Ultrasonic assessment of exercise-induced change in skeletal muscle glycogen content

BACKGROUND: Ultrasound imaging is a valuable tool in exercise and sport science research, and has been used to visualize and track real-time movement of muscles and tendons, estimate hydration status in body tissues, and most recently, quantify skeletal muscle glycogen content. In this validation study, direct glycogen quantification from pre-and post-exercise muscle biopsy samples was compared with glycogen content estimates made through a portable, diagnostic high-frequency ultrasound and cloud-based software system (MuscleSound®, Denver, CO). METHODS: Well-trained cyclists (N = 20, age 38.4 ± 6.0 y, 351 ± 57.6 watts(max)) participated in a 75-km cycling time trial on their own bicycles using CompuTrainer Pro Model 8001 trainers (RacerMate, Seattle, WA). Muscle biopsy samples and ultrasound measurements were acquired pre- and post-exercise. Specific locations on the vastus lateralis were marked, and a trained technician used a 12 MHz linear transducer and a standard diagnostic high resolution GE LOGIQ-e ultrasound machine (GE Healthcare, Milwaukee, WI) to make three ultrasound measurements. Ultrasound images were pre-processed to isolate the muscle area under analysis, with the mean pixel intensity averaged from the three scans and scaled (0 to 100 scale) to create the glycogen score. Pre- and post-exercise muscle biopsy samples were acquired at the vastus lateralis location (2 cm apart) using the suction-modified percutaneous needle biopsy procedure, and analyzed for glycogen content. RESULTS: The 20 cyclists completed the 75-km cycling time trial in 168 ± 26.0 minutes at a power output of 193 ± 57.8 watts (54.2 ± 9.6% watts(max)). Muscle glycogen decreased 77.2 ± 17.4%, with an absolute change of 71.4 ± 23.1 mmol glycogen per kilogram of muscle. The MuscleSound® change score at the vastus lateralis site correlated highly with change in measured muscle glycogen content (R = 0.92, P < 0.001). CONCLUSIONS: MuscleSound® change scores acquired from an average of three ultrasound scans at the vastus lateralis site correlated significantly with change in vastus lateralis muscle glycogen content. These data support the use of the MuscleSound® system for accurately and non-invasively estimating exercise-induced decreases in vastus lateralis skeletal muscle glycogen content.