This study characterized acute changes in thigh muscle cross-sectional area (CSA) in subjects who suffered their first severe knee trauma. Ninety-one males and females who suffered ACL rupture with or without concomitant meniscus injury underwent bilateral mid-thigh axial T1-weighted, fast field echo magnetic resonance imaging early after the index trauma and prior to reconstruction. The extensor and flexor muscle groups were segmented bilaterally, and CSA measured. Within-person injured-to-contralateral normal side differences in CSA (injured minus normal limb) were evaluated. ACL-injured male and female subjects were classified into four meniscal injury groups (lateral, medial, both lateral and medial, or normal meniscus). A separate sample of healthy subjects with no history of knee injury or disease underwent the same protocol to determine the magnitude of side-to-side differences in thigh muscle CSA in uninjured individuals. ACL-injured individuals had a substantially greater between-leg differences in thigh extensor muscle CSA after injury and prior to reconstruction than healthy subjects. The between leg difference in thigh extensor muscle CSA of the injured leg was largest in males with meniscus injury involving both compartments, with males having twice as much atrophy of the extensor muscles in the injured leg compared to females in that group. The sex-specific response of thigh muscle to severe knee trauma involving the ACL and meniscus suggests that males and females may require tailored rehabilitation programs pre- and post-surgery to recover muscle size and improve muscular strength and symmetry.

Knee osteoarthritis (OA) is the leading cause of physical disability in older adults. Total knee arthroplasty (TKA) is a common treatment for advanced stage knee OA that alleviates knee pain, but it is associated with precipitous reductions in physical function early after surgery that can take months or years to recover. Sustaining neuromuscular activation after surgery with neuromuscular electrical stimulation (NMES) can improve recovery of physical function, but the mechanisms underlying its benefits are unclear. To examine the unique effects of NMES on skeletal muscle, we randomized older adult patients (70 % female) to early NMES (n = 11) or no intervention (n = 12) for 5 weeks after surgery. We measured skeletal muscle (vastus lateralis) fiber size, contractility, mitochondrial content, and mRNA abundance pre-surgery and 5 weeks post-surgery. NMES diminished TKA-induced muscle fiber atrophy in fast-twitch, myosin heavy chain (MHC) IIA fibers and improved or preserved single muscle fiber contractility in MHC I and MHC IIA fibers, respectively. In MHC IIA fibers, the beneficial effects of NMES to sustain fiber force production were explained at the molecular level by preservation of strongly bound, myosin-actin crossbridges. Additionally, TKA-induced increases in markers of denervation (CHRNA1 and MYOG) in controls were prevented by NMES. Our results identify beneficial effects of sustaining neuromuscular activation early, post-TKA with NMES on skeletal muscle fiber size and function and potential molecular mechanisms underlying these effects.

Cancer cachexia (CC), a syndrome of skeletal muscle and adipose wasting, reduces responsiveness to therapies and increases mortality. There are no approved treatments for CC, which may relate to discordance between preclinical models and human CC. To address the need for clinically relevant models of lung CC, we generated inducible, lung epithelial cell-specific KrasG12D/+ (G12D) mice. G12D mice develop CC over a protracted time course and phenocopy tissue and tumor, cellular, mutational, transcriptomic, and metabolic characteristics of human lung CC. G12D mice demonstrate early loss of adipose, a phenotype that was apparent across numerous models of CC and translates to patients with lung cancer. Tumor-released factors promote adipocyte lipolysis, a driver of adipose wasting in CC, and adipose wasting was inversely related to tumor burden. Thus, G12D mice model key features of human lung CC and highlight a role for early tumor metabolic reprogramming of adipose tissue in CC.

