Measurement results swimming strokes and turns
Figure 4 shows the acceleration curve for breaststroke. The pulls per lane, push-offs and tilt-turns can be seen well through the acceleration maxima.
Fig. 4: Example extract for swimming strokes and turns (31m lane in Müllersche Volksbad, Munich). Measured acceleration in x-direction (blue in [g]), acceleration in z-direction (green [g]) during a recording of 6x31m breaststroke.
Figure 5 shows a section of a measurement at the time of a push-off after a tilt turn. Here it is clearly visible how the velocity changes due to the push-off from the edge of the pool and the subsequent sliding (red graph). Based on the acceleration maxima in the blue graph, the number of pulls can be determined.
Fig. 5: Enlarged section, example of repulsion (between ~317s and ~ 318s): gray = raw acceleration signal in y-direction (g), red = calculated repulsion velocity (m/s), blue = low-pass filtered acceleration signal in x-direction (g).
As can be seen in Fig. 6, despite ideal water shielding, no valid heart rate data could be measured with the red/infrared LED used. The reason lies in the strong influence of the head movements. In the measurement shown, the heart rate supposedly decreases during the load while swimming, whereas the opposite should be the case.
Fig. 6: Example of a vital parameter measurement. Measurement recording of heart rate (bpm), average speed (m/s) and body temperature (°C).
When the alternative chest technique was used, with the head held above water and no strong head movements, valid heart rate data could be measured:
Fig. 7: Measurement of heart rate during breaststroke with head position above the water. A prototype with a green LED is worn in the left ear and a red/infrared LED in the right ear.
Temperature measurement was possible due to the additional shielding of the ear and the measurements were valid.
The final concept after evaluation of a utility analysis and execution of prototype tests is as follows:
To protect the sensor from moisture, a silicone adhesive is applied to the sensor head opening (a) and the electronics are sealed by a silicone-based casting resin potting (c). A green LED is installed for the PPG sensor (b). Thanks to the 3-axis accelerometer, the recording of movements and accelerations is possible (d). To counteract potential data loss, a memory card is installed for internal data storage (e). Data analysis is based on digital signal processing (f). To optimally protect the ear and the sensor from water, both an ergometric swim cap (g) and an additional sealing of the ear cup with a silicone ear plug (h) are used.