Effects of walking and strength training on walking capacity in individuals with claudication : meta-analysis

Context: Over the past few years, several clinical trials have been performed to analyze the effects of exercise training on walking ability in patients with intermittent claudication (IC). However, it remains unclear which type of physical exercise provides the maximum benefits in terms of walking ability. Objective: To analyze, by means of a meta-analysis, the effects of walking and strength training on the walking capacity in patients with IC. Methods: Papers analyzing the effects of walking and strength training programs in patients with IC were browsed on the Medline, Lilacs, and Cochrane databases. Randomized clinical trials scoring >4 on the Physiotherapy Evidence Database (PEDro) scale and assessing claudication distance (CD) and total walking distance (TWD) were included in the review. Results: Walking and strength training yielded increases in CD and TWD (P < 0.05). However, walking training yielded greater increases than strength training (P = 0.02). Conclusion: Walking and strength training improve walking capacity in patients with IC. However, greater improvements in TWD are obtained with walking training.

inclusion criteria.Figure 1 shows the flowchart of study selection in this meta-analysis.
First, two authors read the study titles to check whether they met the purposes of this meta-analysis.When no decision was reached after reading the title, the abstract and later, if necessary, the whole study was read.This meta-analysis included studies that: (i) were randomized clinical trials; (ii) included a sample of individuals with PAD and symptoms of IC; (iii) analyzed the effects of supervised physical training (walking or strength); (iv) measured claudication distance (CD) or total walking distance (TWD) before and after the intervention; (v) included more than one experimental group; and (vi) had a score equal to or greater than 4 on the Physiotherapy Evidence Database (PEDro) scale, used to measure the quality of methods in clinical studies.

Data extraction
The following data were extracted from the studies that met inclusion criteria:

Data analysis
Mean and standard deviation values were calculated according to mean values in the studies to describe the characteristics of individuals included in the study.For inferential analysis, mean difference and 95% confidence intervals were calculated; the fixed effects model was used when results were homogeneous (P ≤ 0.10); and a random effects model was used when results were heterogeneous (P < 0.10).The Review Manager 5.1 software was used for all analyses.

INTRODUCTION
Peripheral artery disease (PAD), one of the main atherosclerotic diseases, is associated with high morbidity rates among the elderly 1 .The main symptom of PAD is intermittent claudication (IC), characterized by pain in the lower limbs, particularly the calf, when walking 2 .The prevalence of PAD is 3% to 10% in the general population and about 20% in the population older than 70 years 3,4 .
IC is the cause of limitations to walking, which may compromise the performance of physical activities of daily living 5 .In addition, individuals with IC have muscle atrophy and decreased muscle strength 6 , power and resistance in the lower limbs 7 .Supervised physical training combined with changes in life style has proven to be important interventions for the treatment of individuals with IC 8 , and significant increases in their walking ability and muscle and skeletal aptitudes have been found 9,10 .
Currently, there is evidence that supports the use of walking training in patients with PAD 11 .In fact, improvements in fitness and quality of life have been found, in addition to the fact that training is low cost and easy to perform [12][13][14] .Recent studies showed that strength training also improves fitness and quality of life of patients with PAD 10,15-18 .However, it is still unclear which of the two modalities of physical training results in greater increases in walking capacity.
This study conducted a meta-analysis to compare the effects of walking and strength training on the walking capacity of individuals with IC.

Literature review
The MedLine, Lilacs and Cochrane databases were reviewed.First, studies were selected according to their publication date, which was limited to July 1980 to December 2010.
For the search, keywords in Portuguese and their corresponding keywords in English were selected using the DECS and the MeSH databases.The keywords selected were: exercício físico/ physical exercise, aptidão física/fitness, caminhada/ walking, treinamento de força/strength training and claudicação intermitente/intermittent claudication.For the selection of studies, combinations of keywords were used for the search.As a result, 1947 studies were retrieved, but only eight [15][16][17][19][20][21][22][23] met

Study characteristics
Of a total of 424 individuals included, 238 underwent physical training (Table 2).Most participants were men (65%) and elderly (67 ± 4 years).The duration of PAD described in four studies 15,20,22,23 was 3.4 ± 0.8 years.All individuals included in the study had mild to moderate IC, and mean ankle-brachial index (ABI) was 0.64 ± 0.06.

