COMMON assumptions made in geriatric rehabilitation studies are that (a) a direct causal relationship exists between skeletal muscle mass and strength, and (b) interventions designed to improve mass and/or strength should translate into greater physical functional independence. Accordingly, over the past several years much effort has gone into developing interventions to increase skeletal muscle mass and/or strength in older adults. Examples of such interventions include strength training alone (1)(2), in combination with nutritional supplementation (3)(4), or with growth hormone (5)(6) and hormonal therapies alone (7). Although such studies have provided some clarification on the feasibility of different interventions to prevent or attenuate loss of muscle mass and strength in old age, very few studies have actually addressed important questions regarding the “quality” of the remaining muscle. Examples of skeletal muscle properties that could contribute to its overall quality include fiber-type composition, contractile properties, innervation, capillarity, and metabolic capacity. The possibility remains that age-associated changes in muscle quality (MQ), aside from decreased mass, could play important roles in the development of physical functional problems in older adults. In this regard, it would be important to determine how MQ correlates with function, both in terms of muscle tissue and in terms of its effect on an individual's capacity to perform everyday activities. A heightened awareness on the part of researchers to other factors contributing to muscle function (aside from mass and strength) is likely to yield more effective interventions to prevent or slow down physical dysfunction in old age.
The present study by Ivey and colleagues (pp. B152–B157) illustrates the importance of considering the muscle mass–MQ issue with regard to responses of young and older individuals to strength training. The key results of this study were that 9 weeks of strength training resulted in a significantly increased MQ (ratio of muscle strength to volume) of the quadriceps in all age groups, with the largest magnitude of change observed in young women. Following a detraining period of 13 weeks, the strength-training-induced gains in MQ of the quadriceps were significantly preserved in all groups, except older women. The preservation of MQ during detraining was attributable to a lesser decline in strength compared with loss of muscle mass. These results raise some important considerations for future studies of muscle quality in old age.
One of the major points is that how one chooses to define MQ could potentially increase the mechanistic insight gained from a study. For example, Ivey and colleagues defined MQ as muscle function per unit of muscle mass (i.e., ratio of strength measured by 1 RM/muscle volume). Muscle function may be defined in a variety of ways such as 1 RM, muscle peak power, or torque. In the specific case of the 1-RM measurement used in this study, a number of factors should be considered in interpreting the results. Such factors may include initial strength levels of the subjects or the training mode (8). Similarly, muscle mass may be assessed by its component parts, contractile/noncontractile content (9), or examination of specific muscle properties, such as the measurement of myosin heavy chain from other muscle proteins (10), as well as by exploration of changes in muscle endurance capacity through a measurement of mitochondrial protein synthesis (11). In particular, because older individuals can have a greater noncontractile component in their muscle cross-sectional area compared with younger individuals (9), it is important for future studies to differentiate between the contractile and noncontractile components of the muscle volume. Finally, differences in innervation can affect muscle mass, morphology, and function. Therefore, the long-term effects of age-related changes in innervation patterns (12) on MQ have to be determined, as do the reversibility of these effects.
Another factor that should be accounted for in future studies of MQ is the habitual level of physical activity. Clearly an individual's level of physical activity would likely be reflected in his or her functional capacity. Also, it is known that the cumulative effect of physical activity and variations caused by type and/or intensity of training can influence the fiber type composition of muscle (13). Thus, a consideration of physical activity patterns is necessary to accurately interpret the results of studies of muscle quality and aging. The resolution of these muscle mass–MQ issues will have important implications in the development of novel interventions in geriatric rehabilitation. Future interventions for physical functional problems may be targeted toward the improvement of specific muscle properties versus increases in mass or strength.