This paper investigates the effect of user grasping position on the performance of haptic rendering. Two dynamic models, with seven and eleven parameters, respectively, have been used to characterize the PHANToM haptic interface and the user. The parameter variability analysis shows that user grasping position significantly affects system dynamics. This variation also influences the phase margin of the system, leading to different damping factors in response to contacts with rigid virtual objects. To compensate this effect, an adaptive haptic rendering has been developed and successfully implemented, imposing a similar damping factor in the transient responses for all grip positions.