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Effect of Resistance Training on Physical Disability in Chronic Heart Failure


Medicine & Science in Sports & Exercise: August 2011 - Volume 43 - Issue 8 - p 1379-1386
doi: 10.1249/MSS.0b013e31820eeea1
Clinical Sciences

Purpose: Patients with chronic heart failure (CHF) report difficulty performing activities of daily living. To our knowledge, however, no study has directly measured performance in activities of daily living in these patients to systematically assess their level of physical disability. Moreover, the contribution of skeletal muscle weakness to physical disability in CHF remains unclear. Thus, we measured performance in activities of daily living in CHF patients and controls, its relationship to aerobic capacity and muscle strength, and the effect of resistance exercise training to improve muscle strength and physical disability.

Methods: Patients and controls were assessed for performance in activities of daily living, self-reported physical function, peak aerobic capacity, body composition, and muscle strength before and after an 18-wk resistance training program. To remove the confounding effects of several disease-related factors (muscle disuse, hospitalization, acute illness), we recruited controls with similar activity levels as CHF patients and tested patients >6 months after any disease exacerbation/hospitalization.

Results: Performance in activities of daily living was 30% lower (P < 0.05) in CHF patients versus controls and was related to both reduced aerobic capacity (P < 0.001) and muscle strength (P < 0.01). Moreover, resistance training improved (P < 0.05 to P < 0.001) physical function and muscle strength in patients and controls similarly, without altering aerobic capacity.

Conclusions: CHF patients are characterized by marked physical disability compared with age- and physical activity-matched controls, which is related to reduced aerobic capacity and muscle strength. CHF patients respond to resistance training with normal strength/functional adaptations. Our results support muscle weakness as a determinant of physical disability in CHF and show that interventions that increase muscle strength (resistance training) reduce physical disability.

1Department of Medicine, College of Medicine, University of Vermont, Burlington, VT; and 2Department of Molecular Physiology and Biophysics, College of Medicine, University of Vermont, Burlington, VT

Address for correspondence: Michael J. Toth, Ph.D., Health Science Research Facility 126B, 149 Beaumont Ave., University of Vermont, Burlington, VT 05405; E-mail:

Submitted for publication December 2010.

Accepted for publication January 2011.

Patients with chronic heart failure (CHF) have high rates of physical disability on the basis of self-reported difficulty in performing of activities of daily living (29). An impaired ability to perform simple everyday tasks reduces patients' quality of life, increases health care costs by increasing the need for supportive services, and is an independent predictor of mortality (10,11). Knowledge of the factors that determine physical function in CHF patients, therefore, has implications for improving quality of life and prognosis.

The ability to perform daily activities involves a complex interaction of physiological and psychological factors (6). In CHF patients, to our knowledge, no study has directly assessed disability by measuring performance in a range of common activities of daily living that encompass the physical skills required to function independently. The one exception is that many studies have evaluated 6-min walk distance (28), a surrogate of aerobic fitness (13), as an index of functional capacity in daily activities. Arguably, this historical focus on indices of cardiorespiratory fitness is reasonable considering that the hallmark symptoms of CHF-dyspnea and fatigue-are emblematic of a reduction in aerobic capacity. Walking endurance, however, is only one aspect of physical function in daily activities. Moreover, performance in many daily activities has been shown to be poorly correlated to aerobic fitness in some cardiac disease populations (24), arguing that factors other than aerobic fitness likely contribute to physical disability. Compared with the number of studies that have measured indices of aerobic fitness, the relative paucity of studies examining muscle strength is surprising considering evidence for marked muscle weakness in CHF patients (14,38) and the fact that many common daily activities (e.g., lifting objects, rising from a seated position, climbing stairs) are strongly dependent on muscle strength (3,7,30). Despite the importance of muscle strength, its role in determining physical function in real-world activities of daily living in CHF patients has not been examined.

Numerous studies have demonstrated that aerobic training can improve exercise capacity, walking endurance, and clinical status in CHF patients (15,16,25). Despite these beneficial effects, aerobic training generally does not alter muscle strength (22) and, therefore, would not likely remediate aspects of physical disability associated with muscle weakness. Resistance-type exercise, in contrast, improves muscle strength, with little or no effect on aerobic fitness (32). Therefore, resistance exercise training may provide unique functional benefits to CHF patients. In addition, it provides an experimental paradigm to evaluate the unique effects of improvements in muscle strength on physical function. Comparatively few studies, however, have examined the singular effects of resistance training in CHF patients, with most examining the combined effects of resistance and aerobic training (for review, see Braith and Beck (8) and Spruit et al. [34]). Perhaps more importantly, aside from assessment of walking endurance (32), no study to our knowledge has explored the effects of resistance training on directly measured performance of activities of daily living to evaluate the real-world functional benefits or resistance training in CHF patients.

