forces imparted to the skeleton during movement, as well as the effect of increased muscle mass on skeletal loading. While it is difficult to separate those two effects (Beck, 2009), suggestive evidence points to a positive association between measures of musculoskeletal fitness (especially muscle strength and power) and bone health in adults (Ashe et al., 2008; Cooper et al., 2011; von Stengel et al., 2005, 2007). Positive associations also have been reported between muscle strength and power and better quality of life, lower risk of falls and fractures, and reduced morbidity and mortality (Cooper et al., 2011; von Stengel et al., 2005, 2007). Likewise, muscle weakness has been identified as a risk factor for osteoarthritis in this population (Garber et al., 2011).
The validity of the relationships described above is further corroborated by evidence for the effect of resistance training programs on muscle strength, endurance, and power, along with changes in various health outcomes. Resistance training programs now are generally accepted as being effective at improving muscle strength, endurance, and power in both sexes, across all ages during adulthood, and for both healthy adults and those with chronic disease or disability (McCartney and Phillips, 2007; Reid and Fielding, 2012; Williams et al., 2007). These programs also have resulted in a multitude of adaptations that foster better health among adults, such as improved body composition, blood glucose and insulin regulation, systemic arterial blood pressure in prehypertensives, blood lipid and lipoprotein profiles, bone health and management of arthritic pain and disability, and prevention or improved management of the metabolic syndrome (Garber et al., 2011; McCartney and Phillips, 2007; Williams et al., 2007). Similarly, resistance training has resulted in enhanced exercise and functional capacity, improved balance, and decreased falls (Garber et al., 2011; McCartney and Phillips, 2007). Resistance training may also improve quality of life and self-efficacy and moderate levels of depression and anxiety among adults (Garber et al., 2011; McCartney and Phillips, 2007; Williams et al., 2007).
Literature Review Process
The CDC’s systematic review of the literature included muscle strength and muscle endurance, but not muscle power, as components of fitness because they are the dimensions of musculoskeletal fitness that have been used most frequently in fitness test batteries. The muscle strength search screened 2,642 reports, only 63 of which satisfied the CDC search criteria for further consideration and were abstracted. Of this subset of 63 studies, 23 were classified as experimental, 22 as experimental with no control, 12 as quasi-experimental, and 6 as longitudinal.