In this study, we aim to analyze effects of physical motions of human body such as hand movements and speaking during contactless vital signs monitoring. Human respiratory rate (RR) has periodic in nature and its frequency varies between 0.2-0.5Hz for each human. Considering weakness of human vital signs, the analysis of other physical movements of human body on vital signs monitoring should well analyzed to obtain accurate RR results. This situation carries great significance in health-care units which purpose the non-contact and non-invasive monitoring of patients in near future. In order to achieve our goal, an UWB radar system has been used to detect respiratory rate of human body. Mathematical model of human breath signal and theoretical analysis of RR detection using UWB radar has been explained. The spectral estimation of RR has been performed by Fast Fourier Transform (FFT) due to its rapid response and simplicity. The strong noise effects of such physical movements of human body during RR monitoring have been observed and results are presented.