Muscle wasting is common in patients with acute respiratory distress syndrome (ARDS). We have previously shown that acute lung-injured (ALI) mice develop muscle atrophy driven by muscle E3 ubiquitin ligase muscle RING-finger protein 1 (MuRF1). The muscle atrophy response in ALI mice can be partially alleviated by short durations of moderate-intensity treadmill exercise through unclear mechanisms. Glucocorticoid receptor (GR) signaling has been implicated in muscle wasting and repair, and the MuRF1 promoter contains a glucocorticoid response element. We examined the contribution of muscle GR signaling in ALI-associated muscle wasting and the response to exercise. Intratracheal lipopolysaccharides were instilled into wild-type (WT) mice. Mice exercised for prescribed intensity and duration on a treadmill. GR knockdown was achieved through pharmacological inhibition and the use of muscle-specific GR knockout mice. Muscle structure and function was evaluated using physiological and histochemical techniques, and GR activation was assessed under multiple conditions. Muscle wasting in ALI mice was associated with a GR transcriptional response, which was suppressed by exercise. However, neither pharmacological inhibition of muscle GR signaling, nor genetic deletion of muscle GR prevented skeletal muscle wasting or recapitulated the benefits of exercise in WT ALI mice. Moreover, RNAseq of tibialis anterior and diaphragm skeletal muscle in WT mice revealed that exercise influenced genes related to skeletal muscle tissue remodeling, but pathway analysis suggested that this was unrelated to the glucocorticoid axis. GR signaling is dispensable for both ALI muscle wasting and its partial mitigation by exercise in mice.NEW & NOTEWORTHY The endogenous glucocorticoid axis is known to influence skeletal muscle structure and function and is activated during stress. Its role in driving muscle wasting and the response to exercise in the context of lung injury is unknown. Here we find that despite a strong muscle transcriptional glucocorticoid response, this axis appears dispensable for muscle wasting or the favorable response to exercise.

Most patients with lung cancer experience cancer cachexia (CC), a syndrome of skeletal muscle and adipose tissue wasting. Knowledge of body composition changes in patients is limited, however, because most studies have been cross-sectional, comparing patients with noncancer controls or patients with and without CC. Few studies, in contrast, have evaluated body composition in patients with lung cancer over time. This review examines our current understanding of longitudinal body composition changes in patients with lung cancer and identifies modifying factors contributing to variation in muscle and adipose tissue wasting, focusing on biological sex. We identified 32 studies conducting longitudinal measurements of body composition by computed tomography, bioelectrical impedance, dual X-ray absorptiometry, or total body nitrogen, with a total of n = 3,951 patients (35% female). All studies evaluated changes following diagnosis while patients were receiving treatment. Most studies reporting muscle-specific outcomes show decreased skeletal muscle mass, with more pronounced muscle wasting in males and male-enriched populations. In a small number of studies reporting muscle density, the majority show increased myosteatosis. Adiposity changes are less frequently reported, although wasting appears more prevalent in late-stage disease. Further studies are needed to define adipose changes along the lung cancer continuum. Our review emphasizes the need for balanced recruitment based on biological sex and sex-based analyses. In addition, consensus reporting of relevant patient data and outcomes in future studies will allow for meta-analysis and assist in the development of effective treatments for lung CC.

AbstractSkeletal muscle dysfunction in critical illnesses leaves survivors weak and functionally impaired. Macrophages infiltrate muscles; however, their functional role is unclear. We aim to examine muscle leukocyte composition and the effect of macrophages on muscle mass and function in the murine acute lung injury (ALI)‐associated skeletal muscle wasting model. We performed flow cytometry of hindlimb muscle to identify myeloid cells pre‐injury and time points up to 29 days after intratracheal lipopolysaccharide ALI. We evaluated muscle force and morphometrics after systemic and intramuscular clodronate‐induced macrophage depletions between peak lung injury and recovery (day 5–6) versus vehicle control. Our results show muscle leukocytes changed over ALI course with day 3 neutrophil infiltration (130.5 ± 95.6cells/mg control to 236.3 ± 70.6cells/mg day 3) and increased day 10 monocyte abundance (5.0 ± 3.4%CD45+CD11b+ day 3 to 14.0 ± 2.6%CD45+CD11b+ day 10, p = 0.005). Although macrophage count did not significantly change, pro‐inflammatory (27.0 ± 7.2% day 3 to 7.2 ± 3.8% day 10, p = 0.02) and anti‐inflammatory (30.5 ± 11.1% day 3 to 52.7 ± 9.7% day 10, p = 0.09) surface marker expression changed over the course of ALI. Macrophage depletion following peak lung injury increased muscle mass and force generation. These data suggest muscle macrophages beyond peak lung injury limit or delay muscle recovery. Targeting macrophages could augment muscle recovery following lung injury.

