The employee appeals from the compensation judge’s findings that the employee did not sustain an injury to his low back as a result of his work activities on November 14, 2012, and failed to establish that he sustained a new injury or aggravated or accelerated a pre-existing condition on September 23, 2013.  We affirm.

The employee began working for the employer, Mainline Transportation, Inc., (MTI) in April 2011 as a paraprofessional.  He began driving a school bus for the employer in about September 2011.

On November 14, 2012, the employee was seen at the Maple Grove Hospital emergency room.  A nurse noted, upon his arrival at around 10:30 p.m., that he was complaining of back and left hip pain.  The employee reported he drove a school bus and had developed left low back pain radiating down his left leg.  The employee was sent to the waiting room.  At about 12:50 a.m. on November 15, 2012, an MRI scan was taken of the employee’s lumbar spine.  The spine was normal except for a mild, broad-based disc bulge at L4-5 with increased signal intensity in the annulus consistent with an annular tear.  There was no evidence of stenosis or nerve root impingement.  The employee was seen by Dr. Kurt Hessen at 1:30 a.m., following the scan.  Dr. Hessen took a history, recording that the employee was a 24-year old male presenting “with a two day history of back pain.  He states that he did not have any trauma, but over the last 48 hours has been having low back pain that is radiating into his left leg.”  (Jt. Ex. 3D.)  On examination, the employee had diffuse tenderness with palpation of the left paraspinal muscles in the lower lumbar spine.  The doctor noted the employee had subjectively decreased sensation on the left, as well as some questionable weakness secondary to pain.  The employee was given a prescription for Percocet and oral prednisone, and was told to follow up at the Institute for Low Back and Neck Care.

At the hearing, the employee testified that on Wednesday afternoon, November 14, 2012, he was driving children home from Gatewood Elementary School.  He stated that on that Monday, three new children had been added to his route, and Wednesday was the first day they rode his bus.  As he was driving the new section of his route, the employee testified he was going 25 miles an hour and hit an approximately inch-and-a-half to two inch buckle in the pavement.  The employee stated his seat was not working properly, and as he hit the bump, he lifted out of the seat, and as he came down, the seat bottomed out and the metal from the seat hit the metal frame, jarring his spine.  The employee stated he had had plans to go out with his fiancé that evening, but stayed home because his back started hurting.  He tried icing and Tylenol, but it wasn’t improving, so he went to the emergency room.  The employee texted a supervisor stating he was going to the hospital, and called and left a voice mail for the head of dispatching.  The employee asserted he told the emergency room doctor about going over the buckle in the road.  (T. at pp. 64, 66, 70-71, 74, 99.)

A First Report of Injury, completed on November 15, 2012, lists the date and time of injury as November 14, 2012, at 3:15 p.m.  The description of the injury states the employee was “[d]riving bus on roads, hit[t]ing every bump seat bottom out and was putting a lot of blows to back & hip.”  (Ee Ex. 2.)

Gilly Weber, a human resources/safety assistant for the employer, testified at the hearing.  She stated the employer’s school buses are equipped with four interior cameras that turn on as soon as the ignition key is turned and continue to run while the bus is running and for at least ten minutes after the key is turned off.  Ms. Weber’s job includes reviewing the videos recorded on hard drives on the buses.  Software connected to her computer allows her to view the contents of a hard drive.  The screen shows videos from all four cameras along with a GPS, a speed indicator, and on the bottom of the screen, the date the video was taken and a time clock.  Ms. Weber testified she requested the hard drive from the employee’s bus and watched it on the morning of November 15, 2012, due to receipt of the notice of injury.  Ms. Weber testified she watched the video multiple times over the next two weeks, and then burned a copy of the November 14, 2012, video onto a CD on November 27, 2012.  She then wrote “Kyle Johnson 11/14/12” on the CD.  (T. at pp. 133-34; 136-139; 145-48.)

