# Classification of Alpine Skiing Styles Using GNSS and Inertial Measurement Units

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Sensors

#### 2.2. Data

#### 2.2.1. Data Generation

#### 2.2.2. Data Pre-Processing

#### 2.2.3. Training and Testing Data

#### 2.3. Methods

#### 2.3.1. Pre-Classification into Parallel and Non-Parallel

#### 2.3.2. Feature Extraction

#### 2.3.3. Feature Selection

#### 2.3.4. Classification Methods

#### 2.3.5. Performance Measures

#### 2.4. Software

## 3. Results

#### 3.1. Feature Selection

#### 3.2. Important Features for Classifcation of the Alpine Skiing Styles

#### 3.3. Comparison of Model Performance

## 4. Discussion

#### 4.1. Classification Performance

#### 4.2. Limitations

#### 4.3. Application of the Classifier

#### 4.4. Sensor Setup

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A

R Package Name | Version |
---|---|

caret [48] | 6.0–84 |

data.table [49] | 1.12.2 |

tidyr [50] | 1.0.0 |

dplyr [51] | 0.8.3 |

packrat [52] | 0.5.0 |

randomForest [45] | 4.6–14 |

xgboost [39] | 0.90.0.2 |

rpart [38] | 4.1–15 |

rpart.plot [53] | 3.0.8 |

ggplot2 [54] | 3.2.1 |

Feature | Description |
---|---|

sd_TD_Gyro_Yaw | Standard deviation of gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_Speed | Maximum speed of turn (m/s) |

mean_Speed | Mean speed of turn (m/s) |

max_TD_Gyro_Roll | Max gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

min_Speed | Minimum speed of turn (m/s) |

mean_EA_Edge_Angle | Mean estimated inclination angle of turn (mean of left and right boot) (rad/s) |

Turn_DurationSec | Duration of turn (s) |

sd_TD_Gyro_Roll | Standard deviation of gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

sd_TD_Decision_Yaw | Standard deviation of filtered (using a fourth-order, zero-lag, low-pass Butterworth filter with cut-off frequency of 0.5 Hz) yaw axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_TD_Gyro_Yaw | Maximum yaw axis angular velocity of turn (mean of left and right boot) (rad/s) |

mean_GYRO_Z_filt | Mean of the maximum of the gyroscope of the Z-axis of left and right foot of turn (rad/s) |

max_TD_AbsRRate_Roll | Maximum absolute roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_TD_Decision_Roll | Maximum filtered (using a fourth-order, zero-lag, low-pass Butterworth filter with cut-off frequency of 0.5 Hz) roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

Turn_Size | Size of turn |

sd_TD_AbsRRate_Roll | Standard deviation of absolute roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

sd_TD_Decision_Roll | Standard deviation of filtered (using a fourth-order, zero-lag, low-pass Butterworth filter with cut-off frequency of 0.5 Hz) roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

mean_TD_Gyro_Roll_filt | Mean gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_TD_Decision_Yaw | Maximum filtered (using a fourth-order, zero-lag, low-pass Butterworth filter with cut-off frequency of 0.5 Hz) yaw axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_TD_Symmetry_Roll | Maximum symmetry of the roll axis angular velocity of turn between left and right boot (rad/s) |

mean_GYRO_Y_filt | Mean of the maximum of the gyroscope Y-axis of left and right foot of turn (rad/s) |

mean_TD_Gyro_Yaw | Mean gyroscope yaw axis angular velocity of turn (mean of left and right boot) (rad/s) |

sd_TD_Symmetry_Roll | Standard deviation of the symmetry of the roll axis angular velocity of turn between left and right boot (rad/s) |

mean_TD_Gyro_Roll | Mean gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_EA_Edge_Angle | Maximum estimated inclination angle of turn (mean of left and right boot) (degree) |

EADiff_Left_Right | Mean difference of the absolute estimated inclination angle of left and estimated inclination angle of right foot of turn (degree) |

