ST KILDA forward Jack Higgins will miss the next three weeks after failing to either overturn or reduce his suspension for a dangerous tackle at the Tribunal on Tuesday night.

Higgins pleaded not guilty to the rough conduct charge for his tackle on Port Adelaide defender Aliir Aliir, with St Kilda counsel Adrian Anderson arguing the 25-year-old's actions weren't unreasonable in the circumstances.

As a secondary argument, Anderson said there were "exceptional and compelling circumstances" that made it inappropriate to impose a three-match ban, given Aliir's kicking action heavily contributed to the incident.

Anderson said a two-match suspension would be more fitting under those circumstances.

Aliir's head slammed into the turf after being slung to the ground by Higgins, with the subsequent concussion ruling the Port defender out of Thursday night's Showdown against Adelaide.

The three-person Tribunal panel, led by chair Renee Enbom, deliberated for one hour and 40 minutes before quashing St Kilda's two lines of argument.

Although Enbom agreed Aliir's kicking action contributed to his momentum of coming to ground, she said the tackle was unreasonable.

"We find this a clear case of rough conduct," Enbom said.

"Higgins used a poor and dangerous technique in tackling Aliir to ground.

"He grabbed hold of Aliir's left arm and forcefully rotated him to ground. 

"Higgins had hold of Aliir's arm in such a way that Aliir had no or little ability to try to protect himself."

The decision means Higgins will miss matches against North Melbourne, Hawthorn and Fremantle. 

During the hearing on Tuesday night, Anderson called upon sports biomechanics expert Helen Bayne to help explain how Aliir's kicking action was largely to blame for the outcome.

"It increases his angular momentum," Bayne explained.

"The raising of that right leg - aggressively swinging upwards - shifts his mass even more off balance than what it would have been and causes that increase angle that causes that cartwheel motion.

"He makes contact with his shoulder/head region rather than lower down.

"He's rotating faster due to the fact he's swung his leg in a kicking action.

"It makes it more difficult to create any other motions to maybe achieve a safer landing position because he's focused on that sideways rotation."

Anderson also pointed out Higgins is just 177cm and 76kg compared to Aliir's imposing 195cm and 97kg frame.

Higgins said it was Aliir's momentum that largely resulted in the pair hitting the turf.

"I don't think I can tackle a big man like that to the ground," Higgins said during his evidence.

"It didn't feel like I had much control of his body weight on me.

"A Port Adelaide player went to me and told me it was a good tackle and that I was stiff."

AFL counsel Nick Pane said Aliir was in a vulnerable position with little opportunity to protect himself.

"What made this tackle unreasonable was he (Higgins) restricted the use of Aliir's left arm and he rotated Aliir forcefully to the ground," Pane said.

Following the marathon three-and-a-half hour hearing, Saints footy manager David Misson said the club understood the Tribunal's decision.

"Although disappointing to not have Jack's sanction overturned or downgraded, we respect the decision of the Tribunal and accept its findings," he said.

"The head is sacrosanct, and we understand the importance placed on protecting the players."

Higgins said he accepted the Tribunal's decision and wished Aliir well with his recovery.

"I understand and respectfully accept the Tribunal's decision," Higgins said.

"I wish Aliir all the best as he navigates his recovery and hope to see him back playing soon."