H₂/Brγ (top), HeI/Brγ (centre), and [FeII]/Brγ (bottom) line ratios of the galaxies of the sample, ordered by increasing L_IR. The values are measured in the integrated spectra. The weighted mean of each luminosity bin (low, log(L_IR/L_⊙)  <  11.35; intermediate, 11.35  ≤  log(L_IR/L_⊙)  <  12 and high, log(L_IR/L_⊙)  ≥  12) is plotted as a thick line, whereas the box represents the standard deviation of the values. Since all the ULIRGs and one LIRG were not observed in the H-band, [FeII]/Brγ data are presented for only nine LIRGs.