Strength training accords many health benefits and is associated with reducing the risk of a number of disease processes among adults including osteoporosis, cardiovascular disease, and diabetes (26). Strength training activities have also been shown to improve ambulatory ability (7,10), balance (15,25), and function (7) among older adults. In addition to the physical benefits, strength training has an impact on psychological functioning, offering positive changes to mood, anxiety levels, and self-efficacy measures (11,20,23).
Strength training is generally accepted as one component of a healthy and active lifestyle. Strength training is included as an element of the exercise guidelines disseminated by the American College of Sports Medicine (2). In addition, Healthy People 2010, the document that sets health objectives for the United States, incorporates strength training in an objective within the goals to improve the health, fitness, and quality of life through physical activity (24). Thus, there is a national effort to increase “the proportion of adults who perform physical activities that enhance and maintain muscular strength and endurance” (24). The baseline measure that was used to estimate national participation in strength training is derived from data collected in the 1998 National Health Interview Survey (NHIS), which provided the estimate that 18% of adults aged 18 years and older participate in resistance training activities (24). The Healthy People 2010 target is for 30% of adults aged 18 years and older to participate in resistance training activities.
In order to reach the Healthy People 2010 target, more people will need to engage in strength training activities. An analysis of the predictors of participation and nonparticipation in strength training activities provides information about whether there are disparities in participation by demographic factors and profiles the population that may already understand the benefits of strength training. Those adults who are participating are already demonstrating the desired behaviors of the Healthy People 2010 document. The participant population may only need the encouragement of training professionals to continue with these activities. The analysis of those who do not participate in strength training provides information for professionals to target specific subgroups for interventions that encourage participation.
Profiles of persons who participate in strength training have been published by the Centers for Disease Control and Prevention (CDC), both in the form of a trend analysis of NHIS data from the years 1998-2004 (5) and as an analysis of the 2001 NHIS data specific to adults older than the age of 65 (3). The purpose of the 2 CDC analyses was to assess the U.S. population against the Healthy People 2010 goal and to profile participants in strength training by age, race, and ethnicity. Accordingly, CDC analysts found that the prevalence of strength training has only increased slightly in the 1998-2004 period from 17.7 to 19.6% and that 21.9% of men and 17.5% of women were strength training 2 or more times per week in 2004 (5). Among older adult respondents to the 2001 NHIS, 11% engaged in strength training. Older adults who participated in strength training were more likely to be men, more highly educated, in better health, and persons who were categorized as physically active (3). While both of these profiles provide valuable epidemiologic data on strength training, neither employed an inferential analysis to ascertain which of the demographic variables has the greatest influence on participation in strength training.
The purpose of this study is to provide the strength training community with the tools needed to engage in a population public health approach to promoting strength training activities. This study clarifies and further develops a profile of all adults who participate in strength training activities and then analyzes the demographic factors that predict strength training participation. These predictive factors aid in determining whether there are demographically based disparities in participation in strength training.
Experimental Approach to the Problem
To generate population based estimates of strength training participation and to analyze the determinants of strength training in the United States, a secondary analysis of data collected by the National Centers for Health Statistics (NCHS), CDC was conducted. The study employed a retrospective nonexperimental design to generate a profile of persons who participate in strength training activities and to examine demographic predictors of participation. The study comprised an analysis of cross-sectional data collected by the NCHS, CDC for the NHIS. This approach to the analysis of epidemiologic or health services data has been used previously with the NHIS data to study racial and ethnic predictors of leisure time physical activity (1), to profile activity levels of adults trying to lose weight (17), and to estimate the prevalence of adults who were meeting the CDC/ACSM guidelines for physical activity (21).
The final sample for this study consisted of 29,783 persons who were capable of strength training activities and who provided information about their strength training activities. The sample was derived from the 2003 NHIS Sample Adult data file. In 2003, the NHIS had a response rate of 89.2% resulting in a sample of 35,921 households. The response rate for the sample adult component was 74.2%. The 2003 NHIS sample adult file contained 30,852 respondents aged 18 years or older. Respondents were deleted from analysis if they did not provide data on the strength training question or if they responded that they were unable to participate in strength training activities. Six hundred fifty-two respondents replied “unable” to survey questions about strength training, and 417 persons refused or did not provide information to NHIS survey questions about strength training, yielding an unweighted sample of 29,783.
