DOVER, Del. (AP) — An expert in police tactics and use of force said Wednesday that he thinks a white police officer in Delaware delivered a "vicious blow" in kicking a black suspect in the head, breaking his jaw and knocking him out.

But a judge prohibited former San Jose, California, police officer Ron Martinelli from using the word "vicious" while testifying in the assault trial of Dover police officer Thomas Webster IV.

The judge also prohibited Martinelli from giving his opinion on the amount of force delivered to Lateef Dickerson's head, saying his estimate was too imprecise for scientific certainty.

Police dashcam video shows Webster kicking Dickerson in 2013 as Dickerson, who officers believed was armed with a gun, slowly complied with commands to get on the ground.

In front of the jury, Martinelli testified that Webster was not justified in using such force because Dickerson did not present an "objectively reasonable threat" at the time.

"At the time that force was applied, he was not resisting arrest," said Martinelli, who acknowledged that Dickerson was slow to comply with police commands. Officers can be heard ordering Dickerson at least three times to get on the ground before he began to do so.

But Martinelli said that just before the kick, Dickerson did not present a threat to the officer's safety or any indication that he might try to flee.

"He's got both palms on the ground," Martinelli said. "He's got one knee on the ground. ... There is no reasonable threat to the officers."

In his cross-examination, defense attorney James Liguori tried to portray Martinelli as sloppy in reviewing the videotape and other evidence, and in preparing his report. He pointed to several factual inaccuracies in Martinelli's report and testimony and also challenged his credentials, noting that Martinelli received a doctorate from an unaccredited school that is now defunct.

Following Martinelli's testimony, prosecutors rested their case, and the judge denied a defense motion for acquittal.

Liguori has argued that Webster's actions were justified as he and other officers responded to reports of a fight involving a large group of people and a man matching Dickerson's description armed with a gun. His first two defense witnesses were the officer who assisted Webster in taking Dickerson into custody and the officer who first responded to the fight scene.

Liguori also has maintained that Webster's indictment was the result of "state machinations" and an "abuse of power." A grand jury declined to indict the officer after the encounter, but a second grand jury indicted Webster earlier this year.

Liguori has argued in court papers that Democratic Attorney General Matt Denn's decision to take the case to a second grand jury with no new evidence was a politically motivated response to nationwide scrutiny of police encounters with black citizens.

Webster rejected an offer from prosecutors to plead guilty to misdemeanor assault in return for surrendering his certification and never working as a police officer again. If convicted of felony assault, he faces up to eight years in prison.