The employee returned to the Maple Grove Hospital emergency room on November 18, 2012, reporting he had been unable to follow up with the Institute for Low Back and Neck Care.  He was seen by Dr. Katie Vogt and provided a history that on the previous Wednesday, while working as a bus driver, he went over a bump and came down on his buttocks and low back onto a metal portion of the seat and had immediate pain.  Dr. Vogt prescribed Vicodin and a muscle relaxant and took the employee off work.

The employee was seen by Dr. Steven Stulc at the Institute for Low Back and Neck Care on November 26, 2012.  The doctor noted the employee stood 6 foot 4 inches tall and weighed 245 pounds.  He took a history that the employee drove bus and hit “some type of obstruction or pothole” that jarred him up in the air, landing back down on his buttocks, and began to develop pain in his low back area.  Findings on examination were normal except some pain with extension localizing to the L4-5 level.  Dr. Stulc’s impression was axial low back pain likely due to a posterior annular tear at L4-5.  He referred the employee for physical therapy, renewed his prescriptions, and continued the employee off work.  In a follow-up visit on January 3, 2013, Dr. Stulc noted the employee continued to have midline axial low back pain, without radiation into the lower extremities.  The doctor released the employee to return to work driving a short bus which presumably provided a smoother ride.

By letters dated July 8 and July 31, 2013, Dr. Stulc reported he had not seen the employee since January 3, 2013.  The doctor opined the employee had sustained a specific, work-related jarring injury while driving a bus at work, as reported to him by the employee, and that the event on November 14, 2012, was a substantial contributing factor to the symptoms for which he saw the employee.

The employee returned to work for the employer working 20 to 25 hours a week driving a short bus.  He continued to work through the school year and then was off work during the summer months.

The employee was examined by Dr. Edward Szalapski on May 28, 2013, at the request of the employer and insurer.  The employee gave a history of driving elementary school children on Wednesday afternoon, November 14, 2012, when he struck a two-and-a-half inch rise in the pavement which caused him to bounce three to four inches off his seat, landing on his tailbone, causing a sudden sharp pain up his back.  On examination, the doctor noted the employee was a very large individual, poorly conditioned for his age.  Dr. Szalapski noted no abnormal findings other than complaints of tenderness along the midline of the lower back.  Dr. Szalapski reviewed the employee’s medical records and a CD copy of the video of the employee’s bus route on November 14, 2012.  Upon viewing the CD, Dr. Szalapski concluded there was no event of any significance on that date and no evidence of any significant bump in the route.  Dr. Szalapski concluded the employee had very mild degenerative disc disease of the lumbar spine.  He opined the employee did not sustain a specific injury to his low back on November 14, 2012, and did not need any work restrictions.

The employee returned to work for the employer on September 3, 2013, the day after Labor Day, driving his regular bus route.  The employee testified that on September 23, 2013, after dropping off students at West Junior High, he drove to a Holiday station store where bus drivers congregated during their paid break.  After parking the bus, he twisted to get out of his seat, and felt immediate, sharp pain in his back shooting down his left leg.  The employee stated he then went into the store to use the bathroom.  Afterward, he stood for a while and then walked around the store trying to stretch out his back.  The employee stated he asked another driver to assist him back to his bus.  After getting back into the bus, the employee called the dispatcher to get another driver out to cover his route.  When the driver arrived, the employee was driven back to the bus depot and then to North Memorial Clinic in Golden Valley where he was seen by Dr. Daniel Feely.  On examination, there was some tenderness in the lumbar musculature with a little bit of spasm.  The neurological examination was normal.  The doctor prescribed ibuprofen, icing, and a muscle relaxant.  The employee was released to return to work with restrictions through October 3, 2013, but returned to his regular work that afternoon.

CD copies of videos were introduced at the hearing showing the employee’s activities on September 23, 2013.  One was from the employee’s bus and the other from surveillance cameras at the Holiday station store.  There was no dispute that the CDs were authentic copies of the videos on that date.

