  The rotation curves of low surface brightness (LSB) galaxies suggest that
they possess significantly higher mass-to-light (M/L) ratios than their high
surface brightness counterparts, indicating that LSB galaxies may be dark
matter dominated. This interpretation is hampered by the difficulty of
disentangling the disc and dark halo contributions from the disc dynamics of
LSB galaxies. Recently, Fuchs (2002) has attempted such a disentanglement using
spiral arm density wave and swing amplification theory, allowing an independent
measurement of the disc mass; this work suggests that LSB discs are
significantly more massive than previously believed. This would considerably
reduce the amount of matter required in the dark halos in fitting the rotation
curves. Interestingly, the high mass-to-light ratios derived for the discs
appear inconsistent with standard stellar population synthesis models. In this
paper, we investigate whether the high M/L ratios for the Fuchs LSB discs might
be understood by adopting a very ``bottom heavy'' initial mass function (IMF).
We find that an IMF with a power law exponent of around alpha=3.85 (compared to
the standard Salpeter IMF, alpha=2.35) is sufficient to explain the unusually
high M/L ratios of the Fuchs sample. Within the context of the models, the blue
colours ((B-R)_0 < 1.0) of the sample galaxies result from being metal-poor
([Fe/H] = -1.5 ~ -1.0) and having undergone recent (~1-3 Gyr ago) star
formation.
