Skeletal muscle contractility is preserved in COPD patients with normal fat-free mass

A modelling approach to disentangle the factors limiting muscle oxygenation in smokers

Cigarette smoking is associated with a lower exercise capacity and lower muscle fatigue resistance. This is at least partly attributable to carboxyhaemoglobin (HbCO) in the blood that via reduction in the oxygen-carrying capacity, and the left-shift of the Hb-dissociation curve would reduce tissue oxygenation. On the other hand, a reduced oxygen uptake due to mitochondrial dysfunction would result in improved oxygenation. We used previously collected capillarisation, myoglobin and estimated cellular maximal muscle oxygen consumption data derived from succinate dehydrogenase-stained sections from the vastus lateralis muscle from six smokers and five non-smokers. These data were fed into an expanded Krogh tissue oxygenation model to assess whether an impaired muscle fatigue resistance in smokers is primarily due to HbCO or impaired mitochondrial respiration. The model showed that in smokers with 6% and 20% HbCO (causing a left-shift of the Hb-dissociation curve) average muscle oxygenation was reduced by 1.9% and 7.2%, respectively. Muscle oxygenation was increased by 13.3% when maximal mitochondrial respiration was reduced by 29%. A combination of a 29% reduction in maximal mitochondrial respiration and 20% HbCO led to no significant difference in muscle oxygenation from that in non-smokers. This indicates that while HbCO may explain the reduced exercise capacity after just one smoking session, in chronic smokers impaired mitochondrial respiration appears more important in reducing oxygen extraction and exercise capacity with only a small contribution of the left-shift of the Hb-dissociation curve.

Relationship between Volitional and Non-Volitional Quadriceps Muscle Endurance in Patients with Chronic Obstructive Pulmonary Disease

Volitional assessment of quadriceps muscle endurance is clinically relevant in patients with chronic obstructive pulmonary disease (COPD). However, studies that determine the construct validity of volitional tests by comparing them to non-volitional measures are lacking. Therefore, the aim of the current study is to evaluate the correlation between volitional and non-volitional quadriceps muscle endurance in patients with COPD. Quadriceps muscle endurance was evaluated in twenty-six patients with COPD. A volitional isometric and a volitional isokinetic protocol were performed on a computerised dynamometer to determine the isometric time and isokinetic work fatigue index, respectively. Non-volitional assessment of quadriceps muscle endurance was evaluated using repetitive electrical stimulations to establish the isometric muscle force decline. Sixteen patients (61 ± 8 years, 63% male, FEV1 47 (32-53)%) performed all three quadriceps endurance tests conforming to pre-defined test criteria. Both volitional isometric time and isokinetic work fatigue index did not significantly correlate with non-volitional muscle force decline (both p > 0.05). There was a strong correlation between volitional isometric time and isokinetic work fatigue index (rho = -0.716, p = 0.002). To conclude, this study suggests that volitional measures evaluate partly different aspects of quadriceps muscle endurance compared to non-volitional measures. Accordingly, these outcome measures cannot be used interchangeably.

Updated Perspectives on the Role of Biomechanics in COPD: Considerations for the Clinician

Patients with chronic obstructive pulmonary disease (COPD) demonstrate extra-pulmonary functional decline such as an increased prevalence of falls. Biomechanics offers insight into functional decline by examining mechanics of abnormal movement patterns. This review discusses biomechanics of functional outcomes, muscle mechanics, and breathing mechanics in patients with COPD as well as future directions and clinical perspectives. Patients with COPD demonstrate changes in their postural sway during quiet standing compared to controls, and these deficits are exacerbated when sensory information (eg, eyes closed) is manipulated. If standing balance is disrupted with a perturbation, patients with COPD are slower to return to baseline and their muscle activity is differential from controls. When walking, patients with COPD appear to adopt a gait pattern that may increase stability (eg, shorter and wider steps, decreased gait speed) in addition to altered gait variability. Biomechanical muscle mechanics (ie, tension, extensibility, elasticity, and irritability) alterations with COPD are not well documented, with relatively few articles investigating these properties. On the other hand, dyssynchronous motion of the abdomen and rib cage while breathing is well documented in patients with COPD. Newer biomechanical technologies have allowed for estimation of regional, compartmental, lung volumes during activity such as exercise, as well as respiratory muscle activation during breathing. Future directions of biomechanical analyses in COPD are trending toward wearable sensors, big data, and cloud computing. Each of these offers unique opportunities as well as challenges. Advanced analytics of sensor data can offer insight into the health of a system by quantifying complexity or fluctuations in patterns of movement, as healthy systems demonstrate flexibility and are thus adaptable to changing conditions. Biomechanics may offer clinical utility in prediction of 30-day readmissions, identifying disease severity, and patient monitoring. Biomechanics is complementary to other assessments, capturing what patients do, as well as their capability.

