Direction of the force of the hand strokes

/ / SmartPaddle technology

After a swimmer has learned a swimming technique with reasonably low resistance, it’s time to improve the efficiency of the strokes.

There are two main factors influencing the efficiency: timing of the strokes and direction of the force. This time we focus on the latter.

Swimmers produce force in all directions with their hand strokes, but only the propulsive component moves them forward in water. With skillful swimmers, the propulsive component typically makes up more than 60% of the total force. When this is combined with a low resistance, they move seemingly effortlessly through water. The best competitive swimmers can feel when they have a good hold. The rest of us struggle at much lower speed, since we don’t even know what a good hold should feel like.

All of this changes, when you take SmartPaddles into use. You don’t have to rely solely on your feeling, when you can see the force of your strokes. It is even better, when you can also compare your strokes with the advanced swimmers. For recreational swimmers this opens up a possibility to take a huge leap in their technical skills. For the competitive swimmers it’s about shaving the last seconds from their records.

Past few decades underwater video has been the best tool to analyze the stroke technique of the swimmers. In order to make the link between SmartPaddle and video, we imported the force data into the video below. In the embedded diagrams, you can see the propulsive and vertical forces generated by the strokes in green and red respectively.

What you can see from the video alone?

You can see the speed of the hands and their trajectory from the side. It is however very difficult to guess, how much force swimmer uses at any specific moment or even the angle of the hand against swimming direction. You get some qualitative information from the speed of the hands and the bubbles generated, but real values remain unknown.

What you can see, when SmartPaddle data is combined with the video?

You can notice that vertical force dominates in the beginning of the stroke, when the swimmer pushes his hands downwards. Propulsive force peaks later, when the hands are approximately at the level of the shoulders. With careful observation, you can see that the bubbles in the water are formed at the same time with the two force peaks. Ideally, propulsive forces are partly overlapping in the stroke, when one hand is in the beginning of the pull phase and other hand in the end of the push phase. In the video, you can however notice a gap in the propulsion, since the force in the beginning of the stroke is primarily in vertical direction.

In sprint distances, the goal is to maximize the propulsive force regardless whether there is gaps in the propulsion. Powerful leg kicks would partly compensate such deficiencies in the hand strokes. Large vertical force combined with a high stroke rate could even be beneficial when it lifts the swimmer upwards and reduces the form drag. In long distance swimming, vertical force would only cause the swimmer to move up and down and thus increase the wave drag. Gaps in the propulsive force are also much more harmful, because variation in the swimming speed makes the stroke technique uneconomical.

What you can see in the SmartPaddle Analysis center?

The strength of the strokes is represented by the impulse, which combines the magnitude and the duration of the force together. At the top of the page you can see, how the impulse of the strokes varies during one lap (figure 1). The height of the columns shows the magnitude of the impulses, while the colors represents the three different directions. The propulsive, vertical and lateral impulses are presented in green, red and yellow respectively. You can change the lap with just one click and scan, how the strength of the strokes varies during the test.

Figure 1. The impulse of the right hand strokes during one lap.

The detailed force of each stroke can be seen simply by clicking the number of the stroke. In the force graph, you can see not only the overall strength of the stroke but also the timing, when the force is applied (figure 2). E.g. in figure 2, the swimmer applies the force in vertical direction at the beginning of the stroke, while propulsive force peaks at about 0.4 s after the hand has hit the water. Lateral force seems to be rather small and present only at the latter part of the stroke. Instead of a single stroke, you can also see the average stroke by clicking the “Avg” tab presented in figure 1. In figure 2, also the force of the left hand is overlaid on top of the right hand with dashed lines. The balance between the hands seems to be very good, even if the force of the left hand is slightly lower.

Figure 2. The force profile of the right hand strokes showing the magnitude, timing and direction of the force during one stroke. Left hand profile is overlaid in the figure with dashed lines.

We define the efficiency of the stroke by the percentage of the propulsive impulse (green area) of the total impulse. Numerical values can be seen in the summary table presented in figure 3. In this case, the fraction of the propulsive impulse was 55.8% for the right hand and 52.8% for the left in the first measured lap. Both the efficiency and the total impulse of the right hand were thus slightly higher than those of the left. Achieving the target of 60% efficiency while maintaining the high impulse could be set as a goal to the swimmer.

Figure 3: The summary table presenting the average impulse and efficiency of the strokes together with other relevant data from a measurement.

As a summary, SmartPaddles provide visibility to the force of the hand stroke like no system before. You can follow the overall trend of the strokes and their average parameters whenever you wish to do a measurement. The measurements themselves don’t require any special arrangements and can be conducted during a normal training. If needed, you can also easily view the details of each stroke in competition distances or even in complete sets. The results are available at the pool side for fast feedback. The best part of all this is that therefore the rewards to the swimmers are also immediate.

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