The Effect of Increasing Age on the Concentric and Eccentric Contractile Properties of Isolated Mouse Soleus and Extensor Digitorum Longus Muscles

Focused Electric Field Technology: A Novel Myoelectrical Stimulation Technology for Noninvasive Aging Muscle Rejuvenation

BACKGROUND

Myoelectrical stimulation improves muscle function and reduces muscle atrophy and aging. However, research on the mechanism underlying its cosmetic effect remains limited.

AIMS

The aim of this study was to evaluate the cosmetic effects of the myoelectrical stimulation provided by the wearable intelligent flexible beauty device and its focused electric field technology (FEFT) on facial skin and muscle rejuvenation.

PATIENTS/METHODS

We conducted a single-blind, randomized, self-controlled clinical efficacy experiment on 31 female volunteers using the device. Using an FEFT-based platform, mice with d-galactose-induced skeletal muscle aging were subjected to surface myoelectrical stimulation of the gastrocnemius. Immunohistochemical analysis of skeletal muscles and protein immunoblotting were used to analyze the effects of FEFT.

RESULTS

After 14 days of use, facial skin elasticity significantly increased, wrinkle firmness significantly decreased, and the lift height of the upper eyelid and eye corner angle significantly increased in the volunteers. Clinical evaluation showed improvements in the drooping of the upper eyelid and eye bags. Self-evaluation questionnaires indicated alleviation of facial wrinkles. These improvements were more pronounced after 28 days. In mice, FEFT alleviated aging-induced muscle fiber atrophy, muscle fiber cross-sectional area reduction, and muscle satellite cell loss. FEFT also increased the expression of myogenic factors, including myogenic differentiation 1 (MYOD1).

CONCLUSIONS

FEFT exerted a skin-tightening effect by initiating myogenic processes and increasing the transformation of muscle satellite cells. Our research promotes the development of FEFT-based medical rehabilitation or cosmetic anti-aging products and provides a foundation for further application and comprehensive efficacy evaluation in human clinical settings.

Longitudinal Decline of Exercise Capacity in Male and Female Mice

The population of older adults is exponentially expanding. Alongside aging comes the onset of chronic disease, decline of functional capacity, and reduced quality of life. Thus, this population increase will stress the capacity and financial viability of health and long-term care systems. Developing pre-clinical models for age-related functional decline is imperative to advancing therapies that extend healthspan and prolong independence. Previously in a cross-sectional study, we established a powerful composite scoring system we termed CFAB (comprehensive functional assessment battery). CFAB measures physical function and exercise capacity using well-validated determinants to measure overall motor function, fore-limb strength, four-limb strength/endurance, aerobic capacity, and volitional exercise/activity rate. In the current work, we used CFAB to track cohorts of male and female C57BL/6 mice over the lifespan (measuring CFAB at 6, 12, 18, 24, and 28 months of age). Overall, we found statistically significantly declining function as the mice aged, with some differences between males and females in trajectory and slope. We also determined that body mass changes presented differently between sexes, and tracked body composition (fat percentage, using magnetic resonance imagery) in females. In a subset of mice, we tracked in vivo contractile physiology noting declines in plantar flexor maximum isometric torque. In summary, our data suggest that males and females declined at different rates. We confirmed the efficacy of CFAB to track longitudinal changes in exercise capacity and physical fitness in both males and females, further validating the system to track age-related functional decline.

Effect of high-fat diet on isometric, concentric and eccentric contractile performance of skeletal muscle isolated from female CD-1 mice

