Adaptations in human neuromuscular function following prolonged unweighting: I. Skeletal muscle contractile properties and applied ischemia efficacy

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.

A cross-sectional study of ageing at the mouse neuromuscular junction and effects of an experimental therapeutic approach for dynapenia

Despite prior efforts to understand and target dynapenia (age-induced loss of muscle strength), this condition remains a major challenge that reduces the quality of life in the aged population. We have focused on the neuromuscular junction (NMJ) where changes in structure and function have rarely been systematically studied as a dynamic and progressive process. Our cross-sectional study found neurotransmission at the male mouse NMJ to be biphasic, displaying an early increase followed by a later decrease, and this phenotype was associated with structural changes to the NMJ. A cross-sectional characterization showed that age-induced alterations fell into four age groups: young adult (3-6 months), adult (7-18 months), early aged (19-24 months), and later aged (25-30 months). We then utilized a small molecule therapeutic candidate, GV-58, applied acutely during the later aged stage to combat age-induced reductions in transmitter release by increasing calcium influx during an action potential, which resulted in a significant increase in transmitter release. This comprehensive study of neuromuscular ageing at the NMJ will enable future research to target critical time points for therapeutic intervention. KEY POINTS: Age-induced frailty and falls are the leading causes of injury-related death and are caused by an age-induced loss of muscle strength due to a combination of neurological and muscular changes. A cross-sectional approach was used to study age-induced changes to the neuromuscular junction in a mouse model, and physiological changes that were biphasic over the ageing time course were found. Changes in physiology at the neuromuscular junction were correlated with alterations in neuromuscular junction morphology. An acutely applied positive allosteric gating modifier of presynaptic voltage-gated calcium channels was tested as a candidate therapeutic strategy that could increase transmitter release at aged neuromuscular junctions. These results provide a detailed time course of age-induced changes at the neuromuscular junction in a mouse model and test a candidate therapeutic strategy for weakness.

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.

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.

Sex-Related Differences in Functional Fitness Outcomes in Older Adults

Sex-related differences in changes in functional fitness over time were longitudinally assessed in older adults participating in a group-based multimodal exercise program. From a database, functional fitness scores were obtained for 89 older adults (71.6 ± 6.5 years old) who had completed two assessments, 5-8 years apart. Lower body strength, upper body strength, aerobic endurance, flexibility, and change of direction performances were compared over time and with normative values. Females (p = .02), but not males, had an improvement in upper body strength over time. Females were also more flexible than males at both assessments (p ≤ .02). Of those who had five consecutive assessments, females were more flexible than males (p ≤ .05) and had a faster change of direction ability (p < .001). When compared with normative values, our results indicate that typical time-related functional fitness loss can be attenuated with group exercise. Our results further support the need to tailor exercise prescription according to the individual.

Extraterrestrial Gynecology: Could Spaceflight Increase the Risk of Developing Cancer in Female Astronauts? An Updated Review

Outer space is an extremely hostile environment for human life, with ionizing radiation from galactic cosmic rays and microgravity posing the most significant hazards to the health of astronauts. Spaceflight has also been shown to have an impact on established cancer hallmarks, possibly increasing carcinogenic risk. Terrestrially, women have a higher incidence of radiation-induced cancers, largely driven by lung, thyroid, breast, and ovarian cancers, and therefore, historically, they have been permitted to spend significantly less time in space than men. In the present review, we focus on the effects of microgravity and radiation on the female reproductive system, particularly gynecological cancer. The aim is to provide a summary of the research that has been carried out related to the risk of gynecological cancer, highlighting what further studies are needed to pave the way for safer exploration class missions, as well as postflight screening and management of women astronauts following long-duration spaceflight.

A bibliometric analysis study of blood flow restriction using CiteSpace

[Purpose] To assess the current state-of-the-art and the prevailing trends regarding the global use of blood flow restriction (BFR) in the past 20 years. [Participants and Methods] We retrieved literature relating to BFR from 1999 to 2020 using Web of Science. We conducted a bibliometric analysis of countries/institutions, cited journals, authors/cited authors, cited references, and keywords using CiteSpace. An analysis of counts and centrality was used to examine publication output, countries/institutions, core journals, active authors, foundation references, hot topics, and frontiers. [Results] Seven hundred seventy five references were included and the total number of publications has been continually increasing over the investigated period. Representatives of important academic groups are the Japanese scholars from the University of Tokyo as represented by Takashi Abe. Jeremy Paul Loenneke’s article (centrality: 0.15) was the most representative and symbolic reference with the highest centrality. The three topics identified were intervention (intensity resistance exercise, IRE), physiology (ischemia and muscular function) and behavior (adaptation and increase). The four frontier topics were phosphorylation, reduction, low intensity and arterial occlusion. [Conclusion] This study provides an insight into BFR and offers valuable information for BFR researchers to identify new perspectives for potential cooperation with collaborators and their related cooperative institutions.

Dynapaenia and sarcopaenia in chronic haemodialysis patients: do muscle weakness and atrophy similarly influence poor outcome?

BACKGROUND

Sarcopaenia, defined as a decline in both muscle mass and function, has been recognized as a major determinant of poor outcome in haemodialysis (HD) patients. It is generally assumed that sarcopaenia is driven by muscle atrophy related to protein-energy wasting. However, dynapaenia, defined as weakness without atrophy, has been characterized by a different disease phenotype from sarcopaenia. The aim of this study was to compare the characteristics and prognosis of sarcopaenic and dynapaenic patients among a prospective cohort of chronic HD (CHD) patients.

METHODS

Two hundred and thirty-two CHD patients were enrolled from January to July 2016 and then followed prospectively until December 2018. At inclusion, weakness and atrophy were, respectively, evaluated by maximal voluntary force (MVF) and creatinine index (CI). Sarcopaenia was defined as the association of weakness and atrophy (MVF and CI below the median) while dynapaenia was defined as weakness not related to atrophy (MVF below the median, and CI above the median).

RESULTS

From a total of 187 prevalent CHD patients [65% of men, age 65.3 (49.7-82.0) years], 44 died during the follow-up period of 23.7 (12.4-34.9) months. Sarcopaenia and dynapaenia were observed in 33.7 and 16% of the patients, respectively. Compared with patients with sarcopaenia, patients with dynapaenia were younger and with a lower Charlson score. In contrast, mortality rate was similar in both groups (38 and 27%, respectively). After adjustment for age, sex, lean tissue index, serum albumin, high-sensitivity C-reactive protein (hs-CRP), haemoglobin (Hb), normalized protein catabolic rate (nPCR), dialysis vintage and Charlson score, only patients with dynapaenia were at increased risk of death [hazard ratio (HR) = 2.99, confidence interval 1.18-7.61; P = 0.02].

