This issue of the Journal of Cardiopulmonary Rehabilitation features a manuscript that seeks to contribute to the debate on how to best conduct exercise programs that are part of pulmonary rehabilitation.1 Exercise training comes in 2 basic varieties, and just how much of each to include in pulmonary rehabilitation programs is starting to be examined. Endurance training seeks to improve the ability to engage in sustained activities: walking, cycling, and stair climbing, for example. Strength training (also known as resistance training) focuses on explosive activities: rising from a chair, lifting weights, and maintaining balance when standing, for example. Both types of activities are encountered frequently in everyday life. Importantly, as regards to their role in pulmonary rehabilitation, our knowledge base concerning endurance training is much greater than that for strength training.

Endurance exercise training is the central focus of pulmonary rehabilitation and has a better evidence base than any other component. But this was not always so. As recently as the late 1980s, it was posited that exercise programs could have only a psychological benefit. It was argued that exercise limitation was related to pulmonary factors, and it was clear that pulmonary rehabilitation did not improve pulmonary function. As a result, it was felt that exercise programs could have only a psychologic benefit, the essence of which was to lessen the fear of the dyspnea that exercise inevitably provokes.

Through the 1990s and into the new century, a series of scientific investigations have changed our perceptions. The first advancement was the appreciation that endurance exercise programs had to adhere to certain design principles, including adequate frequency, duration, and intensity, to yield physiologic benefits. Next, dysfunction of the muscles of ambulation was described by objective techniques, including muscle biopsy. High-intensity exercise programs were shown to be effective in improving exercise tolerance. Of paramount importance, objective measures of improved muscle function were used. This was a crucial step because effort-dependent measures of exercise tolerance are subject to improvement with increased motivation (ie, as a psychologic benefit of the exercise program). Mechanisms by which improved muscle function translated into improved exercise tolerance were conclusively demonstrated. Today, it is acknowledged that endurance exercise training administered in a program of pulmonary rehabilitation decreases dyspnea on exertion, increases exercise tolerance, and improves health-related quality of life better than any other available intervention.2

In contrast, strength training programs have only relatively recently been studied in clinical trials (with only one appearing before 1999). These studies can be separated into those that allow comparison between a control group (either no exercise or endurance exercise) and a strength-trained group and those that allow comparison between an endurance-trained group and a group receiving a combined endurance and strength training intervention. The latter comparison is especially relevant to rehabilitative practice, where the question is whether the addition of a strength training component to an endurance training program yields additional benefits.

These studies universally show that strength training programs increase measures of muscle strength in patients with chronic obstructive pulmonary disease, whether administered alone or as an additive component to endurance training. The training apparatus, exercise selection, and intensity progression have varied among studies (see Storer3 for a review of suitable strength training strategies). However, the measures of muscle strength improvement have all been effort, motivation, and practice dependent. Do we have any assurance that physiological improvement in strength capabilities has been imparted? The answer comes from 4 studies that included measures of muscle mass of the trained limbs.4-7 All 4 demonstrated increased muscle mass, which can be considered an objective measure of physiologic benefit.

Among the randomized trials investigating the benefits of strength training, program length ranged from 8 to 12 weeks; sessions were held 2 or 3 times per week, and session length was generally from 40 to 90 minutes. These program characteristics seem similar to those known to be effective in healthy subjects. Contrast these characteristics with those used in the study of Phillips et al1 in this issue. The strength training intervention consisted of 10 minutes per session, twice weekly for 8 weeks. The 10 minutes of strength training included 5 exercises spread over several muscle groups. Thus, each muscle group received only a few minutes of training per session. This seems to fall well short of the duration requirements for physiological improvement in muscle strength capability. It seems reasonable to conclude that the improvement in the effort-dependent tasks that were used as outcomes results from improved effort, motivation, and practice and not from physiologic improvements in muscle function.

However, the study of Phillips et al1 does pose a unique question for consideration. In previous studies in which strength training was combined with a program of endurance training, the strength component was added to the endurance component. In the study of Phillips et al, the strength component replaced a portion of the endurance component. This may well more closely represent the real-life situation, in which the practitioner seeks to package the most effective exercise program in a fixed period of time. Spreading the training stimulus over a number of muscle groups and using 2 different training paradigms have the potential to result in a program that yields little if any physiologic improvements.

This becomes all the more apparent when it is considered that most of the laboratory-based studies that were used to develop our understanding of the potential benefits of rehabilitative training in chronic obstructive pulmonary disease (both endurance and strength) focused on a single group of muscles. They also generally used a testing apparatus similar to that used for training so that identical muscle groups were involved. These features were appropriate when being used to demonstrate physiological principles but may be suboptimal for giving guidance on how best to compose a practical program of pulmonary rehabilitation.

What is the optimal composition of a training program where the daily duration of training is limited? Clearly, this must take into account what is important for the patient. There has been the implicit assumption that endurance-related tasks have a higher priority than do strength-related tasks. This has not been evaluated formally, although there seems to be a tendency for quality-of-life measures to be more responsive to endurance than to strength programs. It must also be considered whether upper-extremity exercise has the same priority as lower-extremity exercise. Evidence-based analysis has tended to find more conclusive evidence for the benefits of lower extremity exercise,8 but the individual patient's goals must be taken into account.

Clinical trials examining and seeking to determine the optimal composition of rehabilitative training programs will be welcomed. However, ultimately, it will be in the hands of rehabilitation practitioners to determine the composition of the program that will best benefit the individual patient.

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