Let f : Y --> X be a continuous map between connected CW complexes. The homology H* (F) of the homotopy fibre is then a module over the loop space homology H* (OMEGAX). THEOREM: If H*(F; R) and H*(OMEGAX; R) are R-free (R a principal ideal domain) then for some H*(OMEGAX; R)-projective module P=P(greater-than-or-equal-to 0) and for some m less-than-or-equal-to cat f: Ext(H* (OMEGAX)m(H* (F); P) not-equal 0. Some applications are also given.
Félix, Y., Halperin, S., & Thomas, JC. (1992). The Category of a Map and the Grade of a Module. Israel Journal of Mathematics, 78(2-3), 177-196. https://doi.org/10.1007/BF02808056 (Original work published 1992)