This is an appeal from the judgment entered by the trial court reversing an award of worker’s compensation benefits to Richard Wold (Claimant) by the Deputy Di*113rector of the Department of Labor, Division of Labor and Management (Deputy Director). Claimant, who suffered a myocardial infarction (heart attack), contends that the trial court erred in finding that there was insufficient credible evidence to sustain the Deputy Director’s findings that his heart attack was an injury assignable to a definite time, place and circumstance, and resulting from unusual exertion. We affirm.
Claimant, who was forty-two years old at the time of his heart attack, had spent most of his life working at manual or physical labor jobs. Up until December 9, 1974, he had worked for the last nine years at the same plant for three different employers, the last two years for Meilman Food Industries, Inc. (Meilmans). While employed at the plant, claimant’s duties consisted entirely of assembly line work, primarily at the viscera table, where the internal organs are removed from cattle carcasses, and at the trimming line where foreign substances are removed from cattle carcasses hung on a moving line. During this two-year period, Meilmans ran the line of carcasses at a maximum rate of 130 head an hour, almost double the speed at which the previous owners had operated the chain. Also, when Meilmans increased the speed of the chain, they increased the number of duties the claimant was to perform.
In June, 1974, when the speed of the line at the viscera table was at its peak, and the number of duties required of claimant was also, at its peak, claimant began experiencing pains in his back and in his chest. His request to see a doctor was refused by the foreman. After this experience, claimant requested an easier job and was transferred to the final trim line. Claimant had previously done this job under the old owners and had handled it without difficulty. After claimant had been transferred to the final trim line the task was changed, increasing the difficulty of the job by requiring the claimant to be responsible for the entire carcass rather than only a part of it. Nevertheless, the claimant performed this job nearly six months without complaint. About two weeks prior to the heart attack, the claimant was required to do an additional task, that being the operation of a switch located some five feet from his position. The switch, which would derail condemned cattle, had previously been handled by the government inspector. Shortly thereafter, the claimant complained about the difficulty of the job.
On December 9, 1974, after again experiencing chest and back pains in the morning, claimant requested to see a doctor, but his request was denied and he continued to work the whole day. That evening the pain and dizziness became so bad that claimant went to a hospital. Dr. Michael Ferrell, who specializes in the field of internal medicine and has diagnosed and treated numerous heart attacks, was brought in as a consultant. After examining the claimant, Dr. Ferrell diagnosed a myocardial infarction.
In May, 1977, claimant filed a petition for a hearing before the Division of Labor and Management, Department of Labor, alleging that he had suffered a myocardial infarction (heart attack) arising out of and in the course of his employment with Meil-mans. At the hearing on November 28th, before Deputy Director William Miller, the claimant was the only person to testify. The testimony of Dr. Ferrell, claimant’s attending physician, was offered by deposition.
The Deputy Director made findings of fact and conclusions of law holding that the claimant’s myocardial infarction was a com-pensable injury under SDCL 62-1-1.
Meilmans appealed the Deputy Director’s decision to the Circuit Court of the Second Judicial Circuit. The trial court held that there was not substantial credible evidence to support the Deputy Director’s decision, and claimant appeals.
We first note that the episode occurred before the 1975 amendment to SDCL 62-1-1(2), deleting “by accident” from the definition was effective.
The claimant having been successful before the Deputy Director, the standard for review is as set out in Edge v. City of Pierre (1931) 59 S.D. 193, 200, 239 N.W. 191, 194:
*114A decision of the [director] in favor of a claimant must be supported by affirmatively finding the existence of all material facts as to which claimant has the burden of proof. Such findings are not open to question by the reviewing court if the record exhibits any substantial credible evidence in support thereof.
Further:
The trial forum for ascertaining material facts is the [director] and facts so found-must be accepted by the reviewing court, unless so palpably erroneous upon the record as to be unreasonable; such findings standing substantially upon the same plane as the verdict of a jury, [citations omitted] Upon a question of fact which might conceivably be decided either way on the record, the reviewing court has no right to substitute its view as to the existence or nonexistence of such fact for the view of the [director].