CONCLUSIONS

Screening for muscle functionality is highly warranted to identify patients with muscle functional impairment without muscle atrophy. In contrast to sarcopaenia, dynapaenia should appear as a phenotype induced by uraemic milieu, characterized by young patients with low Charlson score and poor prognosis outcome independently of serum albumin, hs-CRP, Hb, nPCR and dialysis vintage.

Peripheral nerve adaptations to 10 days of horizontal bed rest in healthy young adult males

Space analogs, such as bed rest, are used to reproduce microgravity-induced morphological and physiological changes and can be used as clinical models of prolonged inactivity. Nevertheless, nonuniform decreases in muscle mass and function have been frequently reported, and peripheral nerve adaptations have been poorly studied, although some of these mechanisms may be explained. Ten young healthy males (18-33 yr) underwent 10 days of horizontal bed rest. Peripheral neurophysiological assessments were performed bilaterally for the dominant (DL) and nondominant upper and lower limbs (N-DL) on the 1st and 10th day of bed rest, including ultrasound of the median, deep peroneal nerve (DPN), and common fibular nerve (CFN) , as well as a complete nerve conduction study (NCS) of the upper and lower limbs. Consistently, reduced F waves, suggesting peripheral nerve dysfunction, of both the peroneal (DL: P = 0.005, N-DL: P = 0.013) and tibial nerves (DL: P = 0.037, N-DL: P = 0.005) were found bilaterally, whereas no changes were observed in nerve ultrasound or other parameters of the NCS of both the upper and lower limbs. In these young healthy males, only the F waves, known to respond to postural changes, were significantly affected by short-term bed rest. These preliminary results suggest that during simulated microgravity, most changes occur at the muscle or central nervous system level. Since the assessment of F waves is common in clinical neurophysiological examinations, caution should be used when testing individuals after prolonged immobility.

Motor imagery and action observation following immobilization-induced hypoactivity: a narrative review

BACKGROUND

In sports, the risk of pathology or event that leads to an injury, a cessation of practice or even to an immobilization is high. The subsequent reduction of physical activity, or hypoactivity, induces neural and muscular changes that adversely affect motor skills and functional motor rehabilitation. Because the implementation of physical practice is difficult, if not impossible, during and immediately following injury or immobilization, complementary techniques have been proposed to minimize the deleterious impact of hypoactivity on neuromuscular function.

OBJECTIVE

The current narrative review aimed to discuss the contributions of motor imagery and action observation, which enhance motor (re)learning and induce neural adaptations in both healthy individuals and injured athletes.

METHODS

Online literature research for studies of the effects of motor imagery, action observation and their combination on hypoactivity, extracting relevant publications within the last decade (2009-2020).

RESULTS

From published studies and the authors' knowledge of both motor imagery and action observation, some elements are provided for developing applied protocols during and after the immobilization period. Such interventions consist of associating congruent action observation with kinesthetic motor imagery of different movements, organized in increasing difficulty. The aim is to maintain motor functions and promote motor relearning by activating sensorimotor cortical areas and corticomotor pathways of the injured effector.

CONCLUSION

This narrative review supports the implementation of combined motor imagery and action observation protocols in the context of sports rehabilitation.

Potential Utility of Electrical Impedance Myography in Evaluating Age-Related Skeletal Muscle Function Deficits

Skeletal muscle function deficits associated with advancing age are due to several physiological and morphological changes including loss of muscle size and quality (conceptualized as a reduction in the intrinsic force-generating capacity of a muscle when adjusted for muscle size). Several factors can contribute to loss of muscle quality, including denervation, excitation-contraction uncoupling, increased fibrosis, and myosteatosis (excessive levels of inter- and intramuscular adipose tissue and intramyocellular lipids). These factors also adversely affect metabolic function. There is a major unmet need for tools to rapidly and easily assess muscle mass and quality in clinical settings with minimal patient and provider burden. Herein, we discuss the potential for electrical impedance myography (EIM) as a tool to evaluate muscle mass and quality in older adults. EIM applies weak, non-detectible (e.g., 400 μA), mutifrequency (e.g., 1 kHz–1 MHz) electrical currents to a muscle (or muscle group) through two excitation electrodes, and resulting voltages are measured via two sense electrodes. Measurements are fast (~5 s/muscle), simple to perform, and unaffected by factors such as hydration that may affect other simple measures of muscle status. After nearly 2 decades of study, EIM has been shown to reflect muscle health status, including the presence of atrophy, fibrosis, and fatty infiltration, in a variety of conditions (e.g., developmental growth and maturation, conditioning/deconditioning, and obesity) and neuromuscular diseases states [e.g., amyotrophic lateral sclerosis (ALS) and muscular dystrophies]. In this article, we describe prior work and current evidence of EIM’s potential utility as a measure of muscle health in aging and geriatric medicine.

Surgical results in older patients with lumbar spinal stenosis according to gait speed in relation to the diagnosis for sarcopenia

PURPOSE

The purpose of this study was to evaluate the implication of gait speed, which is a prerequisite for the diagnosis of sarcopenia in older patients with lumbar spinal stenosis (LSS).

METHODS

This study was conducted in a total of 235 patients with LSS who underwent surgical treatment. The state of sarcopenia and pre-sarcopenia, including gait speed, were evaluated before and after the operation.

RESULTS

The proportion of patients with lower than at baseline levels of skeletal muscle mass index (SMI) and gait speed was 27.2% and 17.9%, respectively. Significant changes were observed in gait speed, whereas SMI showed no significant differences between the preoperative and postoperative periods. Sarcopenic patients presented lower levels of activities of daily living preoperatively than pre-sarcopenic patients. However, favorable surgical results were obtained postoperatively. Significant changes were observed in gait speed, whereas grip strength and SMI showed no significant differences between the preoperative and postoperative periods. The postoperative decrease in SMI was not significant.

CONCLUSION

Postoperative gait speed was significantly improved, whereas muscle mass did not increase in the patients in this study. Therefore, low gait speed in patients with LSS seems to be derived from a neurologic disorder. The surgical results in low muscle mass patients without low gait speed were similar to those with low gait speed. Sarcopenia in elderly patients with locomotor disease should be evaluated using muscle mass alone without assessing physical performance.

Prognostic utility of dynapenia in patients with cardiovascular disease

BACKGROUND

Dynapenia, defined as age-associated loss of skeletal muscle strength, is associated with increased mortality rate, poor activities of daily living, and reduced quality of life. Therefore, dynapenia appears to be a better independent predictor of mortality than sarcopenia in the elderly. However, the prognostic utility of dynapenia in patients with cardiovascular disease (CVD) is not clear. This study was performed to examine the prognostic utility of dynapenia defined by the criteria of Manini et al. in patients with CVD.

METHODS

The findings of 4192 consecutive patients ≥30 years old (median [interquartile range (IQR)] age 69 [60-76] years, 2874 males) with CVD were reviewed. Grip strength and quadriceps isometric strength (QIS) were measured just before hospital discharge, and low grip strength (<26 kg in males and <18 kg in females), low QIS (<45.0% body mass [BM] and <35.0% BM in males and females, respectively) were considered to indicate dynapenia. The endpoint was all-cause mortality.

