Where a workers' compensation claim is put as an aggravation or acceleration of a pre-existing condition, the 'substantial contributing factor' requirement under s 9A relates to the aggravation or acceleration, not to the underlying condition. The question of whether employment is a substantial contributing factor is one of fact, and the word 'substantial' is left to the personal judgment of the individual judge according to the circumstances of each case. A trial judge is entitled to prefer one expert's evidence over another where reasons are given, and inferences may legitimately depend in part on intuition provided the conclusion is not merely speculative or idiosyncratic.
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