The effect of higher ATP cost of contraction on the metabolic response to graded exercise in patients with chronic obstructive pulmonary disease

Skeletal Muscle Mitochondrial Dysfunction in Chronic Obstructive Pulmonary Disease: Underlying Mechanisms and Physical Therapy Perspectives

Skeletal muscle dysfunction (SMD) is a prevalent extrapulmonary complication and a significant independent prognostic factor in patients with chronic obstructive pulmonary disease (COPD). Mitochondrial dysfunction is one of the core factors that damage structure and function in COPD skeletal muscle and is closely related to smoke exposure, hypoxia, and insufficient physical activity. The currently known phenotypes of mitochondrial dysfunction are reduced mitochondrial content and biogenesis, impaired activity of mitochondrial respiratory chain complexes, and increased mitochondrial reactive oxygen species production. Significant progress has been made in research on physical therapy (PT), which has broad prospects for treating the abovementioned potential mitochondrial-function changes in COPD skeletal muscle. In terms of specific types of PT, exercise therapy can directly act on mitochondria and improve COPD SMD by increasing mitochondrial density, regulating mitochondrial biogenesis, upregulating mitochondrial respiratory function, and reducing oxidative stress. However, improvements in mitochondrial-dysfunction phenotype in COPD skeletal muscle due to different exercise strategies are not entirely consistent. Therefore, based on the elucidation of this phenotype, in this study, we analyzed the effect of exercise on mitochondrial dysfunction in COPD skeletal muscle and the regulatory mechanism thereof. We also provided a theoretical basis for exercise programs to rehabilitate this condition.

Rates of oxidative ATP synthesis are not augmented beyond the pH threshold in human vastus lateralis muscles during a stepwise contraction protocol

KEY POINTS

The oxygen cost of high-intensity exercise at power outputs above an individual's lactate threshold (LT) is greater than would be predicted by the linear oxygen consumption-power relationship observed below the LT. However, whether these augmentations are caused by an increased ATP cost of force generation (ATP_COST ) or an increased oxygen cost of ATP synthesis is unclear. We used ³¹ P-MRS to measure changes in cytosolic [ADP] (intramyocellular marker of oxidative metabolism), oxidative ATP synthesis (ATP_OX ) and ATP_COST during a 6-stage, stepwise knee extension protocol. ATP_COST was unchanged across stages. The relationship between [ADP] and muscle power output was augmented at workloads above the pH threshold (pH_T ; proxy for LT), whereas increases in ATP_OX were attenuated. These results suggest the greater oxygen cost of contractions at workloads beyond the pH_T is not caused by mechanisms that increase ATP_COST , but rather mechanisms that alter intrinsic mitochondrial function or capacity.

ABSTRACT

Increases in skeletal muscle metabolism and oxygen consumption are linearly related to muscle power output for workloads below the lactate threshold (LT), but are augmented (i.e. greater rate of increase relative to workload) thereafter. Presently, it is unclear whether these metabolic augmentations are caused by increases in the ATP cost of force generation (ATP_COST ) or changes in the efficiency of mitochondrial oxygen consumption and oxidative ATP synthesis (ATP_OX ). To partition these two hypotheses in vivo, we used ³¹ P-MRS to calculate slopes relating step-changes in muscle work to concurrent changes in cytosolic phosphates and ATP_OX before and after the pH threshold (pH_T ; used here as a proxy for LT) within the vastus lateralis muscle of eight young adults during a stepwise knee extension test. Changes in muscle phosphates and ATP_OX were linearly related to workload below the pH_T . However, slopes above the pH_T were greater for muscle phosphates (P < 0.05) and lower for ATP_OX (P < 0.05) than were the slopes observed below the pH_T . The maximal capacity for ATP_OX ( V ̇ max ) and ADP-specific ATP_OX also declined beyond the pH_T (P < 0.05), whereas ATP_COST was unchanged (P = 0.10). These results oppose the hypothesis that high-intensity contractions increase ATP_COST and suggest that greater oxidative metabolism at workloads beyond the pH_T is caused by mechanisms that affect intrinsic mitochondrial function or capacity, such as alterations in substrate selection or electron entry into the electron transport chain, temperature-mediated changes in mitochondrial permeability to protons, or stimulation of mitochondrial uncoupling by reactive oxygen species generation.

