Muscle contractile properties in severely burned rats

Skeletal muscle wasting after burn is regulated by a decrease in anabolic signaling in the early flow phase

Following burns a sustained catabolic stress response is activated, resulting in skeletal muscle wasting. A better understanding of the underlying mechanisms of postburn skeletal muscle wasting is essential for the development of preventive and/or therapeutic strategies. Six weeks old female rats underwent a sham, 10% or 40% total body surface area scald burn. Ten days post-injury, severely burned animals gained significantly less weight compared to sham treated and minor burned animals, reflected in a significantly lower ratio of muscle to total body weight for Soleus (SOL) and Extensor Digitorum Longus (EDL) in the severely burned group. Postburn, total fiber number was significantly lower in EDL, while in SOL the amount of type1 fibers significantly increased and type2 fibers significantly decreased. No signs of mitochondrial dysfunction (COX/SDH) or apoptosis (caspase-3) were found. In SOL and EDL, eEF2 and pAKT expression was significantly lower after severe burn. MURF1,2,3 and Atrogin-1 was significantly higher in SOL, whilst in EDL MURF1,2,3 was significantly lower postburn. In both muscles, FOXO3A was significantly lower postburn. This study identified postburn changes in muscle anthropomorphology and proteins involved in pathways regulating protein synthesis and breakdown, with more pronounced catabolic effects in SOL.

Molecular mechanisms of post-burn muscle wasting and the therapeutic potential of physical exercise

After a severe burn injury, a systemic stress response activates metabolic and inflammatory derangements that, among other, leads to muscle mass loss (muscle wasting). These negative effects on skeletal muscle continue for several months or years and are aggravated by short-term and long-term disuse. The dynamic balance between muscle protein synthesis and muscle protein breakdown (proteolysis) is regulated by complex signalling pathways that leads to an overall negative protein balance in skeletal muscle after a burn injury. Research concerning these molecular mechanisms is still scarce and inconclusive, understanding of which, if any, molecular mechanisms contribute to muscle wasting is of fundamental importance in designing of therapeutic interventions for burn patients as well. This review not only summarizes our present knowledge of the molecular mechanisms that underpin muscle protein balance but also summarizes the effects of exercise on muscle wasting post-burn as promising strategy to counteract the detrimental effects on skeletal muscle. Future research focusing on the pathways causing post-burn muscle wasting and the different effects of exercise on them is needed to confirm this hypothesis and to lay the foundation of therapeutic strategies.

Skeletal muscle wasting after a severe burn is a consequence of cachexia and sarcopenia

Muscle wasting is common and persistent in severely burned patients, worsened by immobilization during treatment. In this review, we posit two major phenotypes of muscle wasting after severe burn, cachexia and sarcopenia, each with distinguishing characteristics to result in muscle atrophy; these characteristics are also likely present in other critically ill populations. An online search was conducted from the PubMed database and other available online resources and we manually extracted published articles in a systematic mini review. We describe the current definitions and characteristics of cachexia and sarcopenia and relate these to muscle wasting after severe burn. We then discuss these putative mechanisms of muscle atrophy in this condition. Severe burn and immobilization have distinctive patterns in mediating muscle wasting and muscle atrophy. In considering these two pathological phenotypes (cachexia and sarcopenia), we propose two independent principal causes and mechanisms of muscle mass loss after burns: (1) inflammation-induced cachexia, leading to proteolysis and protein degradation, and (2) sarcopenia/immobility that signals inhibition of expected increases in protein synthesis in response to protein loss. Because both are present following severe burn, these should be considered independently in devising treatments. Discussing cachexia and sarcopenia as independent mechanisms of severe burn-initiated muscle wasting is explored. Recognition of these associated mechanisms will likely improve outcomes.

