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Mobile gait analysis focuses on the automatic extraction of gait parameters from wearable sensor data. However, development of algorithms for this task requires kinematic data with accurate and highly synchronous ground truth. In this paper we present a wireless trigger system which allows reliable synchronization of wearable sensors to external systems providing ground truth. To demonstrate the applicability of the system for mobile gait analysis, a Shimmer wireless sensor node with inertial sensors was mounted at the heel of a running shoe and synchronized with an external VICON motion capturing system using the wireless trigger system. Inertial sensor data were recorded during walking and running with the shoe, while kinematic and kinetic ground truth was acquired from the synchronized VICON system. Evaluation of delay and jitter of the system showed a mean delay of 2 ms and low jitter of 20 us. Recording was highly synchronous and the collected kinematics had a correlation of up to 0.99. In the future the proposed system will allow the creation of a database of inertial data from human gait with accurate ground truth synchronization.