Activity limitation and breathlessness are the main clinical features of advanced chronic obstructive pulmonary disease (COPD). During the last few years it has become recognised that this activity limitation relates in some way to peripheral muscle dysfunction that can be partly reversed by the process of pulmonary rehabilitation.^1,2 There is still debate about the detailed nature of the peripheral muscle dysfunction but most investigators would agree that deconditioning through inactivity plays a major role. In health, the age related loss of muscle mass can be prevented by maintained activity or reversed by training. In COPD the contribution of other factors to a specific myopathy such as systemic inflammation, hypoxia, or steroid damage remains uncertain. Physical training is the obvious way of improving the function of deconditioned peripheral muscles, although other options such as electrical stimulation or pharmacological treatment can also have an effect. There is now clear evidence that pulmonary rehabilitation programmes that include individually prescribed but generic physical exercise training are capable of improving functional exercise capacity and health status.³ Rehabilitation programmes generally contain a number of therapeutic elements, but it is also clear that the overall benefits of the process do not occur in the absence of some form of physical training. In addition, it has also been a consistent finding that the resulting improvements in general exercise performance and health status appear to relate poorly to each other. In most cases the magnitude of the improvement in quality of life is greater than the more modest improvements in general exercise capacity.

There are several possible explanations for this apparent dissociation. Firstly, exercise training may be the vector for improvement in health status through intermediary mechanisms that involve less tangible factors such as confidence, self-efficacy, or motivation. Another explanation lies in our simplistic assumption that just walking a little bit further improves the quality of life. The frustrations of life for people with COPD are possibly more closely related to the interference by dyspnoea in the basic activities of daily living.⁴ Most rehabilitation programmes concentrate on continuous endurance exercise training that can improve walking or cycling ability but may not be expected to have a direct effect on domestic task performance that involves the upper body. Training for upper and lower limb strength may be expected to have a more direct effect on domestic task performance, although as yet there is no evidence to support this presumption. Even if the optimal mode of physical training is uncertain, there is still further inconsistency in the prescription of the dose of exercise required to have the intended effect. In this respect, two recent systematic reviews have helped to summarise our knowledge in this area. The first examines the overall merits of peripheral muscle strength training and the second, in this issue of Thorax, examines the comparative benefits of different training modalities and effect of intensity.^5,6

DOES PERIPHERAL MUSCLE STRENGTH TRAINING WORK IN COPD?

Most training programmes have previously employed endurance training that improves exercise performance but will not have any effect on muscle mass or strength. To improve the latter, specific resistance training may be required. The systematic review by O’Shea et al⁵ found nine methodologically acceptable clinical trials that examined the effect of upper or lower limb resistance training on a total of 236 patients. The strength training was performed with free weights or exercise equipment and individually prescribed as repetitions based on a variable fraction (50–85%) of the one repetition maximum (1RM). Some studies incremented the training load through the programme. The generally short term outcomes were recorded in terms of impairment (muscle strength, laboratory exercise capacity), activity (walking tests), or participation (quality of life questionnaires).

The pooled results of the training studies show that fairly rigorous strength training can be achieved in people with advanced COPD. They also demonstrated significant improvements in upper and lower body strength in the trained muscle groups. In spite of the findings of one study that demonstrated equivalent benefits of strength and endurance training on exercise capacity, there remains insubstantial evidence of the ability of strength training alone to regularly improve exercise capacity or health status.⁷ The longer term benefits of isolated strength training or sustainability of domestic maintenance training are also unclear. To date there have been no studies of the effect of pure resistance training on activities of daily living or domestic function.

WHAT TRAINING MODALITIES AND PRESCRIPTION INTENSITIES ARE BENEFICIAL?

If the wider advantages of strength training are not clear, then it would be useful to learn how it should be applied or whether it should be combined with endurance training to maximise the benefit. This is examined in the current issue of Thorax in a systematic review of the comparison of exercise training modalities and training intensities in patients with COPD by Puhan et al.⁶ This study identified 15 randomised controlled trials where a head to head comparison of exercise modality or training intensity had been made. In the studies which directly compared endurance with strength training, the results were surprisingly inconsistent. In some cases endurance training produced a greater improvement in physical performance than strength training and in others the reverse occurred. However, there does appear to be a stronger influence of strength training on quality of life as reflected in the Chronic Respiratory Questionnaire (CRQ). These finding are different from the systematic review by O’Shea et al and presumably reflect the selection of head to head study group comparisons rather than a usual care control group.

If both strength and endurance training have merits, then it would be sensible to combine these two modalities to enhance the benefit. In five trials where endurance training was compared with the combination of endurance and strength training, the results were slightly disappointing because the additional benefits of added strength in the combined training were limited to gains in strength only. There did not appear to be any carry over in terms of improved exercise capacity or health status.^8–10 Most endurance training regimes are continuous in format and may not be sustainable by patients with severe impairment. Training efficiency can be improved by breaking the exercise sessions up into intervals.^11,12 In such patients the review suggests that this approach may enhance the benefit of training. Comparative trials of training intensity have been uncommon but suggest that greater physiological benefits can be achieved with high intensity (80% peak oxygen consumption) than with low intensity exercise training.¹³ However, some improvements in health status are still seen with lower intensity training.¹⁴

HOW GOOD IS THE EVIDENCE?

These systematic reviews have helped us to order our knowledge of peripheral muscle training in COPD. However, the pitfalls of systematic reviews and meta-analyses are exposed by comparison. The incomplete agreement is a reflection of the unmatched selection policies of the reviews. Nevertheless, both reviews agree that patients can adhere to physical training programmes and gain benefit from them. Beyond that, the reviews have exposed significant deficiencies in existing evidence and major gaps in our understanding of the field. Most of the included studies contain small numbers of subjects and have methodological flaws, particularly with regard to blinding of interventions and outcome assessment. It is easy to be critical of these shortcomings but any investigator who has undertaken a training study will appreciate the very difficult challenges that these clinical trials present. Patients who are recruited for rehabilitation trials are often vulnerable and expected to make an active commitment to the training programme over a prolonged period without dropping out. It is virtually impossible to blind the training intervention but outcome assessments can and should be made by blinded assessors.

In the athletic domain, physical training is recognised as a highly complex intervention. Although we have seen the benefits of simple training regimes in pulmonary rehabilitation, we have really only begun to explore the complexity of training science in this area. The benefits of physical training accrue with the combined product of training intensity, session frequency, and duration. Most training also results in a training-specific response rather than more general benefit. The interplay of these factors in determining the total training load is largely unknown. In addition, most of the trials have only examined the short term effects of training and have not observed the longer term results or examined the effects or feasibility of maintenance therapy.

We know that physical training can improve general functional exercise performance in COPD but the improvements are relatively modest and non-specific. This offers proof of concept but does not allow us to examine the subtleties of different training modes. These need to be explored in association with the appropriate outcome assessments. Combinations of outcomes that include activities of daily living and reflections of domestic function and participation are more germane to the aims of rehabilitation.