CycleOps: The PowerBeam has strain gauges—ultra-sensitive measurement devices used to read power— integrated into the resistance unit to measure effort. It can also be preset to dictate power levels for a workout. CycleOps reports accuracy of +/-5 percent, twice the margin of error of its road power meters.
LeMond: Instead of using strain gauges to directly measure power, the Revolution’s Power Pilot accessory measures the speed of the resistance fan and barometric pressure to calculate power. In addition to wattage, Power Pilot also records speed, heart rate and cadence.