Erythropoietin Alleviates Burn-induced Muscle Wasting

Background: Burn injury induces long-term skeletal muscle pathology. We hypothesized EPO could attenuate burn-induced muscle fiber atrophy. Methods: Rats were allocated into four groups: a sham burn group, an untreated burn group subjected to third degree hind paw burn, and two burn groups treated with weekly or daily EPO for four weeks. Gastrocnemius muscle was analyzed at four weeks post-burn. Results: EPO attenuated the reduction of mean myofiber cross-sectional area post-burn and the level of the protective effect was no significant difference between two EPO-treated groups (p=0.784). Furthermore, EPO decreased the expression of atrophy-related ubiquitin ligase, atrogin-1, which was up-regulated in response to burn. Compared to untreated burn rats, those receiving weekly or daily EPO groups had less cell apoptosis by TUNEL assay. EPO decreased the expression of cleaved caspase 3 (key factor in the caspase-dependent pathway) and apoptosis-inducing factor (implicated in the caspase-independent pathway) after burn. Furthermore, EPO alleviated connective tissue overproduction following burn via transforming growth factor beta 1-Smad2/3 pathway. Daily EPO group caused significant erythrocytosis compared with untreated burn group but not weekly EPO group. Conclusion: EPO therapy attenuated skeletal muscle apoptosis and fibrosis at four weeks post-burn. Weekly EPO may be a safe and effective option in muscle wasting post-burn.

Inducible satellite cell depletion attenuates skeletal muscle regrowth following a scald-burn injury

KEY POINTS

Severe burns result in significant skeletal muscle cachexia that impedes recovery. Activity of satellite cells, skeletal muscle stem cells, is altered following a burn injury and likely hinders regrowth of muscle. Severe burn injury induces satellite cell proliferation and fusion into myofibres with greater activity in muscles proximal to the injury site. Conditional depletion of satellite cells attenuates recovery of myofibre area and volume following a scald burn injury in mice. Skeletal muscle regrowth following a burn injury requires satellite cell activity, underscoring the therapeutic potential of satellite cells in the prevention of prolonged frailty in burn survivors.

ABSTRACT

Severe burns result in profound skeletal muscle atrophy; persistent muscle atrophy and weakness are major complications that hamper recovery from burn injury. Many factors contribute to the erosion of muscle mass following burn trauma, and we have previously shown concurrent activation and apoptosis of muscle satellite cells following a burn injury in paediatric patients. To determine the necessity of satellite cells during muscle recovery following a burn injury, we utilized a genetically modified mouse model (Pax7CreER -DTA) that allows for the conditional depletion of satellite cells in skeletal muscle. Additionally, mice were provided 5-ethynyl-2'-deoxyuridine to determine satellite cell proliferation, activation and fusion. Juvenile satellite cell-wild-type (SC-WT) and satellite cell-depleted (SC-Dep) mice (8 weeks of age) were randomized to sham or burn injury consisting of a dorsal scald burn injury covering 30% of total body surface area. Both hindlimb and dorsal muscles were studied at 7, 14 and 21 days post-burn. SC-Dep mice had >93% depletion of satellite cells compared to SC-WT (P < 0.05). Burn injury induced robust atrophy in muscles located both proximal and distal to the injury site (∼30% decrease in fibre cross-sectional area, P < 0.05). Additionally, burn injury induced skeletal muscle regeneration, satellite cell proliferation and fusion. Depletion of satellite cells impaired post-burn recovery of both muscle fibre cross-sectional area and volume (P < 0.05). These findings support an integral role for satellite cells in the aetiology of lean tissue recovery following a severe burn injury.

Satellite cell activation and apoptosis in skeletal muscle from severely burned children

KEY POINTS

Severe burns result in profound skeletal muscle atrophy that hampers recovery. The activity of skeletal muscle stem cells, satellite cells, acutely following a severe burn is unknown and may contribute to the recovery of lean muscle. Severe burn injury induces skeletal muscle regeneration and myonuclear apoptosis. Satellite cells undergo concurrent apoptosis and activation acutely following a burn, with a net reduction in satellite cell content compared to healthy controls. The activation and apoptosis of satellite cells probably impacts the recovery of lean tissue following a severe burn, contributing to prolonged frailty in burn survivors.

