Heart rate drifts and is subject to many factors (sleep, enthusiasm level, coffee, time into ride, fatigue level, blah blah blah).
Power is contingent only upon how hard you pedal. Of course power output will be effected by the aforementioned things, but the measurement itself is inherently more quantitative with very low error, provided it's calibrated properly.
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"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste
Last edited by TMonk; 03-25-14 at 06:14 PM.