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Interpretation of the force curve

Interpretation of the force curve

We always try to make the BioRow reports provided after testing simpler and easier to understand, so in order to achieve this we developed an ‘evaluation’ template. The force curve is traditionally considered as one of the most important indicators of rowing technique. In the BioRow evaluation template, it is given in scaled format (Fig.1): both X and Y axes represent percentages of the maximal force and stroke length (total oar angle) for a given rower, not their absolute values. This allows us to compare with the target only the shape of force curve, not the magnitude of force produced. The numerical evaluation of the absolute values is given at the top of the template.

Fig.2 shows the same data as Fig.1, but the X axis is the time of stroke cycle (%). The force starts increasing during the recovery (1), which is measured as an inertia force, where the oar decelerates before the catch. Ideally, the force should continuously increase before and after the catch (as seen in the blue target curve). Correct connection of the blade to the water makes the “Slip” target about 2-3o. A loop in the force curve at the catch (2) means the force decreases immediately after the catch, i.e. the blade creates a negative braking force, which is subtracted from the inertia force and shows a decrease in the measured handle force. Visually, this is accompanied with significant back splash, and the reason for this is inaccurate blade work: when the blade entry happens before the blade acceleration to the stern. This looks like a paradox, but in most cases, blade braking occurs in rowers who try to pull the handle at catch, i.e. their Catch Factor is positive, or close to zero. Rowers who use the “catch through the stretcher” technique very rarely have this problem (RBN 2015/09). In addition, longer catch angles help to improve blade connection to the water, because they give more time for acceleration of the blade entering the water (RBN 2015/06).

If the blade is well connected to the water and a fast leg drive is used for prompt acceleration of the rower’s mass, the force quickly increases (Fig.1, 3) and this “Catch Force Gradient” CFG is measured about 10o after the catch. Observations of the World’s best rowers were confirmed by the highest correlation of CFG (rate of force increase to up to 70% of the peak force) with the first peak of boat acceleration and the “trampoline effect”. Contrarily, slow legs at the catch and an early “opening” of the trunk always makes the force development slower and CFG longer (RBN 2014/04).

After the “transition point” (RBN 2013/07) at about 20% of the drive length (4), the force growth rate decreases because the focus is switching to the stretcher force. At this time, heels are placed on the foot-board and the rower starts utilising hip extension, so the peak force is achieved after about 35% of the drive length. However, rowers who open their trunk too early with a Rowing Style Factor below 70-80% often have an early peak force, followed by a gap in the curve (5), when they try a late acceleration of the legs. This fault could be called “disconnection” between the legs and trunk, which breaks the drive up into two parts and can be visually observed watching the rhythm of the drive.

During the second half of the drive, the force is well maintained (6) by rowers with dynamic hip extension and engagement of powerful glute and hamstring muscles. In this case, Finish Force Gradient FFG is measured at a target level of about 30% of the drive length before the finish. If the hip power is already wasted in early trunk opening (when forces were low), this makes it more difficult to maintain sufficient force during the second half and makes FFG longer. Quite often, these rowers have to bend the arms early before the oar passes the perpendicular position of the boat (0o oar angle, or about 60% of the drive length), which makes hip extension even slower and less productive.

At the finish, the propulsive force of the blade is maintained better if a rower pushes the stretcher as long as possible, then sharply cuts the stretcher force and returns the trunk with a fast “kick” of the elbows to the bow. In this case, the trunk always remains dynamic (like a piston in a cylinder), and the “Wash” stays at the target of about 10o. If the trunk is stopped early and only the arms keep going at the finish, this usually decreases propulsive force at the blade to next to nothing and even may create a negative braking force.

Concluding, the force curve should be considered as a resultant indicator of rowing technique, so it is quite unproductive to ask rowers “please draw us a good force curve”. Instead, proper catch technique, rowing style and blade work should be focused.

©2018 Dr. Valery Kleshnev www.biorow.com