Methods

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Twenty-four sedentary, healthy persons (11 men and 13 women) were randomly assigned within sex groups either to a 12-week resistance-training program or to sedentary control status. The mean age of the participants was 70.4 ± 4 years (range, 65 to 79 years), and no differences in mean age were seen between groups (69.9 ± 4 years in the resistance-training group; 70.7 ± 5 years in the control group [P > 0.2]). The resistance-training group was composed of 6 men and 6 women; the control group consisted of 5 men and 7 women. Participants did symptom-limited treadmill stress tests at baseline, and we collected expired gas to determine peak aerobic capacity. We used a Horizon Metabolic Measurement Cart (Sensormedics, Yorba Linda, California) to analyze expired air. Participants were excluded if they had angina or electrocardiographic ischemia (defined as ST-segment depression more than 1 mm) during the exercise test (n = 2), if their resting blood pressure was higher than 160/90 mm Hg (n = 4), or if they had a noncardiopulmonary limitation of exercise capacity, such as claudication, arthritis, or cerebrovascular disease (n = 1). We tested 29 participants to arrive at the final study sample of 24 persons. No participants were receiving calcium- or ß-blocking medications or estrogen-replacement therapy, and none had diabetes.

We determined body composition by using underwater weighing and correcting for residual volume [6]. Leg-muscle mass was determined by dual-energy x-ray absorptiometry [7]. Strength was evaluated on a Universal Gym apparatus (Universal, Cedar Rapids, Iowa) by measuring the single repetition maximal lift for leg extension, leg flexion, and bench press. We measured submaximal endurance capacity on the treadmill by first having participants walk for 5 minutes at an intensity of 50% of their previously determined peak aerobic capacity. Participants then gradually increased their walking intensity to 80% of peak aerobic capacity by 10 minutes of the continuous protocol and proceeded until exhaustion (n = 24) or 45 minutes, at which time the test was terminated. Because none of the participants had angina, claudication, or exercise-limiting arthritis, exhaustion was defined as a combination of leg fatigue, dyspnea, and overall body fatigue. During endurance testing, heart rate, blood pressure, oxygen uptake, and perceived exertion [8] were measured at 5-minute intervals until exhaustion. After 12 weeks, variables were measured again, and walking endurance was measured at the same absolute workload as before conditioning.

The weight-training regimen consisted of three sets of eight repetitions of seven exercises done using a Universal Gym apparatus 3 days per week. The exercises (and primary muscles exercised) were leg extension (quadriceps), leg curl (hamstrings), arm extension (triceps), arm curl (biceps), lateral pull-down (latissimus dorsi and biceps), bench press (pectoralis major and triceps), and squat (gluteals and quadriceps). A rest period of 1 to 2 minutes was taken between sets. The participants began at a resistance of 50% of their single repetition maximum. The resistance was then increased progressively until participants were exercising at 80% of their single repetition maximum by week 9. Strength was retested intermittently, and the training loads were adjusted as required. The participants did not train aerobically. Controls were instructed not to alter their home activity habits.

A two-factor analysis of variance was done on baseline measures, with sex and training groups as the factors [9]. Women differed from men in baseline measures of fitness, strength, and body composition, but no sex-group interactions were seen. Pre- to postconditioning measures were analyzed using repeated-measure analysis of variance with results separated by sex. Data collected during the submaximal endurance study were analyzed for perceived exertion, heart rate, and systolic blood pressure (excluding rest and exhaustion values) by repeated-measure analysis of variance. Relations between variables were studied by linear regression analysis. Data are presented as mean ±SD. Analyses were done using BMDP New System for Windows, Version 1.0 (BMDP Statistical Software, Los Angeles, California).

Results

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After the 12-week resistance-training program, strength increased substantially in the resistance-training group. The single repetition maximums increased by 29% ± 24% for leg extension, 65% ± 79% for leg flexion, and 29% ± 15% for bench press. When results were analyzed separately by sex and compared with results in controls, single repetition maximums for leg extension increased significantly among women (P < 0.01) but not among men (P = 0.12) (Table 1). Similarly, single repetition maximums for bench press increased significantly among women (P < 0.01) but not among men (P = 0.12). Both women and men had increased single repetition maximums for leg flexion compared with controls (P < 0.01 for women and P < 0.05 for men). It should be noted that the study was not designed to fully address sex-specific outcomes. The control group showed no change in strength measures over the 12-week period, and the strength changes for each of the exercises in the intervention group when sexes were combined were significantly greater than the measures in the controls (P < 0.005) (Table 1).

Women in our study had lower measures of peak aerobic capacity at baseline than men (21.5 ± 1.1 mL/kg · min^–1 compared with 28.8 ± 1.5 mL/kg · min^–1; P = 0.03) and lower measures of strength as measured by single repetition maximum for leg extension (23.5 ± 1.0 kg compared with 42.8 ± 4 kg; P = 0.001), leg curl (5.3 ± 0.4 kg compared with 14.4 ± 1.5 kg; P = 0.003), and bench press (20.9 ± 1.1 kg compared with 31.3 ± 2.2 kg; P = 0.008). Neither women nor men showed changes in peak aerobic capacity after resistance training.

No participant had significant changes in body weight, percentage of body fat, fat mass, or fat-free mass as measured by underwater weighing (Table 1). Analysis of regional body composition measured by dual-energy x-ray absorptiometry scanning showed an increased lean mass of the leg with weight training when compared with controls (P = 0.02) (Table 1). This was due to an increase among women (P < 0.02); no increase was seen among men (P> 0.2). Peak aerobic capacity was unaltered in both groups, regardless of sex.

