I read about it here.
Excerpt from The Guardian:
First, the forces generated by heading back goal-kicks and goalkeeper’s punts were much higher than expected. Some registered at between 50g and 100g – similar to American Football players crashing into each other or punches thrown by boxers. What’s more, as Nauman explains: “The percentages of 100g hits was effectively the same between women’s college soccer and American Football, which really surprised us. And while American Football players tend to take more hits overall in a given practice session and game, the college soccer players were getting hit every day and so it evened out.”