Introduction We examined the effects of aging on passive stiffness, size, quality, and strength characteristics of the posterior hip and thigh muscles. Methods Fifteen young (25 ± 3 years) and 15 old (72 ± 5 years) men participated in this study. Echo intensity (EI) and cross-sectional area (CSA) were determined from ultrasound scans of the hamstrings. Straight-leg raises were used to assess passive stiffness, which was calculated from the slopes of the initial (phase 1) and final (phase 2) portions of the angle–torque curve. Peak torque (PT) and rate of torque development (RTD) were assessed through maximal voluntary contractions. Results Phase 2 slope and EI were higher (P ≤ 0.024), and CSA, PT, and RTD were lower (P ≤ 0.011) in the old compared with the young men; however, no difference (P = 0.145) was observed for phase 1 slope. Conclusions Decreases in muscle quality, as indicated by increases in EI, may contribute to the greater passive stiffness observed in older adults.