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Skeletal Muscle Adaptations to Interval Training in Patients With Advanced COPD^*

^* From the Department of Critical Care Medicine and Pulmonary Services (Drs. Nanas, Stratakos, Simoes, Zakynthinos, and Roussos), Pulmonary Rehabilitation Centre, Evangelismos Hospital, and Thorax Foundation "M. Simou and G.P. Livanos Laboratories"; Department of Physical Education and Sport Science (Drs. Terzis and Vogiatzis, and Ms. Georgiadou), National & Kapodistrian University of Athens, Athens, Greece.

Correspondence to: Ioannis Vogiatzis, PhD, National & Kapodistrian University of Athens, Medical School, Thorax Foundation 3 Ploutarhou Str. 106 75, Athens, Greece; e-mail:gianvog{at}phed.uoa.gr

	Abstract

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Study objectives: To investigate the response to interval exercise (IE) training by looking at changes in morphologic and biochemical characteristics of the vastus lateralis muscle, and to compare these changes to those incurred after constant-load exercise (CLE) training.

Design: Randomized, controlled, parallel, two-group study (IE vs CLE training).

Setting: Multidisciplinary, outpatient, hospital-based, pulmonary rehabilitation program.

Patients: Nineteen patients with stable advanced COPD (mean ± SEM FEV₁, 40 ± 4% predicted).

Interventions: Patients (n = 10) assigned to IE training exercised at a mean intensity of 124 ± 15% of baseline peak exercise capacity (peak work rate [Wpeak]) with 30-s work periods interspersed with 30-s rest periods for 45 min/d. Patients (n = 9) allocated to CLE training exercised at a mean intensity of 75 ± 5% Wpeak for 30 min/d. Patients exercised 3 d/wk for 10 weeks.

Measurements and results: Needle biopsies of the right vastus lateralis muscle were performed before and after rehabilitation. After IE training, the cross-sectional areas of type I and IIa fibers were significantly increased (type I before, 3,972 ± 455 µm²; after, 4,934 ± 467 µm² [p = 0.004]; type IIa before, 3,695 ± 372 µm²; after, 4,486 ± 346 µm² [p = 0.008]), whereas the capillary-to-fiber ratio was significantly enlarged (from 1.13 ± 0.08 to 1.24 ± 0.07 [p = 0.013]). Citrate synthase activity increased (from 14.3 ± 1.4 to 20.5 ± 4.2 µmol/min/g), albeit not significantly (p = 0.097). There was also a significant improvement in Wpeak (by 19 ± 5%; p = 0.04) and in lactate threshold (by 17 ± 5%; p = 0.02). The magnitude of changes in all the above variables was not significantly different compared to that incurred after CLE training. During training sessions, however, ratings of dyspnea and leg discomfort, expressed as fraction of values achieved at baseline Wpeak, were significantly lower (p < 0.05) for IE training (73 ± 9% and 60 ± 8%, respectively) compared to CLE training (83 ± 10% and 87 ± 13%, respectively).

Conclusions: High-intensity IE training is equally effective to moderately intense CLE training in inducing peripheral muscle adaptations; however, IE is associated with fewer training symptoms.

Key Words: interval exercise • obstructive lung disease • pulmonary rehabilitation • skeletal muscle biopsy

	Introduction

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Skeletal muscle dysfunction is common in patients with advanced COPD, and it contributes importantly to limiting functional capacity and quality of life.¹²³ Morphologic and biochemical changes within the vastus lateralis muscle of these patients include abnormal fiber-type proportions, reduced fiber cross-sectional area (CSA), and decreased muscle capillarity and oxidative enzyme activities.³⁴⁵⁶⁷⁸ These changes have been associated with an early activation of anaerobic glycolysis, lactic acidosis, and premature establishment of muscle fatigue during exercise.⁵⁹ Endurance training reverses, at least in part, these abnormalities by inducing significant improvements in fiber CSA and oxidative enzyme activities, thereby enhancing exercise tolerance.¹⁰¹¹¹²

