Background: There is a lack of research investigating the effect of hydration status or intermuscular glycogen content on ultrasound-derived muscle quality imaging in younger adults. Purpose: To examine the effects of varied hydration and muscle glycogen status on ultrasound-measured vastus lateralis (VL) cross-sectional area (CSA) and echo intensity (EI). Materials and Methods: Thirteen young adults (Age: 23 ± 4.2 yrs; Height: 168.5 ± 6.8 cm; Weight: 76.2 ± 17.1 kg) participated in five experimental visits: (1) Baseline (regular dietary intake); (2) hypohydrated and normal carbohydrate (CHO) intake (HH-NC); (3) euhydrated and normal CHO intake (EH-NC); (4) hypohydrated and CHO restriction (HH-CR); and (5) euhydrated and CHO restriction (EH-CR) conditions. Ultrasound imaging of the right VL was used to assess CSA and EI at each visit. A two-way repeated measures ANOVA with Holm adjusted post hoc test was performed for statistical analysis. Results: Body mass was affected by both hydration and carbohydrate restriction (both p < 0.001), while Usg was greater during hypohydration (p < 0.001). VLCSA was lower following carbohydrate restriction (p = 0.010), independent of hydration status, whereas VLEI was not significantly affected by hydration, carbohydrate restriction, or their interaction (all p ≥ 0.063). Conclusions: Overall, these findings suggest that an individual’s hydration and CHO intake may influence ultrasound-derived muscle CSA assessments. Future research should continue to explore the sensitivity of EI and CSA to determine the impact of these findings in both clinical and research environments.

High-intensity interval training (HIIT) can mitigate age-related declines in muscle mass and strength that lead to decreased functional ability and loss of independence. However, previous HIIT protocols are often treadmill or recumbent bike-based with limited whole-body functional exercises and often do not include older adults (OA) beyond 70 years of age. The aim of this study was to examine the effects of a 12-week, Total Body HIIT (TB-HIIT) protocol on muscle size (cross-sectional area [CSA] of the rectus femoris [RF], vastus lateralis [VL], and vastus medialis [VM]), muscle strength (peak toque [pT] and rate of torque development [pRTD]), muscle activation (peak electromyography [pEMG] and rate of EMG rise [pRER]), cardiovascular endurance (6-minute walk test [6MWT]), and quality of life (QOL, 36-Item Short Form [SF-36]) in OA. Older adults, 65-95 years of age, were recruited and completed all outcomes at baseline and following the 12-weeks of TB-HIIT. The TB-HIIT protocol progressively increased the exercise-to-rest ratio every four weeks (20sec:40sec, 30sec:30sec, 40sec:20sec) with target intensity being 85-95% of participants’ heart rate max. Twenty-four OA (74.8□6.37 years, range=65-89) completed this study. Results showed significant increases in VL-CSA (p=0.02, g=-0.77), VM-CSA (p< 0.001, g=-1.03), cardiovascular endurance (p< 0.001, g=-0.95), and health-related QOL (p=0.012, g=-0.47). No significance was revealed in RF-CSA (p=0.068, g=-0.4), pT (p=0.78, g=-0.17), pRTD: p=0.91, g=-0.07), pEMG (p=0.77, g=-0.17) or pRER (p=0.51, g=-0.39). Future research should continue to explore the benefits of TB-HIIT with consideration towards OA subpopulations with comorbidities and greater physical impairment to advance exercise prescription and function for this population.