The opinion of the court was delivered by: Justice Garman
 Docket No. 96857-Agenda 7-March 2004.
 Claimant Sandra Franklin sought compensation under the Workers' Compensation Act (the Act) (820 ILCS 305/1 et seq. (West 2000)), for an injury to her left arm suffered at work during a physical altercation with co-employee Geniver Mohan on February 18, 2001. An arbitrator of the Industrial Commission of Illinois (the Commission) found that the injury did not arise out of claimant's employment and was therefore not compensable. Claimant appealed to the Commission. The Commission also found that the injury did not arise out of claimant's employment, on the ground that claimant was an aggressor in the fight that caused the injury. Claimant appealed to the circuit court of Cook County, which confirmed the decision of the Commission. However, the Industrial Commission Division of the appellate court reversed, holding that the Commission erred when it found that both claimant and Mohan were aggressors. 341 Ill. App. 3d 128. The appellate court remanded the cause to the Commission to find which employee was the aggressor. 341 Ill. App. 3d at 136. We granted employer Carson Pirie Scott & Company's petition for leave to appeal pursuant to Rule 315 (177 Ill. 2d R. 315). We now affirm the judgment of the appellate court.
 Claimant was a cosmetic artist and counter manager in employer's River Oaks store, in Calumet City. She was assigned primarily to sell Elizabeth Arden cosmetics. Her assigned work area was a rectangular counter, or "bay," with an opening at one end. Co-employee Mohan worked in a bay near claimant's, selling Fashion Fair cosmetics. On February 18, 2001, claimant's left arm was seriously injured during a fight between claimant and Mohan.
 Claimant filed a petition for adjustment of claim, and her case was heard by an arbitrator. The arbitrator denied compensation on the ground that the fight between claimant and Mohan was personal in nature or, alternatively, claimant was the aggressor. Claimant appealed to the Commission, which found that the fight was work related. However, the Commission denied compensation on the ground that claimant was an aggressor.
 The evidence heard by the arbitrator and relied upon by the Commission was as follows. Both claimant and Mohan received part of their compensation in the form of commissions. According to employer's policy, cosmetics salespersons were to sell their assigned product lines only, except that if a customer first purchased a product from an employee's line, the employee could follow the customer to another counter and make further sales from other product lines to that customer. Also, employees were permitted to sell products from another employee's counter if the other employee was busy with other customers. Claimant testified that these rules applied to all cosmetics sales persons and that both she and Mohan were aware of the rules.
 Claimant testified that she began having problems with Mohan in August of 2000 when Mohan made sales from claimant's counter in violation of the rules, and claimant complained to management. Her complaint resulted in a meeting between claimant, Mohan, and management. Claimant testified that after that meeting, she had four or five encounters with Mohan in which Mohan bumped claimant in a threatening way. Claimant testified she made written reports of three of these encounters.
 On February 18, 2001, at about 4 p.m. there were three or four customers at claimant's counter. Claimant testified she told the customers that she would take care of each in turn, and the customers agreed to wait for her. Mohan approached one of the customers and asked to help her. Claimant testified that the customer told Mohan that she was going to wait for claimant, and Mohan then became verbally abusive toward the customer. Claimant testified that she called for a manager. Manager Barbara Gerrard arrived and spoke to the customer. Meanwhile, Mohan was shouting threats at claimant and pointing her finger at claimant. Gerrard instructed Mohan to return to her bay. However, when Gerrard left, Mohan exited her bay and approached claimant's bay, shouting at claimant, pacing back and forth, and threatening claimant. Claimant felt frightened, began crying, and left the sales floor. She returned about 20 minutes later with aspirin and picked up a cup to get water.
 Claimant testified that at this point Mohan again approached claimant's bay, shouting at her and threatening her. Claimant went to another bay, called security, and was told to remain at her bay and that security would direct a video camera at claimant's bay. Claimant testified that when she returned to her bay, Mohan was there, shouting and threatening. Claimant told Mohan to "do what you do best." Claimant testified that Mohan then approached with arms outstretched, grabbed claimant by the left arm and hair, and pulled claimant towards her. Claimant testified that she responded by striking Mohan twice "as hard as [she] could" on the head with the cup claimant was still holding in her right hand. Another employee separated the two women. Claimant's left arm, which had previously undergone surgery to treat cancer, was seriously injured.
 The employer called Mohan as a witness but she refused to testify, citing her fifth amendment privilege against self-incrimination.
 Courtney Harris sold Lancome cosmetics from the same bay as claimant. Harris testified that immediately before the fight Mohan approached their bay. Claimant told Mohan that she was not afraid of her. At the same time Mohan was telling claimant that claimant could not order Mohan around. Mohan then moved around a display table to the opening of claimant's bay. Claimant did not advance toward Mohan, but turned to face her. Harris testified that she did not see the fight itself.
 Barbara Gerrard, who was store manager on duty on the day of the fight, testified concerning events prior to the fight. Ellyn Edwards, loss prevention manager for the employer, testified concerning written reports that claimant filed regarding earlier confrontations with Mohan. The testimony of Gerrard and Edwards contradicted claimant's testimony about the events leading to the fight on a number of points. For example, while claimant testified that in November of 2000 she made a written report to a security agent named Haskell of an incident between her and Mohan, Edwards testified that no such report was filed and that Haskell did not work at the River Oaks store during November of 2000.
 A videotape, recorded at the time of the fight by a store security camera, was entered into evidence. The Commission viewed the tape and found that it showed Mohan approaching claimant with her arms folded. It then showed claimant striking Mohan twice with the cup. The camera's view of claimant was partially obscured by a display rack. Claimant testified on rebuttal that the tape did not depict everything that happened during the fight. The Commission found that in the videotape it appears claimant struck the first blow.
 Both the arbitrator and the Commission found claimant was not credible. The Commission cited a number of reasons, including conflicts within claimant's testimony, conflicts between claimant's testimony and other witnesses, and conflicts between claimant's testimony and the videotape. Finally, the Commission found parts of claimant's testimony were implausible, such as her claim that she was initially an innocent bystander in a dispute between Mohan and a customer who preferred to wait to be served by claimant. The Commission relied on the testimony of Harris and on the videotape to find that claimant was an aggressor in the fight along with Mohan. On that basis the Commission denied compensation. The appellate court reversed and remanded, holding that the Commission erred when it denied compensation based on a finding that claimant was an aggressor, as opposed to ...