Determinants of the diminished exercise capacity in patients with chronic obstructive pulmonary disease: looking beyond the lungs

KEY POINTS

Peak oxygen uptake, a primary determinant of prognosis, mortality and quality of life, is diminished in patients with chronic obstructive pulmonary disease (COPD), with mounting evidence supporting an important role for peripheral dysfunction, particularly within skeletal muscle. In patients with severe COPD and activity-matched controls, muscle oxygen transport and utilization were assessed at peak effort during single-leg knee-extensor exercise (KE), where ventilation is assumed to be submaximal. This strategy removes ventilation as the major constraint to exercise capacity in COPD, allowing maximal muscle function to be attained and evaluated. During maximal KE, both convective arterial oxygen delivery to the skeletal muscle microvasculature and subsequent diffusive oxygen delivery to the mitochondria were diminished in patients with COPD compared to control subjects. These findings emphasize the importance of factors, beyond the lungs, that influence exercise capacity in this patient population and may, ultimately, influence the prognosis, mortality and quality of life for patients with COPD.

ABSTRACT

Peak oxygen uptake ( V ̇ O 2 peak ), a primary determinant of prognosis, mortality and quality of life, is diminished in patients with chronic obstructive pulmonary disease (COPD). Mounting evidence supports an important role of the periphery, particularly skeletal muscle, in the diminished V ̇ O 2 peak with COPD. However, the peripheral determinants of V ̇ O 2 peak have not been comprehensively assessed in this cohort. Thus, the hypothesis was tested that both muscle convective and diffusive oxygen (O2 ) transport, and therefore skeletal muscle peak O2 uptake ( V ̇ M O 2 peak ), are diminished in patients with COPD compared to matched healthy controls, even when ventilatory limitations (i.e. attainment of maximal ventilation) are minimized by using small muscle mass exercise. Muscle O2 transport and utilization were assessed at peak exercise from femoral arterial and venous blood samples and leg blood flow (by thermodilution) in eight patients with severe COPD (forced expiratory volume in 1s (FEV1 ) ± SEM = 0.9 ± 0.1 l, 30% of predicted) and eight controls during single-leg knee-extensor exercise. Both muscle convective O2 delivery (0.44 ± 0.06 vs. 0.69 ± 0.07 l min-1 , P < 0.05) and muscle diffusive O2 conductance (6.6 ± 0.8 vs. 10.4 ± 0.9 ml min-1  mmHg-1 , P < 0.05) were ∼1/3 lower in patients with COPD than controls, resulting in an attenuated V ̇ M O 2 peak in the patients (0.27 ± 0.04 vs. 0.42 ± 0.05 l min-1 , P < 0.05). When cardiopulmonary limitations to exercise are minimized, the convective and diffusive determinants of V ̇ M O 2 peak , at the level of the skeletal muscle, are greatly attenuated in patients with COPD. These findings emphasize the importance of factors, beyond the lungs, that may ultimately influence this population's prognosis, mortality and quality of life.

Cardiorespiratory and Muscle Oxygenation Responses to Isokinetic Exercise in Chronic Obstructive Pulmonary Disease

PURPOSE

This study aimed to describe cardiorespiratory, quadriceps oxygenation, and muscle fatigue responses during a one-legged quadriceps isokinetic endurance exercise in chronic obstructive pulmonary disease (COPD) and control subjects.

METHODS

Fourteen patients with COPD and 14 control subjects performed a cardiopulmonary cycling exercise test to exhaustion to assess peak oxygen consumption (V˙O2peak), minute ventilation (V˙Epeak), and heart rate (HRpeak). They also performed a quadriceps isokinetic endurance exercise consisting in 30 maximal knee extensions at 90°·s with continuous monitoring of expired gases, cardiac output, and oxygenation of the quadriceps by near-infrared spectroscopy. Total muscle work and fatigue index were also quantified.

