Human mesenchymal stem cells (hMSCs) were routinely cultured on tissue-culture polystyrene (TCPS) to investigate the in vitro aging and cell stiffening. hMSCs were also cultured on thermoplastic polyurethane (TPU), which is a biocompatible polymer with an elastic modulus of approximately 12.9MPa, to investigate the impact of substrate elastic modulus on cell stiffening and differentiation potential. Cells were passaged over several generations on each material. At each passage, cells were subjected to osteogenic and myogenic differentiation. Local cell elastic modulus was measured at every passage using atomic force microscopy (AFM) indentation. Gene and protein expression was examined using qRT-PCR and immunofluorescent staining, respectively, for osteogenic and myogenic markers. Results show that the success of myogenic differentiation is highly reliant on the elastic modulus of the undifferentiated cells. The success of osteogenic differentiations is most likely somewhat dependent on the cell elastic modulus, as differentiations were more successful in earlier passages, when cells were softer.