A method comprising: receiving, by a portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716), a first set of athletic performance data including a first activity metric associated with a first athletic performance of a user; determining that a first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) to produce a first device assembly comprising additional monitoring functionality, wherein at least a first sensor of the first receiver member provides a second set of athletic performance data from which the first device assembly determines a second activity metric associated with the first athletic performance of the user; determining, using a detector system, whether the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in a first orientation; in response to determining the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in the first orientation, activating, by an activation system, a first function of the portable electronic device that is not available when the first receiver member is not physically engaged with the portable electronic device; and in response to determining that the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in an orientation other than the first orientation, deactivating at least the first function of the portable electronic device, wherein the detector system comprises a piezoelectric system, an accelerometer, a light sensor or a magnetic sensor system. receiving, by a portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716), a first set of athletic performance data including a first activity metric associated with a first athletic performance of a user; determining that a first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) to produce a first device assembly comprising additional monitoring functionality, wherein at least a first sensor of the first receiver member provides a second set of athletic performance data from which the first device assembly determines a second activity metric associated with the first athletic performance of the user; determining, using a detector system, whether the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in a first orientation; in response to determining the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in the first orientation, activating, by an activation system, a first function of the portable electronic device that is not available when the first receiver member is not physically engaged with the portable electronic device; and in response to determining that the first receiver member is physically engaged with the portable electronic device (16, 1101-1104, 110, 1223, 1323, 1516, 1650, 1670, 1675, 1676, 1716) in an orientation other than the first orientation, deactivating at least the first function of the portable electronic device, wherein the detector system comprises a piezoelectric system, an accelerometer, a light sensor or a magnetic sensor system. <EOS>