Musculoskeletal markers are frequently used to reconstruct past lifestyles and activity patterns. Yet the reliability of muscle marker measurements has been called into question because they may be confounded by body size. In this study, an aggregate muscle marker variable was calculated using 20 insertion sites (14 femoral, 6 tibial), and I examined their effects on lower limb size (as a proxy for body size), age, and sex. Analyses were made of a sample of 77 (57 males, 20 females) Native British Columbians (3,500–1,500 years BP) and 18th century Quebec prisoners. Muscle markers were measured using two-point observer rating scales; size was measured by standard methods; and age and sex were determined through pelvic, cranial, and dental morphology. Lower limb muscle markers correlated with: age, r = 0.61; lower limb size, r = 0.52; and sex, r = 0.49; P < 0.001. Older individuals had higher muscle marker scores, as did larger individuals and males. Based on partial correlations and regression analyses, age was the best overall predictor of lower limb muscle markers. Am J Phys Anthropol, 2004. © 2004 Wiley-Liss, Inc.