On long distance journeys passengers at high risk from deep vein thrombosis (DVT) are recommended to exercise on a regular basis to contract the calf muscle pump and encourage venous return. If a passenger fails to complete an exercise program that induces active contraction of the calf muscle pump they will remain at increased risk of DVT. This paper presents a novel inertial and magnetic sensor-based technique for monitoring calf muscle pump activity. The technique could be implemented into a system for monitoring the level of calf muscle pump activity in persons with limited mobility. Such a system could be used to provide a reminder to the user that there is a need to exercise should they have forgotten to exercise, failed to exercise sufficiently or exercised incorrectly. The proposed technique was evaluated by comparison with calf muscle pump activity measured using an electromyography (EMG) sensor. Results show that the technique can be used to monitor calf muscle pump activity over a wide range of leg exercises.