RESULTS

The total muscle work developed during the quadriceps isokinetic endurance exercise was 2.25 ± 0.57 kJ in COPD and 3.12 ± 0.60 kJ in controls, P < 0.001. In absolute terms, there were no between-group differences in V˙O2, V˙E, cardiac output, and HR at the end of quadriceps isokinetic endurance exercise. However, V˙E and HR reported that a fraction of their respective peak values during cardiopulmonary cycling exercise test were higher in COPD (V˙E/V˙Epeak, 69% ± 3%; HR/HRpeak, 82% ± 15%) compared with controls (V˙E/V˙Epeak, 45% ± 2%; HR/HRpeak, 71% ± 13%), all P < 0.05. During quadriceps isokinetic endurance exercise, quadriceps deoxyhemoglobin increased by 47% ± 31% in patients versus 33% ± 41% in controls (P < 0.05 from rest values) with a significant between-group differences (P = 0.025). The fatigue index during the quadriceps exercise was higher in COPD compared with controls.

CONCLUSIONS

Although one-legged quadriceps isokinetic endurance exercise resulted in substantial central cardiorespiratory demands in COPD, this exercise was nevertheless associated with muscle overload as evidenced by muscle deoxygenation and higher muscle fatigue index in COPD compared with controls. These findings may have implications of the design of exercise training programs in COPD.

Exercise-induced calf muscle hyperemia: quantitative mapping with low-dose dynamic contrast enhanced magnetic resonance imaging

Peripheral artery disease (PAD) in the lower extremities often leads to intermittent claudication. In the present study, we proposed a low-dose DCE MRI protocol for quantifying calf muscle perfusion stimulated with plantar flexion and multiple new metrics for interpreting perfusion maps, including the ratio of gastrocnemius over soleus perfusion (G/S; for assessing the vascular redistribution between the two muscles) and muscle perfusion normalized by whole body perfusion (for quantifying the muscle's active hyperemia). Twenty-eight human subjects participated in this Institutional Review Board-approved study, with 10 healthy subjects ( group A) for assessing interday reproducibility and 8 healthy subjects ( group B) for exploring the relationship between plantar-flexion load and induced muscle perfusion. In a pilot group of five elderly healthy subjects and five patients with PAD ( group C), we proposed a protocol that measured perfusion for a low-intensity exercise and for an exhaustion exercise in a single MRI session. In group A, perfusion estimates for calf muscles were highly reproducible, with correlation coefficients of 0.90-0.93. In group B, gastrocnemius perfusion increased linearly with the exercise workload ( P < 0.05). With the low-intensity exercise, patients with PAD in group C showed substantially lower gastrocnemius perfusion compared with elderly healthy subjects [43.4 (SD 23.5) vs. 106.7 (SD 73.2) ml·min^-1·100 g^-1]. With exhaustion exercise, G/S [1.0 (SD 0.4)] for patients with PAD was lower than both its low-intensity level [1.9 (SD 1.3)] and the level in elderly healthy subjects [2.7 (SD 2.1)]. In conclusion, the proposed MRI protocol and the new metrics are feasible for quantifying exercise-induced muscle hyperemia, a promising functional test of PAD. NEW & NOTEWORTHY To quantitatively map exercise-induced hyperemia in calf muscles, we proposed a high-resolution MRI method shown to be highly reproducible and sensitive to exercise load. With the use of low contrast, it is feasible to measure calf muscle hyperemia for both low-intensity and exhaustion exercises in a single MRI session. The newly proposed metrics for interpreting perfusion maps are promising for quantifying intermuscle vascular redistribution or a muscle's active hyperemia.

Cardiorespiratory and muscle oxygenation responses to low-load/high-repetition resistance exercises in COPD and healthy controls

