One of the main challenges in the body-area sensor network domain is to suitably break complex signal processing tasks into manageable parts in order to reduce their algorithmic complexity while retaining their output quality. Our goal is to map some of these tasks onto sensor nodes and the others onto computation platforms or gateways like mobile phones.  We attempt to address this problem in the specific context of physical activity monitoring. To start with, physical activity recognition tasks are carried out on the mobile phone and results are fed back to the sensor nodes for better signal preprocessing. Cooperative signal processing significantly improves the battery life of the mobile phone as well as that of the sensor node.