The text below discusses the details of typical issues we have seen in the force profile. In the end of the blog there is a summary on the benefits of using the force profile in performance analysis.
SmartPaddle force profile
The basic theory on moving in water: pushing against the water creates force and the created force moves the swimmer to the other end of the pool. However, water is a sensitive element and applying more force does not necessarily result in faster swimming. Therefore SmartPaddle displays all the relevant aspects on force production: magnitude, timing and direction. The information is summarised into the force profile. Force profile displays how force is produced during the underwater part of the stroke.
A simple interpretation of the color coding in force profile is
- green means “good” force -> palm pushes the water backwards making swimmer move forward
- red means “bad” force -> palm pushes the water downwards making swimmer move higher in the water
- yellow means “neutral” force -> palm pushes the water sideways
Swimming stroke is 3d motion, so it will never be totally green. Our data indicates that ~60% of green force is optimal.
Vertical force in sprint swimming
Red color in the force profile means that the swimmer is producing force in vertical direction during the stroke (figure 1). Typically, the hand is pushing water down and consequently lifting the swimmer up. For example in figure 1, the lift force is dominant during the first 0.4 s of the stroke. It is very common to see vertical force, when the swimmers arm is extended in front and the pitch angle is positive (figure 2).
Figure 1. The lift force (red) typically dominates the early part of the stroke in sprint technique.
Figure 2. The positive pitch angle of the arm creates lift force as the swimmer moves forward.
Vertical force doesn’t create propulsion and actually resist the movement forward. It is useful however in sprint distances, when the frequency is high and the kicks are powerful. Lifting the body up reduces the form drag and enables the swimmer to achieve greater maximum velocity. The effect is similar to the currently banned technical swimsuits that increased the buoyancy of the swimmer.
Vertical force in distance swimming
At a longer distance swimming the benefit of the lift force is reduced. When the stroke frequency is lower and the kicks are less powerful, the swimmer can’t create enough force to keep the body high in the water. Therefore the result of applying vertical force will be that the body starts moving up and down. Thus, the swimmer generates wave drag in addition to the resistance caused by the hand itself. Usually it is only the upper torso of the swimmer that is lifted up causing the hips and the legs to sink. This means that also the form drag increases rather than decreases. For long distance swimmers it would be beneficial to minimize the red vertical force (figure 3) by keeping the pitch angle close to zero.
Figure 3. Minimal lift force (red) during the glide phase in long distance swimming is beneficial for reducing the resistance.
Dropping elbow is clearly visible
The situation gets even worse, if the swimmers elbow or shoulder drops down. As seen in figure 4, such action not only greatly increase the vertical force and resistance but also substantially decreases and delays the propulsive green force (figure 4).
Figure 4. The lift force (red) increases and the propulsive force (green) decreases, when the swimmers elbow drops down during the stroke.
If the swimmers elbow drops down, it is also very common to see the hand moving back towards the surface at the end of the glide phase. This phenomenon is easy to spot using the trajectory picture from the side (figure 5). It corresponds to the situation shown in figure 6, where the swimmers arm is pointing up at the end of the glide. Understandably, it is rather difficult to turn the hand from such position and start producing propulsive force.
Figure 5. The trajectory of the stroke from the side. The hand is moving back towards the surface at the end of the glide phase, between 80-120 cm after the entry point.
Figure 6. When swimmers elbow drops down and the hand creates lift force, the hips and the legs easily sink increasing the form drag.
Negative forces
Many swimmers try to keep their elbow high during the glide phase by using a negative pitch angle. This easily creates negative lift force as the hand is moving forward and the pressure in the back of the hand becomes greater than on the palm side (figure 7). Effectively the hand not only decreases the swimming velocity but also pushes the swimmer deeper and increases the form drag.
Also in the middle of pull phase it is possible to see sudden droppages in the propulsive force, or even negative forces. This will happen if the hand slows down or palm slices the water (figure 7). These phenomena would be very difficult to see from underwater video, but they are clearly evident in the force profile.
Figure 7. Negative lift force during the glide phase decreases swimmers velocity and pushes him deeper increasing the form drag.
What are the benefits of SmartPaddle analysis compared to video analysis ?
- With SmartPaddle the magnitude of the problem can be easily quantified from force profile
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Results are available immediately at the pool side enabling fast feedback and learning.
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It is easy to compare between the recordings providing great visibility to improvements.
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Swimmers can compare their own performance to their team mates and learn from each other.
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Swimming can be monitored not only during specific drills but also during complete sets in order to see, whether the swimmer is able to maintain the technique.
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Performance can be tracked over months or even years to see the trend in development.