Developments in mRNA/lipid nanoparticle (LNP) technology have advanced the fields of vaccinology and gene therapy, raising questions about immunogenicity. While some mRNA/LNPs generate an adjuvant-like environment in muscle tissue, other mRNA/LNPs are distinct in their capacity for multiple rounds of therapeutic delivery. We evaluate the adjuvancy of components of mRNA/LNPs by phenotyping cellular infiltrate at injection sites, tracking uptake by immune cells, and assessing the inflammatory state. Delivery of 9 common, but chemically distinct, LNPs to muscle revealed two classes of inflammatory gene expression programs: inflammatory (Class A) and noninflammatory (Class B). We find that intramuscular injection with Class A, but not Class B, empty LNPs (eLNPs) induce robust neutrophil infiltration into muscle within 2 h and a diverse myeloid population within 24 h. Single-cell RNA sequencing revealed SM-102-mediated expression of inflammatory chemokines by myeloid infiltrates within muscle 1 day after injection. Surprisingly, we found direct transfection of muscle infiltrating myeloid cells and splenocytes 24 h after intramuscular mRNA/LNP administration. Transfected myeloid cells within the muscle exhibit an activated phenotype 24 h after injection. Similarly, directly transfected splenic lymphocytes and dendritic cells (DCs) are differentially activated by Class A or Class B containing mRNA/LNP. Within the splenic DC compartment, type II conventional DCs (cDC2s) are directly transfected and activated by Class A mRNA/LNP. Together, we show that mRNA and LNPs work synergistically to provide the necessary innate immune stimuli required for effective vaccination. Importantly, this work provides a design framework for vaccines and therapeutics alike.

BACKGROUND:Cancer and its treatment can adversely affect skeletal muscle, impacting physical function, treatment response and survival. No studies, however, have comprehensively characterized these muscle adaptations longitudinally in human patients at the cellular level.METHODS:We examined skeletal muscle size and function from the whole body to the sub-cellular level in 11 patients with non-small cell lung cancer (NSCLC; 6 male/5 female, mean age 58 ± 3 years) studied over a 2-month observation period starting during their first cycle of standard of care cancer treatment and in 11 age- and sex-matched healthy controls (HC) without a current or past history of cancer. Biopsies of the vastus lateralis were performed to assess muscle fibre size, contractility and mitochondrial content, along with assessments of physical function, whole muscle size and function, and circulating cytokines.RESULTS:Body weight, composition and thigh muscle area and density were unaltered over time in patients with NSCLC, while muscle density was lower in patients with NSCLC versus HC (P = 0.03). Skeletal muscle fibre size decreased by 18% over time in patients (all P = 0.02) and was lower than HC (P = 0.02). Mitochondrial fractional area and density did not change over time in patients, but fractional area was lower in patients with NSCLC compared with HC (subsarcolemmal, P = 0.04; intermyofibrillar, P = 0.03). Patients with NSCLC had higher plasma concentrations of IL-6 (HC 1.40 ± 0.50; NSCLC 4.71 ± 4.22; P < 0.01), GDF-15 (HC 569 ± 166; NSCLC 2071 ± 1168; P < 0.01) and IL-8/CXCL8 (HC 4.9 ± 1.8; NSCLC 10.1 ± 6.0; P = 0.02) compared with HC, but there were no changes in inflammatory markers in patients with NSCLC over time. No changes were observed in markers of satellite cell activation or DNA damage in patients and no group differences were noted with HC. Whole-muscle strength was preserved over time in patients with NSCLC coincident with improved single fibre contractility.CONCLUSIONS:This study is the first to comprehensively examine longitudinal alterations in skeletal muscle fibre size and function in patients with NSCLC and suggests that muscle fibre atrophy occurs during cancer treatment despite weight stability and no changes in conventional clinical measurements of whole body or thigh muscle size over this period.

Background: Previous studies identified physical function limitations in older cancer survivors, but few have included objective measures and most focused on breast and prostate cancer survivors. The current study compared patient-reported and objective physical function measures between older adults with and without a cancer history.Methods: Our cross-sectional study used a nationally representative sample of community-dwelling, Medicare beneficiaries from the 2015 National Health and Aging Trends Study (n = 7495). Data collected included patient-reported physical function, including a composite physical capacity score and limitations in strength, mobility, and balance, and objectively measured physical performance metrics, including gait speed, five time sit-to-stand, tandem stand, and grip strength. All analyses were weighted to account for the complex sampling design.Results: Thirteen percent of participants (n = 829) reported a history of cancer, of which more than half (51%) reported a diagnosis other than breast or prostate cancer. In models adjusted for demographics and health history, older cancer survivors had lower Short Physical Performance Battery scores (unstandardized beta [B] = -0.36; 95% CI: -0.64, -0.08), slower gait speed (B = -0.03; 95% CI: -0.05, -0.01), reduced grip strength (B = -0.86; 95% CI: -1.44, -0.27), worse patient-reported composite physical capacity (B = -0.43; 95% CI: -0.67, -0.18) and patient-reported upper extremity strength (B = 1.27; 95% CI: 1.07, 1.50) compared to older adults without cancer. Additionally, the burden of physical function limitations was greater in women than in men, which may be explained by cancer type.Conclusions: Our results extend studies in breast and prostate cancer to show worse objective and patient-reported physical function outcomes in older adults with a range of cancer types compared to those without a cancer history. Moreover, these burdens seem to disproportionately affect older adult women, underscoring the need for interventions to address functional limitations and prevent further health consequences of cancer and its treatment.