The mobility limitation in healthy older people is due to weakness and not slower muscle contractile properties

The maximal power generating capacity of a muscle declines with age and has a negative impact on the performance of daily life activities. As muscle power is the product of force and velocity, we recruited 20 young (10 men, 10 women: 20–31 years) and 20 older (10 men, 10 women: 65–86 years) people to investigate which of these components contributes to the lower power and performance in old age. After determination of the maximal isometric knee extension torque (MVC), they performed a countermovement jump (CMJ) in 1) the normal situation (normal), 2) with an extra load of 15% body weight (loaded) and 3) 15% lower body weight (unloaded with a pulley system), and a timed up-and-go test (TUG) in the normal or loaded condition. The TUG and CMJ performance was lower in old than young participants (p<0.001). Below a critical CMJ peak power of ~23.7 W·kg^-1 TUG showed a progressive decrease. The CMJ take-off velocity (V_off) in the normal condition was lower in old than young participants (p<0.001). However, the V_offvs. body weight/MVC relationship of the normal, loaded and unloaded data combined was similar in the old and young participants and fitted the Hill equation (R² = 0.396). This indicates that 1) only when peak power drops below a critical threshold TUG becomes impaired and 2) there was no evidence for intrinsic slowing of the muscle contractile properties in older people, but rather the older people were working on a slower part of the force-velocity relationship due to weaker muscles.

Fourteen days of smoking cessation improves muscle fatigue resistance and reverses markers of systemic inflammation

Cigarette smoking has a negative effect on respiratory and skeletal muscle function and is a risk factor for various chronic diseases. To assess the effects of 14 days of smoking cessation on respiratory and skeletal muscle function, markers of inflammation and oxidative stress in humans. Spirometry, skeletal muscle function, circulating carboxyhaemoglobin levels, advanced glycation end products (AGEs), markers of oxidative stress and serum cytokines were measured in 38 non-smokers, and in 48 cigarette smokers at baseline and after 14 days of smoking cessation. Peak expiratory flow (p = 0.004) and forced expiratory volume in 1 s/forced vital capacity (p = 0.037) were lower in smokers compared to non-smokers but did not change significantly after smoking cessation. Smoking cessation increased skeletal muscle fatigue resistance (p < 0.001). Haemoglobin content, haematocrit, carboxyhaemoglobin, total AGEs, malondialdehyde, TNF-α, IL-2, IL-4, IL-6 and IL-10 (p < 0.05) levels were higher, and total antioxidant status (TAS), IL-12p70 and eosinophil numbers were lower (p < 0.05) in smokers. IL-4, IL-6, IL-10 and IL-12p70 had returned towards levels seen in non-smokers after 14 days smoking cessation (p < 0.05), and IL-2 and TNF-α showed a similar pattern but had not yet fully returned to levels seen in non-smokers. Haemoglobin, haematocrit, eosinophil count, AGEs, MDA and TAS did not significantly change with smoking cessation. Two weeks of smoking cessation was accompanied with an improved muscle fatigue resistance and a reduction in low-grade systemic inflammation in smokers.

Reduced specific force in patients with mild and severe facioscapulohumeral muscular dystrophy

BACKGROUND

Specific force, that is the amount of force generated per unit of muscle tissue, is reduced in patients with facioscapulohumeral muscular dystrophy (FSHD). The causes of reduced specific force and its relation with FSHD disease severity are unknown.

METHODS

Quantitative muscle magnetic resonance imaging (MRI), measurement of voluntary maximum force generation and quadriceps force-frequency relationship, and vastus lateralis muscle biopsies were performed in 12 genetically confirmed patients with FSHD and 12 controls.

