Ultrasound imaging for monitoring muscle atrophy in rehabilitation patients: A pilot study

Background. Recent research has shown that ultrasound imaging can provide a means of investigating fibre orientation in vivo. Measured by ultrasound, the pennation angle in the quandriceps was found to be highly correlated with the muscle cross-sectional area (CSA). Kawakami et al. (1993) demonstrated that the pennation angle in the triceps brachii muscle of professional body-builders and normal subjects who completed a strengthening programme was significantly greater than that measured in sedentary subjects. In the current research, we were interested in determining whether muscle atrophy would lead to decreases in the pennation angle of muscle fibres and variations in fibre length. If this were so, it would provide rehabilitation specialists with a reliable non-invasive method for monitoring muscle atrophy and changes in muscle architecture during rehabilitation. Methods. Ten male patients with unilateral atrophy of the leg muscles (difference in circumference of at least 1.5 cm between, the two legs at the upper two- thirds of tibial length) due to different causes participated in the study. Scanning was performed with a real time computerized sonograph. A 7-5 MHz 4 cm long ultrasound probe was placed normal to the surface of the gastrocnemius medialis muscle in both the affected and the unaffected leg, and oriented along the median longitudinal axis of the muscle. Results. The results showed that the consistent decreases observed in the pennation angles of the muscle fibres corresponded to the degree of muscle atrophy. Specifically, the decrease in the anatomical CSA of the injured leg of each of our 10 patients was highly correlated to a decrease in pennation angle, which, on average, was 16% smaller than that in the uninjured leg. Atrophy also caused a decrease in fibre length of 13%. Conclusions. Skeletal muscle atrophy involves both structural and functional changes. Variations in muscle size entail not only a decrease in ACSA but also an internal rearrangement of fibre length and pennation angle. A major advantage of this technique is that images can be acquired dynamically and an instant picture of the muscle architecture can be obtained even during a muscle contraction.