Loss of quadriceps strength after total knee arthroplasty (TKA) is most pronounced acutely but persists long‐term, negatively impacting physical function in daily activities. Neuromuscular electrical stimulation (NMES) early after surgery is an effective adjuvant to standard of care rehabilitation (SOC) for attenuating strength loss following TKA, but the mechanisms whereby NMES maintains strength are unclear. This work aimed to determine the effects of early NMES on quadriceps strength and skeletal muscle fiber size 2 weeks after TKA compared to SOC. Patients scheduled for primary, unilateral TKA were enrolled and randomized into SOC (n = 9) or NMES plus SOC (n = 10) groups. NMES was started within 48 h of TKA, with 45‐min sessions twice a day for 2 weeks. Isometric quadriceps strength was assessed preoperatively and 2 weeks following TKA. Vastus lateralis muscle biopsies of the involved leg were performed at the same time points and immunohistochemistry conducted to assess muscle fiber cross‐sectional area and distinguish fiber types. Groups did not differ in age, body mass index, sex distribution, or preoperative strength. Both groups got weaker postoperatively, but the NMES group had higher normalized strength. After 2 weeks, the group receiving NMES and SOC had significantly greater MHC IIA and MHC IIA/IIX fiber size compared to SOC alone, with no group differences in MHC I fiber size. These results suggest that NMES mitigates early muscle weakness following TKA, in part, via effects on fast‐twitch, type II muscle fiber size. This investigation advances our understanding of how adjuvant, early postoperative NMES aids muscle strength recovery.

Objectives: Most lung cancer patients exhibit cancer cachexia (CC), a syndrome of skeletal muscle and fat tissue wasting.No effective therapies for CC have been identified, in part because pathoetiological mechanisms contributing to disease progression are difficult to capture in currently available, rapidly progressing preclinical models.Thus, the goal of this work was to develop and characterize a mouse model of lung CC that better aligns with the clinical syndrome.Methods: Club-cell specific tamoxifen-inducible Kras G12D/+ mice were induced at 4-5 months of age and body weight was monitored over a 12-week period.At 6-and 12-weeks postinduction, tissues were weighed or prepared for histology to measure cell size and tumor burden, and blood was collected for measurement of circulating cytokines.We generated lung organoids from WT and Kras G12D mice and isolated their RNA for RNAseq.Differentiated 3T3L1 adipocytes and C2C12 myotubes were treated with lung organoid conditioned medium (CM) to model the effects of lung tumor-derived factors on peripheral tissues.Results: Kras G12D mice were characterized by ∼15% body weight loss over 12 weeks, yielding a rate of body weight loss (∼1.9%/week) that corresponds favorably with human CC patients (∼1%/week) and is much slower than currently used models (5-10%/week).Body weight loss was accompanied by a marked depletion in adipose tissue mass.At 6 weeks postinduction, when animals are pre-cachectic (<5% body weight loss), we observed loss of fat pad weight and ∼50% lower adipocyte cross-sectional area, while muscle weights were unaltered.Additionally, CM from Kras G12D lung organoids elicited glycerol and IL-6 release from cultured adipocytes when compared to CM from WT organoids.Conclusions: These findings suggest that lung epithelialspecific, inducible Kras G12D mice better model the time course of CC development and progression in humans compared to currently available lung cancer models and highlight fat loss as an early pathological event in CC that may be mediated by tumorderived factors.

