Cohnor Coleman, 24, launched an attack on Andrew Jackson, 43, during an evening when his wife Sarah was also out in the North Yorkshire market town of Richmond.
Coleman, the son of one of her best friends, denies murder but has admitted manslaughter, the jury at Teesside Crown Court has been told.
Pathologist Dr Jennifer Bolton outlined the head injuries Mr Jackson suffered. She said: “The blows have been delivered with a degree of force. These are not trivial blows, these are at least a moderate degree of force, perhaps even a severe degree of force.”
She said there were a minimum of three head blows landed in the attack, which happened outside the Cavern bar on April 25. Witnesses described seeing four or five forceful blows, or possibly more, to the right side of Mr Jackson’s head while he was lying on the left side, on the ground. Dr Bolton said the cause of death was a basal subarachnoid haemorrhage. She explained that was bleeding on the base of the brain caused by a tear in an artery from the neck.
The tear was caused by rapid twisting and tilting of the head, she explained.
Coleman, of Cookson Way, Brough with St Giles, near Catterick, had been in a secret relationship with Mr Jackson’s 38-year-old wife.
The court has heard he shouted “you’re not such a big man now” after the unprovoked attack. Cavern owner Mark O’Grady explained how he had thrown out Coleman and Mrs Jackson before the fatal attack.
The trial continues.