Interesting conclusion from this article:
"Regardless of the approach to quantification employed and the specific phase of the season, our present analysis indicates that cyclists and swimmers perform a lower proportion of Z1 (<72%) and higher proportion on Z2 (>16%) than athletes participating in the triathlon, speed skating, rowing, running, cross-country skiing and the biathlon (all of whom train >80% of the time in Z1 and <12% in Z2).