Examining Muscle Architecture and Muscle Quality Noninvasively
A relatively new noninvasive method to investigate changes in muscle architecture is becoming popular. The use of ultrasonography to image muscle structure has proven to be a valid and reliable method of examining changes in muscle. The validity of ultrasound measures has been compared to that of both computed tomography (CT scans) and magnetic resonance imaging for analyzing large muscle groups (Reeves, Maganaris, and Narici 2004; Thomaes et al. 2012). The benefit of ultrasound compared to these other methods is related to the lower cost of operation and ease of use. With the other methods, physician prescription and the use of radiology technicians are often required.
Ultrasound measures are commonly used to assess muscle thickness, muscle cross-sectional area, fascicle length, and muscle fiber pennation angle. Improvements in our understanding and use of ultrasonography have resulted in a relatively new measure known as echo intensity. Echo intensity is the quantification of muscle quality using grayscale analysis from image analysis software (Cadore et al. 2012). The imaging software analyzes the pixel count that is obtained using the ultrasound image of skeletal muscle (generally the vastus lateralis, rectus femoris, or both) and provides a score that is based on an arbitrary units scale ranging from 0 to 256 (0: black; 256: white). Figure 1.9 provides an example of an echo intensity analysis. A lower number (lower echo intensity) is reflective of an increase in muscle quality. Echo intensity is indicative of the architectural characteristics of muscle, including the infiltration of adipose, connective, and other noncontractile tissue into the muscle. Skeletal muscles with greater lean tissue, less fat, and connective tissue have a lower echo intensity.
Echo intensity of the rectus femoris.
In a study of older (70.4 Â± 5.5 years), healthy Japanese women, echo intensity values were found to significantly correlate with isometric knee extensor strength (r = -0.40, p \lte\ 0.01), independent of age or muscle thickness of the quadriceps femoris (Fukumoto et al. 2012). In addition to its use as a marker of muscle quality, echo intensity may provide a sensitive and reliable measure of muscle damage and recovery. Radaelli and colleagues (2012) assessed echo intensity prior to the onset of exercise and following four sets of 10 repetitions at 80% of the subjects’ maximal elbow flexor strength in the dominant arm at 24, 48, and 72 hours postexercise. Echo intensity values were significantly elevated from preexercise values at all three time points, whereas no change from baseline was seen in the nondominant (no-exercise) arm. These results suggested that echo intensity may be quite useful as a sensitive marker for recovery from resistance exercise. A number of studies on this unique measure of muscle quality will be published over the next few years.