Although high-intensity constant-load exercise (CLE) is generally argued to be needed for an improvement in exercise capacity,¹³ ventilatory-limited patients are usually unable to sustain such intensities for sufficiently long periods.¹⁴ Since trainability in patients with severe COPD is limited, various attempts to improve the outcome of training, such as oxygen supplementation or continuous positive airway pressure, have been employed.¹³ An alternative approach that allows high-intensity exercise to be performed for sufficiently long periods of time is interval exercise (IE). In healthy subjects with this type of exercise, it is possible to impose maximal loads on both muscles and oxygen-transporting organs without significant engagement of anaerobic processes, thus allowing a great amount of work to be performed before exhaustion sets in.¹⁵ In patients with advanced COPD, IE training, consisting of repeated bouts of high or even maximal-intensity work separated by periods of lower intensity work or rest, has been shown to be associated with a small increase in lactate concentration, stable ventilation, and low symptoms of dyspnea and leg discomfort, thus allowing the total amount of work performed to be significantly greater than at CLE training.¹⁶¹⁷

In addition, application of IE training into pulmonary rehabilitation has been shown to be an equally effective alternative to moderately intense CLE training in terms of improving exercise tolerance and quality of life.¹⁸ As there is evidence¹⁹²⁰ that patients with advanced COPD have a significant metabolic reserve capacity that is only evident when muscle activity is somewhat freed from ventilatory constraints, it was hypothesized that application of high-intensity IE training in the rehabilitation of these patients would induce significant peripheral muscle adaptations leading to improvements in exercise tolerance. Consequently, the purpose of the present study was primarily to investigate the response to high-intensity IE training by specifically looking at changes in morphologic and biochemical characteristics of the vastus lateralis muscle. In addition, we compared the magnitude of peripheral muscle adaptations induced by high-intensity IE training to that incurred after implementation of the commonly applied moderately intense CLE training modality.

	Materials and Methods

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Subjects
Patients included 16 men and 3 women with stable, advanced COPD who satisfied the following criteria: (1) postbronchodilator FEV₁ < 50% of predicted and FEV₁/FVC < 70% without significant reversibility (< 12% change of the initial FEV₁ value), (2) optimized medical therapy, and (3) no clinical evidence of exercise-limiting cardiovascular or neuromuscular diseases. Patients signed an informed consent form that was approved by the University Ethics Committee.

Study Design
The study was designed as a randomized, controlled, parallel, two-group study. Once it was verified that patients met the selection criteria, they were randomly assigned to one of the two training modalities: IE or CLE. Stratified randomization was used to achieve approximate balance of certain characteristics (Table 1 ), including FEV₁ ( 40 or > 40% of predicted) and peak work rate (Wpeak) [ 50 or > 50 W] that was assessed by a ramp-incremental cycle ergometer test.

Rehabilitation Program
The rehabilitation program was multidisciplinary and included supervised exercise training, breathing control and relaxation techniques, methods of clearance of pulmonary secretions, disease education, dietary advice, and psychological support on issues relating to chronic disability. Similarly to our previous rehabilitation study,¹⁸ the exercise prescription was designed to present patients with a similar overall training load. Patients assigned to the IE group were instructed to exercise on electromagnetically braked cycle ergometers (Cateye Ergociser, ECl600; Cat Eye; Osaka, Japan) at an intensity initially targeted to 100% of Wpeak, assessed prior to rehabilitation by a ramp-incremental cycle ergometer test (see below), with 30 s of work interspersed with 30-s rest periods for 45 min/d, 3 days per week, for 10 weeks. Patients assigned to the CLE group were instructed to exercise for the same weekly frequency and total duration as the IE group but at an intensity that was initially equivalent to 60% of baseline Wpeak for 30 min each time. Therefore, at the outset of the study, the total amount of work done per session by each member of the IE group was designed to equate the work that these patients would have done had they been assigned to the CLE group. As training principles require the training intensity to parallel the improvement in physical fitness,¹³ the weekly training load for the IE group was designed to represent 100% during weeks 1 to 3, 120% during weeks 4 to 6, and 140% of Wpeak during the last 4 weeks. For the CLE group, the load was designed to represent 60%, 70%, and 80% of Wpeak, respectively, for the three weekly periods. Supervision during training involved measurements of pulse oxygen saturation, heart rate, and sensation of dyspnea and leg discomfort. During training, supplemental oxygen was provided at a rate of 1.5 to 2.0 L/min in six patients in the IE group and four patients in the CLE group.

