A cross-sectional analysis of clinical evaluation in 35 individuals with mutations of the valosin-containing protein gene.

Inclusion body myopathy (IBM) associated with Paget disease of the bone and frontotemporal dementia or IBMPFD is an autosomal dominant degenerative disorder caused by mutations in the valosin-containing protein (VCP) gene. We aim to establish a detailed clinical phenotype of VCP disease amongst 35 (28 affected individuals, 7 presymptomatic gene carriers) individuals versus 14 unaffected first-degree relatives in 14 families to establish useful biomarkers for IBMPFD and identify the most meaningful tests for monitoring disease progression in future clinical trials. Comprehensive studies included the Inclusion Body Myositis Functional Rating Scale (IBMFRS) and fatigue severity scale questionairres, strength measurements using the Manual Muscle Test with Medical Research Council (MRC) scales, hand-held dynamometry using the microFET and Biodex dynamometers, 6 minute walk test (6MWT), and pulmonary function studies. Strong correlation was observed between the IBMFRS and measurements of muscle strength with dynamometry and the other functional tests, indicating that it may be utilized in long-term follow-up assessments due to its relative simplicity. This cross-section study represents the most comprehensive evaluation of individuals with VCP disease to date and provides a useful guide for evaluating and possible monitoring of muscle weakness and pulmonary function progression in this unique cohort of individuals.

Exercise Responses to Gravity-Independent Flywheel Aerobic and Resistance Training.

BACKGROUND: Although several exercise systems have been developed to mitigate the physiological deconditioning that occurs in microgravity, few have the capacity to positively impact multiple physiological systems and still meet the volume/mass requirements needed for missions beyond low Earth orbit. The purpose of this study was to test the gravity-independent Multi-Mode Exercise Device (M-MED) for both resistance (RE) and aerobic (AE) training stimuli. METHODS: Eight men and nine women (mean age 22.0 ± 0.4 yr) completed 5 wk of training on the M-MED: RE 4 × 7 squats 2 d/wk, and AE 4 × 4-min rowing bouts at ∼90% Vo2max 3 d/wk. Pre- and post-training data collection included an aerobic capacity test, MR imaging, strength testing, and vastus lateralis muscle biopsy. RESULTS: Vo2max increased 8%, 3RM strength 18%, and quadriceps femoris cross-sectional area (CSA) 10%. Knee extensor strength increased at all isokinetic speeds tested. Subjects also demonstrated improved fatigue resistance in knee extension. At the cellular and molecular level, the biopsy revealed increases in mixed myofiber CSA (13%), citrate synthase activity (26%), total RNA concentration (24%), IGF-I mRNA (77%), and Type IIa myosin heavy chain (MHC) mRNA (8%), and a concomitant decrease in Type IIx MHC mRNA (-23%). None of the changes were gender-specific. DISCUSSION: Both the functional outcomes and biomarker changes indicate that a very low volume of M-MED exercise results in robust adaptation in the cardiovascular and musculoskeletal systems. The M-MED has the potential to provide a wide range of countermeasure exercises and should be considered for testing in ground-based spaceflight simulation.

Concurrent exercise on a gravity-independent device during simulated microgravity.

PURPOSE: The objective of this study is to examine the effect of a high-intensity concurrent training program using a single gravity-independent device on maintaining skeletal muscle function and aerobic capacity during short-term unilateral lower limb suspension (ULLS). METHODS: Nineteen subjects (10 males and 9 females; 21.0 ± 2.5 yr, 65.4 ± 12.2 kg) were separated into two groups: 1) 10-d ULLS only (n = 9) and 2) 10-d ULLS plus aerobic and resistance training (ULLS + EX, n = 10). Exercise was performed on a single gravity-independent Multi-Mode Exercise Device (M-MED) with alternating days of high-intensity interval aerobic training and maximal exertion resistance training. RESULTS: Aerobic capacity increased by 7% in ULLS + EX (P < 0.05). Knee extensor and ankle plantar flexor three-repetition maximum increased in the ULLS + EX group (P < 0.05), but this change was only different from ULLS in the plantar flexors (P < 0.05). Peak torque levels decreased with ULLS but were increased for the knee extensors and attenuated for the ankle plantar flexors with ULLS + EX (P < 0.05). A shift toward type IIx myosin heavy-chain mRNA occurred with ULLS and was reversed with ULLS + EX in the vastus lateralis (P < 0.05) but not the soleus. Myostatin and atrogin increased with ULLS in both the vastus lateralis and soleus, but this change was mitigated with ULLS + EX only in the vastus lateralis (P = 0.0551 for myostatin, P < 0.05 for atrogin). Citrate synthase was decreased in the soleus during ULLS but was increased with ULLS + EX (P < 0.05). CONCLUSION: These results indicate that an M-MED class countermeasure device appears to be effective at mitigating the deconditioning effects of microgravity simulated during a modified ULLS protocol.