The purpose of this study was twofold: 1) to compare performance in activities of daily living in CHF patients and healthy controls of similar age and habitual physical activity level to evaluate the extent of physical disability and its relationship to aerobic capacity and muscle strength and 2) to determine the effects of 18 wk of resistance training on muscle strength and performance of activities of daily living. We hypothesized that CHF patients would be characterized by considerable physical disability and that this would be related to both diminished aerobic capacity and muscle weakness. In addition, resistance exercise training effectively increases muscle strength and the ability to perform activities of daily living independent of alterations in aerobic fitness. To isolate the effects of CHF, we recruited patients to limit the confounding effects of age, muscle disuse, and acute disease exacerbation. In light of these considerations, our results likely reflect the unique effects of CHF on physical disability, muscle strength, and their response to training rather than the effects of age, muscle disuse, or acute illness.

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Thirteen patients (nine men and four women) with CHF were recruited. Ten patients (seven men and three women; (mean ± SEM) age = 73.4 ± 2.4 yr, height = 170.0 ± 2.7 cm, weight = 95.6 ± 9.4 kg) completed the trial and were included in the analyses. Of these 10 patients, 7 were characterized by systolic failure with diminished ejection fraction (EF; 32% ± 2%, range = 28%-38%), whereas 3 subjects had preserved systolic function (EF > 40; 50% ± 3%, range = 44%-53%). One patient was classified as New York Heart Association class I, six as class II, and three as class III. The cause of CHF was ischemic in four patients and idiopathic in six patients. Patients were excluded if they had: 1) acute myocardial infarction or unstable angina within 3 months, 2) atrioventricular block greater than first degree without a functioning pacemaker, 3) severe hepatic or renal disease, 4) exercise-limiting peripheral vascular disease or orthopedic problems that limit their ability to perform exercise, 5) inflammatory arthritis or autoimmune disease, or 6) an active neoplastic process or history of cancer within 5 yr. To limit any effects of acute disease exacerbation/hospitalization on muscle performance and physical function, we included only patients who were clinically stable and had not been hospitalized for at least 6 months before study. Medications were maintained unchanged during the study and included angiotensin-converting enzyme inhibitors (100%), β-blockers (90%), diuretics (50%), and HMG CoA reductase inhibitors (40%), and one female patient was receiving levothyroxine. The population included three patients with non-insulin-dependent diabetes mellitus. Plasma creatine kinase levels were normal in all patients. All were weight-stable (±2 kg during the previous 6 months), were nonsmokers, and were not taking sex steroid replacement therapy.

All 11 (6 men and 5 women; age = 72.1 ± 2.1 yr, height = 167.3 ± 3.4 cm, weight = 85.5 ± 5.4 kg) healthy volunteers self-reported being sedentary to minimally active (two sessions or fewer of ≥30 min of exercise per week) and were not participating in any exercise or weight loss programs. This recruitment criterion was included to obtain a control group with habitual activity levels that match the reduced level of physical activity in the CHF population (36). Controls were nonsmokers, weight-stable, and not taking sex steroid replacement therapy and had no signs or symptoms of heart failure, CHD, or diabetes. Left ventricular EF (>55%) was normal (62% ± 4%, range = 57%-70%), as were blood counts/biochemical values. Five control subjects had hypertension. Three were treated with diuretics (27%) and two were treated with angiotensin-converting enzyme inhibitors (18%). All were normotensive at testing and showed no evidence of left ventricular hypertrophy or atrial enlargement by echocardiography. Three controls were on stable doses of HMG CoAs (27%), and one female was on levothyroxine. Plasma creatine kinase levels were normal in all controls.

Written informed consent was obtained from all volunteers, and the protocol was approved by the Committees on Human Research at the University of Vermont. Baseline pretraining data from a subset of volunteers in this study have been published in reports of the effects of CHF on whole-muscle and single-fiber protein content and contractile performance and myosin-actin cross-bridge kinetics (20,21,38).