RESULTS

A total of 507 deaths occurred during follow-up (median 2.0 years, IQR 0.8-4.4 years). The overall prevalence of dynapenia was 33.6% and increased with age (p for trend < 0.01). Females showed a significantly higher prevalence rate of dynapenia than males (43.3% vs. 29.2%, respectively; p < 0.01). Patients with dynapenia showed higher all-cause mortality rate than non-dynapenia patients (adjusted hazard ratio: 1.84; 95% confidence interval: 1.51-2.23; p < 0.01).

CONCLUSIONS

Dynapenia has a high prevalence among patients with CVD and is associated with increased mortality rate.

The Effects of Calcium-β-Hydroxy-β-Methylbutyrate on Aging-Associated Apoptotic Signaling and Muscle Mass and Function in Unloaded but Nonatrophied Extensor Digitorum Longus Muscles of Aged Rats

Beta-hydroxy-beta-methylbutyrate (HMB), a naturally occurring leucine metabolite, has been shown to attenuate plantar flexor muscle loss and increase myogenic stem cell activation during reloading after a period of significant muscle wasting by disuse in old rodents. However, it was less clear if HMB would alter dorsiflexor muscle response to unloading or reloading when there was no significant atrophy that was induced by unloading. In this study, we tested if calcium HMB (Ca-HMB) would improve muscle function and alter apoptotic signaling in the extensor digitorum longus (EDL) of aged animals that were unloaded but did not undergo atrophy. The EDL muscle was unloaded for 14 days by hindlimb suspension (HS) in aged (34-36 mo.) male Fisher 344 × Brown Norway rats. The rats were removed from HS and allowed normal cage ambulation for 14 days of reloading (R). Throughout the study, the rats were gavaged daily with 170 mg of Ca-HMB or water 7 days prior to HS, then throughout 14 days of HS and 14 days of recovery after removing HS. The animals' body weights were significantly reduced by ~18% after 14 days of HS and continued to decline by ~22% during R as compared to control conditions; however, despite unloading, EDL did not atrophy by HS, nor did it increase in mass after R. No changes were observed in EDL twitch contraction time, force production, fatigue resistance, fiber cross-sectional area, or markers of nuclear apoptosis (myonuclei + satellite cells) after HS or R. While HS and R increased the proapoptotic Bax protein abundance, BCL-2 abundance was also increased as was the frequency of TUNEL-positive myonuclei and satellite cells, yet muscle mass and fiber cross-sectional area did not change and Ca-HMB treatment had no effect reducing apoptotic signaling. These data indicate that (i) increased apoptotic signaling preceded muscle atrophy or occurred without significant EDL atrophy and (ii) that Ca-HMB treatment did not improve EDL signaling, muscle mass, or muscle function in aged rats, when HS and R did not impact mass or function.

Intracellular Water Content in Lean Mass as an Indicator of Muscle Quality in an Older Obese Population

Background: In aged populations, muscle strength depends more on muscle quality than on muscle quantity, while all three are criteria for the diagnosis of sarcopenia. Intracellular water content (ICW) in lean mass (LM) has been proposed as an indicator of muscle quality related to muscle strength in older people. Objectives: To evaluate the relationship between the ICW/LM ratio, muscle strength and indicators of functional performance in obese older adults, and to assess the value of the ICW/LM ratio as an indicator of muscle quality. Methodology: Design: cross-sectional study. Population: persons aged 65–75 years with a body mass index of 30–39 kg/m². ICW and LM were estimated by bioelectrical impedance. Hand grip, gait speed, unipedal stance test, timed up-and-go (TUG) test, Barthel score and frailty (Fried criteria) were assessed. Sarcopenia was established according to EWGSOP2 criteria. Results: Recruited were 305 subjects (66% women), mean age 68 years. The ICW/LM ratio correlated with the TUG test, gait speed and grip strength, and was also associated with sex, the unipedal stance test and frailty. Independently of age, sex and muscle mass, the ICW/LM ratio was related with gait speed, the TUG test and unipedal stance capacity. One person (0.3%) had sarcopenia defined as low muscle strength and low muscle mass, while 25 people (8.2%) had sarcopenia defined as low muscle strength and poor muscle quality (ICW/LM). With this last definition, sarcopenia was related to frailty, gait speed and the TUG test. Conclusions: ICW content in LM could be a useful muscle quality indicator for defining sarcopenia. However, more studies are required to confirm our findings for other populations.

Effects of blood flow restriction without additional exercise on strength reductions and muscular atrophy following immobilization: A systematic review

PURPOSE

To investigate whether blood flow restriction (BFR) without concomitant exercise mitigated strength reduction and atrophy of thigh muscles in subjects under immobilization for lower limbs.

METHODS

The following databases were searched: PubMed, CINAHL, PEDro, Web of Science, Central, and Scopus.

RESULTS

The search identified 3 eligible studies, and the total sample in the identified studies consisted of 38 participants. Isokinetic and isometric torque of the knee flexors and extensors was examined in 2 studies. Cross-sectional area of thigh muscles was evaluated in 1 study, and thigh girth was measured in 2 studies. The BFR protocol was 5 sets of 5 min of occlusion and 3 min of free flow, twice daily for approximately 2 weeks. As a whole, the included studies indicate that BFR without exercise is able to minimize strength reduction and muscular atrophy after immobilization. It is crucial to emphasize, however, that the included studies showed a high risk of bias, especially regarding allocation concealment, blinding of outcome assessment, intention-to-treat analyses, and group similarity at baseline.

CONCLUSION

Although potentially useful, the high risk of bias presented by original studies limits the indication of BFR without concomitant exercise as an effective countermeasure against strength reduction and atrophy mediated by immobilization.

Lower limb muscle mass is associated with insulin resistance more than lower limb muscle strength in non-diabetic older adults

AIM

We examined the association of muscle mass and muscle strength with insulin resistance, focusing on lower limb muscles.

METHODS

This was a cross-sectional study, and participants were registered in 2017 from the Tanno-Sobetsu Study, a population-based cohort study. After excluding individuals aged <64 years or with a history of diabetes mellitus, 272 non-diabetic older adults (116 men, 156 women) met the inclusion criteria. According to the homeostasis model assessment of insulin resistance, the participants were divided into two groups; that is, the insulin resistance group (homeostasis model assessment of insulin resistance ≥1.73) and non-insulin resistance group (homeostasis model assessment of insulin resistance <1.73). Muscle mass (lower limb, upper limb, appendicular and trunk) and muscle strength (grip strength and knee extension torque) were measured and divided by the weight, and then multiplied by 100 to calculate the weight ratio (%). The adjusted odds ratio (OR) of each muscle index for risk of insulin resistance was calculated separately for both men and women.