The employee was seen a second time by Dr. Szalapski on November 19, 2013.  Dr. Szalapski reviewed the employee’s November 15, 2013, deposition and the bus and Holiday station store CDs.  On examination, the employee walked with a normal gait and had essentially normal findings.  Dr. Szalapski stated, however, that the employee exhibited multiple nonorganic signs which, in the doctor’s opinion, indicated a high degree of symptom amplification.  The doctor again assessed minimal lumbar degenerative disc disease, and opined the employee did not sustain any injury of any kind on September 23, 2013.

The employee filed a claim petition on February 11, 2013, alleging an injury to his back on November 14, 2012, and seeking payment of wage loss benefits and medical expenses.  He subsequently alleged an exacerbation or re-injury to his back on September 23, 2013.  The employer and insurer denied the employee had sustained any work-related injury on either date.  In a findings and order filed March 26, 2014, a compensation judge found the employee did not sustain an injury to his low back as a result of his work activities on November 14, 2012, and that the employee failed to establish that he sustained a new injury or aggravated or accelerated a preexisting condition on September 23, 2013.  We affirm.

The employee contends the compensation judge erred in excluding the testimony of Aaron Geddes.  Counsel asserts he inadvertently failed to disclose Mr. Geddes as a witness until January 17, 2014, when he sent a letter to the court requesting that Mr. Geddes be allowed to testify by telephone because he had moved to western North Dakota.  The employer and insurer apparently objected to any testimony by Mr. Geddes based on late disclosure.  The employee asserts that during a pre-trial telephone hearing on this issue, Judge Brenden sustained the objection and excluded the testimony of Mr. Geddes.

The employer and insurer argue the employee failed to properly preserve the issue of exclusion of the testimony of Mr. Geddes and cannot expand the issues on appeal to include an issue not presented to the judge at the hearing.  The employee counters that a party need not renew an objection to the admission of evidence to preserve a claim of error for appeal following a ruling on a motion.  The employee argues the parties’ respective submissions and arguments at the pre-trial telephone hearing constituted proper preservation of the issue.

A compensation judge is not bound by the rules of evidence or by technical or formal rules of pleading or procedure, Minn. Stat. § 176.411, subd. 1, and has considerable discretion regarding the admission of evidence in a worker’s compensation proceeding.  To warrant a reversal, a compensation judge’s ruling on the admissibility of evidence must be prejudicial as well as erroneous.  See Anderson v. State, Dep’t of Natural Resources, slip op. (W.C.C.A. Oct. 27, 2003); McGuire v. Merillat Indus., Inc., slip op. (W.C.C.A. July 27, 1993).

In his brief, counsel for the employee stated that Mr. Geddes was a former employee of MTI who occasionally drove the employee’s route while the employee was off work after November 14, 2014.  Counsel indicated Mr. Geddes would have testified that he experienced driving over the same buckle in the pavement that the employee described.  The employee contends the testimony of Mr. Geddes was relevant and necessary to corroborate his testimony and to bolster his credibility.

There is nothing in the case file or judgment roll documenting the response of the employer and insurer to the employee’s request, nor is there any record of the pre-trial telephone hearing or of the judge’s ruling.  The employee made no motion or renewal of his request nor any offer of proof at the January 24, 2014, hearing nor at the February 14, 2014, hearing.

The appropriate procedure would have been for the employee to make a motion and formal offer of proof on the record at the hearing.  An offer of proof provides a specific description of the excluded evidence and provides an opportunity for opposing counsel to respond so that this court can review the compensation judge’s ruling and determine whether the ruling prejudiced the employee.  Anderson, id; Singer v. Thermotron, Inc., 51 W.C.D. 259, n.5 (W.C.C.A. 1994).  In the absence of any record on this issue, all that can be said with any certainty is that Mr. Geddes did not testify at the hearing.

It appears, based on the assertions of counsel, that the testimony of Mr. Geddes would have been merely cumulative.  The employee has failed to demonstrate prejudice or an error that warrants reversal.

The compensation judge found the bus route the employee drove on November 14, 2012, was accurately depicted in Employer Exhibit 3.  The employee maintains the bus video introduced into evidence as Joint Exhibit 6/Employer Exhibit 3 does not depict the employee’s bus route on Wednesday, November 14, 2012, and should have been disregarded.