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Body composition.

Total and regional body composition was assessed by dual-energy x-ray absorptiometry (GE Lunar, Madison, WI) as described previously (37). Body composition measurements were not performed on one CHF patient because he exceeded the weight limit of the machine.

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Left ventricular size and function were evaluated by echocardiography (Siemens Accuson Sequoia 512; Siemens Medical Solutions, Malvern, PA). Ventricular volumes and EF were determined by the biplane Simpsons method (17).

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Peak oxygen consumption.

Respiratory gas analysis (Vmax 29c metabolic cart; SensorMedics, Yorba Linda, CA) was performed during treadmill exercise until volitional exhaustion using the protocol of Naughton et al. (23).

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Directly measured performance in activities of daily living.

The Physical Functional Performance Test-10 (PFP-10) is a validated battery of tests based on ordinary activities of daily life, performed at a self-paced, maximal effort (12). The PFP-10 tasks include 1) transferring a weighted pot, 2) putting on and removing a jacket, 3) picking four scarves off the floor, 4) maximal overhead reach, 5) unloading and 6) loading a washing machine and dryer, 7) getting down to a seated position on the floor and returning to a standing position, 8) carrying grocery, 9) stair climbing, and 10) walking for 6 min. All tasks are performed with supervision and are quantified by time, distance, and/or weight carried. Each task is scored 0-100, on the basis of an empirically derived range of individual functional abilities (12). The test yields a total score (range = 0-100) that is the average of the following five separate physical domains scores: 1) upper body strength, 2) lower body strength, 3) balance and coordination, 4) flexibility, and 5) endurance. The PFP-10 has been validated over a broad range of functional levels to assess physical performance in healthy and diseased elderly (12), including older cardiac patients (3).

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Self-reported quality of life/physical function.

The Medical Outcomes Study Short-Form 36 (MOS SF-36) was used to assess "self-reported" physical function and quality of life, as described (2,35), and this has been validated in CHF patients (19).

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Exercise protocol.

Approximately 1 wk after baseline evaluations, volunteers entered an 18-wk resistance training program (three times per week). The resistance exercise training program was designed to improve whole-body skeletal muscle strength with the goal of determining whether improvements in muscle strength can improve functional performance in activities of daily living. The training intensity was set to 80% of one-repetition maximum (1RM) commensurate with guidelines for improving muscle strength and inducing hypertrophy (5). The range of exercises, the progression of exercise volume and intensity, and the length of the program were derived from our previous studies in healthy elderly and those with cardiac disease (1,4,9), and these are supported by data from others (32). Primary goals were to achieve significant strength gains and functional improvements. In addition, we wanted the length of the program to be similar to most cardiac rehabilitation programs. Because this is the first study to assess the effect of resistance training on directly measured physical function in activities of daily living, our data have relevance for the clinical utility of resistance training for improving functional independence.

Subjects were asked not to undertake any additional exercise during the study period. Compliance with the protocol was excellent (91%) and was similar between CHF and control groups (47.6 ± 2.1 vs 48.9 ± 0.9 sessions per patient, respectively, P = 0.55). The original CHF cohort consisted of 13 patients. Three heart failure patients did not complete the training study: one because he became injured in a motor vehicle accident, another because of acute worsening of his heart failure, and the last because of personal reasons.

The resistance training intervention was individually designed on the basis of 1RM (maximum weight an individual can lift once) as described in detail (4). At baseline, 1RM was determined on each of seven exercises including 1) leg extension, 2) leg press, 3) leg curls, 4) shoulder press, 5) bench press, 6) bicep curls, and 7) lateral pull-downs. A "composite 1RM" was calculated by tallying the 1RM for each of the seven different exercises to provide an index of whole-body muscle strength. Each session was supervised by an exercise physiologist or physical therapist. The progression of the program was gradual in both intensity and volume of exercise to orient the volunteers to the resistance training stimulus. The intensity of exercise began at 50% 1RM for one set of 10 repetitions during the first week. On week 2, the intensity was increased to 60% for two sets of eight repetitions. On week 3, the intensity was increased to 70% for three sets of eight repetitions. By week 4, all volunteers were exercising at 80% of 1RM for three sets of eight repetitions. This ensured that the volunteers were exposed to the 80% 1RM stimulus for at least a 3-month period. 1RM was reassessed every 2 wk to account for improvements in strength. At the completion of the training program, all baseline evaluations were repeated, including 1RM measurements. The only exception was echocardiography, which was repeated in CHF patients, but not controls.

