Aim The time‐course for changes in muscle size and strength is not well understood, particularly in women. In addition, contributions of muscle size to strength are commonly assessed utilizing a pre–post change score; however, a more appropriate within‐subject correlational analysis has never been used. Methods To determine the time‐course for thigh muscle size and strength in young (aged 18–25 years) and older (aged 50–65 years) women, and determine the relationship between size and strength after 8 weeks of training carried our three times per week. Results Anterior muscle thickness at the 50% site increased after 1 week of training, and exceeded the measurement error after 2 weeks (mean difference 0.23 cm, 95% CI 0.12–0.34 cm). Anterior muscle thickness at the 70% site increased and exceeded the measurement error after 2 weeks (mean difference 0.30 cm, 95% CI 0.15–0.45 cm). Posterior muscle thickness at the 50% site increased after 1 week of training, and exceeded the measurement error after 2 weeks (mean difference 0.24 cm, 95% CI 0.03–0.44 cm). Posterior muscle thickness at the 70% site increased after 1 week, and exceeded the measurement error after 3 weeks (mean difference 0.21 cm, 95% CI 0.07–0.35 cm). Muscle strength, assessed through one‐repetition maximum, increased by 2 weeks in most measurements, and the within‐subject variance explained by changes in muscle size ranged between 9% and 35%. Conclusions Muscle growth appeared to occur early into a training program in both young and older women. Furthermore, although a large portion remains unexplained, the variance in muscle strength explained by muscle size is greater than previous studies not utilizing within‐subject correlations. Geriatr Gerontol Int 2017; ••: ••–••.