The goal of this study was to analyze the human ability of external force discrimination while actively moving the arm. We give an overview for the whole arm of just-noticeable differences (JNDs) for controlled movements separately executed for wrist, elbow and shoulder joints. With the results of this study a force feedback can be designed which is calibrated according to human force sensitivity. In the experiments presented, ten subjects performed a series of movements facing an opposing force. Force changes had to be detected in a two-alternative forced choice task. For each of the three joints, perceptual thresholds were measured as absolute thresholds (no reference force) using the QUEST procedure and three JNDs corresponding to three reference forces were chosen. Our results demonstrate that different Weber fractions are measured with respect to the joints. These were 0.11, 0.13 and 0.08 for wrist, elbow and shoulder, respectively. The minimum perceivable force on average was 0.04 N for all three joints. The results of this work will be used for delivering appropriate force feedback to a user wearing an exoskeleton in a tele-operation scenario.