What can be seen from the force measurement?
The graphs below show the underwater part of stroke. Graph starts when hand enters the water and ends when hand exits the water.
Force profile has the components of force stacked on top of each other:
- Green: propulsive force
- Red: vertical force
- Yellow: Lateral force
Y-axis shows numerical value (Newtons) of each direction of the force at specific time.
The ability to produce high maximum force is important especially in sprint distances.
The duration of the force production is equally important, in order to maximise the impact of the stroke.
Force x Time = Impulse (total area under the force profile)
In the example below, swimmer A produces a higher maximum force. However, swimmer B generates a higher impulse and is swimming faster, because the duration of the force production is longer.
Swimmer A: Force 42N, Impulse 18.6Ns, Time 1:32 / 100m
Swimmer B. Force 35N, Impulse 24.5Ns, Time 1:11 / 100m
Any breaks in the force production mean that swimmer looses the hold on water and are not able to reach the optimum speed.
In the example below, the drop in the force mean that the impulse of the stroke decreases by 25-30%. It also means that local fatique is building in the same muscles as the gap in the force is compensated with high peak forces.
The drops in the force may be related to non-optimal orientation of the hand. If the palm is not facing the direction of movement, it does not generate force efficiently.
The force is also reduced, if there is a break in the muscle chain causing the speed of the hand to drop.
Force dropping during stroke:
In addition to the magnitude of the force, SmartPaddle displays the direction of the force. Target should be to maximise the magnitude and duration of forward directing force, while keeping force in other directions minimal.
Forward, lateral and vertical directions are presented in green, yellow and red colours respectively. In the example below, the swimmer applies relatively high vertical force (red) especially in the beginning of the stroke. Such stroke typically leads to a non-optimal body posture.
Stroke references found in the Analysis Centre show, how top level swimmers apply the force in different directions. The references help on identifying inefficiencies in the measured strokes.
For all of the swimmers there is some drag created, especially when hand enters the water. Slow hand deep in the water easily causes drag even during the stroke.
The resistance of the hand is visible as negative force in the graph. In this example the resistance is even greater than the impulse of the stroke. The drag force is especially high, when the hand enters the water: pushing water forward after the entry. There are also significant negative values at the end of the stroke, when the swimmer starts the recovery underwater.
Minimizing the drag force may in some cases be even more important than increasing the impulse.
If the strength of the strokes is not evenly balanced between the right and the left hand, it can easily lead to fluctuation in the body speed. Any such deviation from a constant speed increases power consumption. With the SmartPaddle the balance of the hands can be easily made visible.
In the example below the force profile of the left hand is presented with dotted lines on top of the right hand profile. It can be seen that both the maximum force and the impulse of the right hand is substantially larger. In addition, the right hand starts to produce force earlier in the stroke than the left hand.
Typically, the impulse of the hand strokes decreases with increasing speed. It becomes progressively harder to maintain the force of the hand stroke when the body is moving faster to the opposing direction. The decreasing impulse is often the limiting factor for the maximum speed of the swimmer.
Also the balance between the hands may change as the effort is increased: the weaker hand cannot keep up with the stronger one. The example below shows the changes in the impulse when swimming speed is increased. Right hand is displayed with green color and left with orange. It is clearly visible that in this case the difference increases when the swimmer is speeding up.
The same view can be used to compare the dynamics of swimming between different measurements and between different swimmers.
The strength of the hand strokes may vary greatly from one stroke to another, especially if the swimmer has difficulties in maintaining the body posture. For example the breathing rhythm can often be seen from the variation in the stroke impulses.
An example of a significant stroke to stroke variation can be seen in the figure below. The difference between individual strokes causes fluctuation in the swimming speed as well. The mean velocity of the swimmer decreases since part of the power is consumed in accelerating after the bad strokes.
Swimmers can rarely maintain optimal technique when the swimming distance increases. Typically the strength of the strokes decreases as the muscles get tired, even if the swimmer is able to maintain a constant frequency. This change is usually visible also as decreasing stroke length and swimming speed.
The figure below shows the development of the stoke impulse during an 800 m swim. It can be seen that the strength of the hand strokes decreases significantly during the first 100 m. Thereafter the swimmer is able to maintain the impulse on a stable level. It is also evident that the stroke to stroke variation increases when the swimmer gets tired.
The analysis of the swimmers technique is usually based on tests carried out over relatively short distances. This example shows that it does not necessarily help in optimizing the technique for long distance swimmers.