Muscle Biopsy
Within a week before and after the rehabilitation program, percutaneous biopsies of the right vastus lateralis muscle were performed at mid-thigh (15 cm above the patella) as described by Bengstrom.²² Briefly after local anesthesia with 1.5 mL of lidocaine 2%, a 1-cm skin incision was performed and muscle samples were obtained. Samples were placed in embedding compound and immediately frozen in isopentane precooled to its freezing point. All samples were kept at – 80°C until the day of analysis. The pretraining and posttraining muscle biopsy samples were obtained 10-cm apart from each other.

Skeletal Muscle Analysis
Fiber Typing, CSA, and Capillarization:
All muscle specimens were coded and analyzed without knowledge of the clinical data. Samples obtained before and after the training period were analyzed in pairs at the same time. Cryostat transverse sections of 10 µm in thickness were cut at – 20°C and were stained for myofibrillar adenosine triphosphatase after preincubation at pH values of 4.3, 4.6, and 10.3.²³²⁴ A mean of 331 ± 14 muscle fibers were analyzed from each slice and classified as type I, IIa, or IIb. The CSA of a minimum of 200 fibers was measured in each slice with an image analysis system (ImagePro; Media Cybernetics; Silver Spring, MD) at a known and calibrated magnification. A-amylase-periodic acid shift was used to visualize capillaries. The number of capillaries identified in a certain area was divided by the number of fibers found in the corresponding muscle section.²⁵

Enzyme Activity:
The activities of phosphofructokinase (PFK) [enzyme code 2.7.1.11] and citrate synthase (CS) [enzyme code 4.1.3.7] were studied using described spectrophotometric techniques.²⁶

Assessment of Exercise Capacity
Before and after the training program, an incremental protocol was performed on an electromagnetically braked cycle ergometer (Ergoline 800; SensorMedics; Yorba Linda, CA). After 3-min of baseline measurements, followed by 3-min of unloaded pedaling, the work rate was increased every min by 5 or 10 W to the limit of tolerance while patients maintained a pedaling frequency of 60 revolutions/min. Gas exchange and ventilatory variables were recorded breath-by-breath (max 229; SensorMedics). At baseline, during unloaded cycling and incremental exercise, patients performed inspiratory capacity (IC) maneuvers at 3-min intervals according to previously described methods.²⁷ The V-slope technique was used to detect the oxygen uptake (O₂) at which the lactate threshold (LT) occurred.²⁸ The modified Borg Scale²⁹ was used to rate the magnitude of dyspnea and leg discomfort.

Statistical Analysis
Data are presented as mean ± SEM. The within-group and between-group differences were analyzed using repeated-measures analysis of variance. Between-group comparisons of baseline characteristics were carried out by unpaired t test. The level of significance was set at p < 0.05.

	Results

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Patient Characteristics
As shown in Table 1, patients in both groups were characterized by severe airflow limitation, moderate hypoxemia without carbon dioxide retention at rest, and reduced values for DLCO and IC. At the outset of the study pulmonary function, exercise capacity and morphologic characteristics of the vastus lateralis muscle were not significantly different between the two groups, whereas exercise capacity was severely compromised in both groups (Tables 1 23 ).

View larger version (31K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Average values for training intensity sustained during the program sessions (top left, A), cardiac frequency (fc) [bottom left, B], dyspnea (top right, C), and leg discomfort (bottom right, D) for the IE (open circles) and CLE (closed circles) training groups. All parameters are expressed as fractions of the peak values achieved at Wpeak during the baseline incremental test. Data are shown as mean ± SEM.

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Fiber-type distribution (percentage) of the vastus lateralis muscle before (open squares) and after (gray squares) training for the IE group (top, A) and the CLE group (bottom, B). *p < 0.05, comparisons between before (open squares) and after training (gray squares). Data are presented as mean ± SEM.

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 3. Changes in the CSA of the different fiber types of the vastus lateralis muscle following IE training (top, A) and CLE training (bottom, B). *p < 0.05, comparisons between before (open squares) and after training (gray squares). Data are presented as mean ± SEM.

View larger version (13K): [in this window] [in a new window] [Download PPT slide]   Figure 4. Changes in the capillary-to-fiber ratio (top, A), PFK (center, B), and CS activity (bottom, C) of IE training and CLE training. *p < 0.05, comparisons between before (open squares) and after training (gray squares). Data are presented as mean ± SEM.

