Practical recommendations for exercise training in patients with COPD

Assessment tests

Exercise tolerance can be assessed by a cardiopulmonary exercise test using either cycle ergometry or a treadmill, measuring a number of physiological variables, including peak oxygen uptake (V′_O2), peak heart rate and peak work performance. A less complex approach is to use a self-paced, timed walking test (e.g. 6-min walk test (6MWT)). This test requires at least one practice test before data can be interpreted. Shuttle walking tests are also a useful option. They provide more in-depth comprehensive information than an entirely self-paced test, but are easier to perform than a cardiopulmonary exercise test [6]. Additional assessment of muscle strength of the lower and upper extremities also adds important information and provides a comprehensive insight into patients' limitations derived from extrapulmonary manifestations of COPD.

Cardiopulmonary exercise testing

Endurance training

Endurance training is probably the most common exercise modality in patients with COPD. The main objective of endurance training is to improve aerobic exercise capacity as aerobic activities are part of many everyday tasks in these patients. It has been shown that endurance training also improves peripheral muscle function in patients with COPD [21]. In addition, there is some evidence that high-intensity endurance training induces greater physiological benefits than lower-intensity exercise [22]. However, most patients with severe COPD are not able to sustain high-intensity exercise due to serious symptoms, such as dyspnoea and fatigue [23]. Therefore, alternative exercise protocols, such as interval training, have gained increasing interest especially in patients with advanced COPD.

Continuous versus interval training

Historically, the rationale for interval training included the ability to impose high-power bursts of exercise on peripheral muscles without overloading the cardio-respiratory system. As outlined previously, people with COPD respond to training in a different way to healthy subjects as they are restricted by the complex interaction of different determinants of exercise limitation.

A recent systematic review included eight randomised controlled trials with a total of 388 COPD patients and compared the effects of continuous and interval training in a meta-analysis (mean forced expiratory volume in 1 s 33–55% predicted) [24]. The authors summarised that both exercise modalities led to comparable improvements in exercise capacity and health-related quality of life. Continuous and interval endurance training also significantly improved the capillary-to-fibre ratio as well as the fibre-type distribution within the vastus lateralis muscle in similar amounts. Accordingly, there is a significant reduction in the proportion of anaerobic fast-twitch (type IIb) muscle fibres following both training regimes yielding a higher percentage of aerobic slow-twitch (type I) muscle fibres [21]. The benefits in terms of improving exercise tolerance, quality of life and muscle fibre morphology and typology were comparable across patients with COPD in Global Initiative of Chronic Obstructive Lung Disease (GOLD) stages II, III and IV [25].

Nevertheless, in patients with very severe COPD there is evidence that interval training is associated with fewer symptoms of dyspnoea during exercise and fewer unintended breaks [26, 27]. Therefore patients with severe COPD may markedly increase the total exercise duration with significantly lower metabolic and ventilatory stress, as well as lower rates of dynamic hyperinflation when performing interval training compared to continuous training [28].

Although interval training 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 shown to be better than that of constant load exercise [29].

This may indicate a better feasibility of interval training protocols, especially in patients with severe airflow obstruction. In general, many patients are frustrated by the burden of physical limitation in daily life. To avoid frustration during exercise training it may be important to offer exercise protocols that are feasible to each specific patient. It is speculated that this could reveal a psychological advantage to improve the patients' motivation and maybe also increase long-term adherence to exercise training programmes. Nevertheless, especially older, patients with COPD initially have to familiarise themselves with this exercise mode and resting intervals in order to follow the right sequence of work and rest intervals for the required period.

An easy approach to target training intensity for continuous and interval endurance training on a bicycle would be to derive exercise intensity from a certain percentage of the peak work load (e.g. 70%). To further adjust cycling load to an effective, as well as feasible, intensity the patients' perceived exertion on the modified Borg scale (0−10) should be aimed at 4 to 6. table 2 shows some practical recommendations for the implementation of continuous and interval endurance training programmes.

So how to find the right endurance training protocol for your patient? table 3 shows some non-evidence-based indications of when the use of an interval training protocol may be more appropriate. If a patient is in a borderline status at some of these points it is recommended to let the patient decide which exercise protocol they would prefer. The patient could try both modalities on the first days of a pulmonary rehabilitation programme and share their own opinion. Integrating the patient in the planning of their exercise programme may also improve their willingness to adhere to the intervention.

