The measurement of N-15 NMR spin relaxation, which reports the N-15-H-1 vector reorientational dynamics, is a widely used experimental method to assess the motion of the protein backbone. Here, we investigate whether the N-15-H-1 vector motions are representative of the overall backbone motions, by analyzing the temperature dependence of the N-15-H-1 and (COCalpha)-C-13-C-13 reorientational dynamics for the small proteins binase and ubiquitin. The latter dynamics were measured using NMR cross-correlated relaxation experiments. The data show that, on average, the N-15-H-1 order parameters decrease only by 2.5% between 5 and 30 degreesC. In contrast, the (CO)-C-13-C-13(alpha) order parameters decrease by 10% over the same temperature trajectory. This strongly indicates that there are polypeptide-backbone motions activated at room temperature that are not sensed by the N-15-H-1 vector. Our findings are at variance with the common crank-shaft model for protein backbone dynamics, which predicts the opposite behavior. This study suggests that investigation of the N-15 relaxation alone would lead to underestimation of the dynamics of the protein backbone and the entropy contained therein.