it feels like they're pushing their arm back when it straightens out like that, keep working with it.
Also, they alternate with the legs. So when blueleg is going forward, so would red arm, and when (orange? yellow? colorblind, can't tell) leg goes forward, that's when green arm would go forward.
you can see examples of this around on the street when people are walking normally, it's a thing our body does to counterbalance our gait.