Robusticity, as measured by a ratio of bone thickness to bone length, is fundamentally determined by biomechanical stress and the presence of androgens. There is a cline of less robusticity as one goes across the various species of Homo (H. Erectus had the most muscle, H. Sapiens has the least).

Early Modern Homo Sapiens had more robust arms and legs than us today, simply because they walked more, and perhaps on rugged and hilly terrain. They were also engaging in throwing activities (first the boomerang, then the atlatl), which has a certain effect on the humeral head and femur. Domestic activities such as scraping and flintknapping would also have an effect on upper arm strength.

Both early humans (before 100 kya) and Neanderthals seem to have been running even more than today’s marathon runners (ScienceDirect). This same pattern, interestingly, is seen in Khoi San people, who are known to run after animals for hours after being speared or shot with an arrow. Tracking is likely the reason for running in these ancient humans.