RESULTS

Specific force was reduced by ~33% in all FSHD patients independent of disease severity. Quadriceps force-frequency relationship shifted to the right in severe FSHD compared to controls. Fiber type distribution in vastus lateralis muscle biopsies did not differ between groups.

CONCLUSIONS

Reduced quadriceps specific force is present in all FSHD patients regardless of disease severity or fatty infiltration. Early myopathic changes, including fibrosis, and non-muscle factors, such as physical fatigue and musculoskeletal pain, may contribute to reduced specific force.

Muscle fiber dysfunction contributes to weakness in inclusion body myositis

Atrophy and fatty infiltration are important causes of muscle weakness in inclusion body myositis (IBM). Muscle weakness can also be caused by reduced specific force; i.e. the amount of force generated per unit of residual muscle tissue. This study investigates in vivo specific force of the quadriceps and ex vivo specific force of single muscle fibers in patients with IBM. We included 8 participants with IBM and 12 healthy controls, who all underwent quantitative muscle testing, quantitative MRI of the quadriceps and paired muscle biopsies of the quadriceps and tibialis anterior. Single muscle fibers were isolated to measure muscle fiber specific force and contractile properties. Both in vivo quadriceps specific force and ex vivo muscle fiber specific force were reduced. Muscle fiber dysfunction was accompanied by reduced active stiffness, which reflects a decrease in the number of attached actin-myosin cross-bridges during activation. Myosin concentration was reduced in IBM fibers. Because reduced specific force contributes to muscle weakness in patients with IBM, therapeutic strategies that augment muscle fiber strength may provide benefit to patients with IBM.

Oxygen delivery-utilization mismatch in contracting locomotor muscle in COPD: peripheral factors

Central cardiorespiratory and gas exchange limitations imposed by chronic obstructive pulmonary disease (COPD) impair ambulatory skeletal muscle oxygenation during whole body exercise. This investigation tested the hypothesis that peripheral factors per se contribute to impaired contracting lower limb muscle oxygenation in COPD patients. Submaximal neuromuscular electrical stimulation (NMES; 30, 40, and 50 mA at 50 Hz) of the quadriceps femoris was employed to evaluate contracting skeletal muscle oxygenation while minimizing the influence of COPD-related central cardiorespiratory constraints. Fractional O₂ extraction was estimated by near-infrared spectroscopy (deoxyhemoglobin/myoglobin concentration; deoxy-[Hb/Mb]), and torque output was measured by isokinetic dynamometry in 15 nonhypoxemic patients with moderate-to-severe COPD (SpO2 = 94 ± 2%; FEV₁ = 46.4 ± 10.1%; GOLD II and III) and in 10 age- and gender-matched sedentary controls. COPD patients had lower leg muscle mass than controls (LMM = 8.0 ± 0.7 kg vs. 8.9 ± 1.0 kg, respectively; P < 0.05) and produced relatively lower absolute and LMM-normalized torque across the range of NMES intensities (P < 0.05 for all). Despite producing less torque, COPD patients had similar deoxy-[Hb/Mb] amplitudes at 30 and 40 mA (P > 0.05 for both) and higher deoxy-[Hb/Mb] amplitude at 50 mA (P < 0.05). Further analysis indicated that COPD patients required greater fractional O₂ extraction to produce torque (i.e., ↑Δdeoxy-[Hb/Mb]/torque) relative to controls (P < 0.05 for 40 and 50 mA) and as a function of NMES intensity (P < 0.05 for all). The present data obtained during submaximal NMES of small muscle mass indicate that peripheral abnormalities contribute mechanistically to impaired contracting skeletal muscle oxygenation in nonhypoxemic, moderate-to-severe COPD patients.

An official American Thoracic Society/European Respiratory Society statement: update on limb muscle dysfunction in chronic obstructive pulmonary disease

BACKGROUND

Limb muscle dysfunction is prevalent in chronic obstructive pulmonary disease (COPD) and it has important clinical implications, such as reduced exercise tolerance, quality of life, and even survival. Since the previous American Thoracic Society/European Respiratory Society (ATS/ERS) statement on limb muscle dysfunction, important progress has been made on the characterization of this problem and on our understanding of its pathophysiology and clinical implications.