Cycle-based versus walking-based endurance training

Walking is one of the most important activities of daily living in patients with COPD. However, most endurance training programmes are based only on cycle endurance training. In addition to the higher costs and space requirement involving a treadmill in comparison to a cycle ergometer, another possible explanation for this fact could be that patients with COPD exhibit a greater ventilatory response during walking compared to cycling [31]. Thus, minimising dyspnoea sensations and the potential of oxygen desaturation during high intensity exercise are arguments in favour of providing cycling-based endurance training. However, walking-based endurance training programmes are also very effective in improving exercise capacity and quality of life in people with COPD [32, 33]. Compared to equipment-dependent training, such as cycle training, non-treadmill walking is an easily available training modality, particularly for those living in places with limited resources. Furthermore, exercising the patients' walking skills might be more effective to the patient than exercising cycling skills that are unlikely to be essential to everyday life. A recent study has even shown that supervised, progressive walking training resulted in a significantly larger increase in endurance walking capacity compared to supervised, progressive stationary cycle training [34]. Similar effects were found on peak walking and cycling capacity, endurance cycling capacity and health-related quality of life.

Since walking endurance capacity in patients with COPD is especially impaired, this could be the rationale for the implementation of walking-based endurance training to improve the patients walking capabilities.

Up-to-date detailed recommendations for prescribing walking training can rarely be found in the literature. A common reference point that was used in several studies [34, 35] was to set the walking speed at ∼80% of peak V′_O2, which was achieved in a shuttle walking test. To avoid the complex and time consuming procedure of a ergospirometry during the shuttle walk test, peak V′_O2 can be approximately derived by the following equation: peak V′_O2 mL·min⁻¹·kg⁻¹=4.19+0.025×ISWT distance [36]. It has been shown that this intensity is feasible in most patients and is effective in improving exercise capacity [32]. If patients are not able to walk continuously for at least 10 min at the given speed, the intensity could be decreased stepwise by ∼10% until the patient is able to walk without taking a rest.

It is easy to set and control the proper walking speed when patients are exercising on a treadmill. If patients walk on the ground it is much more difficult to accurately stick to the individualised pace. Other methods such as using a metronome or listening to music, the rhythm of which is adjusted to the individual walking speed, might be useful [35, 37]. Also a given track with distance marks could be helpful. If no supportive devices are available to determine walking speed, a perceived exertion on the Borg scale from 4 from 6 could be targeted. Furthermore, an interval approach could also be applied. To date, walking interval training has not been investigated. Due to practical reasons, longer interval walking periods of 1–2 min duration might be more appropriate.

Oxygen supplementation during exercise

The benefits of long-term oxygen therapy (LTOT) in patients with COPD associated with hypoxaemia are well known. In these patients, LTOT prolongs survival and reduces hospitalisations, as well as the risk of comorbidities [38, 39]. More recently, the usefulness of oxygen therapy in improving outcomes from pulmonary rehabilitation in patients with COPD has been evaluated in several studies. In general, a distinction must be made between immediate effects of oxygen on exercise performance and its usefulness in the exercise-training component of pulmonary rehabilitation. As an adjunct to exercise training, supplemental oxygen therapy has been studied in patients who are severely hypoxaemic at rest or with exercise. The rationale for these studies is that supplemental oxygen therapy improves peripheral muscle oxygenation [40], dyspnoea [41] and exercise capacity [42] in patients with COPD and hypoxaemia, possibly allowing them to train at higher intensities. While the use of oxygen improves maximal exercise performance acutely in the laboratory, studies testing its effect in enhancing the exercise-training effects have produced inconsistent results. This may reflect differences in methodology among the studies, especially with regard to training workload [43]. However, the use of continuous supplemental oxygen for patients with COPD and severe resting hypoxaemia is clearly indicated and recommended as a part of routine clinical practice. Oxygen saturation measured by pulse oximetry >90% and/or an arterial oxygen pressure >55 mmHg should be targeted [44].