RESULTS

Lower limb muscle mass (OR 0.72, 95% CI 0.55-0.92 in men; OR 0.69, 95% CI 0.51-0.96 in women, respectively) and appendicular muscle mass (OR 0.75, 95% CI 0.61-0.94 in men; OR 0.72, 95% CI 0.54-0.95 in women, respectively), but not other muscle indexes, were associated with risk of insulin resistance, after adjusting for age, body mass index, highly sensitive C-reacting protein and smoking habits.

CONCLUSIONS

Lower limb muscle mass of non-diabetic older adults is independently related to insulin resistance. Geriatr Gerontol Int 2019; 19: 1254-1259.

The Role of Water Homeostasis in Muscle Function and Frailty: A Review

Water, the main component of the body, is distributed in the extracellular and intracellular compartments. Water exchange between these compartments is mainly governed by osmotic pressure. Extracellular water osmolarity must remain within very narrow limits to be compatible with life. Older adults lose the thirst sensation and the ability to concentrate urine, and this favours increased extracellular osmolarity (hyperosmotic stress). This situation, in turn, leads to cell dehydration, which has severe consequences for the intracellular protein structure and function and, ultimately, results in cell damage. Moreover, the fact that water determines cell volume may act as a metabolic signal, with cell swelling acting as an anabolic signal and cell shrinkage acting as a catabolic signal. Ageing also leads to a progressive loss in muscle mass and strength. Muscle strength is the main determinant of functional capacity, and, in elderly people, depends more on muscle quality than on muscle quantity (or muscle mass). Intracellular water content in lean mass has been related to muscle strength, functional capacity, and frailty risk, and has been proposed as an indicator of muscle quality and cell hydration. This review aims to assess the role of hyperosmotic stress and cell dehydration on muscle function and frailty.

Factors associated with muscle function in patients with hematologic malignancies undergoing chemotherapy

PURPOSE

Muscle dysfunction such as loss of muscle mass and decreased muscle strength is often observed in patients with hematologic malignancies. However, specific factors associated with muscle function have not been identified. The purpose of this study was to identify significant factors affecting muscle function in patients with hematologic malignancies.

METHODS

This was a cross-sectional, observational study. Eighty-eight inpatients with hematologic malignancies undergoing chemotherapy were recruited. Participants were evaluated for muscle thickness and isometric knee extensor strength as indicators of muscle function, physical activity, physical symptoms, psychological distress, and self-efficacy at the start date of rehabilitation. Multiple regression analysis with muscle function as the dependent variable and clinical information and other evaluation items as explanatory variables was performed.

RESULTS

Lymphocyte count, the geriatric nutritional risk index, and physical activity were significant factors associated with muscle thickness, while physical activity and self-efficacy were significant factors associated with isometric knee extensor strength.

CONCLUSIONS

Nutritional status, physical activity, and self-efficacy were significant factors associated with muscle function in patients with hematologic malignancies. Rehabilitation intervention focusing on improving physical activity and nutritional status should be considered necessary for enhancing muscle function in patients with hematologic malignancies.

Short-Term Preconditioning With Blood Flow Restricted Exercise Preserves Quadriceps Muscle Endurance in Patients After Anterior Cruciate Ligament Reconstruction

Surgical ACL reconstruction performed with a tourniquet induces compression and ischemic stress of the quadriceps femoris (QF) muscle which can accelerate postoperative weakness. Given that low-load blood flow restricted (BFR) exercise is potent in enhancing muscle oxygenation and vascular function, we hypothesized that short-term preconditioning with low-load BFR exercise can attenuate QF muscle endurance deterioration in the postoperative period. Twenty subjects undergoing arthroscopic ACL reconstruction performed 5 exercise sessions in the last 8 days prior to surgery. They were assigned into either BFR group, performing low-load BFR knee-extension exercise, or SHAM-BFR group, replicating equal training volume with sham occlusion. Blood flow (near-infrared spectroscopy) and surface EMG of QF muscle during sustained isometric contraction at 30% of maximal voluntary isometric contraction (MVIC) torque performed to volitional failure were measured prior to the intervention and again 4 and 12 weeks after surgery. There was an overall decrease (p = 0.033) in MVIC torque over time, however, no significant time-group interaction was found. The time of sustained QF contraction shortened (p = 0.002) in SHAM-BFR group by 97 ± 85 s at week 4 and returned to preoperative values at week 12. No change in the time of sustained contraction was detected in BFR group at any time point after surgery. RMS EMG amplitude increased (p = 0.009) by 54 ± 58% at week 4 after surgery in BFR group only. BFm increased (p = 0.004) by 52 ± 47% in BFR group, and decreased (p = 0.023) by 32 ± 19% in SHAM-BFR group at week 4 after surgery. Multivariate regression models of postoperative changes in time of sustained QF contraction revealed its high correlation (R2 = 0.838; p < 0.001) with changes in BFm and RMS EMG in the SHAM-BFR group, whereas no such association was found in the BFR group. In conclusion, enhanced endurance of QF muscle was triggered by combination of augmented muscle fiber recruitment and enhanced muscle perfusion. The latter alludes to a preserving effect of preconditioning with BFR exercise on density and function of QF muscle microcirculation within the first 4 weeks after ACL reconstruction.

Effectiveness of Multimodal Training on Functional Capacity in Frail Older People: A Meta-Analysis of Randomized Controlled Trials

In this meta-analysis, we investigated the effect of resistance training (RT) alone or included in a multimodal training on physical frailty outcomes, and whether different variables of RT prescription affect these outcomes. We identified 15 relevant studies searching through MEDLINE, Cochrane Central Register of Controlled Trials, SPORTDiscus, and PEDro database. Postintervention standardized mean difference scores were computed and combined using fixed effects meta-analysis. Analyses have shown positive effects of interventions on maximum strength, gait speed, and Timed Up and Go test. Further analyses have shown significant greater effect of shorter periods on maximum strength. Regarding RT prescription, percentage of one-repetition maximum showed significant effect on physical variables, whereas RT based on rate of perceived effort presented lower effect in the Timed Up and Go test. Although multimodal training is an effective intervention to increase physical capacity, caution should be taken regarding the period and the method to control RT intensity to optimize enhancements in frail older people.

Association between hospitalization-related outcomes, dynapenia and body mass index: The Glisten Study

OBJECTIVE

To compare the prognostic value of dynapenia, as evaluated by handgrip, and body mass index (BMI) on length of stay (LOS), days of bed rest, and other hospitalization-related outcomes in a population of older adults admitted to 12 italian acute care divisions.

METHODS

Data on age, weight, BMI, comorbidities, ADL, physical activity level, muscle strength, were recorded at hospital admission. LOS, days of bed rest, intrahospital falls, and discharge destination were also recorded during the hospitalization. Subjects with BMI <18.5 kg/m² were classified as underweight, subjects with BMI 18.5-24.9 as normal weight, subjects with BMI ≥25 as overweight-obese.

