional areas.

EFFECTS OF EXERCISE ON PHYSICAL FITNESS AND ANTHROPOMETRIC VARIABLES IN EX-CRACK COCAINE USERS

ABSTRACT Introduction: Worldwide cocaine use in all its various forms is increasing; cocaine users exceeded 17 million in the world. In Brazil, this data is also alarming. A survey conducted in 2010 found that the country has more than 900,000 crack-cocaine users. Objective: To evaluate the effects of exercise on anthropometric variables and components of physical fitness in ex-crack cocaine users. Methods: Randomized controlled trial with 20 men, divided into exercise group (n=10) and control group (n=10), admitted to a detoxification center. We assessed the physical fitness components related to health (cardiorespiratory endurance, flexibility, muscular strength/endurance, and body composition) before and after the physical training program. Results: The exercise contributed to the maintenance of anthropometric variables, while the control group had an increased in total body fat and visceral fat. Regarding physical fitness, resistance training led to the increase of most variables studied, particularly strength and cardiorespiratory capacity. On the other hand, the VO2max and the strength of the sedentary subjects were reduced (P<0.05). Conclusion: The exercise showed beneficial effects on the components of physical fitness and maintenance of body composition.

Neuromuscular adaptability of male and female rats to muscle unloading

Previously, it has been shown that following muscle unloading, males and females experience different maladaptations in neuromuscular function. As a follow-up, the present investigation sought to determine if male and female neuromuscular systems demonstrated similar, or disparate morphological adaptations to muscle unloading. Twenty young adult male, and 20 young adult female rats were randomly assigned to one of two treatment protocols: muscle unloading, or control conditions. Following the 2-week intervention period, immunofluorescent procedures were used to quantify pre- and post-synaptic features of neuromuscular junctions (NMJs), and to assess myofiber profiles (size and fiber type composition) of the soleus, plantaris, and EDL muscles. A 2-way ANOVA with main effects for sex and treatment was then used to identify statistically significant (p ≤ .05) differences among structural parameters. Analysis of NMJs showed a consistent lack of differences between males and females. Overall, NMJs were also found to be resistant to the effects of unloading. When examining myofiber profiles, however, male myofibers were revealed to be significantly larger than female ones in each of the muscles examined. Unloading resulted in significant myofiber atrophy only in the primarily weight-bearing soleus muscle. Only the EDL showed unloading-induced differences in myofiber type distribution (Type II → I). These data indicate that different components of the neuromuscular system (NMJs, myofibers) respond uniquely to unloading, and that sex affects myofiber type profiles, but not NMJs. Moreover, it appears that only muscles that have their habitual activity patterns disturbed by unloading (i.e., the soleus, adapt to that intervention).

Human skeletal muscle fibre contractile properties and proteomic profile: adaptations to 3 weeks of unilateral lower limb suspension and active recovery

KEY POINTS

It is generally assumed that muscle fibres go through atrophy following disuse with a loss of specific force and an increase in unloaded shortening velocity. However, the underlying mechanisms remain to be clarified. Most studies have focused on events taking place during the development of disuse, whereas the subsequent recovery phase, which is equally important, has received little attention. Our findings support the hypotheses that the specific force of muscle fibres decreased following unilateral lower limb suspension (ULLS) and returned to normal after 3 weeks of active recovery as a result of a loss and recovery of myosin and actin content. Furthermore, muscle fibres went through extensive qualitative changes in muscle protein pattern following ULLS, and these were reversed by active recovery. Resistance training was very effective in restoring both muscle mass and qualitative muscle changes, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

ABSTRACT

Following disuse, muscle fibre function goes through adaptations such as a loss of specific force (PO /CSA) and an increase in unloaded shortening velocity, which could be a result of both quantitative changes (i.e. atrophy) and qualitative changes in protein pattern. The underlying mechanisms remain to be clarified. In addition, little is known about the recovery of muscle mass and strength following disuse. In the present study, we report an extensive dataset describing, in detail,the functional and protein content adaptations of skeletal muscle in response to both disuse and re-training. Eight young healthy subjects were subjected to 3 weeks of unilateral lower limb suspension (ULLS), a widely used human model of disuse skeletal muscle atrophy. Needle biopsies samples were taken from the vastus lateralis muscle Pre-ULLS, Post-ULLS and after 3 weeks of recovery during which heavy resistance training was performed. After disuse, cross-sectional area (CSA), PO /CSA and myosin concentration (MC) decreased in both type 1 and 2A skinned muscle fibres. After recovery, CSA and MC returned to levels comparable to those observed before disuse, whereas Po/CSA and unloaded shortening velocity reached a higher level. Myosin heavy chain isoform composition of muscle samples did not differ among the experimental groups. To study the mechanisms underlying such adaptations, a two-dimensional proteomic analysis was performed. ULLS induced a reduction of myofibrillar, metabolic (glycolytic and oxidative) and anti-oxidant defence system protein content. Resistance training was very effective in counteracting ULLS-induced alterations, indicating that long-term ULLS did not prevent the positive effect of exercise on human muscle.