PURPOSE

The purpose of this document is to update the 1999 ATS/ERS statement on limb muscle dysfunction in COPD.

METHODS

An interdisciplinary committee of experts from the ATS and ERS Pulmonary Rehabilitation and Clinical Problems assemblies determined that the scope of this document should be limited to limb muscles. Committee members conducted focused reviews of the literature on several topics. A librarian also performed a literature search. An ATS methodologist provided advice to the committee, ensuring that the methodological approach was consistent with ATS standards.

RESULTS

We identified important advances in our understanding of the extent and nature of the structural alterations in limb muscles in patients with COPD. Since the last update, landmark studies were published on the mechanisms of development of limb muscle dysfunction in COPD and on the treatment of this condition. We now have a better understanding of the clinical implications of limb muscle dysfunction. Although exercise training is the most potent intervention to address this condition, other therapies, such as neuromuscular electrical stimulation, are emerging. Assessment of limb muscle function can identify patients who are at increased risk of poor clinical outcomes, such as exercise intolerance and premature mortality.

CONCLUSIONS

Limb muscle dysfunction is a key systemic consequence of COPD. However, there are still important gaps in our knowledge about the mechanisms of development of this problem. Strategies for early detection and specific treatments for this condition are also needed.

Factors associated with impairment of quadriceps muscle function in Chinese patients with chronic obstructive pulmonary disease

Background

Quadriceps muscle dysfunction is well confirmed in chronic obstructive pulmonary disease (COPD) and reported to be related to a higher risk of mortality. Factors contributing to quadriceps dysfunction have been postulated, while not one alone could fully explain it and there are few reports on it in China. This study was aimed to investigate the severity of quadriceps dysfunction in patients with COPD, and to compare quadriceps muscle function in COPD and the healthy elderly.

Methods

Quadriceps strength and endurance capabilities were investigated in 71 COPD patients and 60 age-matched controls; predicted values for quadriceps strength and endurance were calculated using regression equations (incorporating age, gender, anthropometric measurements and physical activities), based on the data from controls. Potential parameters related to quadriceps dysfunction in COPD were identified by stepwise regression analysis.

Results

Mean values of quadriceps strength was 46% and endurance was 38% lower, in patients with COPD relative to controls. Gender, physical activities and anthropometric measurements were predictors to quadriceps function in the controls. While in COPD, forced expiratory volume in 1 second percentage of predicted value (FEV₁% pred), nutritional depletion, gender and physical inactivity were identified as independent factors to quadriceps strength (R² = 0.72); FEV₁%pred, thigh muscle mass, serum levels of tumor necrosis factor-alpha (TNF-α) and gender were correlated to quadriceps endurance variance, with each p<0.05.

Conclusion

Quadriceps strength and endurance capabilities are both substantially impaired in Chinese COPD patients, with strength most affected. For the controls, physical activity is most important for quadriceps function. While for COPD patients, quadriceps dysfunction is related to multiple factors, with airflow limitation, malnutrition and muscle disuse being the main ones.

Determining the role of sarcomeric proteins in facioscapulohumeral muscular dystrophy: a study protocol

Background

Although muscle weakness is a hallmark of facioscapulohumeral muscular dystrophy (FSHD), the molecular mechanisms that lead to weakness in FSHD remain largely unknown. Recent studies suggest aberrant expression of genes involved in skeletal muscle development and sarcomere contractility, and activation of pathways involved in sarcomeric protein degradation. This study will investigate the contribution of sarcomeric protein dysfunction to the pathogenesis of muscle weakness in FSHD.

Methods/Design

Evaluation of sarcomeric function using skinned single muscle fiber contractile studies and protein analysis in muscle biopsies (quadriceps femoris and tibialis anterior) from patients with FSHD and age- and gender-matched healthy controls. Patients with other forms of muscular dystrophy and inflammatory myopathy will be included as disease controls to assess whether results are due to changes specific for FSHD, or a consequence of muscle disease in general. A total of 56 participants will be included. Extensive clinical parameters will be measured using MRI, quantitative muscle studies and physical activity assessments.

