DISSENTING OPINION OF
WITH WHOM MARUMOTO, J., JOINS
LEVINSON, J.This appeal arises out of a claim by the dependents of Edward K. Akamine, deceased, for workmen’s compensation benefits. The issue presented is whether the Labor and Industrial Relations Appeals Board, hereinafter referred to as the appellate board, erred as a matter of law in finding that Mr. Akamine’s death was due exclusively to a preexisting cardiovascular disease and was not attributable in any manner to his employment. More specifically, this appeal calls upon us to determine whether the appellees adduced “substantial evidence” before the appellate board to rebut the presumption created by HRS § 386-85(1) that the claim was for a work covered injury. Contrary to the majority opinion, I believe that such evidence was adduced. I therefore *416dissent.
In Royal State National Insurance Co. v. Labor & Industrial Relations Appeal Board, 53 Haw. 32, 35, 487 P.2d 278, 280 (1971), we said that HRS § 386-85(1)
places on the employer both the burden of going forward with the evidence as well as that of persuasion. Acoustic, Insulation & Drywall, Inc. v. Labor & Industrial Relations Appeal Board, 51 Haw. 312, 316, 459 P.2d 541, 544, rehearing denied, 51 Haw. 632, 466 P.2d 439 (1970). Thus the claimant will win unless the employer produces “relevant and credible evidence of a quality and quantity” sufficient to convince a reasonable man that there exists a non-compensable alternative explanation for the injury. Acoustic, Insulation & Drywall, Inc. v. Labor & Industrial Relations Appeal Board, supra.
The presumption of coverage was designed to give the employee the benefit of any doubt, but it is not irrebuttable.1 Once the trier of fact has determined that the employer has met the substantial evidence test, its task “is to weigh and consider the evidence offered by the employer against the evidence offered by the claimants supportive of the claim.” Acoustic, Insulation & Drywall, Inc. v. Labor & Industrial Relations Appeal Board, supra, 51 Haw. 312, 317, 459 P.2d 541, 544. Where substantial evidence is adduced by both claimant and employer, this court cannot declare that the appellate board is clearly erroneous in its determination that the weight of the evidence lies with one party or the other. See Wheatley v. Adler, 407 F.2d 307, 314 (D.C. Cir. 1968).
The appellate board, in its Decision and Order of November 13, 1970, found that “there is substantial and *417undisputed medical testimony that the work being done by Mr. Akamine at the time of his death most probably did not contribute to or aggravate his coronary disease.” After careful examination of the record, I am persuaded that the appellate board’s finding is consonant with both law and fact.
Dr. Kuramoto, the deceased’s attending physician and the first of the employer’s two medical witnesses,2 testified that Mr. Akamine had probably died of a heart problem and that the most likely cause was ventricular fibrillation or ventricular tachycardia. He stated that he was unable to express an opinion as to whether the heart attack was work-related.3 He testified that although he would be reluctant to permit a heart patient to return to employment involving excessive strain, he did not believe that Mr. Akamine’s employment had been of this type.
Dr. Berk, who had examined Mr. Akamine’s medical records, testified that in his opinion there were five possible causes of Mr. Akamine’s death in descending order of probability: 1) ventricular fibrillation (cardiac arrythmia), 2) coronary thrombosis, 3) cerebral hemmorrhage, 4) rupture of a large blood vessel, 5) pulmonary embolism. Although he acknowledged that some “extremely exertional” work activity would be undesirable for a heart patient to resume, Dr. Berk specifically discounted any connection, other than “very remote” possibilities that were “not very likely,” between Mr. Akamine’s particular work activity and the most likely causes of death. He concluded that Mr. Akamine’s work activity probably prolonged his life, but in any event, he was certain that it did not cut it short. While there existed a degree of exertion which would not be beneficial to the heart muscle, Akamine’s work activity did not reach that level.
The aforementioned testimony constituted relevant and *418credible evidence of a quality and quantity sufficient to convince a reasonable man that Mr. Akamine’s death in fact arose out of non-compensable circumstances. It is true that an injury or death may be compensable even though it occurs in the course of the injured or deceased employee’s routine duties and that the employer must take the employee as he finds him. However, the injury or death must in fact have been precipitated, accelerated, or aggravated in some material degree by the employee’s normal and usual exertions. Holt v. Acme Mattress Co., 40 Haw. 660, 668-70, 672 (1955); Dwyer v. Ford Motor Co., 36 N.J. 487, 493-94, 178. A.2d 161, 164 (1962); see generally, Awai v. Paschoal, 43 Haw. 94 (1959). The thrust of the medical testimony before the appellate board was that Mr. Akamine’s death was not.4 It does not matter that" the two medical witnesses couched their testimony in terms of probabilities. When the issue is one of medical causation, expert testimony may be qualified and less than unequivocal. Dzurik v. Tamura, 44 Haw. 327, 330, 359 P.2d 164, 165-66 (1960); 3 Larson, Workmen’s Compensation Law § 80.32 (1971). Such testimony may constitute “substantial evidence” rebutting the presumption of coverage. See Wheatley v. Adler, supra at 313.5
I would affirm the Decision and Order of the appellate board.
In Holt v. Acme Mattress Co., 40 Haw. 660, 675 (1955), we quoted with approval the following language from Luteran v. Ford Motor Co., 313 Mich. 487, 494, 21 N.W.2d 825, 828 (1946):
The workmen’s compensation law is not designed as a complete substitute for life insurance, or sick and accident insurance, for employees ....
Whatever the merits, as a policy matter, of transmitting workmen’s compensation into such substitute life and health insurance coverage, the legislature clearly has not pursued (his course.
The claimants presented no medical experts at the appellate board hearing.
In this regard, Dr. Kuramoto remarked that it was “very unfortunate” that no autopsy had been performed. However, on cross-examination, he conceded: “Even at autopsies sometimes you can’t determine exactly the mechanism of death .... If it’s arrythmia, you may not find anything there in the heart . . . .”
The majority’s conclusion that “little probative weight should have been attached to Dr. Berk’s testimony . . . that work activity which is not ‘extremely exertional’ will not precipitate a heart attack” is self-serving and conclusory. Is the majority now saying that this court rather than the trier of the facts is the final arbiter on questions involving -the credibility of witnesses in general and Dr. Berk in particular?
I cannot accept the claimants’ suggestion that Drs. Kuramoto and Berk testified in ignorance of the presumption of coverage, the distinction between legal and medical causation, and the fact that Hawaii does not follow the Unusual Strain Rule. The transcript of the hearing before the appellate board reveals that member Saltz carefully explained these elements to the medical witnesses and that each physician indicated that he understood these elements.