A Calgary constable on trial for aggravated assault was justified in his actions though he "unintentionally lost control" during a violent takedown that caused Daniel Haworth to suffer a fractured skull and brain injury, according to a retired city officer who is a use-of-force expert.

Retired inspector Christ Butler was a use-of-force expert for the Calgary Police Service and is now an instructor with the Force Science Institute. He testified for the defence on Day 7 of Trevor Lindsay's trial.

"Violence between humans can have unforeseen, unintended and accidental outcomes," wrote Butler in his report.

"Things often do not always go as planned for police officers, and all uses of force carry the potential for unfortunate outcomes even though those results are not expected or intended."

Court of Queen's Bench Justice Michael Lema will hear final arguments from both Crown and defence on Thursday.

This graphic security footage shows what appears to be a Calgary police officer punching a man repeatedly before throwing him on the ground. 0:42

In May 2015, Lindsay arrested Haworth, suspecting he had stolen sentimental coins from his ex-girlfriend.

In the parking lot of the the arrest processing unit (APU), Lindsay had Haworth pushed up against his cruiser. After a first punch to the head, Haworth began bleeding.

Lindsay testified Haworth then tried to spit blood toward him, which Butler described as "clearly assaultive behaviour" that prompted the officer to throw three more punches.

The video shows Haworth's face bouncing off the roof of the cruiser before the officer throws him onto the pavement head first with Haworth having no ability to break his fall.

The takedown was an "appropriate tactic under the circumstances and consistent with CPS training standards," said Butler under questioning from defence lawyer Don MacLeod.

Daniel Haworth suffered a fractured skull and brain bleed after his arrest by Const. Trevor Lindsay, who is now on trial for aggravated assault. (Court exhibit)

The problem, said the witness, was that Haworth was handcuffed and Lindsay "unintentionally lost control."

While testifying in his own defence, Lindsay said Haworth's hoodie, which he had grabbed, slipped out of his hands.

Under cross-examination by prosecutor John Baharustani, Butler said that Haworth twisting away from Lindsay was considered an assault on the officer.

Butler now works with the Force Science Institute. The centre's director, Bill Lewinski, is a controversial American-based "police psychologist" who has testified in dozens of U.S. cases as well as in several Alberta trials.

Lewinski's critics have called him a "professional witness" who provides testimony for pay in support of officers accused of excessive force.

Following a 2012 CBC investigation that raised questions about Lewinski's credentials and the objectivity of his expert testimony, the B.C. Police Complaints commission said it would never again use the leader of the Force Science Institute as an expert.