Single-limb exercises have been used as a strategy to improve aerobic exercise tolerance in patients with chronic obstructive pulmonary disease (COPD) by alleviating the cardiopulmonary demand. We asked whether this strategy would also apply to cardiorespiratory demand and amount of work performed during single-limb and two-limb low-load/high-repetition resistance exercises in 20 patients with COPD [forced expiratory volume in 1 s (FEV1) = 1.0 liters, 38% of predicted] and 15 age-, sex-, and activity-matched healthy controls. Peak ventilation, peak oxygen consumption (V̇o2), and peak heart rate (HR) were assessed to document cardiorespiratory demand during shoulder flexion and knee extension exercises while exercise tolerance was assessed by the total amount of work achieved. In addition, changes in myoglobin-deoxyhemoglobin level (Δdeoxy-[Hb/Mb]) were measured during single-limb knee extension. In COPD, single-limb shoulder flexion and knee extension elicited higher localized workloads than two-limb exercises (21 and 24% higher workloads for the former exercise) while cardiopulmonary demand was 8–16% higher during two-limb exercises. When expressed as a percentage of peak values achieved during incremental cycle ergometry, peak V̇O2 and HR were similarly high during single-limb shoulder flexion and knee extension exercises, representing 90% of peak HR and 60% of peak V̇O2. Apart from single-limb knee extension, cardiorespiratory demand per kilogram work during low-load/high-repetition knee extension and shoulder flexion exercises was higher in patients with COPD than in healthy controls (range 27–122%, P < 0.0125). Δdeoxy-[Hb/Mb] of the quadriceps during knee extension was similar between the two groups, while Δdeoxy-[Hb/Mb] per kilogram work was higher in patients with COPD. We conclude that 1) in patients with COPD, single-limb exercises resulted in lower peak cardiorespiratory demand as well as higher localized workloads compared with two-limb exercises; 2) compared with healthy controls, the cardiorespiratory demand, either expressed per unit of work or relative to peak capacity, was higher in patients with COPD than in controls during low-load/high-repetition resistance exercises, irrespective of the involvement of one or two limbs or of the upper or lower extremity; 3) quadriceps muscle deoxygenation per unit of work during low-load/high-repetition knee extension was increased in COPD compared with controls; and 4) single- and two-limb low-load/high-repetition knee extension and shoulder flexion resistance exercises imposed a similar burden on the cardiorespiratory system, resulting in a higher cardiorespiratory demand per kilogram work performed during shoulder flexion compared with knee extension, in both COPD and healthy controls.

NEW & NOTEWORTHY In chronic obstructive pulmonary disease (COPD), single-limb knee extension and shoulder flexion resulted in a lower peak cardiorespiratory response as well as larger localized exercise workloads compared with two-limb exercises. Cardiorespiratory and quadriceps deoxygenation cost per kilogram work was greater in COPD compared with healthy controls, despite similar acute responses. Compared with knee extension, shoulder flexion imposed a similar burden on the cardiorespiratory system in patients with COPD and healthy controls.

Muscle dysfunction in chronic obstructive pulmonary disease: update on causes and biological findings

Respiratory and/or limb muscle dysfunction, which are frequently observed in chronic obstructive pulmonary disease (COPD) patients, contribute to their disease prognosis irrespective of the lung function. Muscle dysfunction is caused by the interaction of local and systemic factors. The key deleterious etiologic factors are pulmonary hyperinflation for the respiratory muscles and deconditioning secondary to reduced physical activity for limb muscles. Nonetheless, cigarette smoke, systemic inflammation, nutritional abnormalities, exercise, exacerbations, anabolic insufficiency, drugs and comorbidities also seem to play a relevant role. All these factors modify the phenotype of the muscles, through the induction of several biological phenomena in patients with COPD. While respiratory muscles improve their aerobic phenotype (percentage of oxidative fibers, capillarization, mitochondrial density, enzyme activity in the aerobic pathways, etc.), limb muscles exhibit the opposite phenotype. In addition, both muscle groups show oxidative stress, signs of damage and epigenetic changes. However, fiber atrophy, increased number of inflammatory cells, altered regenerative capacity; signs of apoptosis and autophagy, and an imbalance between protein synthesis and breakdown are rather characteristic features of the limb muscles, mostly in patients with reduced body weight. Despite that significant progress has been achieved in the last decades, full elucidation of the specific roles of the target biological mechanisms involved in COPD muscle dysfunction is still required. Such an achievement will be crucial to adequately tackle with this relevant clinical problem of COPD patients in the near-future.

