A gang beat and tortured a disabled man to death for sport, a court has heard.
Mr Martin suffered from learning difficulties
Brent Martin died in hospital after being found with multiple injuries on Sunderland's Town End Farm estate in August last year.
A youth, aged 17, who cannot be named for legal reasons, denies murdering the 23-year-old, who was found dying next to a parked car.
William Hughes, 21, and a 16-year-old have already admitted Mr Martin's murder, at Newcastle Crown Court.
On Tuesday, at the start of the 17-year-old's trial, the court heard that the attack began at a bus stop when he bet £5 that the 16-year-old could not knock out Mr Martin with his right hand.
Both youngsters were trained as boxers, and their victim was repeatedly punched, kicked, stamped on and head butted, the jury was told.
Prosecutor Toby Hedworth QC said: "That man Brent Martin never lifted an aggressive finger towards them and they did that to him until he was dead. They did that for their own sport."
A post-mortem examination revealed he died from a massive head injury and had suffered at least 18 separate blows to the head and neck.
The court heard that the 17-year-old defendant later told witnesses: "I am not going down for a muppet."
A 47-year-old woman, the mother of the 16-year-old, is also on trial accused of attempting to pervert the course of justice.
She is accused of destroying evidence by washing the blood-stained clothing of her son.
The trial continues.