Matrix metalloproteinase 3 deletion preserves denervated motor endplates after traumatic nerve injury.

OBJECTIVE: Traumatic peripheral nerve injuries often produce permanent functional deficits despite optimal surgical and medical management. One reason for the impaired target organ reinnervation is degradation of motor endplates during prolonged denervation. Here we investigate the effect of preserving agrin on the stability of denervated endplates. Because matrix metalloproteinase 3 (MMP3) is known to degrade agrin, we examined the changes in endplate structure following traumatic nerve injury in MMP3 knockout mice. METHODS: After creation of a critical size nerve defect to preclude reinnervation, we characterized receptor area, receptor density, and endplate morphology in denervated plantaris muscles in wild-type and MMP3 null mice. The level of agrin and muscle-specific kinase (MuSK) was assessed at denervated endplates. In addition, denervated muscles were subjected to ex vivo stimulation with acetylcholine. Finally, reinnervation potential was compared after long-term denervation. RESULTS: In wild-type mice, the endplates demonstrated time-dependent decreases in area and receptor density and conversion to an immature receptor phenotype. In striking contrast, all denervation-induced changes were attenuated in MMP3 null mice, with endplates retaining their differentiated form. Agrin and MuSK were preserved in endplates from denervated MMP3 null animals. Furthermore, denervated muscles from MMP3 null mice demonstrated greater endplate efficacy and reinnervation. INTERPRETATION: These results demonstrate a critical role for MMP3 in motor endplate remodeling, and reveal a potential target for therapeutic intervention to prevent motor endplate degradation following nerve injury.

Satellite cells say NO to radiation.

Skeletal muscles are commonly exposed to radiation for diagnostic procedures and the treatment of cancers and heterotopic bone formation. Few studies have considered the impact of clinical doses of radiation on the ability of satellite cells (myogenic stem cells) to proliferate, differentiate and contribute to recovering/maintaining muscle mass. The primary objective of this study was to determine whether the proliferation of irradiated satellite cells could be rescued by manipulating NO levels via pharmacological approaches and mechanical stretch (which is known to increase NO levels). We used both SNP (NO donor) and PTIO (NO scavenger) to manipulate NO levels in satellite cells. We observed that SNP was highly effective in rescuing the proliferation of irradiated satellite cells, especially at doses less than 5 Gy. The potential importance of NO was further illustrated by the effects of PTIO, which completely inhibited the rescue effect of SNP. Mechanical cyclic stretch was found to produce significant increases in NO levels of irradiated satellite cells, and this was associated with a robust increase in satellite cell proliferation. The effects of both radiation and NO on two key myogenic regulatory factors (MyoD and myogenin) were also explored. Irradiation of satellite cells produced a significant increase in both MyoD and myogenin, effects that were mitigated by manipulating NO levels via SNP. Given the central role of myogenic regulatory factors in the proliferation and differentiation of satellite cells, the findings of the current study underscore the need to more fully understand the relationship between radiation, NO and the functionality of satellite cells.

Interactive effects of corticosteroid and mechanical ventilation on diaphragm muscle function.

Information on the interactive effects of methylprednisolone, controlled mechanical ventilation (CMV), and assisted mechanical ventilation (AMV) on diaphragm function is sparse. Sedated rabbits received 2 days of CMV, AMV, and spontaneous breathing (SB), with either methylprednisolone (MP; 60 mg/kg/day intravenously) or saline. There was also a control group. In vitro diaphragm force, myofibril ultrastructure, αII-spectrin proteins, insulin-like growth factor-1 (IGF-1), and muscle atrophy F-box (MAF-box) mRNA were measured. Maximal tetanic tension (P(o)) decreased significantly with CMV. Combined MP plus CMV did not decrease P(o) further. With AMV, P(o) was similar to SB and controls. Combined MP plus AMV or MP plus SB decreased P(o) substantially. Combined MP plus CMV, MP plus AMV, or MP plus SB induced myofibrillar disruption that correlated with the reduced P(o). αII-spectrin increased, IGF-1 decreased, and MAF-box mRNA increased in both the CMV group and MP plus CMV group. Short-term, high-dose MP had no additive effects on CMV-induced diaphragm dysfunction. Combined MP plus AMV impaired diaphragm function, but AMV alone did not. We found that acute, high-dose MP produces diaphragm dysfunction depending on the mode of mechanical ventilation.