At the hearing, the Deputy Director heard the testimony of the claimant and the deposition of Doctor Ferrell, and from the evidence found, in substance, that after experiencing pain in the back and chest in June of 1974, the claimant requested and was transferred to a different area; that thereafter the tasks assigned to him in that area increased in difficulty; that two weeks prior to the episode, the employer added to his duties the operation of a derailing switch for condemned and retained cattle; that shortly prior to the date of the episode he complained that the job was too much; that midmorning of the date of the episode he complained of back and chest pains and asked to see a doctor, which request was denied; that he continued to work the rest of that day and at 10 o’clock that evening was admitted to the hospital where the myocardial infarction was diagnosed. Upon these findings the Deputy Director concluded that the injury sustained on December 9, 1974, was assignable to a definite time, place and circumstance; that the heart attack resulted from unusual exertion occurring during the prior two-week period; and that the same is a compensable injury under SDCL 62-1-1.
Upon appeal to the circuit court those findings and conclusions were overturned by the trial court, which found, in substance, that the claimant had a preexisting heart condition; that the heart attack was contributed to and/or aggravated by his total period of employment, among other things; and that no substantial credible evidence existed that the myocardial infarction was attributable to the last two weeks of employment. His conclusions of law stated that the decision of the Deputy Director that the heart attack is attributable to the last two weeks of employment is not based on substantial credible evidence, and that the claimant’s heart attack was not caused by unusual exertion attributable to a specific time, place and circumstance, and therefore not compensable.
Before examining the Deputy Director’s findings, it- is necessary to discuss the applicable law. In South Dakota, the pertinent statute is SDCL 62 — 1—1(2),1 which defines “injury” or “personal injury” as the following:
(2) ‘Injury’ or ‘personal injury,’ only injury by accident arising out of and in the course of the employment, and shall not include a disease in any form except as it shall result from the injury. (Emphasis added.)
The key phrase is “injury by accident.” Relying on this phrase, this court has adopted the “unusual exertion” rule in cases involving a preexisting disease. Cooper v. Vinatieri, 1950, 73 S.D. 418, 426, 43 N.W.2d 747, 751, sets forth the rule as follows:
[T]hat an employee with a diseased heart who suffers damage to his heart as a result only of physical and mental exertion commonly incident to the kind of labor to which he has become accustomed has not incurred ‘only injury by accident’ *115within the intendment of that phrase. (Emphasis added.)
See also Tennis v. City of Sturgis, 1953, 75 S.D. 17, 58 N.W.2d 301 and Hanzlik v. Interstate Power Co., 1940, 67 S.D. 128, 289 N.W. 589. Oviatt v. Oviatt Dairy Co., 1963, 80 S.D. 83, 85, 119 N.W.2d 649, 650, which is another worker’s compensation case involving a heart attack, summarizes this court’s holdings that:
It is settled law in this state that disease, or the aggravation of an existing disease, is compensable, but that such disease or aggravation must be assignable to a definite time, place and circumstance, [citation omitted] and that the disease, or aggravation of such disease, must result from unusual exertion, [citations omitted].
Unusual exertion has been found in the following cases: Where the decedent had been employed in a managerial capacity until pressed into service delivering milk on a route for two days and working ten to twelve hours each day in very cold weather, Oviatt v. Oviatt Dairy Inc., supra; Where an operator of a tourist court who was pressed into service as a volunteer fireman and thus helped with the hose throughout the night was considered to have performed unusual exertion, Tennis v. City of Sturgis, supra; When a company serviceman, while exposed to the extreme cold, worked an average of eighteen hours a day for five days to repair the extensive damage done to the company’s lines after a blizzard, Hanzlik v. Interstate Power Co., supra; Where the decedent had to work an extra seven hours in the cold and wet trying to repair a broken main, Campbell v. City of Chamberlain, 1960, 78 S.D. 245, 100 N.W.2d 707.
In the present case, the Deputy Director found as a conclusion of law (which probably should have been a finding of fact as the trial judge points out) that:
[T]he heart attack suffered by the claimant resulted from unusual exertion occurring during the two week period prior to said attack.
The exertion that the Deputy Director found to be unusual apparently consisted of the additional duty of turning on and off the switch which operated the derailment of condemned cattle.