Despite evidence inferring muscle and contractile mode-specific effects of high-fat diet (HFD), no study has yet considered the impact of HFD directly on eccentric muscle function. The present work uniquely examined the effect of 20-week HFD on the isometric, concentric and eccentric muscle function of isolated mouse soleus (SOL) and extensor digitorum longus (EDL) muscles. CD-1 female mice were randomly split into a control (n = 16) or HFD (n = 17) group and for 20 weeks consumed standard lab chow or HFD. Following this period, SOL and EDL muscles were isolated and assessments of maximal isometric force and concentric work loop (WL) power were performed. Each muscle was then subjected to either multiple concentric or eccentric WL activations. Post-fatigue recovery, as an indicator of incurred damage, was measured via assessment of concentric WL power. In the EDL, absolute concentric power and concentric power normalised to muscle mass were reduced in the HFD group (P < 0.038). HFD resulted in faster concentric fatigue and reduced eccentric activity-induced muscle damage (P < 0.05). For the SOL, maximal isometric force was increased, and maximal eccentric power normalised to muscle mass and concentric fatigue were reduced in the HFD group (P < 0.05). HFD effects on eccentric muscle function are muscle-specific and have little relationship with changes in isometric or concentric function. HFD has the potential to negatively affect the intrinsic concentric and eccentric power-producing capacity of skeletal muscle, but a lack of a within-muscle uniform response indicates disparate mechanisms of action which require further investigation.

Mustn1 in Skeletal Muscle: A Novel Regulator?

Skeletal muscle is a complex organ essential for locomotion, posture, and metabolic health. This review explores our current knowledge of Mustn1, particularly in the development and function of skeletal muscle. Mustn1 expression originates from Pax7-positive satellite cells in skeletal muscle, peaks during around the third postnatal month, and is crucial for muscle fiber differentiation, fusion, growth, and regeneration. Clinically, Mustn1 expression is potentially linked to muscle-wasting conditions such as muscular dystrophies. Studies have illustrated that Mustn1 responds dynamically to injury and exercise. Notably, ablation of Mustn1 in skeletal muscle affects a broad spectrum of physiological aspects, including glucose metabolism, grip strength, gait, peak contractile strength, and myofiber composition. This review summarizes our current knowledge of Mustn1's role in skeletal muscle and proposes future research directions, with a goal of elucidating the molecular function of this regulatory gene.

The effects of muscle starting length on work loop power output of isolated mouse soleus and extensor digitorum longus muscle

Force-length relationships derived from isometric activations may not directly apply to muscle force production during dynamic contractions. As such, different muscle starting lengths between isometric and dynamic conditions could be required to achieve maximal force and power. Therefore, this study examined the effects of starting length [±5-10% of length corresponding to maximal twitch force (L0)] on work loop (WL) power output (PO), across a range of cycle frequencies, of the soleus (SOL) and extensor digitorum longus muscle (EDL; N=8-10) isolated from ∼8 week old C57 mice. Furthermore, passive work was examined at a fixed cycle frequency to determine the association of passive work and active net work. Starting length affected maximal WL PO of the SOL and EDL across evaluated cycle frequencies (P<0.030, ηp2>0.494). For the SOL, PO produced at -5% L0 was greater than that at most starting lengths (P<0.015, Cohen's d>0.6), except -10% L0 (P=0.135, d<0.4). However, PO produced at -10% L0 versus L0 did not differ (P=0.138, d=0.35-0.49), indicating -5% L0 is optimal for maximal SOL WL PO. For the EDL, WL PO produced at -10% L0 was lower than that at most starting lengths (P<0.032, d>1.08), except versus -5% L0 (P=0.124, d<0.97). PO produced at other starting lengths did not differ (P>0.163, d<1.04). For the SOL, higher passive work was associated with reduced PO (Spearman's r=0.709, P<0.001), but no relationship was observed between passive work and PO of the EDL (Pearson's r=0.191, r2=0.04, P=0.184). This study suggests that starting length should be optimised for both static and dynamic contractions and confirms that the force-length curve during dynamic contractions is muscle specific.

Understanding the development of sarcopenic obesity

INTRODUCTION

Sarcopenic obesity (SarcO) is defined as the confluence of reduced muscle mass and function and excess body fat. The scientific community is increasingly recognizing this syndrome, which affects a subgroup of persons across their lifespans and places them at synergistically higher risk of significant medical comorbidity and disability than either sarcopenia or obesity alone. Joint efforts in clinical and research settings are imperative to better understand this syndrome and drive the development of urgently needed future interventions.