Discussion

This study is the first to extensively investigate muscle fiber physiology in FSHD following an earlier pilot study suggesting sarcomeric dysfunction in FSHD. The results obtained in this study will increase the understanding of the pathophysiology of muscle weakness in FSHD, and possibly identify novel targets for therapeutic intervention.

Muscle function in COPD: a complex interplay

The skeletal muscles play an essential role in life, providing the mechanical basis for respiration and movement. Skeletal muscle dysfunction is prevalent in all stages of chronic obstructive pulmonary disease (COPD), and significantly influences symptoms, functional capacity, health related quality of life, health resource usage and even mortality. Furthermore, in contrast to the lungs, the skeletal muscles are potentially remedial with existing therapy, namely exercise-training. This review summarizes clinical and laboratory observations of the respiratory and peripheral skeletal muscles (in particular the diaphragm and quadriceps), and current understanding of the underlying etiological processes. As further progress is made in the elucidation of the molecular mechanisms of skeletal muscle dysfunction, new pharmacological therapies are likely to emerge to treat this important extra-pulmonary manifestation of COPD.

Noninvasive estimation of myosin heavy chain composition in human skeletal muscle

PURPOSE

Information on muscle fiber type composition is of great importance in muscle physiology and athletic performance. Because there are only a few techniques available that noninvasively and accurately provide an estimate of muscle fiber type composition, the development of additional and alternative approaches is required.

METHODS

Twenty-seven participants (21 men, 6 women) with an average age of 43 ± 18 yr, height of 175 ± 7 cm, and mass of 74 ± 12 kg participated in the study. Delay, contraction, and half relaxation times were calculated from tensiomyographic radial twitch responses of the vastus lateralis muscle. Univariate and multiple linear regression analyses were used to correlate the proportion of myosin heavy chain I (%MHC-I) in a biopsy obtained from the same muscle with a single and all three radial twitch parameters.

RESULTS

Delay, contraction, and half relaxation times all correlated with %MHC-I (r = 0.612, 0.878, and 0.669, respectively, at P ≤ 0.001). When all three parameters were included in a multiple linear regression, the correlation with the %MHC-I was even better (R = 0.933, P < 0.001).

CONCLUSIONS

These data suggest that time parameters of the skeletal muscle mechanical radial twitch response, measured with a contact linear displacement sensor, can be used as an accurate noninvasive predictor of the %MHC-I in a muscle.

Evidence that a higher ATP cost of muscular contraction contributes to the lower mechanical efficiency associated with COPD: preliminary findings

Impaired metabolism in peripheral skeletal muscles potentially contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). We used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine the energy cost and skeletal muscle energetics in six patients with COPD during dynamic plantar flexion exercise compared with six well-matched healthy control subjects. Patients with COPD displayed a higher energy cost of muscle contraction compared with the controls (control: 6.1 ± 3.1% of rest·min(-1)·W(-1), COPD: 13.6 ± 8.3% of rest·min(-1)·W(-1), P = 0.01). Although, the initial phosphocreatine resynthesis rate was also significantly attenuated in patients with COPD compared with controls (control: 74 ± 17% of rest/min, COPD: 52 ± 13% of rest/min, P = 0.04), when scaled to power output, oxidative ATP synthesis was similar between groups (6.5 ± 2.3% of rest·min(-1)·W(-1) in control and 7.8 ± 3.9% of rest·min(-1)·W(-1) in COPD, P = 0.52). Therefore, our results reveal, for the first time that in a small subset of patients with COPD a higher ATP cost of muscle contraction may substantially contribute to the lower mechanical efficiency previously reported in this population. In addition, it appears that some patients with COPD have preserved mitochondrial function and normal energy supply in lower limb skeletal muscle.