Feature | Description |
---|---|

Turn_DurationSec | Duration of Turn (s) |

mean_TD_Gyro_Roll_filt | Mean gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

max_TD_Symmetry_Roll | Maximum symmetry of the roll axis angular velocity of turn between left and right boot (rad/s) |

max_EA_Edge_Angle | Maximum estimated inclination angle of turn (mean of left and right boot) (degree) |

locEA_Edge_Angle | Position (1–100) of the max estimated inclination angle of the turn |

mean_EA_Edge_Angle | Mean estimated inclination angle of turn (mean of left and right boot) (degree) |

mean_EA_Edge_AngleLeftRight | Mean of the maximum estimated inclination angle of the left and right foot of turn (degree) |

mean_ACC_Z_filt | Mean of the maximum of the acceleration of the Z-axis of left and right foot of turn (m/s^{2}) |

mean_ACC_Y_filt | Mean of the maximum of the acceleration of the Y-axis of left and right foot of turn (m/s^{2}) |

mean_ACC_X_filt | Mean of the maximum of the acceleration of the X-axis of left and right foot of turn (m/s^{2}) |

mean_GF_GForce_PitchYaw | Mean resultant frontal plane acceleration of turn (mean of left and right boot) (m/s^{2}) |

max_GF_GForce_PitchYaw | Maximum resultant frontal plane of turn (mean of left and right boot) (m/s^{2}) |

mean_GF_GForce_PitchYaw_LeftRight | Mean of the maximum resultant frontal plane acceleration of the left and right foot of turn (m/s^{2}) |

Left_Right_Acc_filt | Mean of the maximal acceleration of the left and right foot of each turn (m/s^{2}) |

mean_TD_AbsRRate_Roll | Mean absolute roll axis angular velocity (mean of left and right boot) (rad/s) |

mean_TD_Decision_Roll | Filtered (using a fourth-order, zero-lag, low-pass Butterworth filter with cut-off frequency of 0.5 Hz) roll axis angular velocity (mean of left and right boot) (rad/s) |

mean_TD_Gyro_Roll | Mean gyroscope roll axis angular velocity of turn (mean of left and right boot) (rad/s) |

mean_TD_Symmetry_Roll | Mean symmetry roll axis angular velocity of turn between left and right boot (rad/s) |

sd_TD_Symmetry_Roll | Standard deviation of symmetry of the roll axis angular velocity of turn between left and right boot (rad/s) |

sd_TD_Symmetry_Yaw | Standard deviation of symmetry of the yaw axis angular velocity of turn between left and right boot (rad/s) |

Model | Parameters | |
---|---|---|

Parallel Turns | Non-Parallel Turns | |

Decision tree (package: rpart [38]) | cp = 0.05076923. | cp = 0 |

Random Forest (package: randomForest [45]) | mtry= 2, ntree = 1000 | mtry= 13, ntree = 1000 |

Gradient boosted decision tree (package: xgboost [39]) | max.depth = 3 eta = 0.4 nrounds = 150 gamma =0 colsample_bytree = 0.68 min_child_weight = 1 subsample = 1 | max.depth = 3 eta = 0.3 nrounds = 150 gamma =0 colsample_bytree = 0.6 min_child_weight = 1 subsample = 1 |

ID | Snow Conditions |
---|---|

S01 | hard groomed |

S03 | hard groomed |

S04 | soft (5 cm new snow) |

S05 | hardpack |

S06 | hardpack |

S07 | soft groomed |

S08 | soft groomed |

S09 | soft groomed |

S10 | hard groomed |

S11 | hard groomed |

S12 | hard groomed |

S14 | hardpack |

S15 | hardpack |

S16 | soft (6 cm new snow) |

S17 | hardpack |

S19 | hardpack |

S20 | hardpack |

S21 | hardpack groomed |

S23 | ice |

S24 | refrozen spring snow |

**Figure A2.**Decision tree for the classification of the parallel turns (max_Speed = maximum speed of turn, sd_TD_AbsRRate_Roll = standard deviation of absolute roll axis angular velocity of turn, sd_TD_Gyro_Yaw = standard deviation of gyroscope roll axis angular velocity of the turn).

**Figure A3.**Decision tree for the classification of the non-parallel turns (mean_EA_Edge_AngleLeftRight = mean of the maximum estimated inclination angle of the left and right foot of turn, mean_ACC_X_filt = mean of the maximum of the acceleration of the X-axis of left and right foot of turn, Turn_DurationSec = duration of turn (in seconds)).

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**Figure 6.**Features for the parallel/non-parallel distinction. The features are visualized via boxplots which show the summary statistics. The box represents the interquartile range (IQR = third quartile–first quartile), the thick line the median. The whiskers show the minimum and maximum values without outliers (1.5*IQR), the black dots the outliers which lie outside 1.5*IQR.