There was conflicting evidence regarding the video on the CD submitted at the hearing.  The employee admitted he is the driver depicted on the video and that it depicts his bus route, but denied the video depicted his route on Wednesday, November 14, 2012.  The employee testified that on Monday through Thursday, after dropping off the elementary school children, he drove an activity route for middle school and high school kids.  He stated he did not do his activity route that Wednesday because he had dinner plans with his fiancé, but had arranged for a spare driver named John to cover his activity route.  According to the employee, the video shows him returning to the bus depot and parking the bus in the garage after completing his elementary school route.  He testified that he had instead parked the bus at the pumps outside, gotten out, and the other driver got in to cover the activity route.  He, accordingly, maintained that it was the wrong video, probably Friday instead of Wednesday, because there was no activity route at the end.

Gilly Weber testified that when the hard disc from a bus is connected to her computer, the screen shows all four cameras, and on the bottom of the screen, the date the video was taken and a time clock.  As an example, screen shots of Ms. Weber’s desktop from the September 23, 2013, bus video were submitted as Employer Exhibit 2, with a bar toward the bottom of the screen showing ‘23rd/09/2013” along with the time.  Ms. Weber testified she obtained the hard drive containing the video for November 14, 2012, from the employee’s bus on the morning of November 15, 2012, and reviewed the video that morning.  She reviewed the video five or six times, and when she was satisfied she had everything she needed, she archived it and then burned the video to CD on November 27, 2012.  When shown Employer Exhibit 3, Ms. Weber identified the CD as the one she had created, and that it was her hand writing on the CD stating “Kyle Johnson 11/14/12.”  She stated it was part of the process, when she archived a video, to write on the CD.

The compensation judge found Ms. Weber’s testimony regarding the retrieving and preserving of the video from the employee’s bus convincing, and found it was more likely than not that the exhibit accurately depicted the route driven by the employee on November 14, 2012.  The assessment of a witness’s credibility is the unique function of the trier of fact, and this court must give due weight to the opportunity of the compensation judge to observe the witness and assess his or her credibility.  Even v. Kraft, Inc., 445 N.W.2d 831, 834, 42 W.C.D. 220, 225 (Minn. 1989); Brennan v. Joseph G. Brennan, M.D., P.A., 425 N.W.2d 837, 839‑40, 41 W.C.D. 79, 82 (Minn. 1988).  It is not the role of this court to re-evaluate the credibility and probative value of witnesses’ testimony.  Redgate v. Sroga’s Standard Serv., 421 N.W.2d 729, 734, 40 W.C.D. 948, 957 (Minn. 1988).  The testimony of the witnesses in this case presented a conflict requiring resolution by the compensation judge, and upon review of the evidence as a whole, it cannot be concluded that the the judge erred in accepting the testimony of Ms. Weber.

The compensation judge found the employee visited the emergency room of the Maple Grove Hospital at 10:23 p.m. on November 14, 2012, and that the reason for his visit at that time was low back pain that began two days before the employee’s November 14, 2012, emergency room visit.  In her memorandum, the compensation judge stated she found the employee’s theory of the case unpersuasive, in part, because the notation in the November 14 emergency room history indicated there was no trauma associated with the employee’s onset of low back pain.  The judge drew the inference that there was no specific event during the course of the employee’s work day that triggered his back pain.^[1]

The employee contends the compensation judge’s finding and conclusions are inconsistent with the employee’s testimony and the circumstances surrounding the injury.  The employee asserts he texted his supervisor on November 14, 2014, to report he had injured his back at work, and the nurse’s intake notes at 10:30 p.m. reflect that the employee drove a school bus.  The employee argues that having told his supervisor and the intake nurse that his bus driving activities caused his injury, it makes no sense for the employee to tell Dr. Hessen that he had experienced no specific trauma and that it had started 48 hours earlier.