AREAS COVERED

Herein, we describe the ongoing challenges in defining sarcopenic obesity and the current state of the science regarding its epidemiology and relationship with adverse events. The field has demonstrated an emergence of data over the past decade which we will summarize in this article. While the etiology of sarcopenic obesity is complex, we present data on the underlying pathophysiological mechanisms that are hypothesized to promote its development, including age-related changes in body composition, hormonal changes, chronic inflammation, and genetic predisposition.

EXPERT OPINION

We describe emerging areas of future research that will likely be needed to advance this nascent field, including changes in clinical infrastructure, an enhanced understanding of the lifecourse, and potential treatments.

Effect of stimulation frequency on force, power, and fatigue of isolated mouse extensor digitorum longus muscle

This study examined the effect of stimulation frequency (140, 200, 230 and 260 Hz) on isometric force, work loop (WL) power, and the fatigue resistance of extensor digitorum longus (EDL) muscle (n=32), isolated from 8-10-week-old CD-1 female mice. Stimulation frequency had significant effects on isometric properties of isolated mouse EDL, whereby increasing stimulation frequency evoked increased isometric force, quicker activation, and prolonged relaxation (P <0.047), until 230 Hz and above, thereafter force and activation did not differ (P >0.137). Increasing stimulation frequency increased maximal WL power output (P <0.001; 140 Hz, 71.3±3.5; 200 Hz, 105.4±4.1; 230 Hz, 115.5±4.1; 260 Hz, 121.1±4.1 W.kg-1), but resulted in significantly quicker rates of fatigue during consecutive WL's (P <0.004). WL shapes indicate impaired muscle relaxation at the end of shortening and subsequent increased negative work appeared to contribute to fatigue at 230 and 260 Hz, but not at lower stimulation frequencies. Cumulative work was unaffected by stimulation frequency, except at the start of fatigue protocol where 230 and 260 Hz produced more work than 140 Hz (P <0.039). We demonstrate that stimulation frequency affects force, power, and fatigue, but effects are not uniform between different assessments of contractile performance. Therefore, future work examining contractile properties of isolated skeletal muscle should consider increasing stimulation frequency beyond that needed for maximal force when examining maximal power but utilise a sub-maximal stimulation frequency for fatigue assessments to avoid high degree of negative work atypical of in vivo function.

High-fat diet affects measures of skeletal muscle contractile performance in a temperature specific manner but does not influence regional thermal sensitivity

The present study examined if 20-weeks high-fat diet (HFD) consumption had a temperature specific effect on the contractile performance and regional thermal sensitivity of isolated mouse soleus (SOL) and diaphragm (DIA) muscle. Four-week-old female CD-1 mice were randomly selected to consume either a standard laboratory diet or a standard laboratory diet in conjunction with a HFD for 20-weeks. Peripheral SOL and core DIA were isolated from each animal and maximal isometric force and work loop power were assessed at 20⁰C, 28⁰C, 35⁰C and 40⁰C. Increasing temperature to 35⁰C resulted in greater isometric stress, lower activation and relaxation time and higher work loop power in both muscles. A further increase in temperature to 40⁰C did not affect isometric force but increased work loop power output of the SOL. Conversely, isometric force of the DIA was reduced and work loop power maintained when temperature was increased to 40⁰C. HFD consumption resulted in greater isometric force and absolute work loop power of the SOL and reduced isometric stress of the DIA, effects that were less apparent at lower temperatures. When the relationship between temperature and each measure of contractile function was examined by linear regression, there was no difference in slope between the control or HFD groups for either SOL or DIA. These results indicate that whilst contractile function initially increases with temperature, the temperature to elicit maximal performance is muscle and contractile mode-specific. Furthermore, HFD effects on contractile function are temperature specific, but HFD does not influence the relationship between temperature and performance.