RESULTS

A total of 634 patients, mean age 80.8 ± 6.7 years and 49.4% women, were included in the analysis. Overall dynapenic subjects (D) showed a longer period of LOS and bed rest compared with non-dynapenic (ND). When the study population was divided according to BMI categories, underweight (UW), normal weight (NW), and overweight-obese (OW-OB), no significant differences were observed in hospital LOS and days of bed rest. When analysis of covariance was used to determine the difference of LOS across handgrip/BMI groups, D/OW-OB and D/UW subjects showed significantly longer LOS (11.32 and 10.96 days, both p 0.05) compared to ND/NW subjects (7.69 days), even when controlling for age, gender, baseline ADL, cause of hospitalization and comorbidity. After controlling for the same confounding factors, D/OW-OB, D/NW and D/UW subjects showed significantly longer bed rest (4.7, 4.56, and 4.05 days, respectively, all p 0.05, but D/OW-OB p 0.01) compared to ND/NW subjects (1.59 days).

CONCLUSION

In our study population, LOS is longer in D/UW and D/OW-OB compared to ND/NW subjects and days of bed rest are mainly influenced by dynapenia, and not by BMI class.

Early movement restriction leads to enduring disorders in muscle and locomotion

Motor control and body representation in the central nervous system (CNS) as well as musculoskeletal architecture and physiology are shaped during development by sensorimotor experience and feedback, but the emergence of locomotor disorders during maturation and their persistence over time remain a matter of debate in the absence of brain damage. By using transient immobilization of the hind limbs, we investigated the enduring impact of postnatal sensorimotor restriction (SMR) on gait and posture on treadmill, age-related changes in locomotion, musculoskeletal histopathology and Hoffmann reflex in adult rats without brain damage. SMR degrades most gait parameters and induces overextended knees and ankles, leading to digitigrade locomotion that resembles equinus. Based on variations in gait parameters, SMR appears to alter age-dependent plasticity of treadmill locomotion. SMR also leads to small but significantly decreased tibial bone length, chondromalacia, degenerative changes in the knee joint, gastrocnemius myofiber atrophy and muscle hyperreflexia, suggestive of spasticity. We showed that reduced and atypical patterns of motor outputs, and somatosensory inputs and feedback to the immature CNS, even in the absence of perinatal brain damage, play a pivotal role in the emergence of movement disorders and musculoskeletal pathologies, and in their persistence over time. Understanding how atypical sensorimotor development likely contributes to these degradations may guide effective rehabilitation treatments in children with either acquired (ie, with brain damage) or developmental (ie, without brain injury) motor disabilities.

Dynapenia and Sarcopenia as a Risk Factor for Disability in a Falls and Fractures Clinic in Older Persons

BACKGROUND:

The role of sarcopenia and dynapenia in disability in older persons from falls and bone health clinics remain unknown.

AIM:

This study aims to compare the association of sarcopenia and dynapenia with physical and instrumental disability in a population of older persons attending a falls and fractures clinic.

METHODS:

This is a cross-sectional study in Manizales, Andes Mountains, Colombia. A cohort of 534 subjects (mean age = 74, 75% female) Sarcopenia was measured according to the European Working Group on Sarcopenia in Older People (EWGSOP) including an index of skeletal mass, muscle strength, and gait speed. Dynapenia was defined as a handgrip force ≤ 30 kg for men and ≤ 20 kg for women.

RESULTS:

Dynapenia and sarcopenia were present in 84.6% and 71.2% respectively. Both were more prevalent in older subjects and women than men. While sarcopenia was associated with body mass index and hypertension, dynapenia was associated with hypothyroidism and visual impairment. After controlling for all covariates, sarcopenia was associated with low IADL and mobility disability.

CONCLUSIONS:

Sarcopenia was associated with mobility, ADL and IADL disability. Dynapenia was not associated with disability in this high - risk population. Systematic assessment of sarcopenia should be implemented in falls and fractures clinics to identify sarcopenia and develop interventions to prevent functional decline among elderly individuals.

Loss of maximal explosive power of lower limbs after 2 weeks of disuse and incomplete recovery after retraining in older adults

KEY POINTS

Disuse in older adults can critically decrease lower limb muscle power, leading to compromised mobility and overall quality of life. We studied how muscle power and its determinants (muscle mass, single muscle fibre properties and motor control) adapted to 2 weeks of disuse and subsequent 2 weeks of physical training in young and older people. Disuse decreased lower limb muscle power in both groups; however, different adaptations in single muscle fibre properties and co-contraction of leg muscles were observed between young and older individuals. Six physical training sessions performed after disuse promoted the recovery of muscle mass and power. However, they were not sufficient to restore muscle power to pre-disuse values in older individuals, suggesting that further countermeasures are required to counteract the disuse-induced loss of muscle power in older adults.

ABSTRACT

Disuse-induced loss of muscle power can be detrimental in older individuals, seriously impairing functional capacity. In this study, we examined the changes in maximal explosive power (MEP) of lower limbs induced by a 14-day disuse (bed-rest, BR) and a subsequent 14-day retraining, to assess whether the impact of disuse was greater in older than in young men, and to analyse the causes of such adaptations. Sixteen older adults (Old: 55-65 years) and seven Young (18-30 years) individuals participated in this study. In a subgroup of eight Old subjects, countermeasures based on cognitive training and protein supplementation were applied. MEP was measured with an explosive ergometer, muscle mass was determined by magnetic resonance, motor control was studied by EMG, and single muscle fibres were analysed in vastus lateralis biopsy samples. MEP was ∼33% lower in Old than in Young individuals, and remained significantly lower (-19%) when normalized by muscle volume. BR significantly affected MEP in Old (-15%) but not in Young. Retraining tended to increase MEP; however, this intervention was not sufficient to restore pre-BR values in Old. Ankle co-contraction increased after BR in Old only, and remained elevated after retraining (+30%). Significant atrophy occurred in slow fibres in Old, and in fast fibres in Young. After retraining, the recovery of muscle fibre thickness was partial. The proposed countermeasures were not sufficient to affect muscle mass and power. The greater impact of disuse and smaller retraining-induced recovery observed in Old highlight the importance of designing suitable rehabilitation protocols for older individuals.

Long-term Low-Intensity Endurance Exercise along with Blood-Flow Restriction Improves Muscle Mass and Neuromuscular Junction Compartments in Old Rats

BACKGROUND

During the aging process, muscle atrophy and neuromuscular junction remodeling are inevitable. The present study aimed to clarify whether low-intensity aerobic exercise along with limb blood-flow restriction (BFR) could improve aging-induced muscle atrophy and nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction.