Doxorubicin (Dox) is a commonly used chemotherapeutic that can adversely affect skeletal muscle, including causing muscle atrophy. Dox is known to induce an event known as mitochondrial permeability transition (MPT) in cardiac muscle and this plays an important role in Dox-mediated cardiac toxicity. Further to this, recent evidence identifies MPT as a mechanism of atrophy in skeletal muscle, suggesting that MPT may underlie some of the Dox-related toxicity in skeletal muscle. To test this hypothesis, we used cultured human primary myotubes, C2C12 myotubes, and single adult mouse flexor digitorum brevis (FDB) muscle fibers in experiments involving Dox treatment with or without inhibitors of MPT. Dox treatment of myotubes caused myonuclear translocation of the mitochondrial protein apoptosis inducing factor (AIF) and increased mitochondrial reactive oxygen species (mROS), consistent with the known consequences of MPT. Furthermore, Dox caused atrophy in C2C12 myotubes grown on patterned plates, human primary myotubes, and single muscle fibers from adult mice. Notably, Dox-induced atrophy could be prevented by a wide variety of agents that inhibit MPT, as well as by inhibiting mROS or Caspase 3. In conclusion, our results indicate that MPT plays an important role in driving Dox-mediated skeletal muscle atrophy.

Purpose: Coronary artery bypass graft (CABG) surgery is an important treatment option in patients with coronary artery disease. Despite its beneficial effects, CABG surgery and its subsequent hospitalization may reduce physical functional capacity in patients, contributing to physical disability. Our objective was to assess the early disabling effects of CABG surgery and its subsequent hospitalization using direct measurements of physical function. Methods: Patients (n = 44) were assessed pre-surgery and at hospital discharge for physical function using the Short Physical Performance Battery (SPPB) and self-reported physical and mental health by questionnaire. Results: The total SPPB score ( P < .001) and all of its components ( P < .01-.001) decreased markedly following CABG surgery and hospitalization, with greater reductions in total SPPB score ( P < .05) and gait speed ( P < .01) in patients with higher body mass index. While CABG surgery and hospitalization reduced patient-reported physical function, changes in these indices largely did not correlate with changes in SPPB outcomes. Conclusion: Our results show the early disabling effects of CABG surgery and hospitalization on directly measured physical function, and that patients with higher body mass index had greater reductions. In addition, our results underscore the need to perform direct measurements of physical function to describe reductions in physiological functional capacity. These findings suggest the need for inpatient rehabilitation or early mobility programs to address this decline in physical function.

Background: Anterior cruciate ligament (ACL) trauma and ACL reconstruction (ACLR) are associated with the loss of strength and function of the muscles that span the knee joint. The underlying mechanism associated with this is not completely understood. Purpose: To determine whether the duration of tourniquet use during ACLR has an effect on knee extensor muscle contractile function and size at the cellular (ie, fiber) level 3 weeks after surgery and at the whole-muscle level at 6 months after surgery. Study Design: Descriptive laboratory study and case series; Level of evidence, 4. Methods: Study participants sustained an acute, first-time ACL injury. All participants underwent ACLR with the use of a tourniquet placed in a standardized location on the thigh; the tourniquet was inflated (pressure range, 250-275 mm Hg), and the time of tourniquet use during surgery was documented. Participants were evaluated 1 week before surgery (to measure patient function, strength, and subjective outcome with the Knee injury and Osteoarthritis Outcome Score [KOOS] and International Knee Documentation Committee [IKDC] score), at 3 weeks after ACLR surgery (to obtain muscle biopsy specimens of the vastus lateralis and assess muscle fiber cross-sectional area, contractile function, and mitochondrial content and morphometry), and at 6 months after ACLR (to evaluate patient function, strength, and subjective outcomes via KOOS and IKDC scores). Data were acquired on both the injured/surgical limb and the contralateral, normal side to facilitate the use of a within-subjects study design. Results are based on additional analysis of data acquired from previous research that had common entry criteria, treatments, and follow-up protocols. Results: At 3 weeks after ACLR, the duration of tourniquet use at the time of surgery did not explain the variation in single–muscle fiber contractile function or cross-sectional area (myosin heavy chain [MHC] I and II fibers) or subsarcolemmal and intermyofibrillar mitochondrial content or morphometry. At 6 months after ACLR, the duration of tourniquet use was not associated with the peak isometric and isokinetic torque measurements, patient function, or patient-reported outcomes. Conclusion: The duration of tourniquet use at the time of ACLR surgery did not explain variation in muscle fiber size, contractile function, or mitochondrial content at 3 weeks after surgery or strength of the quadriceps musculature or patient-reported function or quality of life at 6-month follow-up.

