The Need for Standardized Assessment of Muscle Quality in Skeletal Muscle Function Deficit and Other Aging-Related Muscle Dysfunctions: A Symposium Report

Abstract

A growing body of scientific literature suggests that not only changes in skeletal muscle mass, but also other factors underpinning muscle quality, play a role in the decline in skeletal muscle function and impaired mobility associated with aging. A symposium on muscle quality and the need for standardized assessment was held on April 28, 2016 at the International Conference on Frailty and Sarcopenia Research in Philadelphia, Pennsylvania. The purpose of this symposium was to provide a venue for basic science and clinical researchers and expert clinicians to discuss muscle quality in the context of skeletal muscle function deficit and other aging-related muscle dysfunctions. The present article provides an expanded introduction concerning the emerging definitions of muscle quality and a potential framework for scientific inquiry within the field. Changes in muscle tissue composition, based on excessive levels of inter- and intra-muscular adipose tissue and intramyocellular lipids, have been found to adversely impact metabolism and peak force generation. However, methods to easily and rapidly assess muscle tissue composition in multiple clinical settings and with minimal patient burden are needed. Diagnostic ultrasound and other assessment methods continue to be developed for characterizing muscle pathology, and enhanced sonography using sensors to provide user feedback and improve reliability is currently the subject of ongoing investigation and development. In addition, measures of relative muscle force such as specific force or grip strength adjusted for body size have been proposed as methods to assess changes in muscle quality. Furthermore, performance-based assessments of muscle power via timed tests of function and body size estimates, are associated with lower extremity muscle strength may be responsive to age-related changes in muscle quality. Future aims include reaching consensus on the definition and standardized assessments of muscle quality, and providing recommendations to address critical clinical and technology research gaps within the field.

Keywords: imaging; muscle power; muscle quality; muscle strength; myosteatosis; sarcopenia; skeletal muscle function deficit.

Figure 1

Potential mechanisms underlying the effects of myosteatosis. Increased myosteatosis may lead to metabolic and mechanical changes in the muscle through a variety of mechanisms. Changes in muscle cell metabolism can lead to increased insulin resistance and inflammation, aiding in the development of diabetes, and cardiovascular diseases. Alterations in muscle architecture can also lead to muscular dysfunction and functional decline. Both processes may be increased through activation of proteolytic systems, which may also result from increased myosteatosis.

Figure 2

A proposed performance-based index of muscle power. (A–D) The Muscle Quality Index (MQI) is a performance-based functional assessment involving ten repetitions of the sit-to-stand maneuver performed as rapidly as possible. The test requires the use of a scale to record body mass (A), a tape measure to obtain leg length (B), along with a stopwatch and chair and for the timed functional task (C,D). The MQI score is calculated using the following equation: ((leg length × 0.4) × body mass × gravity × 10)/sit-to-stand time.

Figure 3

Relative peak grip force and muscle echogenicity expressed in grayscale units. Grayscale measures derived from the echogenicity of the rectus femoris have an inverse relationship with relative grip strength (peak force scaled to body weight). The filled data markers () represent study participants with Class I sarcopenia (5.76–6.75 kg/m²) based on lean body mass estimates from DXA scanning, and the clear data markers (◦) represent study participants with normal body mass (>6.75 kg/m²). BW, body weight; scaled peak grip force, peak force in kg/body weight in kg; both scaled peak force and grayscale values are unitless measures.

Figure 4

Variation in shear wave elastography secondary to the applied scanning force. Significant variation in shear wave elastography estimates of tissue Young's modulus shown in the figure is a function of preload differences typical of clinical sonography. The varying preload conditions depicted are typical of those seen across a range of operators in routine abdominal sonography and the resultant change in estimated tissue Young's modulus. This variation is explained by the observation that different bias compression levels pre-strain the tissue to different operating points along the tissue's non-linear stress-strain response. Estimated Young's modulus increases from 21.1 to 64.1 kPa in the vastus medialis as applied force (preload) increases from 1 to 18 N.

Figure 5

Ultrasound images of the biceps from a healthy subject at four different forces. Variation in the muscle thickness (denoted by the height of the yellow boxes), based on the measurement from the bone to the subcutaneous fat-muscle separation layer, is highly dependent on the examiner-generated force during scanning.

Figure 6

Sound transducer localization using unique skin features. A freehand ultrasound platform featuring an optical camera kinematically coupled to an ultrasound transducer may be used with software that extracts unique skin features and provides transducer localization relative to the skin features. The ultrasound transducer registration facilitates the acquisition of three-dimensional ultrasound volume estimates derived from a standard optically tracked two-dimensional sonograms. This approach allows for scanned images to be generated and reformatted in any plane to allow for the comparison of matched images across serial examinations.