The First Degree Fitness Challenge AR (Adjustable Resistance) is designed and engineered by professional rowers and craftsman using only the highest degree of quality components – ensuring complete satisfaction in function, performance, and reliability.  With its patented Fluid Technology, First Degree Fitness has incorporated the truest emulation of sensation, sight, and sound of real on-water rowing captured in the private setting of your own home.
In the empty-lung technique, rowers inhale during the drive, and exhale during the recovery so that they have empty lungs at the catch. Because the knees come up to the chest when the lungs are empty, this technique allows the rower to reach a little bit further than if the lungs were full of air. Full lungs at the release also can help the rower to maintain a straighter back, a style encouraged by many coaches.
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.  
Another negative point for me is the angle of the seat. Unlike the Concept2, it has a slight backward angle, which makes it easier to maintain good form at the end of the stroke. Given that the Waterrower has greater resistance at the catch, it puts more pressure on the lower back at this point making it uncomfortable for people like myself with lumbar spine problems.
The distinction between rowing and other forms of water transport, such as canoeing or kayaking, is that in rowing the oars are held in place at a pivot point that is in a fixed position relative to the boat, this point is the load point for the oar to act as a second class lever (the blade fixed in the water is the fulcrum). In flatwater rowing, the boat (also called a shell or fine boat) is narrow to avoid drag, and the oars are attached to oarlocks ( also called gates ) at the end of outriggers extending from the sides of the boat.[8] Racing boats also have sliding seats to allow the use of the legs in addition to the body to apply power to the oar.
For a third option, you can look to hydraulic machines, which use pistons to generate resistance. Hydraulic rowers are quiet, and they also tend to be smaller and cheaper than other kinds of rowing machines, but you won’t get the same smooth rowing feel, or the consistency of resistance, that you would get with an air or water rower. What's more, reviewer after reviewer has found them unreliable and high-maintenance.
There is some minor maintenance. Because it's made out of wood, which can expand and contract, the bolts need tightening every few months or so. It's easy and takes only a couple of minutes. That, and putting in a fresh water purification tablet every six to twelve months, are about it for regular maintenance. The only problem I've had with the rower (the reason for 4 stars instead of 5) was a squeak that developed after about 6 weeks. It took me some time to determine the source of the squeak, which was a metal bracket connecting the footrest board to the horizontal boards above the drum. I had to partially disassemble the top section and tighten four bolts, but that fixed the problem and it's been quiet ever since.
In sweep or sweep-oar rowing, each rower has one oar, held with both hands. This is generally done in pairs, fours, and eights. In some regions of the world, each rower in a sweep boat is referred to either as port or starboard, depending on which side of the boat the rower's oar extends to. In other regions, the port side is referred to as stroke side, and the starboard side as bow side; this applies even if the stroke oarsman is rowing on bow side and/or the bow oarsman on stroke side.
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