Walking ability before intervention
CD was reported in eight studies [15][16][17][19][20][21][22][23] . Mean C before intervention was 203 ± 126m and 197 ± 124m in the studies that used walking and strength training.In all studies, CD was similar between experimental and control groups before intervention.
Mean TWD before intervention was 365 ± 182 m and 329 ± 171m in the studies that used walking and strength training.In all studies, TWD was similar between experimental and control groups before intervention.

DISCUSSION
This study compared the effects of walking training and strength training on the walking capacity of individuals with IC using data in the literature.For that purpose, a meta-analysis was conducted.Most studies included in this meta-analysis used walking training [15][16][17][19][20][21][22] . This my be partly explained by the fact that several Vascular Surgery Associations 8,11 , in their official guidelines, recommend walking as the most important exercise for patients with PAD.Recent recommendations have included strength training as part of training for individuals with PAD, although only a few studies using strength training have been published.In fact, our analysis included only four studies that evaluated the effects of strength training on the walking ability of patients with PAD [15][16][17]23 .Furthermore, one of these studies had a weight greater than 70% in the metaanalysis because of the high number of individual included in its sample. Therfore, further studies about this topic should be conducted.
The effects of walking and strength training on CD were similar, but TWD increased more after from 11 to 14 16,17 , and perception of claudication pain, with scores ranging from 3 to 4 19 .Peak oxygen consumption (peak VO 2 ) was used in one study, at an intensity of 80% of peak VO 2
Strength training was prescribed according to perception of exertion, with Borg scores ranging from 11 to 13 16,17 , and tests of 6 15 and 15 23 maximum repetitions.

Effects of training on walking ability
The comparison of the effects of walking training and control intervention on CD (Table 3    walking training.This can be explained by the fact that the mechanisms of increase in walking ability differ between walking and strength training.The increases in walking ability after walking training have been assigned to: angiogenesis 24 ; improvement of endothelial function; increase of oxidative enzyme concentrations 13 ; and improvement of walking efficiency.In contrast, the increases obtained with strength training have been basically assigned to angiogenesis and improvements on walking efficiency.Therefore, the effects of walking training on oxidative metabolism seem to explain the differences between the effects of walking and strength training on the walking ability of patients with PAD.One of the limitations of this study was the inclusion of studies only in Portuguese or English.Another important aspect was the fact that, although only studies that measured walking ability using treadmills were included, there was some variation in the protocols used.Therefore, results between studies should be compared cautiously.However, individual studies used the same protocol to measure walking ability, and we were, therefore, able to assess the effects of training between groups.

CONCLUSION
Walking and strength training improve the walking capacity of patients with PAD, but walking training results in greater increases of TWD.
(a) publication year; (b) groups; (c) number of individuals in each group; (d) type of physical exercises; (e) duration of intervention; (f) frequency (times a week); (g) volume of training session; (h) method used to measure intensity; (i) intensity prescribed; (j) initial CD or TWD; (k) final CD or TWD.

Figure 1 .
Figure 1.Flowchart of inclusion of studies in the meta-analysis CD -claudication distance; TWD -total walking distance.
Results showed that: (i) walking and strength training increased the walking capacity of patients with IC; (ii) the effects of strength and walking training on CD are similar; (iii) walking training resulted in greater TWD increases than strength training.

Table 1 .
Quality of studies included in meta-analysis.

Table 2 .
Characteristics of the studies that met inclusion criteria.

Table 3 .
Effects of walking and strength training on claudication distance.