**Figure 7.**Decision tree for pre-classification (max_TD_AbsRate_Roll = maximum absolute roll axis angular velocity, max_TD_Symmetry_Roll = maximum symmetry of the roll axis angular velocity).

Parallel Turns | Actual | ||
---|---|---|---|

Drifting | Carving | ||

Predicted | Drifting | True drifting turns (tp) | False carving turns (fn) |

Carving | False drifting turns (fp) | True carving turns (tn) |

Non-Parallel Turns | Actual | ||
---|---|---|---|

Snowplow | Snowplow-Steering | ||

Predicted | Snowplow | True snowplow turns (tp) | False snowplow-steering turns (fn) |

Snowplow-Steering | False snowplow turns (fp) | True snowplow-steering turns (tn) |

Metrics | Formula |
---|---|

Accuracy (acc) | $Accuracy=\frac{tp+tn}{tp+tn+fp+fn}$ |

Sensitivity (sn) | $Sensitivity=\frac{tp}{tp+fn}$ |

Specificity (sp) | $Specificity=\frac{tn}{tn+fp}$ |

Geometric mean | $Geometric\text{}Mean=\sqrt{sn\ast sp}$ |

Accuracy | Sensitivity | Specificity | Geometric Mean | |
---|---|---|---|---|

Decision Tree | 0.885 | 0.901 | 0.866 | 0.883 |

Random Forest | 0.948 | 0.938 | 0.960 | 0.949 |

Boosted Tree | 0.953 | 0.959 | 0.945 | 0.951 |

Accuracy | Sensitivity | Specificity | Geometric Mean | |
---|---|---|---|---|

Decision Tree | 0.822 | 0.688 | 0.860 | 0.769 |

Random Forest | 0.890 | 0.688 | 0.947 | 0.807 |

Boosted Tree | 0.877 | 0.688 | 0.930 | 0.800 |

Parallel Turns | Actual | ||
---|---|---|---|

Carving | Drifting | ||

Predicted | Carving | 218 (90.1%) | 27 (13.4%) |

Drifting | 24 (9.9%) | 174 (86.6%) |

Parallel Turns | Actual | ||
---|---|---|---|

Carving | Drifting | ||

Predicted | Carving | 227 (93.8%) | 8 (4.0%) |

Drifting | 15 (6.2%) | 193 (96.0%) |

Parallel Turns | Actual | ||
---|---|---|---|

Carving | Drifting | ||

Predicted | Carving | 232 (95.6%) | 11 (5.5%) |

Drifting | 10 (4.1%) | 190 (94.5%) |

Non-Parallel Turns | Actual | ||
---|---|---|---|

Snowplow-Steering | Snowplow | ||

Predicted | Snowplow-Steering | 11 (68.8%) | 8 (14.0%) |

Snowplow | 5 (31.2%) | 49 (86.0%) |

Non-Parallel Turns | Actual | ||
---|---|---|---|

Snowplow-Steering | Snowplow | ||

Predicted | Snowplow-Steering | 11 (68.5%) | 3 (5.3%) |

Snowplow | 5 (31.2%) | 54 (94.7%) |

Non-Parallel Turns | Actual | ||
---|---|---|---|

Snowplow-Steering | Snowplow | ||

Predicted | Snowplow-Steering | 11 (68.5%) | 4 (7.0%) |

Snowplow | 5 (31.2%) | 53 (93.0%) |

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## Share and Cite

**MDPI and ACS Style**

Neuwirth, C.; Snyder, C.; Kremser, W.; Brunauer, R.; Holzer, H.; Stöggl, T.
Classification of Alpine Skiing Styles Using GNSS and Inertial Measurement Units. *Sensors* **2020**, *20*, 4232.
https://doi.org/10.3390/s20154232

**AMA Style**

Neuwirth C, Snyder C, Kremser W, Brunauer R, Holzer H, Stöggl T.
Classification of Alpine Skiing Styles Using GNSS and Inertial Measurement Units. *Sensors*. 2020; 20(15):4232.
https://doi.org/10.3390/s20154232

**Chicago/Turabian Style**

Neuwirth, Christina, Cory Snyder, Wolfgang Kremser, Richard Brunauer, Helmut Holzer, and Thomas Stöggl.
2020. "Classification of Alpine Skiing Styles Using GNSS and Inertial Measurement Units" *Sensors* 20, no. 15: 4232.
https://doi.org/10.3390/s20154232