The contemporaneous reports of the employee’s injury on November 14 and 15, including the emergency room chart notes and the First Report of Injury, do reference the employee’s bus driving, but do not describe a specific injury on the afternoon of November 14.  The most specific history is that related by Dr. Hessen stating the employee presented with a two-day history of back pain and that the employee denied any trauma but, over the last 48 hours, had been having low back pain and pain radiating into his leg.  It was not until the November 18, 2012, return visit to the emergency room, after the insurer denied authorization for further medical treatment, that the employee described a specific incident on November 14, going over a bump and coming down on his buttocks onto a metal portion of his seat.

This is again a question of credibility, and the compensation judge found the contemporaneous medical records more persuasive than the employee’s subsequent recitations describing the onset of his symptoms.  We find no basis for reversal on these facts.

The employee additionally argues that Dr. Szalapski’s opinion lacks adequate foundation and the compensation judge erred by accepting the independent medical examiner’s opinion over that of Dr. Stulc.  Dr. Szalapski examined the employee on two occasions, took a history from the employee, and reviewed the employee’s treatment records, a deposition, and three CDs of videos taken from the employee’s bus and the Holiday station store.  There is adequate foundation for his opinions.

Dr. Szalapski based his opinion that the employee did not sustain a work-related injury on November 14, 2012, on several factors.  First, he stated the employee had essentially no objective findings on examination and very minimal degenerative disc disease.  Second, the doctor noted the initial history recorded in the emergency room chart notes of November 14-15, stating that in his experience the most reliable history is the history the patient gives the first time.  Third, Dr. Szalapski watched the video of the November 14, 2012, bus route and saw no evidence of any jolts or bumps or changes in velocity or direction.  The employee, in fact, agreed that he did not see any significant bumps on that video that would correlate to what he experienced.  Finally, Dr. Szalapski opined that even if such an incident had occurred, he did not believe that sufficient force would be generated driving a school bus over a one-and-a half to two inch rise to cause a spinal injury.

This court will uphold a judge’s choice between conflicting medical expert opinions unless the facts assumed by the expert in rendering an opinion are not supported by the evidence.  See Nord v. City of Cook, 360 N.W.2d 337, 342-43, 37 W.C.D. 364, 372-73 (Minn. 1985).  The compensation judge properly relied upon the opinions of Dr. Szalapski in finding the employee did not sustain a work-related injury on November 14, 2012.  We, therefore, affirm.

Finally, the employee contends the compensation judge’s finding that the employee did not sustain a work-related injury, or aggravation or acceleration of a preexisting condition, on September 23, 2013, is clearly erroneous.  The employee argues that even if the court were to conclude the employee did not sustain a work injury on November 14, 2012, but instead had mild lumbar degenerative disc disease, the only logical conclusion is that the employee’s twisting injury on September 23, 2013, caused an aggravation of his preexisting condition.

As with the injury on November 14, 2012, the evidence was conflicting as to whether an injury occurred on September 23, 2013.  The employee testified he twisted as he got up from his seat after parking the bus and felt an immediate sharp pain in his low back radiating into his left leg.  The First Report of Injury on that date indicated the employee twisted his lower back wrong as he was getting up out of the seat and his entire back spasmed causing extreme low back pain so that he could barely walk.

Video of the employee’s bus route and video from the Holiday station store surveillance cameras taken that day cast significant doubt on the employee’s claim of a significant injury to his back that morning.  Additionally, Dr. Szalapski, relied upon by the compensation judge, concluded the the employee did not sustain a work-related injury on September 23, 2013.

There is substantial evidence of record to support the compensation judge’s determination that the employee failed to establish that he sustained a work-related injury on September 23, 2013, and we, accordingly, affirm.

The employer and insurer contend the compensation judge erroneously stated in Stipulation 2 that if the employee sustained a compensable work injury on November 14, 2012, the parties agreed the employer and insurer were liable for temporary total disability benefits during the period November 14, 2012, through January 12, 2013.

Since this court has affirmed the compensation judge’s determination that the employee did not sustain a work-related injury on November 14, 2012, the cross-appeal is moot and need not be addressed.^[2]