The primary outcome variable of interest was walking endurance time. After the resistance-training program, mean walking time until exhaustion for the resistance-training group increased by 38%, from 25 ± 4 minutes to 34 ± 9 minutes. In contrast, no change was seen in the control group (20 ± 5 minutes to 19 ± 10 minutes; P = 0.005 for comparison between groups). The increase in endurance time was due primarily to increases seen among men (P < 0.02) rather than among women (P> 0.2). Mean exercise intensity, measured at 10 minutes of the endurance protocol at baseline, was 82% of peak aerobic capacity in both the training group and the control group (P> 0.2 between groups). Before the study intervention, none of the 24 participants completed the full 45-minute endurance protocol. After conditioning, 3 of the 12 participants who received resistance training and 1 of 12 controls completed the entire protocol. After conditioning, mean scores for perceived exertion for all submaximal data points from 10 minutes until the end of the walking endurance protocol (not including the exhaustion point) were lower in the resistance-training group than in the control group (P = 0.03) (Figure 1). Mean submaximal heart rate and systolic blood pressure measures were lower after conditioning; however, when groups were compared by repeated-measure analysis of variance, differences between groups were not significant (Figure 1). Submaximal or peak exercise oxygen consumption did not change during the endurance testing.

View larger version (27K): [in this window] [in a new window]   Figure 1. Conditioning data in the resistance-training group. Open symbols indicate values before conditioning; closed symbols indicate values after conditioning.

For all 24 participants, significant relations were seen between change in walking endurance and change in strength measures for leg extension (r = 0.48 [95% CI, 0.10 to 0.74]; P = 0.02) and leg flexion (r = 0.46 [CI, 0.07 to 0.73]; P = 0.03). Baseline peak aerobic capacity (r = –0.10 [CI, –0.48 to 0.3]; P > 0.2) and baseline endurance time (r = 0.41 [CI, –0.22 to 0.73]; P = 0.13) were not significantly related to change in endurance capacity in the resistance-training group of 12 participants.

Discussion

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We show that healthy, community-dwelling elders can improve walking endurance and leg strength by participating in a resistance-training program. This finding is relevant to persons at risk for disability because walking endurance and leg strength are important components of physical functioning [3].

Resistance training improves walking endurance in at least two ways. First, our data show a relation between changes in leg strength and change in walking endurance; thus, increased leg strength is a factor. Second, in view of the measured increase in leg strength, participants worked at lower percentages of their peak strength at a given workload, thereby using anaerobic mechanisms to a lesser degree. This is confirmed by Marcinik and colleagues [10], who documented reduced plasma lactate levels and a higher lactate threshold during steady-state submaximal exercise after a strength-training program in young (25 to 34 years of age) healthy men. We did not monitor the physical activity of study participants at home, but it is unlikely that an increase in aerobic activity at home contributed to the increase in endurance capacity, because the aerobic capacity of the participants did not change.

Sex differences were prominent both at baseline and in response to conditioning. At baseline, women had lower strength measures, lower fat-free mass, greater fat mass, and lower peak aerobic capacity. In response to resistance training, women had improvements in strength and fat-free mass of the leg that were somewhat greater than those of men, yet men increased their submaximal endurance to a greater degree. However, our study was not designed to fully evaluate sex differences in response to training.

Several investigators have documented the utility of resistance training in elderly persons of both sexes living both in institutions and in the community [4, 5, 10, 11]. Fiatorone and colleagues [4, 5] observed frail, institutionalized patients with mean ages of 87 and 90 years in two studies. Their patients participated in a 10-week program of lower-extremity resistance training at an intensity of 80% of single repetition maximum. They found that the strength of the quadriceps correlated with walking speed over a 6-meter walking course and that muscle strength and short-course walking speeds increased by 113% and 12%, respectively, after resistance training [5]. Our study, by using a more intensive evaluation of endurance walking capacity, extends these findings into the much broader population of younger, community-dwelling, healthy elderly persons. Furthermore, we suggest a preventive intervention to improve mobility and strength in younger, nondisabled elderly persons in contrast to a treatment program for older, frail, institutionalized elderly persons. The mechanism of increased walking endurance after resistance training is more clearly defined, suggesting that change in the strength of quadriceps is a primary determinant. The magnitude of improvement in walking endurance seen in this study of healthy elderly persons was similar to that seen in older patients with coronary disease who participated in a 12-week aerobic exercise program consisting primarily of treadmill walking [12]. Although the patients with coronary disease did not have strength testing, our resistance-training group almost certainly had greater increases in leg strength measures than the aerobically trained patients with coronary disease.

As the population ages, disability in older persons will increasingly affect quality of life in addition to increasing requirements for home care [13]. Physical performance testing can now be used to predict which "healthy" elderly persons are at highest risk for the development of disability; an important use of such data is in the planning of preventive health care. Interventions can therefore be considered for patients at risk for disability [14]. We believe that resistance training should be considered as a treatment option for increasing walking endurance in this population. This is an important area for future intervention trials on preventing disability in high-risk elderly persons.

Dr. Ballor: 477 North Sheridan Avenue, Tacoma, WA 98403.

Dr. Ashikaga: 24C Hill Building, Biometry and Biostatistics, University of Vermont, Burlington, VT 05405.

Ms. Utton: 4 Mohawk Avenue, Essex Junction, VT 05452.

Dr. Nair: Joseph 5-169, Endocrinology Division, Mayo Clinic, Rochester, MN 55905.