I've heard of things like this, specifically a tribe where the mothers will hang their babies upside down by their ankles and give their legs a deep massage every day. This produces kids with long, strong legs who are good at running.
Swinging the baby like that or not, the point is to stimulate the baby's muscles and joints, which we Westerners don't do much of. Parents tend to stick their babies in carriers for hours and give them maybe 20 minutes of "tummy time" a day.
You lousy kids! Get off my savannah!