Title: Neuron Activation Analysis for Multi-Joint Robot Reinforcement Learning. Abstract: Recent experiments indicate that pre-training of end-to-end Reinforcement Learning neural networks on general tasks can speed up the training process for specific robotic applications. However, it remains open if these networks form general feature extractors and a hierarchical organization that are reused as apparent e.g. in Convolutional Neural Networks. In this paper we analyze the intrinsic neuron activation in networks trained for target reaching of robot manipulators with increasing joint number in a vertical plane. We analyze the individual neuron activity distribution in the network, introduce a pruning algorithm to reduce network size keeping the performance, and with these dense network representations we spot correlations of neuron activity patterns among networks trained for robot manipulators with different joint number. We show that the input and output network layers have more distinct neuron activation in contrast to inner layers. Our pruning algorithm reduces the network size significantly, increases the distance of neuron activation while keeping a high performance in training and evaluation. Our results demonstrate that neuron activity can be mapped among networks trained for robots with different complexity. Hereby, robots with small joint difference show higher layer-wise projection accuracy whereas more different robots mostly show projections to the first layer.