Fitbit Inc. and Apple Inc. picked up the pace in their race to put health monitoring apps on wrists everywhere with Fitbit gaining 510(k) clearance from the U.S. FDA for its new ECG app for the Fitbit Sense and Apple revealing the blood oxygen sensor built into its Series 6 watch this week. The new apps join a growing array of technological advances that permit wearable devices to track and record a range of health metrics.