Reasonable force? You're joking aren't you? Sprinting & cracking someone over the head from the rear isn't reasonable force! The lad may of done something to warrant arrest beforehand but if a group of burly coppers can't take down a scrawny unarmed kid without a brutal assault then there's something seriously wrong. The resulting injury to the lads head is shocking & being struck like that could also leave the lad with future problems like seizures or epilepsy. Gross negligence on that coppers part. If any of us struck someone with a club on the street like that we'd be facing serious jail time.