The radiosensitivity of satellite cells: cell cycle regulation, apoptosis and oxidative stress.

Skeletal muscles are the organ of movement, and their growth, regeneration and maintenance are dependent in large part on a population of myogenic stem cells known as satellite cells. Skeletal muscles and these resident myogenic stem cells (i.e., satellite cells) are commonly exposed to significant doses of radiation during diagnostic procedures and/or during the radiotherapeutic management of cancer. The main objective of this study was to examine the effects of clinically relevant doses of γ radiation on satellite cell survival and proliferation, cell cycle regulation, apoptosis, DNA double-strand break repair, oxidative stress and NO production. Overall, our findings demonstrate that doses of γ radiation ≥5 Gy reduced satellite cell numbers by at least 70% due in part to elevated apoptosis and the inhibition of cell cycle progression. Radiation was also found to cause a significant and persistent increase in the level of reactive oxygen and nitrogen species. Interestingly, and within this backdrop of elevated oxidative stress, similar doses were found to produce substantial reductions in the levels of nitric oxide (NO). Proliferation of satellite cells has been shown to depend in part on the production of NO, and our findings give rise to the possibility that radiation-induced reductions in NO levels may provide a mechanism for the inhibition of satellite cell proliferation in vitro and possibly the regrowth of skeletal muscle exposed during clinical irradiation procedures.

Bench-to-bedside review: Diaphragm muscle function in disuse and acute high-dose corticosteroid treatment.

Critically ill patients may require mechanical ventilatory support and short-term high-dose corticosteroid to treat some specific underlying disease processes. Diaphragm muscle inactivity induced by controlled mechanical ventilation produces dramatic alterations in diaphragm muscle structure and significant losses in function. Although the exact mechanisms responsible for losses in diaphragm muscle function are still unknown, recent studies have highlighted the importance of proteolysis and oxidative stress. In experimental animals, short-term strategies that maintain partial diaphragm muscle neuromechanical activation mitigate diaphragmatic force loss. In animal models, studies on the influence of combined controlled mechanical ventilation and short-term high-dose methylprednisolone have given inconsistent results in regard to the effects on diaphragm muscle function. In the critically ill patient, further research is needed to establish the prevalence and mechanisms of ventilator-induced diaphragm muscle dysfunction, and the possible interaction between mechanical ventilation and the administration of high-dose corticosteroid. Until then, in caring for these patients, it is imperative to allow partial activation of the diaphragm, and to administer the lowest dose of corticosteroid for the shortest duration possible.

Lactate threshold at the same fat-free mass and age is larger in men than women.

The lactate threshold (LT) represents the onset of a metabolic acidosis during graded exercise testing (GXT). It is typically measured as an oxygen uptake (VO(2)) but then ratio scaled by body mass or VO(2) peak to make comparisons among subjects. Theoretical considerations and empirical evidence suggest that this type of ratio scaling is not valid. A method that allows a dependent variable to be compared between groups at that same value of one or more covariates is analysis of covariance (ANCOVA). Our purpose was to compare the LT, estimated non-invasively by gas exchange (LT(GE)), at the same fat-free mass (FFM) and age in 203 sedentary subjects (102 men) aged 20-70 years. Each subject underwent cycle ergometer GXT with LT(GE) measured by the V-slope method. In model development, we discovered an interaction term between sex and age. As dimensional analysis predicts a log-linear relationship between LT(GE) and FFM, two of the model terms were ln LT(GE) and ln FFM. The ANCOVA model was then as follows: dependent variable = ln LT(GE), fixed factor = sex, covariates = ln FFM, age, and sex x age. Sex made a significant contribution to the model (F = 30.7, P < 0.001). At the mean FFM (56.32 kg) and age (44.01 years) of both sexes combined, the LT(GE) was 29% larger in males (1,307 ml min(-1) versus 1,011 ml min(-1)). The model's interaction term resulted in larger differences at younger ages and smaller differences at older ages. We conclude that LT(GE) at the same FFM and age is larger in sedentary men compared to sedentary women.

Acute effects of high-dose methylprednisolone on diaphragm muscle function.