METHODS

Forty-eight male Wistar rats, aged 23-24 months, were randomly divided into control, sham (Sh: subjected to surgery without BFR), BFR (subjected to BFR), exercise (Ex: subjected to 10 weeks of low-intensity exercise), Sh+Ex, and BFR+Ex groups. Forty-eight hours after the last training session, the animals were sacrificed and their soleus and extensor digitorum longus (EDL) muscles were removed. The hypertrophy index was calculated, and molecular parameters were measured using western blotting. Statistical analysis was done with ANOVA using SPSS (version 20), with a P<0.05 as the level of significance.

RESULTS

The control and Sh groups showed weight gain (P=0.001), whereas the Ex, Sh+Ex, and BFR+Ex groups had significant weight loss (P<0.001). The hypertrophy index of the soleus was significantly higher in the BFR+Ex group than in the control, Sh, and BFR groups (P<0.001). BFR+Ex induced significant hypertrophic effects on the EDL (P<0.001 vs. the control, Sh, Ex, and Sh+Ex groups, and P=0.006 vs. the BFR group). BFR+Ex also increased nAChRs in the soleus (P=0.02 vs. the control and Sh groups) and the EDL (P=0.008 vs. the control and Sh groups).

CONCLUSION

BFR plus mild exercise is a safe method with potential beneficial effects in protecting and augmenting muscle mass and nAChR clustering at the neuromuscular junction in old rats.

Are higher blood flow restriction pressures more beneficial when lower loads are used?

The application of blood flow restriction during low-load resistance exercise has been shown to induce muscle growth with high or low restriction pressures, however, loads lower than 20% one-repetition maximum (1RM) remain unexplored. Fourteen trained individuals completed six elbow flexion protocols involving three different loads (10%, 15%, and 20% 1RM) each of which was performed with either a low (40% arterial occlusion) or high (80% arterial occlusion) pressure. Pre- and post-measurements of surface electromyography (sEMG), isometric torque, and muscle thickness were analyzed. An interaction was present for torque (p < 0.001) and muscle thickness (p < 0.001) illustrating that all increases in pressure and/or load resulted in a greater fatigue and muscle thickness. There was no interaction for sEMG (p = 0.832); however, there were main effects of condition (p = 0.002) and time (p = 0.019) illustrating greater sEMG in the 20% 1RM conditions. Higher blood flow restriction pressures may be more beneficial for muscle growth when very low loads are used.

Omega-3 Fatty Acids and Vitamin D in Immobilisation: Part B- Modulation of Muscle Functional, Vascular and Activation Profiles

OBJECTIVES

This study set out to determine whether two potential protein-sparing modulators (eicosapentaenoic acid and vitamin D) would modulate the anticipated muscle functional and related blood vessels function deleterious effects of immobilisation.

DESIGN

The study used a randomised, double-blind, placebo-controlled design.

SETTING

The study took part in a laboratory setting.

PARTICIPANTS

Twenty-four male and female healthy participants, aged 23.0±5.8 years.

INTERVENTION

The non-dominant arm was immobilised in a sling for a period of nine waking hours a day over two continuous weeks. Participants were randomly assigned to one of three groups: placebo (n=8, Lecithin, 2400 mg daily), omega-3 (ω-3) fatty acids (n=8, eicosapentaenoic acid (EPA); 1770 mg, and docosahexaenoic acid (DHA); 390 mg, daily) or vitamin D (n=8, 1,000 IU daily).

MEASUREMENTS

Isometric and isokinetic torque, antagonist muscle co-contraction (activation profile), muscle fatigability indices, and arterial resting blood flow were measured before, at the end of the immobilisation period, and two weeks after re-mobilisation.

RESULTS

Muscle elbow flexion and extension isometric and isokinetic torque decreased significantly with limb immobilisation in the placebo group (P<0.05). Despite no significant effect of supplementation, ω-3 and vitamin D supplementation showed trends (P>0.05) towards attenuating the decreases observed in the placebo group. There was no significant change in muscle fatigue parameters or co-contraction values with immobilisation and no effect of supplementation group (P>0.05). Similarly, this immobilisation model had no impact on the assessed blood flow characteristics. All parameters had returned to baseline values at the re-mobilisation phase of the study.

CONCLUSION

Overall, at the current doses, neither ω-3 nor vitamin D supplementation significantly attenuated declines in torque associated with immobilisation. It would appear that muscle function (described here in Part B) might not be as useful a marker of the effectiveness of a supplement against the impact of immobilisation compared to tissue composition changes (described in Part A).

Dissociation between short-term unloading and resistance training effects on skeletal muscle Na+,K+-ATPase, muscle function, and fatigue in humans

Physical training increases skeletal muscle Na⁺,K⁺-ATPase content (NKA) and improves exercise performance, but the effects of inactivity per se on NKA content and isoform abundance in human muscle are unknown. We investigated the effects of 23-day unilateral lower limb suspension (ULLS) and subsequent 4-wk resistance training (RT) on muscle function and NKA in 6 healthy adults, measuring quadriceps muscle peak torque; fatigue and venous [K⁺] during intense one-legged cycling exercise; and skeletal muscle NKA content ([³H]ouabain binding) and NKA isoform abundances (immunoblotting) in muscle homogenates (α_1-3, β_1-2) and in single fibers (α_1-3, β₁). In the unloaded leg after ULLS, quadriceps peak torque and cycling time to fatigue declined by 22 and 23%, respectively, which were restored with RT. Whole muscle NKA content and homogenate NKA α_1-3 and β_1-2 isoform abundances were unchanged with ULLS or RT. However, in single muscle fibers, NKA α₃ in type I (-66%, P = 0.006) and β₁ in type II fibers (-40%, P = 0.016) decreased after ULLS, with other NKA isoforms unchanged. After RT, NKA α₁ (79%, P = 0.004) and β₁ (35%, P = 0.01) increased in type II fibers, while α₂ (76%, P = 0.028) and α₃ (142%, P = 0.004) increased in type I fibers compared with post-ULLS. Despite considerably impaired muscle function and earlier fatigue onset, muscle NKA content and homogenate α₁ and α₂ abundances were unchanged, thus being resilient to inactivity induced by ULLS. Nonetheless, fiber type-specific downregulation with inactivity and upregulation with RT of several NKA isoforms indicate complex regulation of muscle NKA expression in humans.

Sensory-motor training targeting motor dysfunction and muscle weakness in long-term care elderly combined with motivational strategies: a single blind randomized controlled study

BACKGROUND

This study evaluated the effects of a combined innovative training regime consisting of stochastic resonance whole-body vibration (SR-WBV) and a dance video game (DVG) on physical performance and muscle strength in long-term-care dwelling elderly.

