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. The goal is to map some of these tasks onto sensor nodes and the others onto computation platforms or gateways like mobile phones. In this paper 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. But as soon as a steady-state (e.g., walking or running at constant speed) is detected, this information is transmitted to the sensor node. At this stage, the sensor node monitors the known physical activity, which entails relatively simpler algorithms. In the event of a change in activity pattern, it switches back to raw data transmission and hands over processing to the mobile phone. Such cooperative signal processing significantly improves the battery life of the mobile phone as well as that of the sensor node. We present the main principles behind such distributed physical activity monitoring algorithms and compare their output quality with those from standard processing done entirely on the mobile phone.