Exercise Capacity
After rehabilitation, there were significant improvements for both groups in Wpeak (IE, from 53 ± 9 to 63 ± 10 W [p = 0.04]; CLE, from 61 ± 8 to 70 ± 9 W [p = 0.001]) and in LT (IE, from 0.64 ± 0.05 to 0.75 ± 0.07 L/min [p = 0.01]; CLE, from 0.68 ± 0.03 to 0.80 ± 0.06 L/min [p = 0.03]), whereas there was a tendency of improvement in peak O₂ in both groups (IE by 9%; CLE by 5%). At an identical work rate during the incremental test, there was a significant reduction in minute ventilation (by 16%; from 37.9 ± 4.2 to 31.8 ± 2.9 L/min; [p = 0.009]) only in the IE group, whereas the reduction in IC changes from rest (from – 0.49 ± 0.09 to – 0.35 ± 0.13 L) was not significant (p = 0.16). Furthermore, at an identical work rate, there were also significant reductions in perception of dyspnea (from 4.1 ± 0.5 to 2.6 ± 0.3 [p = 0.04]) and leg discomfort (from 4.8 ± 0.7 to 2.8 ± 0.3 [p = 0.007]) only after IE training.

	Discussion

TOP Abstract Introduction Materials and Methods Results Discussion References

 
The impetus to study the efficacy of IE training in patients with advanced COPD derives from studies¹⁶¹⁷¹⁸ demonstrating that IE allows the application of intense loads on peripheral muscles without the attainment of the reduced ventilatory ceiling. As there is evidence¹⁹²⁰ that patients with advanced COPD have a significant metabolic reserve capacity that is only evident when muscular work is not influenced by central ventilatory limitations, it was hypothesized that implementation of IE would allow significant peripheral muscle adaptations to take place leading to physiologic training responses. The present study reveals that in patients with advanced COPD, IE training induces significant changes in the structure and function of the vastus lateralis muscle. These changes, although not significantly different from those induced by the implementation of moderately intense CLE, were achieved with fewer training symptoms compared to CLE training.

At baseline, structural and functional abnormalities within our patients’ vastus lateralis muscles (Table 2) are consistent with earlier reports⁴⁵²⁰ comparing patients with advanced COPD with age-matched healthy subjects. These abnormalities, including abnormal proportion of type I fibers, muscle capillarity, and CS activity, were accompanied by a reciprocal high frequency of type II fibers and PFK activity.⁴³⁰ Furthermore, the observed values for CSA of both type I and IIa fibers are in line with previously published reports,⁴⁸ demonstrating that the CSA of these fiber types in COPD patients are lower compared to those of age-matched healthy subjects.⁴⁵²⁰ Such changes within the muscles explain, at least in part, the substantially reduced capacity for aerobic work during the baseline incremental exercise test (Table 3). Besides these skeletal muscle abnormalities, dynamic hyperinflation and its detrimental consequences to perceived dyspnea also contributed to exercise intolerance.¹⁵

Application of rehabilitative IE training yielded morphologic and physiologic changes within the vastus lateralis muscle that increased its oxidative capacity. This can be conveyed by the significantly enhanced capillary-to-fiber ratio and the increased activity of CS after training. These improvements in oxidative capacity within the vastus lateralis muscles are likely to be associated with the significant changes that occurred in the LT after training. Previous studies¹⁰¹¹¹² have shown that the increase in the LT following exercise training in COPD patients is indicative of a smaller increase in blood lactic acid concentration for a given level of exercise, which, in part, reflects the improved capacity for oxidative metabolism within the trained muscles. Additionally, an important physiologic training effect that was likely mediated by the shift in the LT is that, at a given level of exercise during the incremental test after IE training, ventilatory requirement was significantly reduced.⁹¹⁴ This ventilatory adaptation is likely to have contributed to the lower degree of dynamic hyperinflation and perception of dyspnea. Furthermore, an improvement in muscle oxidative metabolism would justify the significantly reduced degree of muscle fatigability that we observed at an identical workload during the incremental test.

