A medical examiner who performed an autopsy on a man killed in a fight with a fellow hockey dad said it would require "substantial force" to inflict the injuries that caused the victim's death.

Dr. Stanton Kessler, the medical examiner who performed an autopsy on Michael Costin, testified on Friday as prosecutors began presenting their case in the manslaughter trial of Thomas Junta, 42.

Junta is accused of beating the 40-year-old Costin to death after the two argued over rough play at their sons' July 5, 2000 hockey practice. Junta says he acted in self-defense. The trial has focused a national spotlight on the issue of parental violence at youth sports.

Kessler, along with a rink worker and a police officer who came to the scene after the fight, were the witnesses as testimony began Friday.

When Middlesex Assistant District Attorney Sheila Calkins asked him what kind of force was required to inflict Costin's injuries, Kessler said: "This is a substantial force injury. It takes a lot of trauma to tear ligaments. The ligaments on the back of the skull were torn and hemorrhaging."

Kessler described Costin's injuries as "almost tearing the head from the neck."

Both sides in the case agree Junta argued with Costin, then left the rink with his 10-year-old son, returned, and a second fight ensued. Prosecutors say Junta, a 6-foot-1, 270-pound truck driver, used his size to overpower the 160-pound Costin, then pounded his head on the floor until he lost consciousness.

Costin never regained consciousness and was declared dead the day after the fight.

On cross examination by Junta's attorney, Thomas Orlandi Jr., Kessler acknowledged that a brain injury could also have been the outcome if Costin were on Junta's back and Junta flipped him over, causing him to hit his head on the floor. That is what the defense claims happened.

Another witness, rink worker Nancy Blanchard, saw the two men fighting and called 911. Blanchard said she went to an alcove between two locker rooms and saw Junta and Costin "pushing each other back and forth."

"There were young children out there ... one of the little boys was crying," she said. "He was grabbing at the two men."

She said she saw Costin with his back against the wall and Junta with his back to her holding on to Costin. She saw Costin sliding down the wall and kicking his legs as if he were trying to get back up, she said.

"I had the young boy. He was crying. He said 'It's my daddy,"' Blanchard testified. It was unclear which of the men's sons she was referring to.

Testimony is scheduled to resume Monday.