Summary
Background
We have a limited understanding of the association between behavioural participation in muscle-strengthening activities (MSA) and all-cause mortality.
Aim
To determine the effect of MSA on all-cause mortality, and examine a potential dose-response relationship between the frequency with which MSA are performed and the incidence of all-cause mortality.
Methods
Individuals (8772 adults aged ≥ 20 years) from the 2003–2006 National Health and Nutritional Examination Survey were evaluated for baseline characteristics, then followed for an average of 6.7 years. MSA were assessed at baseline as the number of self-reported sessions completed within the past 30 days. Analyses were performed in 2015.
Results
Only 18.6% of individuals met MSA guidelines (2–3 MSA sessions/week) at baseline, while those performing any form of MSA had a 23% reduced risk of all-cause mortality (hazard ratio [HR]: 0.77; 95% confidence interval: 0.60–0.98; P = 0.04). Additionally, we created a five-category variable to determine whether a dose-response relationship existed between MSA and premature mortality; only individuals performing 8–14 sessions over a 30-day period (current MSA guidelines) had a reduced risk of all-cause mortality (HR: 0.70; P = 0.02). Results were similar for CVD-specific mortality.
Conclusion
The national recommendations that 2–3 MSA sessions be performed per week appear to be most effective at reducing the risk of premature all-cause mortality; however, despite these recommendations, the majority of the adult population in the USA still fails to perform any MSA. Future studies should determine strategies for increasing adherence to these established guidelines.
Résumé
Justification
Les données disponibles concernant l’association entre les activités physiques avec renforcement musculaire et le risque de mortalité toutes causes sont plus nombreuses.
Objectifs
Déterminer l’effet de l’activité musculaire en force sur la mortalité toutes causes et examiner le lien dépendant de la charge entre la fréquence de l’exercice physique en force et l’incidence de la mortalité toutes causes.
Méthode
Un total de 8727 adultes âgés de plus de 20 ans ont été évalués entre 2003 et 2006 par l’Institut de la santé et de la nutrition américaine pour ce qui concerne les caractéristiques de base, puis ces sujets ont été suivis pendant une durée moyenne de 6,7 ans. L’exercice physique a été évalué à l’état basal en prenant en compte le nombre de sessions rapportées par le sujet dans les 30 jours précédents. Les analyses ont été réalisées en 2015.
Résultats
Seulement 18,6 % des sujets ont atteint les recommandations pour la charge d’exercice physique en force (2 à 3 sessions par semaine) à l’état basal, tandis que ceux réalisant n’importe quelle forme d’exercice physique en force avaient une réduction de la mortalité toutes causes de 23 % pour ce risque (hazard ratio : 0,77 ; IC 95 % : 0,60–0,98 ; p = 0,04). De plus, nous avons créé une variable afin de déterminer la relation dose-réponse entre l’exercice physique en force et la mortalité prématurée. Seuls les sujets réalisant 8 à 14 sessions dans un période de 30 jours correspondant aux recommandation actuelles de la société nationale américaine avaient une réduction du risque de mortalité toutes causes (hazard ratio : 0,70 ; p = 0,02). Les résultats étaient similaires pour ce qui concerne la mortalité cardiovasculaire.
Conclusion
Les recommandations américaines indiquant que 2 à 3 sessions d’exercice physique en force doivent être réalisées chaque semaine apparaissent efficaces pour réduire le risque de mortalité prématurée toutes causes ; cependant, malgré des recommandations, la majorité des adultes américains n’atteignent pas cet objectif. Des études ultérieures pourraient déterminer quelles stratégies pourraient contribuer à augmenter l’observance de ces recommandations.
Background
The American College of Sports Medicine (ACSM) and the United States Department of Health and Human Services (USDHHS) recommend that muscle-strengthening activities (MSA) be performed 2–3 times per week as part of a comprehensive exercise programme . The most commonly documented adaptations resulting from MSA involve increased muscle size and strength, both of which have been shown to be inversely associated with all-cause mortality . In addition to local muscle adaptations, MSA have been shown to provide a wide range of health benefits, including improvements in glucose metabolism , cholesterol concentrations , body composition , blood pressure and other cardiovascular variables that have been shown to help to prevent or attenuate risks associated with a wide range of chronic diseases (e.g. cancer, diabetes) .
While an inverse association appears to exist between aerobic-based physical activity and all-cause mortality , much less is known about the potential attenuating effects of behavioural participation in MSA, and its association with all-cause mortality. The majority of studies illustrating that muscle strength is associated with a decreased risk of all-cause mortality , along with the beneficial effects of MSA on various chronic diseases , would seem to suggest that participation in MSA may help to reduce the risk of premature mortality. Although strength is influenced by MSA, a large number of studies (aside from those previously mentioned) have illustrated an association between strength and mortality centred on hand-grip strength , which has been shown to be unaffected by participation in a full-body MSA intervention , and thus these studies may not be representative of behavioural participation in MSA. Additionally, all studies associating muscle strength with all-cause mortality are highly dependent on the mode of strength testing, which probably modulates the association between strength and all-cause mortality. Therefore, studies specifically examining the association between behavioural participation in MSA and mortality are needed, as MSA may be associated with mortality from factors other than strength gains (e.g. contraction-facilitated glycaemic control). The importance of behavioural participation in MSA and its effect on all-cause mortality may differ from that of strength because of the variability in baseline strength levels, as well as the variations in individual responses to MSA . To illustrate, some individuals completing 12 weeks of MSA increased muscle strength by 250%, while others saw minimal to no increase in strength ; however, these “non-responders” may have still benefited from the various other positive effects of MSA mentioned previously (e.g. reduced blood pressure). Therefore, the importance of examining behavioural participation in MSA is critical for a more precise measure of the true beneficial effects of MSA; after all, the nationally accredited government-issued guidelines are not based on muscle strength, but are rather centred on the behavioural participation in MSA .
