Far-infrared cyclotron resonance measurements have been used to investigate the effective mass in the strained silicon channels of modulation-doped, two-dimensional electron gases grown on relaxed Si1-xGex. By using a range of Ge fractions x, the effect of strain was investigated. Consistent results were obtained when the resonance positions were fitted to a model for zero-dimensional confinement, yielding m*approximate to 0.196 m(e) for most samples. The use of this formula was justified by invoking electron localization due to a disorder potential. The observed confinement effect was strongest in two samples where the Si channel was partially relaxed, suggesting this to be a possible mechanism. Qualitatively different results were obtained for a sample with a high background concentration of donor impurities, indicating that the type of disorder present can affect the nature of the resonances.