Skeletal muscle properties and fatigue resistance in relation to smoking history

Although smoking-related diseases, such as chronic obstructive pulmonary disease (COPD), are often accompanied by increased peripheral muscle fatigability, the extent to which this is a feature of the disease or a direct effect of smoking per se is not known. Skeletal muscle function was investigated in terms of maximal voluntary isometric torque, activation, contractile properties and fatigability, using electrically evoked contractions of the quadriceps muscle of 40 smokers [19 men and 21 women; mean (SD) cigarette pack years: 9.9 (10.7)] and age- and physical activity level matched non-smokers (22 men and 23 women). Maximal strength and isometric contractile speed did not differ significantly between smokers and non-smokers. Muscle fatigue (measured as torque decline during a series of repetitive contractions) was greater in smokers (P = 0.014), but did not correlate with cigarette pack years (r = 0.094, P = 0.615), cigarettes smoked per day (r = 10.092, P = 0.628), respiratory function (%FEV_1pred) (r = −0.187, P = 0.416), or physical activity level (r = −0.029, P = 0.877). While muscle mass and contractile properties are similar in smokers and non-smokers, smokers do suffer from greater peripheral muscle fatigue. The observation that the cigarette smoking history did not correlate with fatigability suggests that the effect is either acute and/or reaches a ceiling, rather than being cumulative. An acute and reversible effect of smoking could be caused by carbon monoxide and/or other substances in smoke hampering oxygen delivery and mitochondrial function.

Carbon monoxide inhalation reduces skeletal muscle fatigue resistance

AIM

To determine whether inhalation of carbon monoxide (CO), resulting in carboxyhaemoglobin (COHb) levels observed in smokers, had an effect on muscle fatigue during electrically evoked and voluntary muscle contractions.

METHODS

Young non-smoking males inspired CO from a Douglas bag until their COHb level reached 6%. During the control condition the same participants inspired ambient air from a Douglas bag for 6 min. Fatigue was assessed as the decline in torque in isometric knee extensions, during 2 min of electrically evoked contractions (30 Hz, 1 s on, 1 s off) and during 2 min of maximal isometric voluntary contractions (1 s on, 1 s off). A fatigue index (FI) was calculated as the ratio of final torque : initial torque. Time to peak torque (TPT) and half relaxation time ((1/2)RT) were also determined for the electrically evoked contractions.

RESULTS

The FI during both the voluntary fatigue test (control: 0.80 +/- 0.09 vs. CO: 0.70 +/- 0.08; mean +/- SD) and that of the fatigue test with electrically evoked contractions (control: 0.61 +/- 0.09 vs. CO: 0.53 +/- 0.12) was significantly lower after CO inhalation than after inhalation of ambient air (P < 0.05). There was, however, no effect of CO on the changes in TPT or (1/2)RT during the fatigue test.

CONCLUSION

Carbon monoxide inhalation resulting in COHb levels found in smokers has an acute impact on the ability of the muscle to resist fatigue.

Clinical value of anthropometric estimates of leg lean volume in nutritionally depleted and non-depleted patients with chronic obstructive pulmonary disease

This study aimed to investigate the clinical usefulness of an anthropometrically based method for estimating leg lean volume (LLV) in patients with chronic obstructive pulmonary disease (COPD) who presented or not with nutritional depletion. We prospectively evaluated a group of forty-eight patients (thirty-eight males) with moderate to severe COPD (Global Initiative for Chronic Obstructive Lung disease stages II-IV) who underwent a 6 min walking test and knee isokinetic dynamometry. Leg lean mass (muscle mass plus bone) was determined by dual-energy X-ray absorptiometry (DEXA) with derivation of its respective volume: these values were compared with those obtained by the truncated cones method first described by Jones and Pearson in 1969. As expected, depleted patients (n 19) had reduced exercise capacity and impaired muscle performance as compared to non-depleted subjects (P < 0.01). The mean bias of the LLV differences between anthropometry and DEXA were 0.40 litre (95 % CI - 0.59, 1.39) and 0.50 litre (95 % CI - 1.08, 2.08) for depleted and non-depleted patients, respectively. Anthropometrically and DEXA-based estimates correlated similarly with muscle functional attributes. A ROC curve analysis revealed that leg height-corrected LLV values had acceptable sensitivity and specificity to identify depleted patients (area under the curve 0.93 (range 0.86-1.00); P < 0.001). Moreover, patients with LLV <or= 9.2 litres/m (the best cut-off value according to the ROC curve) had significantly lower exercise capacity and muscle performance than their counterparts (P < 0.05). In conclusion, an anthropometrically based method of estimating LLV (Jones and Pearson method) was shown to present with clinically acceptable accuracy and external validity in depleted and non-depleted patients with stable COPD.

