Monday, January 29, 2007

More From Concept II

After reading the most recent erg posts, Dick Dreissigacker at Concept II had a few more comments. After agreeing that a dynamic erg is certainly more like rowing than a static erg, he went on to say:

I think there is a realistic way to isolate the difference between modes and experience the magnitude of the forces being discussed. Sit on a stationary erg, do not pick up the handle, go through the motion of rowing at the desired stroke rate (be careful as it can be easy to row too high without the handle). You will feel the forces required to move your body mass back and forth. (Probably the most dramatic thing you will feel is at the end of the drive movement as you reverse the motion of your torso, which in practice is supported by the arms pulling the handle). Now do the same in dynamic mode. The forces you feel are those required to move the erg parts, and they will be lower than on the stationary erg. You will have just experienced the differences this discussion is talking about.

The forces you generate in spinning the flywheel are in addition to these “movement forces”. The forces you apply to the flywheel are significantly greater than the “movement forces”, are adjustable through machine settings, and are the same regardless of stationary or dynamic mode.

While you are at it, here is another experiment that will demonstrate the dramatic effect that stroke rate has on the force you will be feel, in either stationary or dynamic mode. Start out rowing at 30 SPM and settle in to a comfortable pace, about 10 seconds slower than your race pace. After 10 strokes or so drop to 20 SPM but maintain the same pace/500 (be careful not to hurt yourself when you do this!). I think you will notice a much heavier force at the start of your stroke while rowing at 20 SPM. Now bring your stroke rate back to 30 at the same pace and get a feel of the force at the catch. A low stroke rate means low flywheel speed at the catch therefore a heavier load can be generated. This is the same thing that is experienced on the water, and why high power, low stroke rate workouts, on or off the water, should be done with care…and perhaps not overdone.

5 comments:

KC said...

I'm amazed that Dr. Dudhia, Dr. Kleshnev, Ivan Hooper, Dick Dreissigacker, Mark Campbell, and even Xeno all forgot to account for the effects that hydrodynamic drag has on the hull of a boat when it is moving. The suggested example (from Anu Dudhia's rowing FAQ and Ivan Hooper's paper) of sitting on a static erg at the catch and watching how much your body moves when you extend your legs, versus sitting in a boat and doing the same (blades out of the water) totally neglects the fact that when the water is MOVING under the hull, there is a significant amount of resistance to the hull's bow-ward motion.

One of the main claims to fame by Row Perfect is that the moving head of their erg is roughly the same mass as a single scull. Simply matching the mass of a boat does not come near a full simulation of the complex dynamic system that is on-water-rowing.

Let's consider two situations where a dynamic erg is supposedly better than a static erg: First the release/recovery, then the catch/drive:

At the release, on a dynamic erg like the RP, as the rower comes up the slide during the recovery, their body remains almost stationary, while the erg head moves toward them. The RP erg head has very little resistance as it slides along its track toward the rower. In a *moving* boat, the hull has a very significant amount of force acting across its entire hull: hydrodynamic drag from the flow of water. So, the hull is not going to travel "toward the rower" the same amount that the RP erg head did, even though it is roughly the same mass as the RP erg head. So, the rower has to do more work during the recovery while rowing on water, than they do while on an RP ergometer (but definitely MUCH less work than they must do on a static erg). Making the mass of the moving head of the RP erg more or less wouldn’t fix this problem, either, since the resistance to the hull’s motion is not affected (much) by its mass; it is affected by it’s speed through the water, and the surface area of the hull that is underwater. So, a C2 on slides is probably just as good as an RP in this regard.