The time- and dose-dependent effects of acute high-dose corticosteroids on the diaphragm muscle are poorly defined. This study aimed to examine in rabbits the temporal relationships and dose-response effects of acute high-dose methylprednisolone succinate on diaphragmatic contractile and structural properties. Animals were assigned to groups receiving: (1) 80 mg/kg/day methylprednisolone (MP80) intramuscularly for 1, 2, and 3 days; (2) 10 mg/kg/day methylprednisolone (MP10, pulse-dose) for 3 days; or (3) saline (placebo) for 3 days; and (4) a control group. Diaphragmatic in vitro force-frequency and force-velocity relationships, myosin heavy chain (MyHC) isoform protein and mRNA, insulin-like growth factor-1 (IGF-1), muscle atrophy F-box (MAF-box) mRNA, and volume density of abnormal myofibrils were measured at each time-point. MP80 did not affect animal nutritional state or fiber cross-sectional area as assessed in separate pair-fed groups receiving methylprednisolone or saline for 3 days. Compared with control values, MP80 decreased diaphragmatic maximum tetanic tension (Po) by 19%, 24%, and 34% after 1, 2, and 3 days (P < 0.05), respectively, whereas MP10 decreased Po modestly (12%; P > 0.05). Vmax and MyHC protein proportions were unchanged in both the MP80 and MP10 groups. Maximum power output decreased after 2 and 3 days of MP80. Suppression of IGF-1 and overexpression of MAF-box mRNA occurred in both MP groups. Significant myofibrillar disarray was also observed in both MP groups. The decline in Po was significantly associated with the increased volume density of abnormal myofibrils. Thus, very high-dose methylprednisolone (MP80) can produce rapid reductions in diaphragmatic function, whereas pulse-dose methylprednisolone (MP10) produces only modest functional loss.

Hypergravity resistance exercise: the use of artificial gravity as potential countermeasure to microgravity.

The aims of this study were to 1) determine if hypergravity (HG) squats can produce foot forces similar to those measured during 10-repetition maximum (10RM) squats using weights under normal 1-G(z) condition, and 2) compare the kinematics (duration and goniometry) and EMG activities of selected joints and muscles between 10RM and HG squats of similar total foot forces. Eight men and six women [27 yr (SD 4), 66 kg (SD 10)] completed ten 10RM [83 kg (SD 23)] and 10 HG squats (2.25-3.75 G(z)). HG squats were performed on a human-powered short-arm centrifuge. Foot forces were measured using insole force sensors. Hip, knee, and ankle angles were measured using electrogoniometers. EMG activities of the erector spinae, biceps femoris, rectus femoris, and gastrocnemius were also recorded during both squats. All subjects were able to achieve similar or higher average total foot forces during HG squats compared with those obtained during 10RM squats. There were no differences in total duration per set, average duration per repetition, and goniometry and EMG activities of the selected joints and muscles, respectively, between 10RM and HG squats. These results demonstrate that HG squats can produce very high foot forces that are comparable to those produced during 10RM squats at 1 G(z). In addition, the technique and muscle activation are similar between the two types of squats. This observation supports the view that HG resistance training may represent an important countermeasure to microgravity.

Repair of spinal cord transection and its effects on muscle mass and myosin heavy chain isoform phenotype.

A number of significant advances have been developed for treating spinal cord injury during the past two decades. The combination of peripheral nerve grafts and acidic fibroblast growth factor (hereafter referred to as PNG) has been shown to partially restore hindlimb function. However, very little is known about the effects of such treatments in restoring normal muscle phenotype. The primary goal of the current study was to test the hypothesis that PNG would completely or partially restore 1) muscle mass and muscle fiber cross-sectional area and 2) the slow myosin heavy chain phenotype of the soleus muscle. To test this hypothesis, we assigned female Sprague-Dawley rats to three groups: 1) sham control, 2) spinal cord transection (Tx), and 3) spinal cord transection plus PNG (Tx+PNG). Six months following spinal cord transection, the open-field test was performed to assess locomotor function, and then the soleus muscles were harvested and analyzed. SDS-PAGE for single muscle fiber was used to evaluate the myosin heavy chain (MHC) isoform expression pattern following the injury and treatment. Immunohistochemistry was used to identify serotonin (5-HT) fibers in the spinal cord. Compared with the Tx group, the Tx+PNG group showed 1) significantly improved Basso, Beattie, and Bresnahan scores (hindlimb locomotion test), 2) less muscle atrophy, 3) a higher percentage of slow type I fibers, and 4) 5-HT fibers distal to the lesion site. We conclude that the combined treatment of PNG is partially effective in restoring the muscle mass and slow phenotype of the soleus muscle in a T-8 spinal cord-transected rat model.

