A method includes: supporting a normal operation mode during which functionalities of a portable apparatus are available through an operating system of the apparatus, wherein the operating system includes a plurality of layers including a kernel and library functions-layer; supporting a limited operation mode during which the apparatus is configured to execute a physical activity algorithm based on physical activity data corresponding to a physical activity session performed by a user of the apparatus, wherein the physical activity algorithm applies a direct low-level hardware access bypassing at least the layers above the kernel and the library functions-layer; and switching between the normal operation mode and the limited operation mode