Muscle function is determined by structure and morphology at the architectural level. In response to resistance training, older adults have demonstrated that the neuromuscular system has a substantial adaptability, which may compensate for muscle size and quality and lead to improved functional capacities and higher quality of life. PURPOSE: The purpose of this study was to examine the effect of six weeks of progressive resistance exercise on muscle morphology and architecture in healthy older adults. METHODS: Twenty- five healthy men and women were randomly assigned to either six weeks of progressive resistance training (RT) (n=13; age = 71.08 ± 6.75, BMI = 28.5 ± 5.22) or to serve as a control (CON) (n = 12; age = 70.17 ± 5.58, BMI = 27.52 ± 5.6). Fat mass (FM), lean mass (LM), and lean thigh mass (LTM) were evaluated using dual x-ray absorptiometry. Lower body strength was estimated by predicting maximal knee extensor strength (1RM). Muscle quality (MQ) was evaluated as strength per unit mass (kg/kg). Cross-sectional area (CSA), muscle thickness (MT), fascicle length (Lf), pennation angle (cosΘ), and echo intensity (EI) of the rectus femoris (RF) and vastus lateralis (VL) were collected using B-mode ultrasound and extended field of view (FOV) ultrasound. EI was quantified using grayscale analysis software. Strength per unit of echo intensity (REI) was determined by dividing 1RM by EI of the thigh. Physiological cross-sectional area (PCSA) was calculated as the ratio of (CSA x cosΘ) / (EI x Lf). A 2x2 (group [exercise vs. control] x time [pre vs. post]) repeated measures ANOVA was used to identify group differences and group x time interactions and stepwise regression was performed to assess variables related to strength. RESULTS: 1RM increased by 31.9% (p ≤ 0.01) in the RT group and was significantly correlated to PCSA of the thigh (r = .579; p = .003) at baseline. MQ increased 31.4% (p ≤ 0.01) in the RT group consistent iv with an REI increase of 33.3% (p ≤ 0.01). There were no significant changes in LTM in either group. VL CSA increased 7.4%, (p ≤ 0.05) and demonstrated a significant interaction (p ≤ 0.05) in the RT group. There were no significant changes in the CON group for 1RM, MQ, REI or VL CSA. PCSA demonstrated a significant (p ≤ 0.05) group x time interaction but did not significantly change in either group. EI did not significantly change in the RT or CON groups. CONCLUSION: Calculated PCSA of the thigh assessed by ultrasound was related to the force producing capacity of muscle and demonstrated a significant interaction following resistance training. Short term resistance exercise training was effective in increasing 1RM, muscle quality as relative strength, muscle quality as relative echo intensity, and muscle morphology, but not EI. In addition, ultrasonography appears to be a safe, feasible, informative and sensitive clinical technique to aid in our understanding of muscle strength, function, and quality.