This analysis employed public use data from the 2003 NHIS. The NHIS is a national probability survey conducted by the NCHS. NHIS data are collected through personal interviews conducted by the U.S. Bureau of the Census. The NHIS is one of the tools used to study and measure the health status and health behaviors of the civilian, noninstitutionalized population of the United States. The survey consists of a core module that includes a family core survey, a survey of a sample adult randomly selected from each household, and a survey of a sample child randomly selected from each household. In addition, there are survey supplements on special health topics that change on an annual basis.
Public use data files are released annually and made available to researchers via the CDC file transfer protocol site on the NCHS Web site. NCHS data are publicly available to encourage and allow nongovernment-based researchers and analysts the opportunity to conduct independent analyses and research. Detailed information about the NHIS and the methodology used in data collection may be found at the NCHS Web site or in published literature (6,14,18).
The Sample Adult data file was the source of this study population. The Sample Adult file included demographic information and survey questions that were asked about leisure time activities related to exercise. The data file contained weighting, primary sampling unit, and strata variables to account for the complex survey design of the NHIS.
The measure of participation in strength training activities was derived from several questions asked in the NHIS regarding these activities. In the NHIS, respondents were asked to report the frequency of “physical activities specifically designed to strengthen your muscles such as lifting weights or doing calisthenics.” Using the ACSM guidelines (2), which recommend a frequency of strength training on 2-3 days per week, a dichotomous variable was created in which a respondent was categorized as a participant in a strength training program when the reported frequency of activities to strengthen muscles was at least 2 times per week.
To examine the determinants of strength training activities, demographic variables, and a variable that described respondents' leisure time physical activity were included in the final analysis file. Demographic variables were directly derived from the NHIS Sample Adult data file. These variables included age, sex, race, level of education, family size, census region, and marital status. The leisure time physical activity variable was derived from questions about physical activity classified as aerobic in nature (activities that produced increases in heart rate or breathing).
All analyses and data management were completed using Stata/SE 8.2 for Windows (Stata Corp., College Station TX) “Survey” commands, which employ a Taylor Series Linearization technique to account for the complex survey design effect of the NHIS. To produce the national estimates of adults who participate in strength training activities and the SEs associated with these estimates, a weighting variable and the complex survey variables supplied by the CDC within the data file were used.
The χ2 statistic was used for the univariate analysis. The constructed dichotomous variable that defined whether a respondent participated in strength training activities on ≥2 days of the week was the outcome variable while each of the respondent characteristics were used as independent variables.
Multivariate modeling was performed using binary logistic regression with the strength training variable as the dependent variable and the respondent characteristics as the independent variables. The “Survey” commands in Stata do not incorporate the additional tests and diagnostics used in logistic regression model building. Accordingly, analysis was completed in 2 stages, a model-based analysis and a design-based analysis (16). In the model-based analysis, the complex survey variables were not used, so that logistic regression diagnostic statistics could be calculated to assess model fit (16). Changes incorporated in the model-based analysis were then carried into a design-based analysis that used the “Survey” commands and the complex survey variables to produce the actual estimates that are used in the presentation of the regression results. The α level was set at 0.05 for all analyses.
Figure 1 shows that in 2003, 21% of adults participated in strength training activities on at least 2 days of the week. This figure equates to an estimate of 433.2 million adults in the United States who are participants in strength training. Both population and prevalence estimates for the frequency of strength training activities on a weekly basis by U.S. adults are presented in Table 1. Among those who participated in strength training, 3 sessions per week was the modal category.
Table 2 contains the results of the univariate analysis comparing respondents who participated in strength training at least twice weekly to those who did not. Significant differences were found among all variables except race. The modal categories among the persons who did participate in strength training included being male and the age group of 25-44 years. In addition, strength training was associated with having at least a high school degree.
The final multivariate model developed to analyze the determinants of strength training included the variables age, sex, marital status, level of education attained, and aerobic exercise status. This model is presented in Table 3.