METHODS

Thirthy long-term-care elderly were randomly allocated to an intervention group (IG; n = 16) receiving combined SR-WBV training and DVG, or a sham group (SG; n = 14). IG performed five sets one minute of SR-WBV, with one minute rest between sets (base frequency 3 Hz up to 6 Hz, Noise 4) during the first five weeks on three days per week. From week five to eight a DVG was added to SR-WBV for IG on three days per week. SG performed a five-set SR-WBV program (1 Hz, Noise 1) lasting five times one minute, with one minute rest in between, three days a week. From week five to eight stepping exercises on a trampoline were added on three days per week.

PRIMARY OUTCOME

Short physical performance battery (SPPB). Secondary outcome: isometric maximal voluntary contraction (IMVC), and sub phases of IMVC (Fsub), isometric rate of force development (IRFD) and sub time phases of IRFD (IRFDsub) were measured at baseline, after four and eight weeks. ANOVA with repeated measures was used for analyses of time and interaction effects and MANOVA determined between group intervention effects.

RESULTS

Between group effects revealed significant effects on the SPPB primary outcome after four weeks F(1, 27) = 6.17; p = 0.02) and after eight weeks F(1,27) = 11.8; p = 0.002). Secondary muscle function related outcome showed significant between group effects in IG on IRFD, Fsub 30 ms, 100 ms, 200 ms and IRFDsub 0-30 ms, 0-50 ms, 0-100 ms and 100-200 ms compared to SG (all p < 0.05).

CONCLUSIONS

Eight weeks SR-WBV and DVG intervention improved lower extremity physical function and muscle strength compared to a sham intervention in long-term-care elderly. SR-WBV and DVG seems to be effective as a training regime for skilling up in long-term-care elderly.

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.

Live strong and prosper: the importance of skeletal muscle strength for healthy ageing

Due to improved health care, diet and infrastructure in developed countries, since 1840 life expectancy has increased by approximately 2 years per decade. Accordingly, by 2050, a quarter of Europe’s population will be over 65 years, representing a 10 % rise in half a century. With this rapid rise comes an increased prevalence of diseases of ageing and associated healthcare expenditure. To address the health consequences of global ageing, research in model systems (worms, flies and mice) has indicated that reducing the rate of organ growth, via reductions in protein synthetic rates, has multi-organ health benefits that collectively lead to improvements in lifespan. In contrast, human pre-clinical, clinical and large cohort prospective studies demonstrate that ageing leads to anabolic (i.e. growth) impairments in skeletal muscle, which in turn leads to reductions in muscle mass and strength, factors directly associated with mortality rates in the elderly. As such, increasing muscle protein synthesis via exercise or protein-based nutrition maintains a strong, healthy muscle mass, which in turn leads to improved health, independence and functionality. The aim of this review is to critique current literature relating to the maintenance of muscle mass across lifespan and discuss whether maintaining or reducing protein synthesis is the most logical approach to support musculoskeletal function and by extension healthy human ageing.

Muscle power and nutrition

Sarcopenia, as defined by the European working group on sarcopenia in older people (EWGSOP), is a highly prevalent syndrome characterized by age-related loss of muscle mass and muscle strength/power with impacts on physical function, health and quality of life in older people. The complex, multifaceted and still not completely elucidated etiology of sarcopenia and loss of muscle function (dynapenia) poses challenges for the design of interventional studies to combat loss of muscle strength. Several factors, however, have been demonstrated to have major impacts for maintenance of physiological muscle functioning, including nutrition and in particular specific nutrients. For example, proteins, amino acids and micronutrients have been extensively studied regarding their impact on muscle synthesis and metabolism. This literature review focuses on the impact of nutrition on muscle strength and power as it relates to older people given that muscle changes with age can have important implications for health.

Age-related neuromuscular changes affecting human vastus lateralis

KEY POINTS

Skeletal muscle size and strength decline in older age. The vastus lateralis, a large thigh muscle, undergoes extensive neuromuscular remodelling in healthy ageing, as characterized by a loss of motor neurons, enlargement of surviving motor units and instability of neuromuscular junction transmission. The loss of motor axons and changes to motor unit potential transmission precede a clinically-relevant loss of muscle mass and function.

ABSTRACT

The anterior thigh muscles are particularly susceptible to muscle loss and weakness during ageing, although how this is associated with changes to neuromuscular structure and function in terms of motor unit (MU) number, size and MU potential (MUP) stability remains unclear. Intramuscular (I.M.) and surface electromyographic signals were recorded from the vastus lateralis (VL) during voluntary contractions held at 25% maximal knee extensor strength in 22 young (mean ± SD, 25.3 ± 4.8 years) and 20 physically active older men (71.4 ± 6.2 years). MUP size, firing rates, phases, turns and near fibre (NF) jiggle were determined and MU number estimates (MUNEs) were made by comparing average surface MUP with maximal electrically-evoked compound muscle action potentials. Quadriceps cross-sectional area was measured by magnetic resonance imaging. In total, 379 individual MUs were sampled in younger men and 346 in older men. Compared to the MU in younger participants, those in older participants had 8% lower firing rates and larger MUP size (+25%), as well as increased complexity, as indicated by phases (+13%), turns (+20%) and NF jiggle (+11%) (all P < 0.0005). The MUNE values (derived from the area of muscle in range of the surface-electrode) in older participants were ∼70% of those in the young (P < 0.05). Taking into consideration the 30% smaller cross-sectional area of the VL, the total number of MUs in the older muscles was between 50% and 60% lower compared to in young muscles (P < 0.0005). A large portion of the VL MU pool is lost in older men and those recruited during moderate intensity contractions were enlarged and less stable. These MU changes were evident before clinically relevant changes to muscle function were apparent; nevertheless, the changes in MU number and size are probably a prelude to future movement problems.

Effects of sex and gender on adaptation to space: musculoskeletal health

There is considerable variability among individuals in musculoskeletal response to long-duration spaceflight. The specific origin of the individual variability is unknown but is almost certainly influenced by the details of other mission conditions such as individual differences in exercise countermeasures, particularly intensity of exercise, dietary intake, medication use, stress, sleep, psychological profiles, and actual mission task demands. In addition to variations in mission conditions, genetic differences may account for some aspect of individual variability. Generally, this individual variability exceeds the variability between sexes that adds to the complexity of understanding sex differences alone. Research specifically related to sex differences of the musculoskeletal system during unloading is presented and discussed.

