Dynapenia and aging: an update

Abstract

In 2008, we published an article arguing that the age-related loss of muscle strength is only partially explained by the reduction in muscle mass and that other physiologic factors explain muscle weakness in older adults (Clark BC, Manini TM. Sarcopenia =/= dynapenia. J Gerontol A Biol Sci Med Sci. 2008;63:829-834). Accordingly, we proposed that these events (strength and mass loss) be defined independently, leaving the term "sarcopenia" to be used in its original context to describe the age-related loss of muscle mass. We subsequently coined the term "dynapenia" to describe the age-related loss of muscle strength and power. This article will give an update on both the biological and clinical literature on dynapenia-serving to best synthesize this translational topic. Additionally, we propose a working decision algorithm for defining dynapenia. This algorithm is specific to screening for and defining dynapenia using age, presence or absence of risk factors, a grip strength screening, and if warranted a test for knee extension strength. A definition for a single risk factor such as dynapenia will provide information in building a risk profile for the complex etiology of physical disability. As such, this approach mimics the development of risk profiles for cardiovascular disease that include such factors as hypercholesterolemia, hypertension, hyperglycemia, etc. Because of a lack of data, the working decision algorithm remains to be fully developed and evaluated. However, these efforts are expected to provide a specific understanding of the role that dynapenia plays in the loss of physical function and increased risk for disability among older adults.

Figure 1.

The age-related loss of muscle strength is weakly associated with the loss of muscle mass. These figures were adapted from published data obtained from the Health ABC Study to examine the relationship between changes in knee extensor strength and quadriceps femoris cross-sectional area muscle (measured via computed tomography) in a 5-y longitudinal study of older adults (21). These data represent the annualized rate of loss more than 5 y in older adults that lost body weight (left panel; n = 309 men) and gained body weight (right panel; n = 143 men). Note that (a) muscle strength is lost at a substantially faster rate than muscle mass and (b) that gaining muscle mass does not prevent the aging-related loss of muscle strength (right panel). Adapted from Delmonico and colleagues (21). Created figure approved by the corresponding author (M. J. Delmonico).

Figure 2.

Relative risk of poor physical performance, functional limitation, or physical disability in older adults with dynapenia (low muscle strength), or sarcopenia (low muscle mass). The counterfactuals are older adults with normal muscle strength or mass. Studies investigating multiple outcomes or expressing findings by sex are repeated. The author of each study is followed with whether the relative risk was estimated in men (M), women (W), or both (M/W). Symbols indicate whether outcome was self-report physical function/disability (*) or observed physical performance (^). Specific information on each study is provided in the Supplementary Table.

Figure 3.

Theoretical model of the potential neurologic and muscular factors leading to dynapenia. Reprinted with permission from (47).

Figure 4.

Older adults exhibit impairment in their nervous systems ability to fully activate the knee extensor muscles. This cross-sectional data are the largest study to date examining differences between young and old in central activation and represent data from 46 young (18–32 y) and 46 older (64–84 y) humans. Adapted from Stevens and colleagues (64). Created figure approved by the corresponding author (S. K. Stackhouse).

Figure 5.

Older animal skeletal muscle exhibits a reduction in it's intrinsic force-generating capacity, particularly at high stimulation frequencies. This cross-sectional data were obtained from young (6–8 mos) and older (24 mo) rats by normalizing the electrically stimulated plantarflexor maximal force to the muscle weight. At the higher stimulation frequencies the older—but not yet senescent—muscle exhibited a greater than 30% reduction in its intrinsic force capacity. It should be noted that this reduction occurred in the presence of a very limited amount of atrophy. Modified with permission from Russ and colleagues (102) Modified figure approved by the corresponding author (D. W. Russ).

Figure 6.

Working decision algorithm to define dynapenia. In an attempt to facilitate discussions on dynapenia, we have established a Web site blog where we invite comments and input on general and specific components of the algorithm: http://dynapenia.blogspot.com.