ABSTRACT

Severe burns result in profound skeletal muscle atrophy; persistent muscle loss and weakness are major complications that hamper recovery from burn injury. Many factors contribute to the erosion of muscle mass following burn trauma and we propose that an impaired muscle satellite cell response is key in the aetiology of burn-induced cachexia. Muscle biopsies from the m. vastus lateralis were obtained from 12 male pediatric burn patients (>30% total body surface area burn) and 12 young, healthy male subjects. Satellite cell content, activation and apoptosis were determined via immunohistochemistry, as were muscle fibre regeneration and myonuclear apoptosis. Embryonic myosin heavy chain expression and central nucleation, indices of skeletal muscle regeneration, were elevated in burn patients (P < 0.05). Myonuclear apoptosis, quantified by TUNEL positive myonuclei and cleaved caspase-3 positive myonuclei, was also elevated in burn patients (P < 0.05). Satellite cell content was reduced in burn patients, with approximately 20% of satellite cells positive for TUNEL staining, indicating DNA damage associated with apoptosis (P < 0.05). Additionally, a significant percentage of satellite cells in burn patients expressed Ki67, a marker for cellular proliferation (P < 0.05). Satellite cell activation was also observed in burn patients with increased expression of MyoD compared to healthy controls (P < 0.05). Robust skeletal muscle atrophy occurs after burn injury, even in muscles located distally to the site of injury. The activation and apoptosis of satellite cells probably impacts the recovery of lean tissue following a severe burn, contributing to prolonged frailty in burn survivors.

Burn injury induces skeletal muscle degeneration, inflammatory host response, and oxidative stress in wistar rats

Burn injuries (BIs) result in both local and systemic responses distant from the site of thermal injury, such as skeletal muscle. The purpose of this study was to investigate the expression of cyclooxygenase-2 (COX-2) and hydroxy-2'-deoxyguanosine (8-OHdG) as a result of inflammation and reactive oxygen species production, respectively. A total of 16 male rats were distributed into two groups: control (C) and submitted to BI. The medial part of gastrocnemius muscle formed the specimens, which were stained with hematoxylin and eosin and were evaluated. COX-2 and 8-OHdG expressions were assessed by immunohistochemistry, and cell profile area and density of muscle fibers (number of fibers per square millimeter) were evaluated by morphometric methods. The results revealed inflammatory infiltrate associated with COX-2 immunoexpression in BI-gastrocnemius muscle. Furthermore, a substantial decrease in the muscle cell profile area of BI group was noticed when compared with the control group, whereas the density of muscle fibers was higher in the BI group. 8-OHdG expression in numerous skeletal muscle nuclei was detected in the BI group. In conclusion, the BI group is able to induce skeletal muscle degeneration as a result of systemic host response closely related to reactive oxygen species production and inflammatory process.

Effects of exercise on soleus in severe burn and muscle disuse atrophy

BACKGROUND

Muscle loss is a sequela of severe burn and critical illness with bed rest contributing significantly to atrophy. We hypothesize that exercise will mitigate muscle loss after burn with bed rest.

MATERIALS AND METHODS

Male rats were assigned to sham ambulatory (S/A), burn ambulatory (B/A), sham hindlimb unloading (S/H), or burn hindlimb unloading (B/H). Rats received a 40% scald burn or sham and were ambulatory or placed in hindlimb unloading, a model of bed rest. Half from each group performed twice daily resistance climbing. Hindlimb isometric forces were measured on day 14.

RESULTS

Soleus mass and muscle function were not affected by burn alone. Mass was significantly lower in hindlimb unloading (79 versus 139 mg, P < 0.001) and no exercise (103 versus 115 mg, P < 0.01). Exercise significantly increased soleus mass in B/H (86 versus 77 mg, P < 0.01). Hindlimb unloading significantly decreased muscle force in the twitch (12 versus 31 g, P < 0.001), tetanic (55 versus 148 g, P < 0.001), and specific tetanic measurements (12 versus 22 N/cm(2), P < 0.001). Effects of exercise on force depended on other factors. In B/H, exercise significantly increased twitch (14 versus 8 g, P < 0.05) and specific tetanic force (14 versus 7 N/cm(2), P < 0.01). Fatigue index was lower in ambulatory (55%) and exercise (52%) versus hindlimb (69%, P = 0.03) and no exercise (73%, P = 0.002).

CONCLUSIONS

Hindlimb unloading is a significant factor in muscle atrophy. Exercise increased the soleus muscle mass, twitch, and specific force in this model. However, the fatigue index decreased with exercise in all groups. This suggests exercise contributes to functional muscle change in this model of disuse and critical illness.