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Statistic analysis.

Unpaired t-tests were used to compare baseline values between groups. Baseline 1RM was compared between groups after statistical control for body weight using ANCOVA. Pearson correlations were used to measure associations between variables. Repeated-measures ANOVA was used to assess group, training, and group × training interaction effects. Paired t-tests were used to evaluate the effect of training on cardiac function in CHF patients. All statistical analyses were carried out using Stat View 5.01 (SAS Institute, Cary, NC).

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Baseline data.

At baseline, study groups were similar by age, sex distribution, body size and composition, and physical activity, whereas, as expected, CHF patients had lower EF and peak oxygen consumption (V˙O2peak) and greater left ventricular end-systolic volume (all P < 0.001; Table 1).



Muscle strength (1RM) was similar between groups for all exercises, as was the composite 1RM (Table 2). The lack of difference in muscle strength between groups was explained by variation among groups in body size. When composite 1RM was statistically controlled for body mass, whole-body muscle strength was significantly reduced by 24% in CHF patients versus controls (222.9 ± 20.2 vs 293.5 ± 19.3 kg, P = 0.023). Similarly, lower (25%) muscle strength was found in CHF patients when leg extension measurements were adjusted for body mass (38.0 ± 4.1 vs 50.6 ± 3.9 kg, P = 0.042), in keeping with our previous report examining knee extensor strength in a subset of this cohort using dynamometry (38). In addition, after adjusting for body weight, 1RM for bench press (−28.9%, P = 0.031), shoulder press (−34%, P = 0.05), and leg press (−15%, P = 0.05) were lower in CHF patients, whereas there was a strong trend toward a lower 1RM for leg curl (−37%, P = 0.061) and no differences were found for 1RM values for lat pull-down (P = 0.125) or arm curl (P = 0.172).



CHF patients had a lower (P < 0.01) total PFP-10 score at baseline (Table 3) because of reduced (P < 0.05 to P < 0.01) lower body strength, balance, and endurance, whereas upper body strength and flexibility were similar between groups. In addition, baseline 6-min walk distance was reduced (P < 0.001) in CHF patients. CHF patients self-reported lower physical and mental component summary scores as a result of lower (P < 0.05 to P < 0.001) scores for physical function, role-physical, bodily pain, general health, vitality, and social function (P < 0.05 to P < 0.001; Table 4).





Correlation analysis evaluated which physiological and psychological factors predict reduced physical function (Table 5). We examined both measured (i.e., PFP-10) and self-reported physical function (i.e., MOS SF-36 subdomain) as indices of true physiological capacity for daily activities and self-perceived functional capacity, respectively, and also evaluated predictors of 6-min walk performance because it is a commonly used index of functional capacity in CHF populations. For directly measured functional capacity (i.e., PFP-10 total score), V˙O2peak (P < 0.001), 1RM composite (P < 0.01), and MOS SF-36 mental component score (P < 0.05) were significant correlates. For 6-min walk distance, the correlates were similar to PFP-10 total score but also included EF (P < 0.01). For self-reported physical function, V˙O2peak, measured physical functional capacity (i.e., PFP-10 total score), MOS SF-36 mental component score, and EF (all, P < 0.01) were significant correlates, whereas 1RM composite was not.



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Resistance training data.

No training or group × training effects were observed for body weight, adiposity, fat-free mass, leg fat-free mass, appendicular skeletal muscle mass, or V˙O2peak (Table 1 and Fig. 1). Resistance training increased (P < 0.01, training effect) arm fat-free mass. For CHF subjects, there were no changes in LV volumes or EF with training.



Resistance training resulted in significant improvements in all 1RM measures, including composite 1RM (all, P < 0.001 training effect; Table 2 and Fig. 1), and these improvements did not differ between groups (i.e., no group × training effect).

Total PFP-10 score improved with training in both groups (P < 0.001 training effect; Table 3 and Fig. 1), as a result of improvements in upper and lower body strength and balance scores (P ≤ 0.02 to 0.001), whereas no improvements in upper body flexibility or endurance were noted. Training-induced increases in performance of activities of daily living were similar between groups (i.e., no group × training effect). Training increased 6-min walk distance in both groups similarly (P < 0.05). Conversely, there was no training effect on MOS SF-36 scores (Table 4), although there were trends toward improvement in the physical function and mental health subdomains (P = 0.06 and P = 0.08, respectively).