Quadriceps exercise intolerance in patients with chronic obstructive pulmonary disease: the potential role of altered skeletal muscle mitochondrial respiration

This study sought to determine if qualitative alterations in skeletal muscle mitochondrial respiration, associated with decreased mitochondrial efficiency, contribute to exercise intolerance in patients with chronic obstructive pulmonary disease (COPD). Using permeabilized muscle fibers from the vastus lateralis of 13 patients with COPD and 12 healthy controls, complex I (CI) and complex II (CII)-driven State 3 mitochondrial respiration were measured separately (State 3:CI and State 3:CII) and in combination (State 3:CI+CII). State 2 respiration was also measured. Exercise tolerance was assessed by knee extensor exercise (KE) time to fatigue. Per milligram of muscle, State 3:CI+CII and State 3:CI were reduced in COPD (P < 0.05), while State 3:CII and State 2 were not different between groups. To determine if this altered pattern of respiration represented qualitative changes in mitochondrial function, respiration states were examined as percentages of peak respiration (State 3:CI+CII), which revealed altered contributions from State 3:CI (Con 83.7 ± 3.4, COPD 72.1 ± 2.4%Peak, P < 0.05) and State 3:CII (Con 64.9 ± 3.2, COPD 79.5 ± 3.0%Peak, P < 0.05) respiration, but not State 2 respiration in COPD. Importantly, a diminished contribution of CI-driven respiration relative to the metabolically less-efficient CII-driven respiration (CI/CII) was also observed in COPD (Con 1.28 ± 0.09, COPD 0.81 ± 0.05, P < 0.05), which was related to exercise tolerance of the patients (r = 0.64, P < 0.05). Overall, this study indicates that COPD is associated with qualitative alterations in skeletal muscle mitochondria that affect the contribution of CI and CII-driven respiration, which potentially contributes to the exercise intolerance associated with this disease.

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.

In vivo evidence of an age-related increase in ATP cost of contraction in the plantar flexor muscles

Impaired skeletal muscle efficiency potentially contributes to the age-related decline in exercise capacity and may explain the altered haemodynamic response to exercise in the elderly. Thus we examined whether (i) the ATP cost of contraction increases with age, and (ii) this results in altered convective O(2) delivery to maintain microvascular oxygenation in the calf muscle. To this aim, we used an integrative experimental approach combining (31)P-MRS (magnetic resonance spectroscopy), Doppler ultrasound imaging and NIRS (near-IR spectroscopy) during dynamic plantar flexion exercise at 40% of WR(max) (maximal power output) in 20 healthy young and 20 older subjects matched for physical activity. The ATP cost of contraction was significantly higher in the old (7.2±4.1 mM/min per W) compared with the young (2.4±1.9 mM/min per W; P<0.05) and this was only significantly correlated with the plantar flexion WR(max) value in the old subjects (r=-0.52; P<0.05). Even when differences in power output were taken into account, end-exercise blood flow (old, 259±168 ml/min per W and young, 134±40 ml/min per W; P<0.05) and convective O(2) delivery (old, 0.048±0.031 l/min per W and young, 0.026±0.008 l/min per W; P<0.05) were greater in the old in comparison with the young subjects. In contrast, the NIRS oxyhaemoglobin, deoxyhaemoglobin and microvascular oxygenation indices were not significantly different between the groups (P>0.05). Therefore the present study reveals that, although the peripheral haemodynamic responses to plantar flexion exercise appear to be appropriate, the elevated energy cost of contraction and associated reduction in the WR(max) value in this muscle group may play a role in limiting exercise capacity with age.

Effects of nutraceutical diet integration, with coenzyme Q10 (Q-Ter multicomposite) and creatine, on dyspnea, exercise tolerance, and quality of life in COPD patients with chronic respiratory failure

BACKGROUND

The protein-calorie malnutrition, resulting in muscle mass loss, frequently occurs in severe COPD patients with chronic respiratory failure (CRF), causing dyspnea, reduced exercise tolerance and impaired quality of life.The cause of this occurrence is an intake-output energy imbalance. A documented deficit of phosphocreatine and reduced mithocondrial energy production can contribute to this imbalance.Aim of this study is to verify whether a dietary supplementation with creatine and coenzyme Q10, important mitochondrial function factors, is able to influence this mechanism leading to a dyspnea reduction and improving exercise tolerance and quality of life.