With this conclusion (finding) the trial court disagreed, stating that such finding was not based on substantial credible evidence. We agree with the trial court. Reviewing the record in accordance with the scope of review previously noted, we find that the Deputy Director’s finding was not supported by substantial credible evidence even when viewed in a light most favorable to the finding.
We recognize that there is evidence showing that claimant’s employment caused him much stress and strain, and that the company had been requiring the claimant to do more and more tasks than were usually required by other employers. However, the law requires unusual exertion that is assignable to a definite time, place and circumstance. In looking at the evidence most favorably to the claimant, the claimant’s testimony of his additional duty of operating the switch hardly constitutes an unusual exertion such as that found in Oviatt, Tennis and Hanzlik, referred to above. Furthermore, claimant’s expert medical testimony utterly failed to bolster his contention.
The testimony2 of Dr. Ferrell, claimant’s doctor, is crucial in that this court has held that to establish the causal .relationship between one’s employment and his subsequent heart attack, a finding must rest on the testimony of professionals because the field is one in which laymen are not qualified to express an opinion. Podio v. American Colloid Co., 1968, 83 S.D. 528, 162 N.W.2d 385. Dr. Ferrell, in answer to a lengthy hypothetical question, did state or imply that the claimant’s employment with Meilman Foods was a competent producing cause of the mild cardiac infarction that he *116suffered on December 9, 1974. However, there was no direct testimony relating the cause of the heart attack to the two-week period prior to the attack, which the claimant argues is the period of unusual exertion.
In fact, during cross-examination, Dr. Ferrell stated that in finding the claimant’s employment was a contributing cause, it was more in the range of possibility than probability. He did state on redirect that the tension and the strenuous physical exertion of the claimant’s employment was a contributory or aggravating factor in claimant’s condition, but again failed to relate it to the crucial two-week period. It was held in Rose v. John Deere Ottumwa Works, (1956) 247 Iowa 900, 76 N.W.2d 756, that medical testimony to the effect that it is possible that a given injury caused a subsequent disability is insufficient, standing alone, to establish the causal relation under worker’s compensation statutes. In Holmes v. Bruce Motor Freight, Inc., (Iowa 1974), 215 N.W.2d 296, the court held that the claimant has the burden of proving by a preponderance of evidence that some employment incident or activity brought about the disability on which the worker’s compensation claim is based; a possibility is insufficient and a probability is necessary. In Ruderman v. Forman Bros., 1963, 157 Neb. 605, 609, 60 N.W.2d 658, 661, the Nebraska Supreme Court stated:
‘A compensation award cannot be based on possibilities or probabilities, but must be based on sufficient evidence that the claimant incurred a disability arising out of and in the course of his employment.’ [Citations omitted.]
Therefore, we hold that the expert medical testimony is defective on two counts. First, Dr. Ferrell does not assign the contributing or aggravating cause of the heart attack to the two-week period prior to the attack. Secondly, he terms the employment as possibly a contributing cause and not a probability.
This court has recognized that worker’s compensation laws are remedial in character and are entitled to a liberal construction. Meyer v. Roettele, 1935, 64 S.D. 36, 264 N.W. 191; Oviatt v. Oviatt Dairy Inc., supra. However, this rule of liberal construction applies only to the law, not to the evidence offered to support a claim, and does not permit a court to award compensation where the requisite proof is lacking. Podio v. American Colloid Company, supra. See also Eschenbrenner v. Employers Mutual Casualty Co., 1957, 165 Neb. 32, 84 N.W.2d 169.
We hold that there is not substantial credible evidence to support the Deputy Director’s decision, and the judgment of the trial court is affirmed.
WOLLMAN and PORTER, JJ., concur. DUNN, C. J., and ZASTROW, J., dissent.. As noted, SDCL 62-1-1(2) was amended in 1975 (S.L.1975 Ch. 322, § 1) in which the phrase “by accident” was deleted. However, since the injury in question occurred on December 9, 1974, under the old law, we at this time need not interpret the amended statute for its purported intent.
. The testimony of Dr. Ferrell having been submitted by deposition can be reviewed by both the trial court and this court unhampered by the clearly erroneous rule. Ayres v. Junek, 1976, S.D., 247 N.W.2d 488.