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Our study is novel because it is the first to directly measure the extent of physical disability in common activities of daily living in CHF patients and its response to resistance exercise training. Our results demonstrate that CHF patients are markedly impaired in their physiological capacity to perform activities of daily living compared with controls of similar age and habitual physical activity level and that this impairment is associated with both reduced aerobic capacity and muscle weakness. Furthermore, resistance training is an effective intervention to enhance muscle strength and performance in activities of daily living. In light of our careful selection of volunteers to control for a variety of confounding disease-related factors, we are confident that group differences in physical function and muscle performance and their response to training reflect the unique effects of heart failure. Collectively, our findings support a role for muscle strength as a determinant of the physiological capacity to perform daily activities in CHF patients.

Although many studies have reported reduced walking endurance in CHF patients (i.e., 6-min walk test [28]), this measure reflects but one facet of daily physical functioning. In the current study, we directly measured performance in a variety of real-world daily activities that better reflect the range of physical attributes (i.e., strength, endurance, balance, and flexibility) required to function independently. Because CHF patients are profoundly inactive (27,36), it could be argued that the extent of physical disability we observed is related to the secondary effects of chronic muscle disuse. This is unlikely because we recruited controls with similar habitual physical activity levels as CHF patients (Table 1) and tested patients distal to periods of acute muscle disuse (i.e., hospitalization). Thus, we are confident that the observed functional differences are attributable to the effects of the CHF syndrome rather than muscle disuse.

From a physiological standpoint, two of the most important factors determining physical function in the elderly are muscle strength and aerobic capacity (31). Although numerous studies have evaluated reduced aerobic capacity in CHF, the role of muscle weakness in physical disability, to our knowledge, has not been investigated. The focus on aerobic capacity is likely because patients perceive their physical limitations through symptoms such as dyspnea and fatigue, which are emblematic of reduced aerobic fitness. In support of this notion, we found that self-reported functional capacity, as measured by the MOS SF-36, was correlated to aerobic capacity but not to muscle strength. In contrast, our direct measurement of performance in activities of daily living (i.e., PFP-10 score) was related to both aerobic capacity and muscle strength, substantiating a role for muscle strength as a determinant of physical function. These results highlight an often overlooked limitation of indices of self-reported physical function, which have formed the basis for nearly all of our current estimates of physical disability in the CHF population, namely, that they measure the patient's perception of their limitations rather than the patient's actual capacity to perform activities. Perhaps more importantly, although all of the metrics used in this study suggest a similar relative degree of physical disability (∼30% lower functional capacity in CHF patients), the fact that their underlying determinants differ substantially (Table 5) demonstrates that each measure reflects a unique mix of the physiological and psychological determinants of physical disability. Our study provides the most rigorous assessment of the determinants of disability in CHF to date because we used direct assessments of performance in activities of daily living as an index of physical disability. Our findings demonstrate that, as in populations of elderly without cardiac disease (31), muscle weakness conspires with diminished aerobic fitness to limit the capacity to perform daily activities in CHF.

Although it was not a goal of our study, the relative contribution of aerobic exercise intolerance and muscle weakness to physical disability deserves comment. Based solely on the strength of the statistical relationships, reduced aerobic capacity seems to be the more important determinant. However, V˙O2peak measurements reflect more than just aerobic fitness level. For instance, muscle strength is a strong correlate of V˙O2peak (14), which is likely explained by the fact that muscle force production is a determinant of power output during treadmill exercise (i.e., contractile force × velocity = muscle power output). In this sense, the correlation between V˙O2peak and physical function reflects, in part, the effects of muscle strength. In addition, because V˙O2peak serves as a physiological index of disease severity in CHF patients (18,39), its correlation to PFP-10 reflects, in part, the fact that heart failure patients were more functionally impaired. Both of these examples highlight the fact that V˙O2peak has a high degree of colinearity with other determinants of physical disability, which complicates our ability to define its true contribution. There are statistical approaches to account for these problems of colinearity (33), and a partial correlation analysis shows that control for the effects of 1RM composite score diminishes the relationship between V˙O2peak and PFP-10 score (partial r = 0.695, P < 0.01). However, caution is urged with drawing physiological conclusions from such statistical analysis that seeks to circumvent the fact that our study was not designed specifically to address this issue. Instead, we believe that delineation of the unique contributions of aerobic fitness and muscle strength to physical disability should be derived from studies that are carefully designed for this purpose.