Despite the importance of determining an association between MSA and all-cause mortality, to our knowledge, only three published studies have examined this association, demonstrating that individuals who engaged in ≥ 2 MSA sessions per week were at a small, non-significant, reduced risk of premature all-cause mortality after adjustment for potential confounding variables . These studies, however, applied dichotomous variables to determine the effectiveness of MSA, and therefore could not delineate whether additional participation in MSA, above meeting established guidelines, had a more protective effect against premature mortality. Therefore, the purpose of our study was to determine the effect of MSA on all-cause mortality, and to examine a potential dose-response relationship between the frequency with which MSA is performed and the incidence of all-cause mortality.
Methods
Study design
The 2003–2006 National Health and Nutritional Examination Survey (NHANES) data were used for the calculation of baseline characteristics of 8772 adults (age ≥ 20 years). The NHANES is an ongoing survey conducted by the Center for Disease Control and Prevention, designed to evaluate the health of citizens of the USA through a complex, multistage, stratified, clustered probability design. All individuals included in the analyses provided informed consent, and all study procedures were approved by the ethics review board of the National Center for Health Statistics. Participants were followed for a period of up to 9 years, with the average follow-up being 6.7 years (standard error: 0.20 years).
Muscle-strengthening activities
Individuals self-reported their involvement in MSA at the initial baseline assessment by responding to two questions: “Over the past 30 days, did you do any physical activities specifically designed to strengthen your muscles, such as lifting weight, push-ups or sit-ups?”; and if so “Over the past 30 days, how many times did you do these activities designed to strengthen your muscles, such as lifting weights, push-ups or sit-ups?” Those answering “no” to the first question were coded as engaging in “0” MSA in the past 30 days. Evidence of the validity of self-reported participation in MSA has been demonstrated .
All-cause mortality
All-cause mortality was determined as the number of individuals who died during the follow-up period, by matching the individual’s identification information with the National Death Index of the National Center for Health Statistics. Individuals from the NHANES were linked to the National Death Index using common identifiers (e.g. name, age, sex, date of birth, etc.), and positive matches were then examined manually for further verification, while individuals who had no signs of mortality were considered alive at the census measure. CVD-specific mortality was assessed from diseases of the heart, spanning International Classification of Diseases (ICD)-9 and ICD-10 coding (I00–I09, I11, I13, I20–I51).
Statistical analysis
Statistical analyses were performed using Stata, version 12.0 (StataCorp, College Station, TX, USA), and all analyses accounted for the complex survey design employed in the NHANES (analysed in 2015). An adjusted Wald test was used to examine statistical differences for continuous variables (e.g. age), and a design-based likelihood ratio test was used for categorical variables (e.g. sex) ( Table 1 ). To examine the associations between engagement in MSA and all-cause mortality, a weighted Cox proportional hazard model was employed to calculate hazard ratios (HRs) and 95% confidence intervals (CIs), while adjusting for the following covariates ( Table 2 ): age (years; continuous); sex; ethnicity (white/other); education (some college or more/high school or less); self-reported physical activity (see Table 2 for categorization); serum cotinine (ng/mL; continuous); serum C-reactive protein (mg/dL; continuous); overweight/obese (body mass index ≥ 25.0 kg/m 2 ); and comorbid illness. With regard to the comorbid illness variable , participants were classified as having 0–8 of the following physician-diagnosed conditions: arthritis; chronic bronchitis; congestive heart failure; coronary artery disease; emphysema; heart attack; hypertension; and stroke. Additional analyses were computed with several of the morbidities split out as independent covariates, as opposed to using a morbidity index, and the results were unchanged.
| Variable | Baseline point estimates (95% CI) | P | |
|---|---|---|---|
| Alive at follow-up ( n = 7915) | Dead at follow-up ( n = 857) | ||
| Demographics | |||
| Mean age (years) | 46.6 (46.2–47.0) | 72.7 (71.8–73.6) | < 0.001 |
| Women (%) | 52.7 (51.6–53.8) | 42.2 (38.9–45.5) | < 0.001 |
| Non-Hispanic white (%) | 50.5 (49.4–51.6) | 64.9 (61.7–68.1) | < 0.001 |
| Some college education (%) | 48.9 (47.8–50.0) | 33.0 (29.8–36.1) | < 0.001 |
| Participation in MSA | |||
| Engaged in MSA in past 30 days (%) | 26.7 (25.8–27.7) | 12.9 (10.7–15.2) | < 0.001 |
| Mean number of MSA sessions in past 30 days | 3.85 (3.64–4.06) | 2.46 (1.78–3.15) | < 0.001 |
| Average level of daily physical activity (%) | |||
| Sits during the day without walking about very much | 21.6 (20.7–22.6) | 44.5 (41.2–47.9) | |
| Stands or walks about a lot during the day, but does not have to carry or lift things often | 52.9 (51.8–54.0) | 43.9 (40.6–47.3) | |
| Lifts light loads or climbs stairs or hills often | 17.3 (16.4–18.1) | 9.1 (7.1–11.0) | |
| Does heavy work or carries heavy loads | 8.0 (7.4–8.6) | 2.3 (1.3–3.3) | |
| Health variables | |||
| Mean CRP (mg/dL) | 0.45 (0.43–0.47) | 0.71 (0.61–0.81) | < 0.001 |
| Overweight/obese (%) | 69.6 (68.6–70.6) | 65.2 (62.0–68.4) | 0.01 |
| Mean number of comorbidities | 0.72 (0.70–0.74) | 1.88 (1.78–1.98) | < 0.001 |
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