METHODS

55 COPD patients with chronic respiratory failure (in long term O2 therapy), in stable phase of the disease and without severe comorbidities were assigned (double-blind, randomized) to: group A (30 patients) with daily dietary supplementation with Creatine 340 mg + 320 mg Coenzyme Q-Ter (Eufortyn®, Scharper Therapeutics Srl) for 2 months whereas Group B (25 patients) received placebo.All patients continued the same diet, rehabilitation and therapy during the study. At recruitment (T0) and after 2 months (T1), patients were submitted to medical history, anthropometry (BMI), bioelectrical impedance, arterial blood gas analysis, evaluation of dyspnea (VAS, Borg, BDI, MRC) and functional independence (ADL), 6-minute walk test (6MWT) and quality of life questionnaire (SGRQ). At 6 months and 1 year, a telephone follow up was conducted on exacerbations number.

RESULTS

No significant difference was detected at baseline (T0) in the 2 groups. After 2 months of therapy (T1) the FFMI increased in the daily dietary supplementation group (+ 3.7 %) and decreased in the placebo group (- 0.6 %), resulting in a statistically significant (p < 0.001) treatment difference. Statistically significant treatment differences, favouring daily dietary supplementation group, were also seen for the 6MWT comparison. Group A patients also showed significant: 1) improvement in the degree of dyspnea (VAS: p < 0.05; Borg: p < 0.05; MRC: p < 0.001; BDI1: p < 0.05; BDI3: p < 0.03), and independence level in activities of daily living (p < 0.03); 2) improvement in quality of life in activity section (- 6.63 pt) and in total score (- 5.43 pt); 3) exacerbation number decrease (p < 0.02). No significant differences were found (end of study vs baseline) in group B.

CONCLUSIONS

The nutraceutical diet integration with Q-Ter and creatine, in COPD patients with CRF in O2TLT induced an increasing lean body mass and exercise tolerance, reducing dyspnea, quality of life and exacerbations. These results provide a first demonstration that acting on protein synthesis and muscular efficiency can significantly modify the systemic consequences of the disease.

Disability in COPD and Chronic Heart Failure Is the Skeletal Muscle the Final Common Pathway?

Chronic Obstructive Pulmonary Disease (COPD) and Chronic Heart Failure (CHF), two major causes of worldwide morbidity and mortality have important systemic components, affecting additional tissues, other than the lung or the heart, such as the skeletal muscle. Muscle function (or dysfunction) may not only influence the symptoms that limit exercise, but may contribute directly to the poor exercise performance, health status and increased healthcare utilization.The present review tries to summarize the muscular abnormalities in COPD and CHF and the mechanisms underlying these alterations, which are strikingly similar, despite the obvious differences concerning the primary impairment in these two chronic diseases.The muscles therefore represent a potential site to improve patients' functioning level and quality of life of COPD and CHF. Only one practical therapeutic intervention currently exists that can reverse some of the muscle abnormalities observed in COPD and CHF, namely exercise training, which becomes nowadays the "cornerstone" of the whole rehabilitation.

Pathophysiology of muscle dysfunction in COPD

Muscle dysfunction often occurs in patients with chronic obstructive pulmonary disease (COPD) and may involve both respiratory and locomotor (peripheral) muscles. The loss of strength and/or endurance in the former can lead to ventilatory insufficiency, whereas in the latter it limits exercise capacity and activities of daily life. Muscle dysfunction is the consequence of complex interactions between local and systemic factors, frequently coexisting in COPD patients. Pulmonary hyperinflation along with the increase in work of breathing that occur in COPD appear as the main contributing factors to respiratory muscle dysfunction. By contrast, deconditioning seems to play a key role in peripheral muscle dysfunction. However, additional systemic factors, including tobacco smoking, systemic inflammation, exercise, exacerbations, nutritional and gas exchange abnormalities, anabolic insufficiency, comorbidities and drugs, can also influence the function of both respiratory and peripheral muscles, by inducing modifications in their local microenvironment. Under all these circumstances, protein metabolism imbalance, oxidative stress, inflammatory events, as well as muscle injury may occur, determining the final structure and modulating the function of different muscle groups. Respiratory muscles show signs of injury as well as an increase in several elements involved in aerobic metabolism (proportion of type I fibers, capillary density, and aerobic enzyme activity) whereas limb muscles exhibit a loss of the same elements, injury, and a reduction in fiber size. In the present review we examine the current state of the art of the pathophysiology of muscle dysfunction in COPD.