Macrophages are important mediators of skeletal muscle function in both healthy and diseased states. In vivo specific depletion of macrophages provides an experimental method to understand physiological and pathophysiological effects of macrophages. Systemic depletion of macrophages can deplete skeletal muscle macrophages but also alters systemic inflammatory responses and metabolism, which confounds the muscle specific effects of macrophage depletion. The primary aim of this manuscript is to evaluate two methods of murine intramuscular macrophage depletion in an acute lung injury-associated indirect skeletal muscle wasting mouse model. Adult C57BL/6 (WT) and Macrophage Fas-Induced Apoptosis (MaFIA, C57BL/6-Tg) mice received clodronate liposomes or the dimerization drug AP20187 through intramuscular injection of the tibialis anterior muscle compartment, respectively. Vehicle control was injected in the contralateral muscle. We demonstrate intramuscular AP20187 in the MaFIA mouse depletes macrophages but causes an infiltration of CD45 intermediate neutrophils. In contrast, intramuscular clodronate liposomes successfully depletes macrophages without an associated increase in CD45 intermediate cells. In conclusion, intramuscular clodronate is effective for selective depletion of muscle macrophages without eliciting acute inflammation seen with AP20187 in MaFIA mice. This technique is an important tool to study the functional roles of macrophages in skeletal muscle.

The abundance, anatomical distribution, and vascularity of skeletal muscle make it a potentially important contributor to local cytokine production and systemic cytokine abundance during inflammatory events. An orchestrated balance between the production of pro- and anti-inflammatory mediators is necessary for proper immune function, yet the contribution of the body’s largest organ system, comprised primarily of skeletal muscle myocytes that fuse to form myofibers, to this process is largely unknown. Endotoxin (lipopolysaccharide, LPS) stimulates toll-like receptor 4 (TLR4) to induce the production of several pro-inflammatory cytokines, including interleukin-6 (IL-6) and C-C motif chemokine ligand 2 (CCL2), by a of myriad cell types. We sought to quantify the influence of myofibers on systemic cytokine concentrations following an acute endotoxemia challenge. To accomplish this, we generated muscle specific conditional knockouts for TLR4 (TLR4SMKO), IL-6 (IL6SMKO), and CCL2 (CCL2SMKO). We administered low concentrations of intravenous LPS (IV LPS) to these receptor and effector knockout mice and collected samples after 3 h. Using gene expression analysis of gastrocnemius muscle and serum cytokine measurements after IV LPS, we determined that deletion of myofiber IL-6 or CCL2 led to a 93% and 57% reduction of these specific cytokines in the systemic circulation, respectively. Myofiber specific TLR4 deletion decreased the expression of IL-6, CCL2, and C-X-C motif chemokine ligand 1 (CXCL1) in the gastrocnemius muscle. These data indicate the critical involvement and direct contribution of myofibers during the early systemic inflammatory cytokine response to endotoxin.

Total knee arthroplasty (TKA) is an important treatment option for knee osteoarthritis (OA) that improves self-reported pain and physical function, but objectively measured physical function typically remains reduced for years after surgery due, in part, to precipitous reductions in lower extremity neuromuscular function early after surgery. The present study examined intrinsic skeletal muscle adaptations during the first 5 weeks post-TKA to identify skeletal muscle attributes that may contribute to functional disability. Patients with advanced stage knee OA were evaluated prior to TKA and 5 weeks after surgery. Biopsies of the vastus lateralis were performed to assess muscle fiber size, contractility, and mitochondrial content, along with assessments of whole muscle size and function. TKA was accompanied by marked reductions in whole muscle size and strength. At the fiber (i.e., cellular) level, TKA caused profound muscle atrophy that was approximately twofold higher than that observed at the whole muscle level. TKA markedly reduced muscle fiber force production, contractile velocity, and power production, with force deficits persisting in myosin heavy chain (MHC) II fibers after expression relative to fiber size. Molecular level assessments suggest reduced strongly bound myosin-actin cross bridges and myofilament lattice stiffness as a mechanism underlying reduced force per unit fiber size. Finally, marked reductions in mitochondrial content were apparent and more prominent in the subsarcolemmal compartment. Our study represents the most comprehensive evaluation of skeletal muscle cellular adaptations to TKA and uncovers novel effects of TKA on muscle fiber size and intrinsic contractility early after surgery that may contribute to functional disability.NEW & NOTEWORTHY We report the first evaluation of the effects of total knee arthroplasty (TKA) on skeletal muscle at the cellular and subcellular levels. We found marked effects of TKA to cause skeletal muscle fiber atrophy and contractile dysfunction in older adults, as well as molecular mechanisms underlying impaired contractility. Our results reveal profound effects of TKA on muscle fiber size and intrinsic contractility early after surgery that may contribute to functional disability.