The impact of sex and gender on adaptation to space: executive summary

This review article is a compendium of six individual manuscripts, a Commentary, and an Executive Summary. This body of work is entitled "The Impact of Sex and Gender on Adaptation to Space" and was developed in response to a recommendation from the 2011 National Academy of Sciences Decadal Survey, "Recapturing a Future for Space Exploration: Life and Physical Sciences for a New Era," which emphasized the need to fully understand sex and gender differences in space. To ensure the health and safety of male and female astronauts during long-duration space missions, it is imperative to examine and understand the influences that sex and gender have on physiological and psychological changes that occur during spaceflight. In this collection of manuscripts, six workgroups investigated and summarized the current body of published and unpublished human and animal research performed to date related to sex- and gender-based differences in the areas of cardiovascular, immunological, sensorimotor, musculoskeletal, reproductive, and behavioral adaptations to human spaceflight. Each workgroup consisted of scientists and clinicians from academia, the National Aeronautics and Space Administration (NASA), and other federal agencies and was co-chaired by one representative from NASA and one from the external scientific community. The workgroups met via telephone and e-mail over 6 months to review literature and data from space- and ground-based studies to identify sex and gender factors affecting crew health. In particular, the Life Sciences Data Archive and the Lifetime Surveillance of Astronaut Health were extensively mined. The groups identified certain sex-related differences that impact the risks and the optimal medical care required by space-faring women and men. It represents innovative research in sex and gender-based biology that impacts those individuals that are at the forefront of space exploration.

Maximal explosive power of the lower limbs before and after 35 days of bed rest under different diet energy intake

PURPOSE

Microgravity leads to a decline of muscle power especially in the postural muscles of the lower limb. Muscle atrophy primarily contributes to this negative adaptation. Nutritional countermeasures during unloading were shown to possibly mitigate the loss of muscle mass and strength. The aim of this study was to investigate the effects of different diet energy intakes during prolonged inactivity on body composition and lower limbs power output.

METHODS

The effects of lower or higher diet energy intake on the decline of maximal explosive power of the lower limbs, as determined on a sledge ergometer before and after 35 days of bed rest, were investigated on two matched groups of young healthy volunteers. Body composition and lean volume of the lower limb were also measured.

RESULTS

After bed rest, fat mass increased (+20.5 %) in the higher energy intake group (N = 9), while it decreased (-4.8 %) in the lower energy intake group (N = 10). Also, the loss of body fat-free mass and lean volume of the lower limb was significantly greater in the higher (-4.6 and -10.8 %, respectively) as compared to the lower (-2.4 and -3.7 %, respectively) diet energy intake group. However, the loss of maximal explosive power was similar between the two groups (-25.2 and -29.5 % in the higher and lower energy intake group, respectively; P = 0.440).

CONCLUSIONS

The mitigation of loss of muscle mass by means of a moderate caloric diet restriction during prolonged inactivity was not sufficient for reducing the loss of maximal explosive power of the lower limbs.

The power of the mind: the cortex as a critical determinant of muscle strength/weakness

We tested the hypothesis that the nervous system, and the cortex in particular, is a critical determinant of muscle strength/weakness and that a high level of corticospinal inhibition is an important neurophysiological factor regulating force generation. A group of healthy individuals underwent 4 wk of wrist-hand immobilization to induce weakness. Another group also underwent 4 wk of immobilization, but they also performed mental imagery of strong muscle contractions 5 days/wk. Mental imagery has been shown to activate several cortical areas that are involved with actual motor behaviors, including premotor and M1 regions. A control group, who underwent no interventions, also participated in this study. Before, immediately after, and 1 wk following immobilization, we measured wrist flexor strength, voluntary activation (VA), and the cortical silent period (SP; a measure that reflect corticospinal inhibition quantified via transcranial magnetic stimulation). Immobilization decreased strength 45.1 ± 5.0%, impaired VA 23.2 ± 5.8%, and prolonged the SP 13.5 ± 2.6%. Mental imagery training, however, attenuated the loss of strength and VA by ∼50% (23.8 ± 5.6% and 12.9 ± 3.2% reductions, respectively) and eliminated prolongation of the SP (4.8 ± 2.8% reduction). Significant associations were observed between the changes in muscle strength and VA (r = 0.56) and SP (r = -0.39). These findings suggest neurological mechanisms, most likely at the cortical level, contribute significantly to disuse-induced weakness, and that regular activation of the cortical regions via imagery attenuates weakness and VA by maintaining normal levels of inhibition.

Neuromuscular function following muscular unloading and blood flow restricted exercise

PURPOSE

The aim of the study is to evaluate central and peripheral neuromuscular function in the knee extensors (KE) and plantar flexors (PF) after 30 days of unilateral lower limb suspension (ULLS) and to examine the effects of low-load blood flow restricted (BFR) resistance training on the KE during ULLS.

METHODS

Strength, cross-sectional area (CSA), central activation, evoked force, and rates of force development and relaxation were assessed in the KE and PF before and after ULLS in sixteen subjects (9 M, 7F; 18-49 years). Eight of those subjects participated in BFR on the KE three times per week during ULLS (ULLS + Exercise).

RESULTS

The ULLS group had decrements in strength and CSA of the KE (16 and 7 %, respectively) and PF (27 and 8 %, respectively) and the ULLS + Exercise maintained strength and CSA of the KE (P > 0.05), but significantly lost strength and CSA in the PF (21 and 5 %; P > 0.05). KE central activation declined 6 % in the ULLS group and was maintained in the ULLS + Exercise group, but a time × group interaction was not evident (P = 0.31). PF central activation was reduced in both groups (ULLS: -7.6 ± 9.9 and -7.9 ±11.6 %; time main effect P = 0.01). A time × group interaction for KE-evoked twitch force (P = 0.04) demonstrated a 9 % decline in the ULLS + Exercise group following the intervention. Evoked PF doublet torque decreased 12 % in both groups (P = 0.002).

CONCLUSION

Central and peripheral neuromuscular function is compromised during unloading. While BFR resistance training on the KE during unloading can maintain muscle mass and strength, it may only partially attenuate neuromuscular dysfunction.

Hypo-activity induced skeletal muscle atrophy and potential nutritional interventions: A review

Periods of hypo-activity result in profound changes in skeletal muscle morphology and strength. This review primarily addresses the differential impact of de-training, bed-rest, limb immobilisation and unilateral lower limb suspension on muscle morphology, strength and fatigability. The degree of muscle atrophy differs depending on the hypo-activity model and the muscles in question, with the leg and postural muscles being the most susceptible to atrophy. Hypo-activity also results in the dramatic loss of strength that often surpasses the loss of muscle mass, and consequently, the nervous system and contractile properties adapt to adjust for this excessive loss of strength. In addition, the degree of muscle strength loss is different depending on the hypo-activity model, with immobilisation appearing to have a greater impact on strength than unloaded models. There is a step-wise difference in the magnitude of muscle loss so that, even after accounting for differential durations of interventions immobilisation ≥ unilateral lower limb suspension ≥ bed-rest ≥ de-training. Muscle fatigability varies between hypo-activity models but the results are equivocal and this may be due to task-specific adaptations. This review also addresses potential nutritional interventions for attenuating hypo-activity induced muscle atrophy and strength declines, in the absence of exercise. Essential amino acid supplementation stands as a strong candidate but other supplements are good contenders for attenuating hypo-activity induced atrophy and strength losses. Several potential nutritional supplements are highlighted that could be used to combat muscle atrophy but extensive research is needed to determine the most effective.