The use of supplemental oxygen as an adjunct to exercise training could also be useful in patients who do not meet inclusion criteria for LTOT and do not experience exercise-induced hypoxaemia. In a double-blinded randomised trial, patients without significant exercise-induced oxygen desaturation were randomised to receive either room air or oxygen during high-intensity exercise training [45]. Exercise performance improved significantly more in the group receiving oxygen. This improvement was accompanied by a reduction in respiratory rate.

The long-term effects when supplemental oxygen is discontinued and the effect on other outcomes such as health-related quality of life remain to be determined.

Strength training

Peripheral muscle dysfunction and muscle weakness are highly prevalent comorbidities of COPD, contributing to exercise intolerance and symptom intensity [46, 47]. It is assumed that resistance training can reverse peripheral muscle dysfunction and thereby reduce, at least in part, the burden of COPD impairment [48]. Resistance training as an adjunct to endurance is recommended in all patients especially those with peripheral muscle weakness. Because strength training has a greater potential to improve muscle mass and strength than endurance training [49, 50], a combination of these two exercise modalities is highly recommended. Strength training also provokes less dyspnoea during exercise, which most probably makes it easier to tolerate than aerobic training [51]. Therefore, a combination of resistance training with interval endurance training can be a useful alternative training strategy in patients severely restricted in their ability to perform endurance training due to marked ventilatory limitation.

A systematic review that included 18 randomised controlled trials showed consistent improvements in muscle strength despite a large variation of exercise characteristics, such as number of repetitions, exercise intensity or the method of strength training itself (e.g. conventional strength training machines, pulleys, free weights, Thera-Band; Hygenic Corporation, Akron, OH, USA) [52]. Therefore, the ideal exercise modalities for strength training in patients with COPD remain unknown. It appears that there is more than one mechanism to improve strength skills. Another important and clinically relevant finding of this article was that the effects of strength training may also be translated into meaningful changes in functional performance such as climbing stairs, standing up or arm elevation activities [52].

The ATS/ERS statement on pulmonary rehabilitation recommends performing two to four sets of six to 12 repetitions at intensities ranging from 50% to 85% of the one repetition maximum (1RM) 2−3 days per week (table 4) [53]. However, the principle of deriving strength training intensities from the 1RM must be critically considered. Due to a large inter-individual discrepancy and a wide range of variability of the number of possible repetitions at a certain percentage of the 1RM, the ideal resistance for many individuals may be over- or underestimated [54, 55]. Therefore, the main focus could rather be on targeting local muscular exhaustion within the range of six to 12 repetitions (in other words determine a six to 12 repetition maximum) rather than reaching a certain percentage of the 1RM. Of course the patient must be instructed on this obligatory goal of achieving muscular exhaustion, otherwise they may not be willing to expose themselves to this exertion. An easy and very practical approach to determine optimal resistance for strength training is that the physiotherapist sets a training load so the patient is able to repeat an exercise six to maximally 12 times and has to stop due to muscle fatigue. With a little experience from the physiotherapist this is a very quick and useful approach that does not necessarily require determining the 1RM. However, it is recommended to use both upper and lower limb resistance weight training conducted with moderate speed at 1–2 s for both concentric and eccentric. When the subject can perform the current workload for one or two repetitions over the desired number of six to 12 repetitions on two consecutive training sessions it is recommended to apply a 2–10% increase in load [56].

Inspiratory muscle training

As a consequence of COPD, strength and endurance [57] of the diaphragm can also be reduced and contribute to hypercapnia, dyspnoea and reduced walking capacity [58]. Inspiratory muscle training may enhance the dysfunction of the diaphragm and improve some of its consequent burden. Various meta-analyses [59, 60] have shown that inspiratory muscle training can improve inspiratory muscle strength and endurance, as well as reduce dyspnoea. Scientific data on its benefits on functional exercise capacity and quality of life are less consistent, but evidence is emerging [60]. In patients suffering from inspiratory muscle weakness, defined as maximal inspiratory pressure (P_Imax) <60 cmH₂O, the addition of inspiratory muscle training to a general exercise training programme improved P_Imax and tended to improve exercise performance [61]. More studies that investigate inspiratory muscle training, especially in patients with inspiratory muscle weakness, are needed.

