Performance fatigability during the 30-s sit-to-stand (STS) test is not well characterized despite its potential to detect early functional decline. Therefore, this study aimed to quantify temporal changes in power, trunk flexion and movement subphase durations during the 30sSTS, and to examine differences by age and sex. 93 middle-aged adults (50 males and 43 females; mean age 60.5 ± 3.0 years) and 102 older adults (48 males and 54 females; mean age 71.5 ± 5.0 years) performed a 30sSTS. Inertial measurement units (IMUs) mounted over the L4/L5 vertebral level were used to capture sit-to-stand power, trunk flexion and subphase durations in the first and last 10 s. Linear mixed-effects models evaluated temporal changes and group effects. Mean power declined (-11.9 W, d = -0.66), trunk flexion increased (+1.35°, d = 0.42), sit-to-stand duration lengthened and stand-to-sit duration decreased throughout the test (all p < 0.001). The within-test decrease in stand-to-sit duration was less pronounced in older compared to middle-aged adults (d = 0.42, p = .039). Older adults generated less power and spent more time in all subphases (p < 0.05). Females produced less power and greater trunk flexion (p < 0.001). The 30sSTS captures modest performance fatigability; but longer protocols may better reveal clinically meaningful decline. Future research should investigate mobility-limited individuals or examine associations with functional outcomes (frailty, mobility, balance) to provide additional insight.