Building on the last point, with respect to the unique effects of muscle strength on physical functional performance (i.e., PFP-10 score), if the aforementioned correlation between muscle strength and function in daily activities reflects an underlying cause-effect relationship, we would predict that interventions that increase muscle strength, such as resistance training, would improve physical function. Consistent with this hypothesis, performance of activities of daily living was increased in CHF patients after 18 wk of resistance training (Fig. 1). In fact, improvements were similar to controls, suggesting that CHF does not impair neuromuscular adaptations to training in patients with mild to moderate disease. Perhaps more importantly, because V˙O2peak was not altered with training, improvements in physical function are likely explained primarily by enhanced muscle strength rather than altered aerobic capacity. We acknowledge that our resistance training program may have improved other factors that determine physical function, such as balance and flexibility. Thus, we cannot completely ascribe the improvements to improvements in muscle strength alone. Nonetheless, together with our correlation analyses, these findings strongly support a role for skeletal muscle strength as a determinant of performance in activities of daily living.

Participants with CHF had baseline total PFP-10 scores of 48.5, which is well below the score of 57 suggested by Cress et al. (12) as a threshold for physical disability. Although physical function improved with training in CHF patients to 53.9, it remained below this putative threshold. The inability of our training regimen to correct physical disability may relate to its short-term nature (4 months). The extent of muscle hypertrophy achieved during this period was minimal. Programs of longer duration that stimulate hypertrophy would certainly yield greater strength and functional benefits. In addition, a singular intervention of resistance training may not be sufficient to correct disability in CHF patients because physical function is partially dependent on cardiorespiratory fitness (Table 5), which was not altered by our training program. Exercise regimens that combine both resistance and aerobic training may be needed to optimize functional improvements. Parenthetically, the fact that we observed no training-induced increase in physical activity level in CHF patients (via accelerometry) suggests that patients did not increase the amount of activity in their daily life despite their improved physiological capacity (i.e., PFP-10 score). This result may be explained by psychological barriers to performing daily activities in CHF patients (26) and suggests the need for cognitive behavioral training to accompany exercise interventions so that patients can exploit their improved physiological capacity to reduce physical disability in their everyday life.

Limitations to the current study include the small number of subjects and a lack of participants randomized to a nonexercise control group. Importantly, our intent was not to conduct a randomized controlled trial of the clinical efficacy of resistance training, as numerous studies have examined resistance training in CHF (8,34). Instead, we sought to examine the role of muscle strength as a determinant of physical disability in CHF patients and the effects of an exercise intervention that specifically enhances muscle strength to improve physical function. Although we compromised statistical power with our approach, the rigorous selection criteria allowed us to experimentally remove several confounding factors from baseline disability levels and their responsiveness to training, a stronger experimental approach than post hoc statistical adjustment. Lending credibility to our results, the strength and functional (e.g., 6-min walk) improvements observed in our study were similar to those found in randomized controlled trials using similar intensity resistance training in CHF patients (32). Nonetheless, larger randomized controlled trials are needed to assess the clinical utility of resistance training, alone and in combination with aerobic exercise, in reversing or delaying the onset of disability and improving clinical outcomes in CHF patients. In addition, future studies should assess physical function and clinical outcomes after cessation of resistance exercise interventions to assess its long-term effects.

In summary, the ability to perform necessary activities of daily living is markedly impaired in CHF patients and is related to reduced aerobic capacity and muscle weakness. Resistance training is an effective intervention to improve strength and direct measures of physical function in CHF patients, and these effects are independent of alterations in aerobic fitness. Collectively, these findings strongly support a role for muscle strength in determining the physiological capacity to perform activities of daily living. From a clinical perspective, our results suggest that interventions designed to lessen physical disability in CHF patients should consider improving muscle strength as one of their goals.

This study was funded by grants from the National Institutes of Health (HL-077418 and RR-00109).

The authors thank all the volunteers who dedicated their valuable time to this study.

The authors recognize no conflicts of interest. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Heart Lung and Blood Institute or the National Institutes of Health, nor do the results of the study constitute endorsement by the American College of Sports Medicine.

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