Some athletic gizmos measure steps. Others measure your leaping ability, or speed, or heart rate. But only the Under Armour Armour39 ($150) promises to measure your willpower. Or rather, it measures the effort you put forth during an activity by using an algorithm that takes into account your activity’s duration, body position, heart rate, intensity, and calories burned, then considers it in light of your individual profile. How does it get all this info? From a sensor that straps to your chest and transmits the data to your smartphone — and you can expect to start checking out your own stats when it ships in March.