Most U.S. adults do not participate in strength training activities. This is not surprising given that a majority of adults are classified as “not physically active” on a regular basis in government reports (4). The figure of 21% falls short of the Healthy People 2010 target but is certainly approaching it. Greater public health efforts are needed to focus not only on the need for exercise and strength training but also the benefits. With a population trend that shows an increase in the proportion of adults participating in strength training (5), public health officials and policy decision makers may believe that this aspect of exercise warrants less attention. However, given the health benefits of strength training, the government target of 30% is probably a lower figure than that desired by strength training professionals.
Factors that have been found to correlate with participation in physical activity are manifold and include personal, social, and environmental variables (22). Among demographic factors, those negatively associated with physical activity participation include increasing age, female sex, and lower levels of education (22). It appears from the current study that the demographic correlates negatively associated with participation in strength training activities are identical to those associated with physical activity in general.
Determinants of strength training in this analysis were similar to some of those found to be associated with strength training among older adults. Women were less likely than men to participate in strength training. Persons with higher levels of education were more likely to participate in strength training. In fact, a person with a college degree is twice as likely to participate in strength training as a person who did not complete high school. Education on strength training among adults with lower levels of formal education is imperative. Strength training professionals should consider this group a priority population, especially since education can be used as a proxy measure for class or socioeconomic position and there are strong health measures that show lower health status among U.S. adults with lower levels of education (19). Since health status could be mitigated with exercise and strength training, attention is warranted to this subpopulation.
Although the benefits of strength training have been demonstrated in studies of older adults (7,10,15,25), increasing age reduced the likelihood of strength training participation. This finding warrants substantial attention from the community of strength training professionals. Given the age-related decline that occurs in muscle strength and muscle mass after the age of 60 (8,12,13), ideally an increase rather than a decrease in the persons participating in strength training should be seen. The public health questions that need to be resolved and addressed are the reasons for the decline in strength training participation with age and the methods that best target adults to either continue participation from younger years or to start participation in strength training as they approach their 50s and 60s. Some of the barriers to participation in physical activity that have been previously identified include lack of time, fatigue, a feeling that the activity is too tiring, fear of falling, bad weather, lack of access to facilities, and a lack of partners with whom to exercise (22). If these barriers are identical for strength training activities, then strength training professionals should devote their efforts to not only education, but also to developing home-based programs and improving facility access.
The strongest of the determinants of strength training is participation in other aerobic activities. Persons who are already active and exercising are 5-15 times more likely to also participate in strength training activities. This finding is similar to the finding among older adults (3) and indicates that adults who exercise in one manner are considering all aspects of fitness when they approach their activity. The finding should encourage training professionals to promote strength training among persons participating in other aerobic activities and to continue to advocate for participation in full fitness programs among all potential client populations.
A number of limitations were present in both the design of this study and the data source for the study. The study design was an analysis of relationships with the intention of demonstrating the presence or absence of an association between demographic factors and participation in strength training. This association, while modeled to be meaningful, does not imply causality. Thus, an analysis of this type is only a stepping off point to ascertain whether and, even potentially, where a problem may exist. The study cannot, however, answer the question of what stimulates a person to participate or not to participate in strength training activities.
The use of a secondary data source also posed its own set of limitations. The source may have contained inaccurate data or insufficient data to measure the constructs under study. It is possible that data represented inaccurate answers or misinterpretations of the questions asked by interviewers. In addition, the data in this study were cross-sectional in nature, which exposes some limitations to the study in terms of its conclusions. Cross-sectional data can measure the prevalence of persons participating in strength training at a moment in time but not the ongoing efforts and adherence to strength training regimens in the population. Data on adherence to exercise programs indicate that 50% of adults who start a program will drop out within a few months (9). The 50% figure speaks to the need to remove barriers to activity participation and to the need for longitudinal studies of physical activity and strength training combined with efforts to improve access and adherence.
This study is meant to draw the attention of strength and conditioning professionals to the populations they are serving but more importantly to the populations among which they need to promote the benefits of strength training activities. The study provides impetus for strength and conditioning professionals to join public health efforts to increase the proportion of adults who participate in strength training and provides some information about targeting these efforts among certain subgroups including women and older adults.
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