The Challenge AR has the capability to allow the rower to control and adjust the resistance from “Feather light” to “Olympic sprint” delivering a silky smooth resistance to the user’s effort and instantaneous response to input with no “flat sport”.  With water resistance being the most accurate manner in which to simulate actual outdoor rowing, the Challenge AR provides a smooth and consistent sensation throughout the range of motion of the exercise experience.  
The model E’s seat stands 6 inches higher. And while the model D’s monitor rests on an adjustable arm, the E’s sits on unbending metal. These few technical differences do nothing to impact ride feel. We recommend the $200-cheaper model D as the best buy, but the Concept 2 E will appeal to anyone who values a higher seat and more solid construction.
As far as “racing”, I said an air rowing machine is better because the Concept2 is the only model used for indoor rowing competitions, setting world records, and entering your actual scores online. It is because their monitor can calculate the drag factor of the flywheel in real time and accurately calculate distance and time. Small changes such as dust build up, air temperature, and humidity will not change the times between different machines because the drag factor is calculated every stroke.
The Verified Sustainable Hardwoods used for the WaterRower are sourced from the Appalachian Mountain region of the eastern United States, where planting rates exceed harvest rates by an average multiple of 2.29. The Appalachian Hardwood Timber resource includes more than 65.4 million acres in the eastern United States and provides the world with a bounty of the finest hardwood species.

The WaterRower Natural is outfitted with a Series 4 performance monitor that's designed to balance technical sophistication with user-friendliness. The monitor--which includes six information and programming windows, six QuickSelection buttons, and three navigation buttons--displays your workout intensity, stroke rate, heart rate, zone bar, duration, and distance. Plus, the monitor is compatible with an optional heart rate chest strap and receiver, which helps you optimize your workout and achieve your exercise objectives.

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