Applications of Eccentric Exercise to Improve Muscle and Mobility Function in Older Adults

Muscle aging ultimately leads to the deterioration of human physiological functioning, including declining muscle strength, loss of muscle mass, and decreased quality of life in advanced age. Eccentric exercise is a key intervention that has the potential to ameliorate this problem. Recent studies have focused on evidence-based exercise interventions to prevent declines in muscle strength and physical function in older adults. This paper reviewed relevant literature on the use of eccentric exercise to improve muscle and mobility function in older adults. We explained not only the changes in mobility that occur with aging but also the rationale for and positive effects of eccentric intervention in older adults. We also explored several proposed mechanisms for the intramuscular changes caused by eccentric muscle contraction and considered the safety and side effects accompanying eccentric training. We concluded by suggesting that eccentric exercise is an exercise modality that can potentially improve muscle strength and enhance mobility in older adults.

Age-dependent mechanical and microstructural properties of the rabbit soleus muscle

During growth there are serious changes in the skeletal muscles to compensate for the changed requirements in terms of body weight and size. In this study, the age-dependent (between 21 and 100 days) mechanical and microstructural properties of rabbit soleus muscle tissue were investigated. For this purpose, morphological properties (animal mass, soleus muscle mass, tibial length) were measured at 5 different times during aging. On the other hand, fibre orientation-dependent axial and semi-confined compression experiments were realised. In addition, the essential components (muscle fibres, extracellular matrix, remaining components), dominating the microstructure of muscle tissue, were analysed. While the mechanical results show hardly any age-dependent differences, the morphological and microstructural results show clear age-dependent differences. All morphological parameters increase significantly (animal mass by 839.2%, muscle mass 1050.6%, tibial length 233.6%). In contrast, microstructural parameters change differently. The percentage of fibres (divided into slow-twitch (ST) and fast-twitch (FT) fibres) increases significantly (137.6%), while the proportions of the extracellular matrix and the remaining components (48.2% and 46.1%) decrease. At the same time, the cross-sectional area of the fibres increases significantly (697.9%). The totality of this age-dependent information provides a deeper understanding of age-related changes in muscle structure and function and may contribute to successful development and validation of growth models in the future. STATEMENT OF SIGNIFICANCE: This article reports the first comprehensive data set on age-dependent morphological (animal mass, soleus muscle mass, tibial length), mechanical (axial and semi-confined compression), and microstructural (muscle fibres, extracellular matrix, remaining components) properties of the rabbit soleus muscle. On the one hand, the results of this study contribute to the understanding of muscle mechanics and thus to understanding of load transfer mechanisms inside the muscle tissue during growth. On the other hand, these results are relevant to the fields of constitutive formulation of age-dependent muscle tissue.

A High-Fat Diet Aggravates the Age-Related Decline in Skeletal Muscle Structure and Function

The age-related decline in muscle function is aggravated by a high-fat diet (HFD)-induced increase in fat mass. The hypothesis is that an HFD leads to a faster accumulation of intramyocellular lipids (IMCL) and an earlier onset of muscle dysfunction in old than in young-adult individuals. The IMCL accumulation is attenuated in young-adult organisms by an elevated oxidative capacity. Methionine restriction enhances mitochondrial biogenesis and is promising to combat obesity across the ages.

The effects of high adiposity on concentric and eccentric muscle performance of upper and lower limb musculature in young and older adults

The present study uniquely examined the influence of old age and adiposity on maximal concentric and eccentric torque and fatigue of the elbow and knee (KF, KE) flexors and extensors. Forty males were recruited and categorised into young (n = 21, 23.7 ± 3.4) and old (n = 19, 68.3 ± 6.1) and then further into normal (young = 16.9 ± 2.5%, old = 20.6 ± 3.1%) and high adiposity (young = 28.9 ± 5.0%, old = 31.3 ± 4.2%) groups. Handgrip strength, sit-to-stand performance, and isokinetic assessments of peak torque at 60°, 120° and 180°·s^-1 were measured. Older men produced significantly less concentric and eccentric peak torque (P < 0.016) but this was not influenced by adiposity (P > 0.055). For KE and KF, high adiposity groups demonstrated reduced peak torque normalised to body mass (P < 0.021), and muscle and contractile mode specific reduction in torque normalised to segmental lean mass. Eccentric fatigue resistance was unaffected by both age and adiposity (P > 0.30) and perceived muscle soreness, measured up to 72 hours after, was only enhanced in the upper body of the young group following eccentric fatigue (P = 0.009). Despite the impact of adiposity on skeletal muscle function being comparable between ages, these results suggest high adiposity will have greater impact on functional performance of older adults. Novelty: Irrespective of age, high adiposity may negatively impact force to body mass ratio and muscle quality in a muscle and contractile mode specific manner. Whilst the magnitude of adiposity effects is similar across ages, the impact for older adults will be more substantial given the age-related decline in muscle function.

