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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) is not physically engaged with the portable electronic device; and in response to determining that the first receiver member (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) 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 (40, 1105, 1107, 1216, 1316, 1540) is not physically engaged with the portable electronic device; and in response to determining that the first receiver member (40, 1105, 1107, 1216, 1316, 1540) 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.