There are a lot of discussions online about dynamic and static ergs (indoor rowing machines). We clarify using physics and some basic engineering what the differences are.
The fundamental difference between the mechanics of a static ergometer (such as Concept 2) and a dynamic ergometer (or a boat on water) can be found by taking the following test.
- Place yourself at front-stops and exert pressure on the footstretcher, you move backwards in proportion to the force exerted.
- However if you sit at the frontstops in a boat and do the same, you only move backwards relative to the bank by an amount ~20% of your leg length. The rest of the motion is taken by the boat moving away from you.
This is a result of Newton’s 3rd law of action-reaction. The force exerted by your legs on the flywheel acts equally on both you and the flywheel. In the case of a static erg, the flywheel remains stationary as it is mounted to the ground and instead you do all the moving. In case of a dynamic erg (or a boat on water), the mass of the boat is typically 10-20% lighter than you, making it mover further than you.
In the static ergometer you are actually putting in more effort to accelerating your body weight compared to the dynamic ergometer, which splits the acceleration of your body weight and the boat in opposite directions.
A dynamic erg such as Rowperfect, attempts to simulate the reaction effect by having the flywheel moving on the rail to absorb most of the motion. The Rowperfect weighs approximately the same as a sculling boat providing you with an experience of rowing and not sliding.
The designer of Rowperfect Casper Rekers performed a test comparing the power output for an erg with and without a fixed flywheel. The test results showed the subject gaining about 10-20% power output without a fixed flywheel. This additional power could be used to accelerate the flywheel instead of one’s bodyweight. This is also one reason why the catch of the static erg feels ‘slack’ compared to the boat.
To refer to the maths behind these principles click here.
Other articles about static versus dynamic ergs
- Static versus dynamic ergs – Concept 2 response
- Five reasons why I choose a dynamic erg
- The Catch on dynamic erg versus static erg
- Discussion on Rowing Illustrated Dynamic Erg – Jan 2011