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The ratio of $\alpha$-elements to iron in galaxies holds valuable informationabout the star formation history since their enrichment occurs on differenttimescales. The fossil record of stars in galaxies has mostly been excavatedfor passive galaxies, since the light of star-forming galaxies is dominated byyoung stars which have much weaker atmospheric absorption features. Here we usethe largest reference cosmological simulation of the EAGLE project toinvestigate the origin of variations in stellar $\alpha$-enhancement amongstar-forming galaxies at $z=0$, and their impact on integrated spectra. Thedefinition of $\alpha$-enhancement in a composite stellar population isambiguous. We elucidate two definitions - termed `mean' and `galactic'$\alpha$-enhancement - in more detail. While a star-forming galaxy has a high`mean' $\alpha$-enhancement when its stars formed rapidly, a galaxy with alarge `galactic' $\alpha$-enhancement generally had a delayed star formationhistory. We find that absorption-line strengths of Mg and Fe correlate withvariations in $\alpha$-enhancement. These correlations are strongest for the`galactic' $\alpha$-enhancement. However, we show that these are mostly causedby other effects which are cross-correlated with $\alpha$-enhancement, such asvariations in the light-weighted age. This severely complicates the retrievalof $\alpha$-enhancements in star-forming galaxies. The ambiguity is not severefor passive galaxies and we confirm that spectral variations in these galaxiesare caused by measurable variations in $\alpha$-enhancements. We suggest thatthis more complex coupling between $\alpha$-enhancement and star formationhistories can guide the interpretation of new observations of star-forminggalaxies.

On the variation in stellar $\alpha$-enhancements of star-forming galaxies in the EAGLE simulation