Recently there has been a growing interest in sensing by means of optical microring resonators in photonic integrated circuits (PIC) that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et. al. proposed the use of a waveguide based ring resonator as a strain gauge. However, the strong lateral confinement of the light in SOI waveguides and its corresponding modal dispersion was not taken into account. We present experimental results and understanding of the effects of an applied strain in the effective index in a SOI-PIC. In addition, we also investigated the influence of the waveguide geometry.