Losartan administration reduces fibrosis but hinders functional recovery after volumetric muscle loss injury

Losartan is a Food and Drug Administration approved antihypertensive medication that is recently emerging as an antifibrotic therapy. Previously, losartan has been successfully used to reduce fibrosis and improve both muscle regeneration and function in several models of recoverable skeletal muscle injuries, such as contusion and laceration. In this study, the efficacy of losartan treatment in reducing fibrosis and improving regeneration was determined in a Lewis rat model of volumetric muscle loss (VML) injury. VML has been defined as the traumatic or surgical loss of skeletal muscle with resultant functional impairment. It is among the top 10 causes for wounded service members to be medically retired from the military. This study shows that, after several weeks of recovery, VML injury results in little to no muscle regeneration, but is marked by persistent inflammation, chronic upregulation of profibrotic markers and extracellular matrix (i.e., collagen type I), and fat deposition at the defect site, which manifest irrecoverable deficits in force production. Losartan administration at 10 mg·kg(-1)·day(-1) was able to modulate the gene expression of fibrotic markers and was also effective at reducing fibrosis (i.e., the deposition of collagen type I) in the injured muscle. However, there were no improvements in muscle regeneration, and deleterious effects on muscle function were observed instead. We propose that, in the absence of regeneration, reduction in fibrosis worsens the ability of the VML injured muscle to transmit forces, which ultimately results in decreased muscle function.

Insulin effects on glucose tolerance, hypermetabolic response, and circadian-metabolic protein expression in a rat burn and disuse model

Insulin controls hyperglycemia after severe burns, and its use opposes the hypermetabolic response. The underlying molecular mechanisms are poorly understood, and previous research in this area has been limited because of the inadequacy of animal models to mimic the physiological effects seen in humans with burns. Using a recently published rat model that combines both burn and disuse components, we compare the effects of insulin treatment vs. vehicle on glucose tolerance, hypermetabolic response, muscle loss, and circadian-metabolic protein expression after burns. Male Sprague-Dawley rats were assigned to three groups: cage controls (n = 6); vehicle-treated burn and hindlimb unloading (VBH; n = 11), and insulin-treated burn and hindlimb unloading (IBH; n = 9). With the exception of cage controls, rats underwent a 40% total body surface area burn with hindlimb unloading, then IBH rats received 12 days of subcutaneous insulin injections (5 units·kg(-1)·day(-1)), and VBH rats received an equivalent dose of vehicle. Glucose tolerance testing was performed on day 14, after which blood and tissues were collected for analysis. Body mass loss was attenuated by insulin treatment (VBH = 265 ± 17 g vs. IBH = 283 ± 14 g, P = 0.016), and glucose clearance capacity was increased. Soleus and gastrocnemius muscle loss was decreased in the IBH group. Insulin receptor substrate-1, AKT, FOXO-1, caspase-3, and PER1 phosphorylation was altered by injury and disuse, with levels restored by insulin treatment in almost all cases. Insulin treatment after burn and during disuse attenuated the hypermetabolic response, increased glucose clearance, and normalized circadian-metabolic protein expression patterns. Therapies aimed at targeting downstream effectors may provide the beneficial effects of insulin without hypoglycemic risk.

Animal models in burn research

Burn injury is a severe form of trauma affecting more than 2 million people in North America each year. Burn trauma is not a single pathophysiological event but a devastating injury that causes structural and functional deficits in numerous organ systems. Due to its complexity and the involvement of multiple organs, in vitro experiments cannot capture this complexity nor address the pathophysiology. In the past two decades, a number of burn animal models have been developed to replicate the various aspects of burn injury, to elucidate the pathophysiology, and to explore potential treatment interventions. Understanding the advantages and limitations of these animal models is essential for the design and development of treatments that are clinically relevant to humans. This review aims to highlight the common animal models of burn injury in order to provide investigators with a better understanding of the benefits and limitations of these models for translational applications. While many animal models of burn exist, we limit our discussion to the skin healing of mouse, rat, and pig. Additionally, we briefly explain hypermetabolic characteristics of burn injury and the animal model utilized to study this phenomena. Finally, we discuss the economic costs associated with each of these models in order to guide decisions of choosing the appropriate animal model for burn research.

Intramuscular transplantation and survival of freshly isolated bone marrow cells following skeletal muscle ischemia-reperfusion injury

BACKGROUND

Delayed treatment cellular therapies offer an attractive means to treat extremity injuries involving acute skeletal muscle ischemia-reperfusion injury (I/R). Bone marrow is a rich source of stem and progenitor cells with the potential to improve skeletal muscle regeneration. The extent to which bone marrow cells (BMCs) may be useful for I/R is not known. The purposes of this study were twofold: (1) to evaluate BMC survival following intramuscular injection 0, 2, 7, and 14 days after injury and (2) to determine whether BMCs improve functional recovery following I/R.