Age-Related Skeletal Muscle Dysfunction Is Aggravated by Obesity: An Investigation of Contractile Function, Implications and Treatment

Obesity is a global epidemic and coupled with the unprecedented growth of the world's older adult population, a growing number of individuals are both old and obese. Whilst both ageing and obesity are associated with an increased prevalence of chronic health conditions and a substantial economic burden, evidence suggests that the coincident effects exacerbate negative health outcomes. A significant contributor to such detrimental effects may be the reduction in the contractile performance of skeletal muscle, given that poor muscle function is related to chronic disease, poor quality of life and all-cause mortality. Whilst the effects of ageing and obesity independently on skeletal muscle function have been investigated, the combined effects are yet to be thoroughly explored. Given the importance of skeletal muscle to whole-body health and physical function, the present study sought to provide a review of the literature to: (1) summarise the effect of obesity on the age-induced reduction in skeletal muscle contractile function; (2) understand whether obesity effects on skeletal muscle are similar in young and old muscle; (3) consider the consequences of these changes to whole-body functional performance; (4) outline important future work along with the potential for targeted intervention strategies to mitigate potential detrimental effects.

Comparative analysis of the transcriptomes of EDL, psoas, and soleus muscles from mice

BACKGROUND

Individual skeletal muscles have evolved to perform specific tasks based on their molecular composition. In general, muscle fibers are characterized as either fast-twitch or slow-twitch based on their myosin heavy chain isoform profiles. This approach made sense in the early days of muscle studies when SDS-PAGE was the primary tool for mapping fiber type. However, Next Generation Sequencing tools permit analysis of the entire muscle transcriptome in a single sample, which allows for more precise characterization of differences among fiber types, including distinguishing between different isoforms of specific proteins. We demonstrate the power of this approach by comparing the differential gene expression patterns of extensor digitorum longus (EDL), psoas, and soleus from mice using high throughput RNA sequencing.

RESULTS

EDL and psoas are typically classified as fast-twitch muscles based on their myosin expression pattern, while soleus is considered a slow-twitch muscle. The majority of the transcriptomic variability aligns with the fast-twitch and slow-twitch characterization. However, psoas and EDL exhibit unique expression patterns associated with the genes coding for extracellular matrix, myofibril, transcription, translation, striated muscle adaptation, mitochondrion distribution, and metabolism. Furthermore, significant expression differences between psoas and EDL were observed in genes coding for myosin light chain, troponin, tropomyosin isoforms, and several genes encoding the constituents of the Z-disk.

CONCLUSIONS

The observations highlight the intricate molecular nature of skeletal muscles and demonstrate the importance of utilizing transcriptomic information as a tool for skeletal muscle characterization.