The mechanical behavior of activated skeletal muscle during stretch: effects of muscle unloading and MyHC isoform shifts.

The response of activated skeletal muscle to a ramp stretch is complex. Force rises rapidly above the isometric plateau during the initial phase of stretch. However, after a strain of approximately 1-2%, force yields and continues to rise but with a slower slope. The resistance to stretch during the initial phase can be characterized by the stiffness of the muscle and/or the preyield modulus (E(pre)). Similarly, a measure of modulus also can be used to characterize the postyield modulus response (E(post)). This study examined the effects of muscle atrophy and altered myosin heavy chain (MyHC) isoform composition on both E(pre) and E(post). Female Sprague-Dawley rats were assigned to 1) control group, 2) a hypothyroid group, 3) a hyperthyroid group, 4) a hindlimb suspension group, and 5) a hindlimb suspension + hyperthyroid group. These interventions were used either to alter the MyHC isoform composition of the muscle or to induce atrophy. Soleus muscles were stretched at strain rates that ranged from approximately 0.15 to 1.25 muscle length/s. The findings of this study demonstrate that 4 wk of hindlimb suspension can produce a large (i.e., 40-60%) reduction in E(pre). Hindlimb suspension did not produce a proportional change in E(post). Analyses of the E(pre)-strain rate relationship demonstrated that there was little dependence on MyHC isoform composition. In summary, the disproportionate decrease in E(pre) of atrophied muscle has important implications with respect to issues related to joint stability, especially under dynamic conditions and conditions where the static joint stabilizers (i.e., ligaments) have been compromised by injury.

Scaling of lactate threshold by peak oxygen uptake and by fat-free mass 0 x 67.

The lactate threshold (LT) represents the onset of metabolic acidosis during cardiopulmonary exercise testing (CPET). It is measured as a O(2) in the units of ml min(-1). In order to make comparisons among subjects, LT is often scaled or normalized by O(2) peak resulting in the LT/O(2) peak ratio. Ratio variables have underlying assumptions. One assumption is that the relationship between the numerator and denominator is linear with a zero y-intercept. If the relationship has a positive y-intercept, then the ratio will decrease with increasing values of the scaling variable thereby penalizing subjects with larger values of the scaling variable. Our purpose was to examine the validity of scaling LT by O(2) peak and by fat-free mass raised to 0 x 67 power (FFM(0 x 67)) as dimensional analysis predicts that LT is proportional to FFM(0 x 67). Cycle ergometer CPET was administered to 204 healthy, sedentary subjects (103 males) to the limit of tolerance. Lactate threshold was estimated noninvasively using the V-slope technique. Fat-free mass was assessed by skinfolds. The relationship of LT versus O(2) peak was linear with a positive y-intercept for both sexes. Consequently, the LT/O(2) peak ratio decreased as O(2) peak increased for both sexes. The relationship of LT versus FFM(0 x 67)was linear with a zero y-intercept for both sexes. Consequently, the plot of the LT/FFM(0 x 67) ratio versus FFM resulted in a straight line with a slope of zero for both sexes. The results of this study support the conclusion that FFM(0 x 67), but not O(2) peak, is a valid scaling variable for LT.

Maintenance of slow type I myosin protein and mRNA expression in overwintering prairie dogs (Cynomys leucurus and ludovicianus) and black bears (Ursus americanus).

Hibernating mammals have the remarkable ability to withstand long periods of fasting and reduced activity with dramatic maintenance of skeletal muscle function and protein composition. We investigated several hindlimb muscles of white-tailed prairie dogs (Cynomys leucurus) and black bears (Ursus americanus), two very different hibernators who are dormant and fasting during winter. The black-tailed prairie dog (C. ludovicianus) remains active during winter, but suffers minor skeletal muscle atrophy; nevertheless, they also demonstrate apparent skeletal muscle adaptations. Using SDS-PAGE, we measured myosin protein isoform profiles before and after the hibernation season. All species maintained or increased levels of slow myosin, despite the collective physiological challenges of hypophagia and reduced activity. This contrasts markedly with standard mammalian models of skeletal muscle inactivity and atrophy predicting significant loss of slow myosin. A mechanism for changes in myosin isoforms was investigated using reverse-transcription PCR, following partial sequencing of the adult MHC isoforms in C. leucurus and U. americanus. However, mRNA expression was not well correlated with changes in MHC protein isoforms, and other synthesis and degradation pathways may be involved besides transcriptional control. The muscles of hibernating mammals demonstrate surprising and varied physiological responses to inactivity and atrophy with respect to slow MHC expression.

