Researchers here propose an approach to measure the progression of sarcopenia, the loss of muscle mass and strength with aging, via ultrasound assessment of muscle structure. The present most widely practiced approaches involve assessment of muscle mass, grip strength, walking speed, ability to stand up from a chair, and the like. As understanding of the underlying mechanisms of the condition grow, a more rigorous form of assessment becomes desirable, one that can hopefully be extended into detecting the earliest stages of sarcopenia, with an eye towards prevention.
While the definition of sarcopenia is an evolving concept that started with a classification based on muscle mass alone, this has progressively moved to a more operational definition that includes the loss not only of muscle mass but also of muscle strength, with a risk of adverse outcomes such as physical disability, poor quality of life, and even death. However, as recognized by the latest definition of sarcopenia, low muscle mass or quality is a determinant factor for confirming sarcopenia in the presence of low muscle strength, otherwise known as dynapenia. Hence, the measurement of muscle mass remains a key requirement for the clinical diagnosis of sarcopenia. Typically, this has been achieved using dual X-ray absorptiometry (DXA), MRI, or bioelectrical impedance. The use of these methods has been widespread and has been instrumental for the diagnosis of sarcopenia in clinical settings.
In 2003, we reported for the first time that the loss of muscle mass associated with sarcopenia not only entails a decrease in muscle cross-sectional area and volume but also alterations in the spatial arrangement of muscle fibres within the muscle. Knowledge of the spatial arrangement of muscle fibres within a muscle is particularly important because muscle architecture is one of the most important determinants of muscle force and velocity. Using ultrasonography, we were able to show, for several locomotor muscles, that the key parameters of muscle architecture are significantly altered in sarcopenic muscle.
If changes in muscle architecture were to scale harmonically with the decrease in muscle volume due to sarcopenia, one would expect the ratio of fascicle length (Lf) to muscle thickness (Tm) to remain constant. However, muscle length (and thus fascicle length) is constrained by its connections into the proximal and distal tendons that insert into bony structures. Although fascicle length has been found to decrease with ageing, this effect should be limited by the proximal and distal tendon insertions into bone, unless tendons were to elongate, which is most unlikely. Hence, the reduction in muscle mass with ageing should be due more to a decrease in muscle thickness than in fascicle length, that is, it should involve a greater loss of sarcomeres in parallel than in series. Recent observations made in our laboratory in different populations of older individuals (active, sedentary, and mobility impaired) seem to confirm this assumption: with increasing degree of sarcopenia, the decrease in muscle thickness (Tm) exceeds that of fascicle length (Lf).
These findings prompted us to formulate the hypothesis that the Lf/Tm ratio, which we shall refer to as 'ultrasound sarcopenia index' (USI), may be used as a marker of the loss of muscle mass associated with sarcopenia. An important advantage of using a marker based on an anatomical ratio rather than on absolute values is its independence from gender and body dimensions. Further, there are several important advantages to be considered regarding the use of ultrasound for assessing muscle architecture, both for clinical and practical purposes. Ultrasound can be delivered at a fraction of the cost of MRI, and has a very good reliability and reproducibility when performed by properly trained personnel.