Unilateral lower limb suspension: From subject selection to “omic” responses

The unilateral lower limb suspension (ULLS) method was developed, introduced, and validated in the quest for a simple, effective, and highly reliable human analog to study the consequences of spaceflight on muscle size and function. Because withdrawal of weight bearing for no more than 2–3 days is sufficient to inflict disturbances in protein metabolism of postural muscles, it is imperative ULLS serves as a very powerful method to manifest skeletal muscle adaptations similar to those experienced in 0 g. Thus the rate of global muscle loss appears rather constant over the first 2 mo, amounting to about 2–3% per week. At the microscopic level, these changes are accompanied by a corresponding decrease in individual muscle fiber size. ULLS alters metabolism favoring more carbohydrate over fat substrate utilization. Altogether, these changes result in impaired work and endurance capacity of muscles being subjected to ULLS. Maximal voluntary force decreases out of proportion to the muscle loss, suggesting motor control is modified. Past reviews offer near exhaustive information on ULLS-induced responses with regard to the above changes. Hence, the current brief review describes more broadly the evolution of the ULLS model, from issues of subject recruitment and compliance control, to recent advances unraveling molecular mechanisms facilitating unloading-induced muscle wasting. Such knowledge is critical in designing future studies aimed at exploring and developing exercise countermeasures or other means to combat the debilitating effects on muscle experienced by astronauts during long-haul missions in Orbit.

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

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

Impact of creatine on muscle performance and phosphagen stores after immobilization

PURPOSE

This study investigated the effect of creatine (CR) supplementation during cast-immobilization to preserve skeletal muscle total work, power and intramuscular phosphocreatine (PCr) kinetics during dynamic exercise.

METHODS

Twenty-five active individuals (24 ± 4 years,) performed wrist flexion exercise within a 1.9 Tesla superconducting magnet before and after 1 week of cast-immobilization. An incremental protocol to fatigue and two constant load (CL1 and CL2) exercise bouts were performed. While casted, participants consumed either 20 g day(-1) of CR or a placebo (PLA). (31)P magnetic resonance spectroscopy was used to quantify in vivo intramuscular PCr levels.

RESULTS

No significant group × time interaction effects were found for work or power throughout all exercise bouts. Total work was significantly reduced over time in both groups (p = 0.049) during the incremental exercise bout. Work production in CL1 tended (p = 0.073) to attenuate in the CR group, compared to PLA. No changes were observed in CL2. Baseline PCr significantly decreased with casting in PLA (PRE: 26.6 ± 6.3 vs. POST: 22.5 ± 5.6 mM kg(-1) wet muscle, p = 0.003). No change (p = 0.31) was observed in the CR group. Changes in work production were significantly correlated with changes in resting PCr in CR (r = -0.63, p = 0.021) but not PLA (r = -0.36, p = 0.26) group.

CONCLUSIONS

Results suggest decreases in short-term endurance may be due to alternations of PCr status and/or metabolism. More research is needed to fully determine the efficacy of CR supplementation during short-term immobilization.

Alterations in bone mineral density and lower extremity lean mass after hip arthroscopy in a professional female Ironman triathlete: a case study

INTRODUCTION

Femoroacetabular impingement is a pathomechanical condition of the hip, which is often treated through arthroscopic techniques. The ensuing immobilization period is associated with decreases in muscle mass and bone mass. To date, minimal knowledge is present about the development of tissue mass during the considerably short rehabilitation period before returning to competition in elite endurance athletes.

CASE DESCRIPTION

Before and after surgery, a professional female Ironman triathlete underwent dual-energy X-ray absorptiometry and peripheral quantitative computed tomography measurements.

DISCUSSION AND EVALUATION

Areal bone mineral density (aBMD) of the proximal femur and lower extremity lean mass decreased in the surgically treated lower extremity during the two-month period of immobilization after the hip arthroscopy. These losses were compensated for after only six weeks of rehabilitation. A similar progression of aBMD values was observed in the lumbar spine. The adaptational pattern in volumetric BMD (vBMD) and volumetric bone mineral content (vBMC) of the tibiae were more complex, but attained pre-immobilization values for most variables also after six weeks of rehabilitation. All other variables attained pre-immobilization values no later than nine months after the surgical intervention.