Maximal oxygen uptake at the same fat-free mass is greater in men than women.

Maximal oxygen uptake (VO(2max)) is commonly divided by body mass or fat-free mass (body mass minus fat mass) in order to make it size independent so that comparisons among persons of different size can be made. However, numerous studies have shown that the ratio created is not size-independent. Analysis of covariance (ANCOVA) allows a dependent variable to be compared between groups at a common value of a covariate. The purpose of this study was to compare VO(2max) at the same fat-free mass (FFM) in 230 sedentary subjects (half men) who ranged in age from 20 to 70 years. The subjects underwent maximal cardiopulmonary exercise testing on a cycle ergometer as ventilation and the expired gas fractions were being measured. Two ANCOVA models were evaluated. The dependent variable, fixed factor and covariate(s) in the linear model were VO(2max), sex and FFM, respectively. The corresponding terms in the log-linear model were ln VO(2max), sex, and ln FFM and age. Sex made a significant contribution to both models. In the linear model, the mean VO(2max) at the same FFM was 27% higher in men (2,444 versus 1,929 ml min(-1); P<0.001). In the log-linear model, the corresponding value at the same FFM and age was 32% higher in men (2,368 versus 1,794 ml min(-1); P<0.001). The goodness of fit indices of squared multiple correlation coefficient and standard error of estimate were significantly better for the log-linear model. We conclude that VO(2max) at the same FFM is considerably higher in men than in women who have a sedentary lifestyle.

Pathogenic accumulation of APP in fast twitch muscle of IBM patients and a transgenic model.

Inclusion body myositis (IBM) is the most common age-related degenerative skeletal muscle disorder. The aberrant intracellular accumulation of the beta-amyloid (Abeta) peptide within skeletal muscle is a pathological hallmark of IBM. Skeletal muscle is comprised of both slow and fast twitch fibers, which are present in different proportions in various muscles. It remains unclear if fast and/or slow twitch fibers are differentially involved in IBM pathogenesis. To better understand the molecular pathogenesis of IBM, we analyzed human IBM muscle biopsies and muscle from a transgenic mouse model of IBM (MCK-betaAPP). Here we report that the majority of histopathologically-affected fibers in human IBM biopsies were type II fast fibers. Skeletal muscle from MCK-betaAPP mice exhibited higher transgene expression and steady-state levels of human betaAPP in fast type IIB fibers compared to slow type I fibers. These findings indicate that fast twitch fibers may selectively accumulate and be more vulnerable to betaAPP- and Abeta-mediated damage in IBM. These findings also highlight parallels between the MCK-betaAPP mice and the human IBM condition.

Early effects of mechanical ventilation on isotonic contractile properties and MAF-box gene expression in the diaphragm.

This study aimed to determine the time-dependent effects of diaphragmatic inactivity on its maximum shortening velocity (V(max)) and the muscle atrophy F-box (MAF-box, atrogin-1) gene expression during controlled mechanical ventilation (CMV). Twenty-four New Zealand White rabbits were grouped into 1 day, 2 days, and 3 days of CMV and controls in equal numbers. The in vitro isotonic contractile properties of the diaphragm were determined. In addition, myosin heavy chain protein and mRNA, myosin light chain, MAF-box mRNA, and volume density of abnormal myofibrils were measured. Tetanic force decreased, and V(max) increased from control of 6.4 to 6.6, 7.7, and 8.1 muscle lengths per second after 1, 2, and 3 days of CMV, respectively (P < 0.02). The increased V(max) compensated for the decreased tetanic force; consequently, compared with the controls, maximum power output was unchanged after 3 days of CMV. V(max) correlated with the volume density of abnormal myofibrils [y = 0.1x + 5.7 (r = 0.87, P < 0.01)]. In the diaphragm, MAF-box was overexpressed (355% of control) after 1 day of CMV, before the evidence of structural myofibril disarray. In conclusion, CMV produced a time-dependent increase in V(max) that was associated with the degree of myofibrillar disarray and independent of changes in myosin isoform expression. Furthermore, CMV produced an increase in MAF-box mRNA levels that may be partially or completely responsible for the degree of myofibrillar disarray resulting from CMV.