METHODS

Magnetic-activated cell sorting was used to isolate lineage-negative (Lin⁻) BMCs and enrich for stem and progenitor cells. To evaluate in vivo cell survival following I/R, Lin⁻ BMCs were injected intramuscularly 0, 2, 7, and 14 days after I/R, and bioluminescent imaging was performed for up to 28 days after cell injections. To assess their ability to improve muscle regeneration, intramuscular injections were performed 2 days after injury, and in vivo muscle function was assessed 14 days later.

RESULTS

Lin⁻ BMCs survived throughout the study period regardless of the timing of delivery. Intramuscular injection of Lin⁻ BMCs did not improve maximal isometric torque (300 Hz); however, both saline-injected and Lin⁻ BMC-injected muscles exhibited an increase in the twitch-tetanus ratio, suggesting that damage incurred with the intramuscular injections may have had deleterious consequences for functional recovery.

CONCLUSION

Although BMCs injected intramuscularly survived cell transplantation, they failed to improve muscle function following I/R. The ability of BMCs to persist in injured muscle following I/R lends to the possibility that with further development, their full potential can be realized.

Satellite cell functional alterations following cutaneous burn in rats include an increase in their osteogenic potential

BACKGROUND

Significant consequences of severe burn include skeletal muscle atrophy and heterotopic ossification (HO). The cellular mechanisms underlying either of these conditions are not known. Whether the functionality of satellite cells stem cells resident in skeletal muscle is affected by changes in circulatory factors following burn was determined to better understand their role in atrophy and HO.

MATERIALS AND METHODS

Serum (20%) from sham-treated animals or burned animals (40% total body surface area full-thickness burn) was used to culture satellite cells isolated from either sham or burn animals. Satellite cells were separated based on fiber type (i.e., fast-twitch or slow-twitch in some cases). To gain greater insight into the potential role for satellite cells in controlling muscle mass following burn, the effect of serum taken from burn animals on satellite cell proliferation, migration, and myogenic differentiation was evaluated. Osteogenic differentiation was assessed to evaluate the potential of satellite cells to contribute to HO.

RESULTS

Burn serum (BS) increased the proliferative capacity of cells from fast-twitch muscle, and the migratory capacity of satellite cells taken from both fast- and slow-twitch muscles. BS increased both the myogenic and osteogenic differentiation of satellite cells taken from both sham and burn animals.

CONCLUSIONS

The unexpected increase in myogenic functionality of satellite cells with BS is difficult to rectify, given the degree of atrophy that occurs. However, the increased osteogenic capacity of satellite cells with BS suggests they may play a role in burn-induced HO.

A standardized rat model of volumetric muscle loss injury for the development of tissue engineering therapies

Soft tissue injuries involving volumetric muscle loss (VML) are defined as the traumatic or surgical loss of skeletal muscle with resultant functional impairment and represent a challenging clinical problem for both military and civilian medicine. In response, a variety of tissue engineering and regenerative medicine treatments are under preclinical development. A wide variety of animal models are being used, all with critical limitations. The objective of this study was to develop a model of VML that was reproducible and technically uncomplicated to provide a standardized platform for the development of tissue engineering and regenerative medicine solutions to VML repair. A rat model of VML involving excision of ∼20% of the muscle's mass from the superficial portion of the middle third of the tibialis anterior (TA) muscle was developed and was functionally characterized. The contralateral TA muscle served as the uninjured control. Additionally, uninjured age-matched control rats were also tested to determine the effect of VML on the contralateral limb. TA muscles were assessed at 2 and 4 months postinjury. VML muscles weighed 22.7% and 19.5% less than contralateral muscles at 2 and 4 months postinjury, respectively. These differences were accompanied by a reduction in peak isometric tetanic force (Po) of 28.4% and 32.5% at 2 and 4 months. Importantly, Po corrected for differences in body weight and muscle wet weights were similar between contralateral and age-matched control muscles, indicating that VML did not have a significant impact on the contralateral limb. Lastly, repair of the injury with a biological scaffold resulted in rapid vascularization and integration with the wound. The technical simplicity, reliability, and clinical relevance of the VML model developed in this study make it ideal as a standard model for the development of tissue engineering solutions for VML.