Muscle fatigue resistance during stimulated contractions is reduced in young male smokers

AIM

To determine whether muscle function is compromised in healthy smokers in comparison with activity-matched non-smokers.

METHODS

Nine male smokers (aged 22.2 +/- 2.5 years: mean +/- SD) with a smoking history of 2.5 +/- 3.1 pack years, and ten male control participants (25.4 +/- 2.9 years) matched for physical activity level participated in this study. Knee extensor strength was measured using isometric maximal voluntary contractions. Voluntary activation of the quadriceps and co-activation of the biceps femoris were determined using interpolated twitches and surface electromyography respectively. The frequency-torque relationship and fatigue resistance were assessed with electrically evoked contractions. A fatigue index was determined as the ratio of final torque to initial torque during a series of isometric contractions (2 min; 30 Hz; 1 s contraction/1 s rest). Quadriceps anatomical cross sectional area was measured with MRI at 50% of femur length.

RESULTS

Maximal voluntary contraction torque, quadriceps anatomical cross sectional area, knee extensor torque/quadriceps cross sectional area, activation, co-activation and force-frequency relationship were similar, whereas the fatigue index was 17% lower in smokers than non-smokers.

CONCLUSION

In young men smoking does not significantly affect quadriceps muscle mass and contractile properties, but does reduce fatigue resistance of the quadriceps muscle, which was not attributable to differences in physical activity.

Apoptosis and Id2 expression in diaphragm and soleus muscle from the emphysematous hamster

During chronic obstructive pulmonary disease (COPD) diaphragm and peripheral muscle weakness occur. Muscle remodeling and wasting may be a result of apoptosis and changes in muscle-specific transcription factors, such as MyoD, altering muscle-specific gene transcription and muscle regenerative capacity. To investigate this, we instilled under ketamine/xylazine anesthesia porcine elastase in the lungs of hamsters to induce emphysema. The emphysematous hamster is an accepted model for COPD. In the diaphragm and peripheral muscles we assessed the occurrence of apoptosis, and in the diaphragm and soleus also the expression of MyoD and inhibitor of differentiation protein 2 (Id2). There was no significant muscle atrophy in emphysematous hamsters. The mRNA levels of TNF-alpha and markers of apoptosis were significantly elevated in the diaphragm and soleus muscles during emphysema. This was accompanied by an increased presence of nucleosomes in the cytosol. Caspase 3 activity and the DNA-binding activity of the p65 subunit of NF-kappaB, however, were unaltered in all muscles. The protein expression of MyoD and Id2 were decreased and increased in the diaphragm and the soleus muscle, respectively. Thus, despite the absence of muscle atrophy in emphysematous hamsters, there was evidence of increased TNF-alpha expression, apoptosis, and altered muscle-specific transcriptional regulation as reflected by decreased MyoD and elevated Id2 levels at least in the soleus and diaphragm muscle. These alterations may impair the regenerative capacity of skeletal muscles and ultimately contribute to muscle wasting.

Acute hypoxia limits endurance but does not affect muscle contractile properties

Acute hypoxia causes skeletal muscle dysfunction in vitro, but little is known about its effect on muscle function in vivo. In 10 healthy male subjects, isometric contractile properties and fatigue resistance of the quadriceps muscle were determined during normoxia and hypoxia using electrically evoked and voluntary contractions. The oxygen saturation (SaO(2); 96.9 +/- 0.7 vs. 79.9 +/- 3.0%; P < 0.001) was reduced during hypoxia. The maximal voluntary contraction (MVC), force-frequency relation, and contraction and relaxation times were unaffected by hypoxia. The endurance time of a sustained 30% MVC was reduced in hypoxia (248 +/- 104 vs. 217 +/- 76 s; P < 0.05), but not that of a sustained 70% MVC. Fatigue induced by electrically evoked intermittent contractions was unaltered. Thus, acute hypoxia has no significant impact on contractile properties of skeletal muscle in vivo but causes reduced endurance during low-level sustained voluntary contractions. This indicates that skeletal muscle dysfunction during conditions associated with prolonged hypoxemia, except for limited endurance, is not due to acute effects of hypoxemia.