By far, the most commonly used inspiratory muscle training technique is the one of “threshold loading” devices (table 5). These devices generally have a spring-loaded valve requiring the patient to inhale strongly enough to open the valve and to breathe in against an individualised load. The optimal training intensity is still unknown but an initial resistance of ≥30% of P_Imax is recommended [60]. Resistance should then be increased stepwise, as tolerated. At present the optimal exercise duration is uncertain, most studies provided a total exercise time of 15−20 min per day [63]. It might be helpful to split inspiratory muscle training into two to three short exercise sessions per day.

Neuromuscular electrical stimulation

During neuromuscular electrical stimulation (NMES) the muscles get stimulated electronically via adhesive electrodes placed on the skin. As the metabolic response during a NMES session is significantly lower compared to a resistance exercise training session in patients with COPD [64], this technique may be particularly relevant to severely deconditioned or bed-bound patients [65]. The most consistent finding of NMES training in COPD is a 20−30% gain in quadriceps strength as compared with control subjects [66, 67]. Most exercise protocols aim for the muscles of the thigh and the calf muscle. In NMES studies the duty cycle ranges between 2 to 10 s on and 4 to 50 s off for 20−60 min and one to two sessions per day 3−7 days per week (table 6) [68]. To set the initial stimulus the intensity will be increased until a visible strong muscle contraction occurs or to the maximum level of the patients' toleration. A novel finding is that it seems that there is a certain threshold for responders and non-responders to NMES. The gains in functional exercise capacity were heavily dependent on the ability of the patient to increase and sustain a progressively higher current intensity. The relationship between the change in walking test distance was characterised by a threshold of 10 mA increase from the beginning until the end of a 6-week NMES intervention below which changes in exercise capacity with NMES were practically absent [69].

Whole body vibration training

During whole body vibration training subjects exercise on a vibrating platform that produces sinusoidal oscillations. At high intensities this vibration evokes muscle contractions on the entire flexor and extensor chain of muscles in the legs and all the way up to the trunk. Instead of voluntary muscle control like in common resistance training, the muscle contractions during high-intensity vibration training are caused by stretch reflexes [70]. The user has no direct influence on muscle activity itself and can only control body posture movement.

To date there is some evidence that whole body vibration training yields improvements of similar magnitude in regards of exercise capacity, muscle force and quality of life in comparison to conventional strength training [71]. Another study concluded that in patients with advanced COPD, whole body vibration training seems to be an effective and feasible exercise modality that may even enhance functional exercise capacity significantly more when performed in addition to endurance and strength training [72]. The underlying mechanisms are not yet investigated, but it is speculated that improvements in neuromuscular activation may play an important role. table 7 provides some practical guidelines for the use of whole body vibration training.

Breathing retraining (or breathing exercises)

Breathing retraining is a simple approach aiming to alter respiratory muscle recruitment in order to reduce dyspnoea and improve respiratory muscle performance. Breathing retraining techniques described over time have mainly included pursed-lip breathing, diaphragmatic breathing and expiratory muscle strengthening. The improvements with breathing retraining discussed in the literature have included improvements in dyspnoea, work on breathing, ventilation, lung volume, functional performance and activities in daily living. However, many argue the improvements may not be due to breathing retraining alone but rather due to the adjunctive therapies such as medications, use of oxygen, and exercise training itself [73]. However, in a meta-analysis by Holland et al. [74] it was concluded that breathing exercises improve functional exercise capacity in patients with COPD compared to no intervention despite the fact that there are no consistent effects on dyspnoea or health-related quality of life. Breathing exercises may be useful to improve exercise tolerance in selected individuals with COPD who are unable to undertake exercise training [74].

Assessing the effectiveness of the exercise training programme

Any pulmonary rehabilitation programme should include given outcome assessments, required for the objective evaluation of programme effectiveness, and of patient progress during the testing period. As currently practiced, pulmonary rehabilitation typically includes several different components, including not only exercise training but also education, nutritional and psychosocial support, and instruction in various respiratory and chest physiotherapy techniques. As an example, different outcomes to be assessed concerning their effectiveness after pulmonary rehabilitation include (but are not restricted to): overall exercise capacity (exercise tests), upper and lower limbs strength, symptoms (breathlessness and fatigue), health-related quality of life (specific questionnaires), and the improvement in physical activity performed in daily life not only in exercise tests.