(dissenting). I respectfully dissent from the majority's conclusion that LIRC's factual finding is not supported by the evidence. The evidence is disputed as to whether the dramatic change in Lange's spine from the slip and fall was caused by a weakened condition from the earlier work-related injury. I would conclude that there is sufficient evidence from which LIRC could reasonably infer that the slip and fall alone was responsible for the dramatic change in Lange's spine.
There is no contention that LIRC used an improper standard under the holdings in Western Lime & Cement Co. v. Boll, 194 Wis. 606, 608-09, 217 N.W. 303, 304 (1928), and Burton v. DILHR, 43 Wis. 2d 218, 228-28a, 168 N.W.2d 196, 201 (1969). Essentially, these cases hold that a person's subsequent non-work injury may be compensable under worker's compensation if it is caused by the weakened condition of the worker from the earlier work-related injury. Said in another way, the earlier work-related injury must have made the worker more vulnerable to re-injury.
Professor Arthur Larson helps us understand this area of the law where he states that an off-duty injury is compensable when the "episode is some nonemployment exertion like raising a window or hanging up a suit, so long as it is clear that the real operative factor is the progression of the compensable injury." 1 ARTHUR LARSON, WORKER'S COMPENSATION § 13.11(a) (1997). Neal and Danas describe it another way by saying that if medical proof establishes that the residual effects of a compensable injury cause a subsequent off-the-job re-injury, the employer is liable for the subsequent reinjury. JOHN D. Neal & JOSEPH Danas, Jr, Worker's Compensation Handbook § 3.37 (4th ed. 1997). Therefore, LIRC's duty in this case was *574to examine whether Lange's earlier work injury was a substantial factor in his later injury from the slip and fall. LIRC concluded it was not, and I agree there is sufficient evidence for it to reasonably reach this conclusion.
We review a commission's factual findings with great deference. Princess House, Inc. v. DILHR, 111 Wis. 2d 46, 54, 330 N.W.2d 169, 173 (1983). In fact, the factual findings are conclusive so long as they are supported by credible and substantial evidence. Brakebush Bros. v. LIRC, 210 Wis. 2d 624, 630-31, 563 N.W.2d 512, 515 (1997); § 102.23(6), Stats. This is so even if we believe that the weight of the evidence supports a contrary determination. Hagen v. LIRC, 210 Wis. 2d 12, 24, 563 N.W.2d 454, 459 (1997). We are also to consider conclusive any finding of the commission that is based upon a reasonable inference from the evidence. Kraynick v. Industrial Comm'n, 34 Wis. 2d 107, 111, 148 N.W.2d 668, 670 (1967).
Whether an injury is a proximate result of a compensable injury under the worker's compensation laws is a question of fact for the commission. Harnischfeger Corp. v. Industrial Comm'n, 253 Wis. 613, 615-16, 34 N.W.2d 678, 679 (1948). Additionally, the weight and credibility to be accorded to both witnesses and medical evidence are functions left to the commission. Brakebush, 210 Wis. 2d at 631, 563 N.W.2d at 515.
Here, LIRC accepted Dr. David Ketroser's report as most credible. Based primarily on his report, LIRC noted that prior to the slip and fall, Lange's back was relatively stable, if not improving. His back symptoms were decreasing and lifting restrictions had been loosened. After the slip and fall, Lange experienced a number of new symptoms, including foot drop and *575radiation of pain to the right leg. This was a dramatic change from Lange's prior condition.
LIRC inferred from Lange's prior condition, and then the subsequent dramatic change after the slip and fall, that the second accident alone was responsible for Lange's present injury. Unlike the worker in Burton who reinjured himself after a sneezing attack because of his weakened condition from the work injury, LIRC concluded that Lange's present injury was not a result of the progression of the initial work injury. This is a reasonable factual inference from the evidence, and we should not disturb this factual finding, even if we may not agree with its conclusion. Although one could certainly argue that the second injury only occurred because the original injury weakened Lange, LIRC is entitled to reject this inference.
The majority reasons that an aggravation of a preexisting injury, by definition, links the two injuries. I disagree. Under the rationale of Western Lime and Burton, the second injury is compensable only if it is caused by the weakened condition from the work injury or where the work-related injury made the worker more vulnerable to reinjury. If a worker has a bad back from a work-related injury and then seriously injures himself in a later non-work-related injury, the fact that the work injury is aggravated does not mean the work injury was a cause of the subsequent non-work injury or the result of a weakened condition. Nor does the fact that the work injury is "significantly worsened" mean that there is a causal relationship. It simply means what the doctor said. His spine condition is now significantly worse; it does not necessarily follow that it is a progression of the work injury. This was the issue faced by LIRC, which concluded that Lange's injury from the slip and fall was so dramatic a change from his *576prior condition that the injuries were unrelated. It is a reasonable inference, and it is not our role to change this factual finding. Because of my above rationale, I would not address LIRC's finding that Lange's act of drinking and then walking on ice acted as an intervening cause.