Obesity has become one of the most pressing public health issues of the 21st century and currently affects a substantial proportion of the older adult population. Although the cardiometabolic complications are well documented, research from the past 20 years has drawn attention to the detrimental effects of obesity on physical performance in older adults. Obesity-related declines in physical performance are due, in part, to compromised muscle strength and power. Recent evidence suggests there are a number of mechanisms potentially underlying reduced whole muscle function, including alterations in myofilament protein function and cellular contractile properties, and these may be related to morphological adaptations, such as shifts in fiber type composition and increased intramyocellular lipid content within skeletal muscle. To date, even less research has focused on how exercise and weight loss interventions for obese older adults affect these mechanisms. In light of this work, we provide an update on the current knowledge related to obesity and skeletal muscle contractile function and highlight a number of questions to address potential etiologic mechanisms as well as intervention strategies, which may help advance our understanding of how physical performance can be improved among obese older adults.

Muscle dysfunction following anterior cruciate ligament reconstruction (ACLR) may evolve from alterations in muscle contractility at the myofilament protein level. Using a prospective, within‐subject case–control design, we evaluated cellular‐level contractility, cross‐sectional area (CSA), and myosin heavy chain (MHC) isoform expression on single muscle fibers 3 weeks post ACLR, and evaluated their relationship to whole muscle strength and patient‐oriented outcomes 6 months post operation. Biopsies of the vastus lateralis were performed 3 weeks post ACLR in 11 subjects (5 females, mean age ± SD = 24.7 ± 6.5 years, height = 172.7 ± 8.2 cm, mass = 75.7 ± 12.5 kg) following first‐time ACL rupture and whole muscle strength and self‐reported pain, function, and quality of life assessed 6 months post ACLR. At 3 weeks post ACLR, force production was reduced (p < 0.01) in MHC I (−36%) and IIA (−48%) fibers compared with the non‐injured leg. When force production was expressed relative to CSA to account for fiber atrophy, reductions remained in MHC IIA fibers (−40%; p < 0.001), but MHC I fibers showed only a trend toward being lower (−13%; p = 0.09). Finally, skeletal muscle fiber functional deficits at 3 weeks post ACLR were associated with whole muscle weakness and less favorable patient‐reported outcomes at 6‐month follow‐up. Thus, ACLR promotes early cellular contractile dysfunction that may contribute to decreased whole muscle strength and patient function, and increased patient‐reported symptoms, at 6‐month follow‐up.

Muscle may contribute to the systemic inflammatory environment during critical illness, but leukocyte interaction and cytokine influence on muscle and its response has not been fully explored in this context. Using an in vivo model of intratracheal lipopolysaccharide (IT LPS)-induced acute lung injury, we show that skeletal muscle rapidly responds with expression of proinflammatory genes, which may be explained by migration of LPS into the circulation. Treatment of mature C2C12 myotubes with LPS at a level achieved in the circulation following IT LPS elicited a proinflammatory cytokine expression profile similar to that of in vivo murine muscle following IT LPS. Stimulation with toll-like receptor (TLR) 2 and 3 agonists provoked comparable responses in C2C12 myotubes. Additionally, co-cultures of C2C12 myotubes and bone marrow-derived macrophages (BMDM) identified the capacity of macrophages to increase myotube proinflammatory gene expression, with tumor necrosis factor-alpha (TNF alpha) gene and protein expression largely attributable to BMDM. To investigate the contribution of TNF alpha in the synergy of the co-culture environment, C2C12 myotubes were treated with recombinant TNF alpha, co-cultures were established using TNF-deficient BMDM, and co-cultures were also depleted of TNF alpha using antibodies. To determine whether the in vitro observations were relevant in vivo, mice received intramuscular administration of LPS +/- TNF alpha or TNF alpha-neutralizing antibodies and showed that TNF alpha is both sufficient and necessary to induce synergistic cytokine release from muscle. Taken together, these data demonstrate how skeletal muscle tissue may contribute proinflammatory cytokines following acute endotoxin injury and the potential of leukocytes to augment this response via TNF alpha secretion.