Assessing the qualitative validity of a non-invasive ultrasound technique for estimating muscle glycogen change
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Intramuscular glycogen is a primary fuel source for intense exercise of any substantial duration, and the use of an invasive muscle biopsy is still the gold-standard of measurement. The purpose of this study is to assess the qualitative validity of a proprietary algorithm (MuscleSound®) for determination of intramuscular glycogen concentration against the body of literature on muscle glycogen depletion from exercise and subsequent recovery. To test whether this new technology appears valid, 12 healthy recreational students (n=1 female) (25.5 ± 1.25y, 74.03 ± 2.58kg, 174 ± 4.48cm) were recruited from the University of Texas at Austin campus for participation in this study. Subjects performed six bouts of six-minute efforts on a stationary cycle ergometer at 84% of their VO2peak; an exercise protocol designed to elicit substantial muscle glycogen depletion. Care was taken to maintain body hydration status to further elucidate whether the device was truly measuring glycogen or a proxy such as intramuscular water. Each interval was followed by a six-minute rest period, during which time ultrasound images were taken of the vastus lateralis (VL), rectus femoris (RF) and gastrocnemius/soleus (GS) muscles. Images were processed using the MuscleSound® (MS) algorithm which assigns a MS score (0-100) based on the opacity of the image. Following the exercise protocol, subjects remained in the laboratory for a six-hour recovery phase, wherein images were taken at the same three sites at two-hour intervals. A high carbohydrate (CHO) and protein (PRO) beverage was consumed immediately following exercise, and after two and four hours in order to assess the MS algorithm’s ability to capture the qualitative pattern of glycogen resynthesis established in the literature. It was hypothesized that the VL and RF muscles would decrease in a curvilinear fashion during the intervals to a greater extent than the GS and that during the recovery phase these muscles would show an increase in MS units every two hours for six hours. In support of our hypothesis, we found that during exercise, MS values decreased in a curvilinear pattern in the VL with significantly lower values after bouts 3 through 6 versus baseline. The RF showed a similar pattern with bouts 4 through 6 being significantly lower than baseline, while the GS remained constant throughout. After 120 minutes of recovery the MS values of the VL and RF had returned to baseline levels. It is established in the literature that a nearly depleted muscle should take approximately 24 hours to return to capacity (36); therefore it is unclear whether the MS technology is truly capturing muscle glycogen in its images. While an effective qualitative tool, more work should be done to determine if MS is a valid quantitative measurement of muscle glycogen.