Matrix metalloproteinase inhibition negatively affects muscle stem cell behavior

Skeletal muscle is a large and complex system that is crucial for structural support, movement and function. When injured, the repair of skeletal muscle undergoes three phases: inflammation and degeneration, regeneration and fibrosis formation in severe injuries. During fibrosis formation, muscle healing is impaired because of the accumulation of excess collagen. A group of zinc-dependent endopeptidases that have been found to aid in the repair of skeletal muscle are matrix metalloproteinases (MMPs). MMPs are able to assist in tissue remodeling through the regulation of extracellular matrix (ECM) components, as well as contributing to cell migration, proliferation, differentiation and angiogenesis. In the present study, the effect of GM6001, a broad-spectrum MMP inhibitor, on muscle-derived stem cells (MDSCs) is investigated. We find that MMP inhibition negatively impacts skeletal muscle healing by impairing MDSCs in migratory and multiple differentiation abilities. These results indicate that MMP signaling plays an essential role in the wound healing of muscle tissue because their inhibition is detrimental to stem cells residing in skeletal muscle.

Skeletal muscle satellite cell activation following cutaneous burn in rats

BACKGROUND

Cutaneous burn distant from skeletal muscles induces atrophy; however, its effect on muscle stem cells resident in skeletal muscle (satellite cells) distal to burn is not known.

METHODS

Satellite cell activation was measured in predominantly fast-twitch [tibialis anterior, extensor digitorum longus (EDL), plantaris, and gastrocnemius] and slow-twitch (soleus) muscles of rats that received either 40% total body surface area full-thickness scald burn or sham burn to the trunk area by determining bromodeoxyuridine incorporation, MyoD, and Pax7 immunohistochemistry in vivo ≤48 h after burn. To determine the effects of circulating factors on satellite cell activation, satellite cell cultures were treated with serum from sham or burn rats.

RESULTS

In vivo activation of satellite cells was increased in fast muscles isolated from burn as compared to sham animals, whereas a significant response was not seen in slow muscles. Serum taken from animals in the burn group increased the activation of satellite cells isolated from both sham and burn animals in vitro, suggesting that circulating factors have the potential to increase satellite cell activation following burn.

CONCLUSIONS

Increases in satellite cell activation in muscles distal to burn are fiber-type-dependent, and circulating factors may play a role in the activation of satellite cells following burn. A better understanding of the impact of burn on satellite cell functionality will allow us to identify the cellular mechanisms of long-term muscle atrophy.

The impact of muscle disuse on muscle atrophy in severely burned rats

BACKGROUND

Severe burn induces a sustained hypermetabolic response, which causes long-term loss of muscle mass and decrease in muscle strength. In this study, we sought to determine whether muscle disuse has additional impact on muscle atrophy after severe burn using a rat model combining severe cutaneous burn and hindlimb unloading.

METHODS

Forty Sprague-Dawley rats (≈ 300 g) were randomly assigned to sham ambulatory (S/A), sham hindlimb unloading (S/HLU), burn ambulatory (B/A), or burn hindlimb unloading (B/HLU) groups. Rats received a 40% total body surface (TBSA) full thickness scald burn, and rats with hindlimb unloading were placed in a tail traction system. At d 14, lean body mass (LBM) was determined using DEXA scan, followed by measurement of the isometric mechanical properties in the predominantly fast-twitch plantaris muscle (PL) and the predominantly slow-twitch soleus muscle (SL). Muscle weight (wt), protein wt, and wet/dry wt were determined.

RESULTS

At d 14, body weight had decreased significantly in all treatment groups; B/HLU resulted in significantly greater loss compared with the B/A, S/HLU, and S/A. The losses could be attributed to loss of LBM. PL muscle wt and Po were lowest in the B/HLU group (<0.05 versus S/A, S/HLU, or B/A). SL muscle wt and Po were significantly less in both S/HLU and B/HLU compared with that of S/A and B/A; no significant difference was found between S/HLU and B/HLU.

CONCLUSIONS

Cutaneous burn and hindlimb unloading have an additive effect on muscle atrophy, characterized by loss of muscle mass and decrease in muscle strength in both fast (PL) and slow (SL) twitch muscles. Of the two, disuse appeared to be the dominant factor for continuous muscle wasting after acute burn in this model.