Both the air rower and water rower machines are very popular choices of rowing equipment, particularly the former type. They have been around in the 80s, and many consumers and those that have tried them in gyms consider the equipment to be the best type ever. These air resistance equipment, often regarded as Ergometers, is the standard base model for indoor rowing sport.
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 WaterRower Classic Rowing Machine is an amazing and long-term investment in your health, as well as the beauty of your home décor. For the money, the durability, quality and guarantee to have a great rowing machine for a very long time. On top of that, this machine provides a level of comfort that will allow you to extend your workouts for an ultimate experience. Highly recommended.
From a fitness perspective, the WaterRower Club works 84 percent of your muscle mass, helping tone and strengthen your muscles while burning far more calories than most other aerobic machines. The exercise is also low impact, as it removes all the body weight from the ankles, knees, and hips, but still moves the limbs and joints through a full range of motion--from completely extended to completely contracted.
I teach pilates and also have several pieces of cardio equipment for my clients to warm up on. I bought a Hi, Rise Club Water Rower model a few years ago . as it turns out, my hard core bikers and elliptical walkers really did not use the Rower. SO it is in great shape. A beautiful piece.  I could not get a good photo in my house as there was not enough light, but if you Google this, you can see just what it looks like. 
There is much to love about the WaterRower--and I do love it--but I would echo others' comments that although the seat rolls solidly and smoothly on the wood rails, the seat itself is very hard (I use a gel seat pad I bought for my hard fiberglass kayak seat), and the footpads are in need up rethinking and upgrading--the cheap plastic doesn't let you row in socks or barefoot and is not really worthy of a machine that is otherwise a stunning piece of engineering and a beautiful one as well. As one other person noted about his machine, my machine made a clicking noise on the return stroke, so I had to adjust the wheel underneath the top rail that connects to the footpad and pull it away gently from where it was rubbing against another component. Also, be warned: the instruction booklet is in the DVD case. I did not see the little sticker on the case telling me that, thinking I'd wait to watch the DVD until after I'd assembled it. But WaterRower has a copy of the assembly instructions on their website, along with a video (I found the written ones better and easier to follow), so I was able to assemble it with no difficulty.
Of the four types of rowing machines or ergometers, water rowers are best at reproducing the sensation of on-water rowing. Their stroke cycles hold true to the dynamics of real paddling, and each stroke is punctuated with a splash! Water rowers have sliding seats to allow full-body cardiovascular workouts. (With cheaper rowing machines, in contrast, the seats don’t move.) These fitness machines tend to feature high quality parts and can last a lifetime.
Meticulous care is taken in the creation of each rowing machine, ensures the WaterRower ethos of elegant design and American craftsmanship is evident whether in use or in storage. The soothing sound of our patented water resistance is both blissful and motivational. Creature-comforts can be found throughout, from the ergonomic handle, remarkably comfortable seat cushion and user-friendly performance monitor, you will find yourself searching out reasons to "sneak in" an added WaterRower workout into your day. When finished, the WaterRower stores with ease, while blending seamlessly into any environment, ensuring your design and décor values are never compromised by your fitness lifestyle.
The WaterRower Natural rowing machine is handcrafted in solid ash wood, stained Honey Oak, and finished with Danish oil. Wood is an excellent material for this application due to its ability to absorb sound and vibration enhancing the WaterRower's smooth, quiet operation. The Natural, as with all WaterRowers, features patented WaterFlywheel technology, unrivaled in its replication of the resistance felt in on the water rowing.

Water rowers are just a recent invention followed by the air rower, which utilizes paddles and water in order to create the resistance. While there are a lot of people who prefer the air rower compared to this one, there are also people who prefer the water rower because it has a smoother, quieter feel of the action, especially when emulating the movement of water rowing. Like the air rower, this machine is able to burn calories and work up your muscles through offering resistance to your movement while sitting on the sliding seat. The more resistance there is, the more it makes it harder to work on the machine and your workout becomes more challenging.
Here’s a secret, because the rowing machine provides an intense full body workout, the exercise burns a high amount of calories not only during the actual activity but also for a period after you’ve stopped. This is known as the “after-burn effect”. Essentially, you’ve worked your body so hard that it is forced to keep burning calories and fat even when you’re not rowing. The scientific jargon for this is “excess post-exercise oxygen consumption” or “EPOC” for short. But all you need to know is that if you’re doing an intense rowing workout, the benefits continue after you’re done. How great is that?
In the rowing machine category, there are a lot of models to chose from. They range in price from under $100 to well over $1500.  As you can imagine, you’re getting a different type of row machine at those ends of the spectrum, but within that variety there’s something for everyone.  To help guide you, we’ve listed our favorite row machine models and brands by price below:
If you are anything like me, you read the negative reviews to see what some of the complaints could potentially be...I have found none that match with my machine or experience so far. The boxes it came in were a little bunged-up but everything on the inside was perfect. Only took about 15-20 minutes to build (first time building a row machine) and another 5-10 to fill with water. We used it for a little while before we added the danish oil.
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.
Rowing has long been recognized as the perfect aerobic pursuit, with naturally smooth and flowing movements that don't tax the joints but do boost the heart rate. Now you can take your rowing experience to the next level with the WaterRower Natural rowing machine. Using the same principles that govern the dynamics of a boat in water, the WaterRower Natural is outfitted with a "water flywheel" that consists of two paddles in an enclosed tank of water that provide smooth, quiet resistance, just like the paddles in an actual body of water. As a result, the machine has no moving parts that can wear out over time (even the recoil belt and pulleys don't require lubricating or maintaining). More significantly, the water tank and flywheel create a self-regulating resistance system that eliminates the need for a motor. As with real rowing, when you paddle faster, the increased drag provides more resistance. When you paddle slower, the resistance is less intense. The only limit to how fast you can row is your strength and your ability to overcome drag. And unlike conventional rowing machines, which tend to be jerky and jarring, the WaterRower Natural is remarkably smooth and fluid.
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