Age-related changes in isolated mouse skeletal muscle function are dependent on sex, muscle, and contractility mode

The present study aimed to simultaneously examine the age-related, muscle-specific, sex-specific, and contractile mode-specific changes in isolated mouse skeletal muscle function and morphology across multiple ages. Measurements of mammalian muscle morphology, isometric force and stress (force/cross-sectional area), absolute and normalized (power/muscle mass) work-loop power across a range of contractile velocities, fatigue resistance, and myosin heavy chain (MHC) isoform concentration were measured in 232 isolated mouse (CD-1) soleus, extensor digitorum longus (EDL), and diaphragm from male and female animals aged 3, 10, 30, 52, and 78 wk. Aging resulted in increased body mass and increased soleus and EDL muscle mass, with atrophy only present for female EDL by 78 wk despite no change in MHC isoform concentration. Absolute force and power output increased up to 52 wk and to a higher level for males. A 23–36% loss of isometric stress exceeded the 14–27% loss of power normalized to muscle mass between 10 wk and 52 wk, although the loss of normalized power between 52 and 78 wk continued without further changes in stress ( P > 0.23). Males had lower power normalized to muscle mass than females by 78 wk, with the greatest decline observed for male soleus. Aging did not cause a shift toward slower contractile characteristics, with reduced fatigue resistance observed in male EDL and female diaphragm. Our findings show that the loss of muscle quality precedes the loss of absolute performance as CD-1 mice age, with the greatest effect seen in male soleus, and in most instances without muscle atrophy or an alteration in MHC isoforms.

The impact of a high-fat diet in mice is dependent on duration and age, and differs between muscles

Prolonged high-fat diets (HFDs) can cause intramyocellular lipid (IMCL) accumulation that may negatively affect muscle function. We investigated the duration of a HFD required to instigate these changes, and whether the effects are muscle specific and aggravated in older age. Muscle morphology was determined in the soleus, extensor digitorum longus (EDL) and diaphragm muscles of female CD-1 mice from 5 groups: young fed a HFD for 8 weeks (YS-HFD, n=16), young fed a HFD for 16 weeks (YL-HFD, n=28) and young control (Y-Con, n=28). The young animals were 20 weeks old at the end of the experiment. Old (70 weeks) female CD-1 mice received either a normal diet (O-Con, n=30) or a HFD for 9 weeks (OS-HFD, n=30). Body mass, body mass index and intramyocellular lipid (IMCL) content increased in OS-HFD (P≤0.003). In the young mice, this increase was seen in YL-HFD and not YS-HFD (P≤0.006). The soleus and diaphragm fibre cross-sectional area (FCSA) in YL-HFD was larger than that in Y-Con (P≤0.004) while OS-HFD had a larger soleus FCSA compared with that of O-Con after only 9 weeks on a HFD (P<0.001). The FCSA of the EDL muscle did not differ significantly between groups. The oxidative capacity of fibres increased in young mice only, irrespective of HFD duration (P<0.001). High-fat diet-induced morphological changes occurred earlier in the old animals than in the young, and adaptations to HFD were muscle specific, with the EDL being least responsive.

Does Dietary-Induced Obesity in Old Age Impair the Contractile Performance of Isolated Mouse Soleus, Extensor Digitorum Longus and Diaphragm Skeletal Muscles?

Ageing and obesity independently have been shown to significantly impair isolated muscle contractile properties, though their synergistic effects are poorly understood. We uniquely examined the effects of 9 weeks of a high-fat diet (HFD) on isometric force, work loop power output (PO) across a range of contractile velocities, and fatigability of 79-week-old soleus, extensor digitorum longus (EDL) and diaphragm compared with age-matched lean controls. The dietary intervention resulted in a significant increase in body mass and gonadal fat pad mass compared to the control group. Despite increased muscle mass for HFD soleus and EDL, absolute isometric force, isometric stress (force/CSA), PO normalised to muscle mass and fatigability was unchanged, although absolute PO was significantly greater. Obesity did not cause an alteration in the contractile velocity that elicited maximal PO. In the obese group, normalised diaphragm PO was significantly reduced, with a tendency for reduced isometric stress and fatigability was unchanged. HFD soleus isolated from larger animals produced lower maximal PO which may relate to impaired balance in older, larger adults. The increase in absolute PO is smaller than the magnitude of weight gain, meaning in vivo locomotor function is likely to be impaired in old obese adults, with an association between greater body mass and poorer normalised power output for the soleus. An obesity-induced reduction in diaphragm contractility will likely impair in vivo respiratory function and consequently contribute further to the negative cycle of obesity.