CONCLUSIONS

The athlete showed a high plasticity of bone and lean tissue with an optimal short- and midterm outcome. Following a two months immobilization period after a hip arthroscopy, aBMD, vBMD and vBMC achieved pre-surgical levels after four months of rehabilitation in a female Ironman triathlete. A nine-month follow-up measurement confirmed the safety of the fast return to sport.

Mechanisms modulating skeletal muscle phenotype

Mammalian skeletal muscles are composed of a variety of highly specialized fibers whose selective recruitment allows muscles to fulfill their diverse functional tasks. In addition, skeletal muscle fibers can change their structural and functional properties to perform new tasks or respond to new conditions. The adaptive changes of muscle fibers can occur in response to variations in the pattern of neural stimulation, loading conditions, availability of substrates, and hormonal signals. The new conditions can be detected by multiple sensors, from membrane receptors for hormones and cytokines, to metabolic sensors, which detect high-energy phosphate concentration, oxygen and oxygen free radicals, to calcium binding proteins, which sense variations in intracellular calcium induced by nerve activity, to load sensors located in the sarcomeric and sarcolemmal cytoskeleton. These sensors trigger cascades of signaling pathways which may ultimately lead to changes in fiber size and fiber type. Changes in fiber size reflect an imbalance in protein turnover with either protein accumulation, leading to muscle hypertrophy, or protein loss, with consequent muscle atrophy. Changes in fiber type reflect a reprogramming of gene transcription leading to a remodeling of fiber contractile properties (slow-fast transitions) or metabolic profile (glycolytic-oxidative transitions). While myonuclei are in postmitotic state, satellite cells represent a reserve of new nuclei and can be involved in the adaptive response.

Isotopic decay of urinary or plasma 3-methylhistidine as a potential biomarker of pathologic skeletal muscle loss

BACKGROUND

Skeletal muscle loss accompanying aging or cancer is associated with reduced physical function and predicts morbidity and mortality. 3-Methylhistidine (3MH) has been proposed as a biomarker of myofibrillar proteolysis, which may contribute to skeletal muscle loss.

METHODS

We hypothesized that the terminal portion of the isotope decay curve following an oral dose of isotopically labeled 3MH can be measured non-invasively from timed spot urine samples. We investigated the feasibility of this approach by determining isotope enrichment in spot urine samples and corresponding plasma samples and whether meat intake up to the time of dosing influences the isotope decay.

RESULTS

Isotope decay constants (k) were similar in plasma and urine, regardless of diet. Post hoc comparison of hourly sampling over 10 h with three samples distributed over 10 or fewer hours suggests that three distributed samples over 5-6 h of plasma or urine sampling yield decay constants similar to those obtained over 10 h of hourly sampling.

CONCLUSION

The findings from this study suggest that an index of 3MH production can be obtained from an easily administered test involving oral administration of a stable isotope tracer of 3MH followed by three plasma or urine samples collected over 5-6 h the next day.

Skeletal muscle fatigue

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

Hemin, heme oxygenase-1 inducer, attenuates immobilization-induced skeletal muscle atrophy in mice

AIMS

The present study examined the effect of the heme oxygenase (HO)-1 inducer hemin on skeletal muscle atrophy induced by single limb immobilization in mice.

MAIN METHODS

Immobilization was conducted in the left hindlimb of C57BL/6 mice for 1 week and the right hindlimb was used as a control. Hemin (30 mg/kg) was administered intraperitoneally once a day during the immobilization period. Gastrocnemius muscles were used for analysis. Muscle weight was measured to quantify degree of atrophy, and exhaustion treadmill test was performed to assess muscle function.

KEY FINDINGS

Immobilization increased HO-1 protein levels in skeletal muscle, which was further increased by hemin treatment. Immobilization induced weight loss and a functional reduction in skeletal muscle, which were attenuated by hemin treatment. Gene expression and protein levels of MuRF1 and atrogin-1 were increased by immobilization and hemin treatment attenuated the increment. The phosphorylation of mTOR and p70S6k was decreased by immobilization in skeletal muscle and hemin had no effect on mTOR and p70S6k phosphorylation. Gene expression of the antioxidants superoxide dismutase and glutathione peroxidase 1 in skeletal muscle was reduced by immobilization and hemin treatment recovered the reduction. Immobilization increased levels of carbonylated protein and nitrotyrosine in skeletal muscle, which was reversed by hemin treatment. Gene expression of inflammatory cytokines was increased by immobilization and was normalized as a result of hemin treatment.

SIGNIFICANCE

These results suggest that hemin attenuates immobilization-induced skeletal muscle atrophy through the suppression of protein degradation via its anti-oxidant and anti-inflammatory properties.