The magnitude of improvement in muscle capillarity, CS activity, and LT following implementation of IE is very similar to that induced by the moderately intense CLE regime, which is in accordance with other studies⁴¹⁰¹² employing CLE training in patients with severe COPD. In fact, the majority of changes that took place in the vastus lateralis muscles following IE training are consistent with those described in previous studies for CLE training. Accordingly, after IE training the magnitude of enlargement in the CSA of both type I and IIa fibers is similar to that observed by Whittom et al⁴ after implementation of CLE training in patients with severe airway obstruction and did not differ from that demonstrated in our CLE training group. In addition, our findings on the effects of IE are in line with the findings from studies^31323334 that employed high-intensity IE training in healthy subjects, demonstrating a significant increase in the CSA of both type I and IIa fibers and in the activity of CS.

However, the present study is the first in patients with COPD to demonstrate a significant improvement in the capillary-to-fiber ratio after both exercise training modalities. Furthermore, this is the first study to demonstrate a significant alteration in the fiber-type distribution within the vastus lateralis muscle after training in patients with COPD. Accordingly, there was a significant reduction in the proportion of type IIb fibers following both training regimes. These findings are in accordance with studies³¹³²³⁴ in healthy subjects demonstrating that it is possible to induce shifts in the distribution of the subgroups of the fast twitch fibers with training. The similarity of changes in peripheral muscle characteristics induced by the two different modalities extends our previous findings¹⁸ by demonstrating that in COPD patients IE is equally effective to CLE in inducing significant physiologic effects, providing that the imposed training load remains comparable between training regimes.

Although our patients in both groups exhibited significant improvements in the structure and function of their muscles, this did not directly translate into significant improvements in peak O₂, possibly because of the early attainment of their reduced ventilatory ceiling at high work rates.¹⁹ Therefore, constant-load submaximal exercise and isolated dynamic²⁰ or isometric⁸ knee-extensor exercise might constitute more appropriate testing tools in order to evaluate the improvement in muscle function irrespective of the decrement in lung function.

Classical studies³⁵³⁶ on muscle fiber metabolism utilizing the glycogen depletion technique have shown that with IE glycogen depletion is similar between type I and II fibers, hence suggesting that both fiber types are recruited to a similar degree during IE. These studies³⁶ have also shown that the capacity to reload the myoglobin stores and partially restore the phosphocreatine levels during the recovery phases³¹ allows a more oxidative degradation of glycogen; this has been proposed as the principal mechanism for the slowed glycolysis and the relatively low accumulation of lactate during IE. Consequently, IE appears to be a more suitable mode of exercise for the severe COPD patient, as it relies on the periodic recruitment of both fast and slow twitch fibers.³⁵ As lactic acidosis puts particular stress on the ventilatory system and is associated with the premature onset of muscle fatigue, the small increase in lactate typically observed during IE¹⁶¹⁷ appears to be beneficial to the COPD patient by allowing lower sensation of dyspnea and leg discomfort during training compared to CLE. Although IE consists of a sequence of on-and-off high-intensity muscular loads, its tolerability in the context of perceived respiratory and peripheral muscle discomfort has been proven to be better than that of CLE.¹⁷ Nevertheless, older patients with COPD have to initially familiarize themselves with the exercise and rest intervals in order to follow the right sequence of work and rest intervals for the required period. When the exercise apparatus provides a display of time and power output, patients have to initially dedicate few sessions to get accustomed with the training protocol.

In summary, we have shown that in patients with advanced COPD, implementation of brief, high-intensity bouts of exercise, alternated with equally brief periods of rest, yields significant adaptations in the vastus lateralis muscles that are similar to those obtained by CLE. However, IE training induces lower training symptoms than CLE training, and hence it may provide a good alternative strategy in patients with severe COPD.

	Acknowledgements

 
We thank Drs. E. Kosmas and E. Kastanakis for their contribution to the study. We are grateful to Professor P. Manta from the Department of Neurology, Eginiteion Hospital, for providing the necessary facilities to analyze the muscle specimens.

	Footnotes

 
Abbreviations: CLE = constant-load exercise; CS = citrate synthase; CSA = cross sectional area; DLCO = diffusion capacity of the lung for carbon monoxide; IC = inspiratory capacity; IE = interval exercise; LT = lactate threshold; PFK = phosphofructokinase; O₂ = oxygen uptake; Wpeak = peak work rate

This work was supported in part by the European Community CARED FP5 project (No. QLG5-CT-2002–0893) and by the Thorax Foundation.

Received for publication March 4, 2005. Accepted for publication September 26, 2005.

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