The human hand is a versatile and complex system with dexterous manipulation capabilities. For the transfer of human grasping capabilities to humanoid robotic and prosthetic hands, an understanding of the dynamic characteristics of grasp motions is fundamental. Although the analysis of grasp synergies, especially for kinematic hand postures, is a very active field of research, the description and transfer of grasp forces is still a challenging task. In this work, we introduce a novel representation of grasp synergies in the force space, so-called force synergies, which describe forces applied at contact locations in a low dimensional space and are inspired by the correlations between grasp forces in fingers and palm. To evaluate this novel representation, we conduct a human grasping study with eight subjects performing handover and tool use tasks on 14 objects with varying content and weight using 16 different grasp types. We capture contact forces at 18 locations within the hand together with the joint angle values of a data glove with 22 degrees of freedom. We identify correlations between contact forces and derive force synergies using dimensionality reduction techniques, which allow to represent grasp forces applied during grasping with only eight parameters.