Next, the catch:
On an RP erg, again there is virtually no resistance for the RP erg head to slide on its track. So, as the rower drives his/her legs down, the rower’s center of gravity remains virtually stationary, while the RP erg head moves away from the rower easily. In a *moving* boat, hopefully the rower has adeptly placed the oar in the water, and is pulling on it with good "connection" to the water (I'll not go into the hydrodynamics of the blade here). Since the blade is "locked on" so to speak, then as the rower drives his/her legs, the hull will have to accelerate through the water, increasing its bow-ward velocity. The rower must travel with the boat, so the rower has to accelerate his/her mass in the bow-ward direction too, but even more so than the boat, which is not simulated AT ALL by an RP erg (or C2/slides). In this case, a static erg would actually be a better simulation of what happens during the drive of an on-water stroke.

So, what it comes down to, is that the recovery on a dynamic erg is a *better* simulation of on-water rowing than a static erg, but by no means is it perfect, and the fact that an RP’s moving mass is similar to that of a single scull doesn’t really matter. On the other hand, a dynamic erg is actually a WORSE simulation of the drive of real on-water rowing, than is a static erg, since in real rowing, the rower must accelerate their own body away from an also accelerating boat, whereas on a dynamic erg, the rower need not accelerate their body much at all.

So, which is better? I prefer training on the dynamic erg, mostly for the reasons that Dick Dreissigacker outlined: the recovery is much easier on a dynamic erg, thus a higher stroke rate can be maintained. With a higher stroke rate, you can keep the wheel spinning faster, thus the load at the catch is lighter, and feels more like real rowing, even though the rower's body is not accelerated during the drive like it is on the water. Dreissigacker suggested using a lower damper setting on a static erg to get the lighter feel at the catch that a higher rate would otherwise allow. This only solves half the problem: the static erg still has the significant work requirement of hauling one's body from release to catch during the recovery. But I'm fairly certain that the lower incidences of back problems with dynamic ergs is largely due to the lower loading thanks to the higher stroke rates. Also the fact that you don't have to accelerate your torso during the drive as much on a dynamic erg helps reduce the loads on the back.

Furthermore, the higher rates achievable on a dynamic erg better simulate the muscle requirements (speed of contraction) of real rowing. And, since the rower does not have to really slow his/her body down as they approach the catch on a dynamic erg, the position a rower takes at the catch on a dynamic erg is probably more like that which he/she takes in a boat, than that done on a static erg.

-Kieran Coghlan
Rower, coach, engineer.

JW Burk said...

An innocent (I think) question: Doesn't the fact that the rower is sitting in a moving boat, and moving along with it, neutralize any effect that the movement of the boat has on the rower? In other words, if you throw a baseball inside a moving airplane, it doesn't take more energy to throw it from back to front then from front to back. The movement of the airplane has no effect on the movement of the baseball. The fact, then, that during the drive the rower's body moves faster (in absolute terms) than the boat doesn't mean that it takes even more effort than if the boat were stationary. As for the recovery, doesn't this also mean that the coach's admonition to let the movement of the boat pull you up the slide is impossible? Of course, this would seem to argue for the static erg being the better simulation, and that just doesn't seem (or feel) correct. I think I'm missing something obvious here, but I can't figure out what it is. Kleshnev's measurements suggest that there is a difference... Perhaps you're right and it really all comes down to stroke rate.

KC said...

Your example of an airplane assumes that the airplane is a non-accelerating reference frame. In the boat, the boat is accelerating. For example, say you're a flight attendant in an airliner. If the plane is at constant speed, then moving the cart up and down the isle is no different from moving the cart with the plane stationary, on the ground. However, say the pilot accelerates the aircraft from 400mph to 600mph very quickly. During this acceleration, passengers will feel their bodies pressed against the seat, and you will feel that it is much more difficult to push the cart toward the front of the plane (and the cart may even move on its own toward the back of the plane.)

In the boat, as soon as the blades "lock on" and you begin the drive, the boat is accelerating, and the rower is accelerating with it. If the rower is then accelerating relative to the boat as well, then even more force is required to move the rowers body toward bow than on a static erg (let alone a dynamic one).

Hope this clears up the confusion.

-Kieran

KC said...

"isle" should be "aisle" duh. :-P
-Kieran

JW Burk said...

Oh yes, of course. Thanks.