Effect of BMI Group and Walking Modes on Accuracy

The present study has achieved an equivalent level of speed estimation accuracy with that of previous research for the combined BMI groups, and for the obese and normal groups in isolation. Within the study there are, however, a notable differences between the accuracies obtained for each BMI group and for each mode of walking.

The speed estimation models in the present study were derived from data containing both walking modes and both BMI groups and were chosen to give the best overall performance for the entire dataset of mixed BMI and walking mode data. A model derived this way necessitates a compromise in accuracy for individual walking modes and BMI groups in favour of overall accuracy.

When considering the best performing models based on combined BMI group and combined walking modes the RMSE for the obese group is generally higher than the normal group. Figure 18 shows a plot of the RMSE values for each BMI group for a selection of one thousand of the linear models generated by the analysis. The models are ranked in order of RMSE for the combined BMI group data. It can be seen that the majority of the obese RMSE

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values (o) are greater than the normal BMI values (x). There lower differences in RMSE between the two BMI groups for overground walking, though the RMSE remains generally higher for the obese group (Figure 19). For treadmill walking in isolation there is a distinct difference in RMSE values between groups (Figure 20).

The brute-force search technique that was implemented in this study returned accuracy metrics for over one million linear speed estimation models. These one million results were also subdivided by BMI group and walking mode. This means that accuracy performance between BMI groups could be compared on a larger scale. A sample of 1,000 of the best performing linear models for overground walking in isolation, ranked in order of performance for the obese group, returns varying results for the normal BMI group (as shown in Figure 21); though the normal BMI group has generally higher RMSE values than the obese group for this subset of data. When a sample of 1,000 of the best performing linear models for overground walking are ranked according to the normal BMI group performance, the RMSE values of the obese group are distinctly higher (Figure 22). When these comparisons are made for treadmill walking as shown in Figure 23, the normal BMI group RMSE values are distributed relatively evenly above and below the line representing the best ranking obese group models. In contrast, the highest ranking models for the normal BMI group return distinctly lower RMSE values when applied to the obese group (Figure 24). These results imply that there is an inherent difference between the accelerometer output collected from obese and normal individuals while performing walking activities. There are two explanations for this: either there is an intrinsic difference between the walking styles of obese and normal BMI individuals; or the accelerometer movement is affected by a factor, other than gait, which differs between the BMI groups, such as the increased adipose tissue at the accelerometer site for the obese group. The difference is particularly evident in treadmill walking for the obese group (Figure 24). The results imply that differences in gait between BMI groups are responsible for this difference in accuracy; the accelerometer placement is a constant between the two walking modes as it was not removed between walking trials, yet results differ according to walking mode, which suggests that the differences are not caused by an alternative effect on accelerometer movement. This also apparently implies there may be a difference in walking styles between treadmill and overground walking for one or both of the BMI groups. However, this conclusion cannot be made for certain as the observed

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differences may have been influenced by the different speeds that were performed between the obese and the normal BMI groups. This is discussed further in section 4.4.7.

When speeds were divided into two bands (slower speeds between 1.0ms-1 and 1.3ms-1, and faster speeds between 1.4ms-1 and 1.7ms-1) and analysed separately, the walking speed estimation accuracies improved (Table 19 and Table 20). MAPE values were well under 4% for nearly all models. The difference in estimation accuracy between BMI groups was also affected. For the slower walking band, model 3 returned good accuracies across BMI groups, though the obese group showed better results for treadmill walking than the normal BMI group (RMSE was 0.66ms-1 and 0.71ms-1 respectively), and the normal BMI group showed better accuracy for overground walking than the obese group (RMSE was 0.62ms-1 and 0.88ms-1 respectively). For the faster walking band, RMSE values were generally similar for all combinations of model, BMI group and walking mode, except for overground walking which showed a greater disparity between BMI group accuracy for models 1 and 2. For overground walking in the faster speed band, model 1 returned RMSE values of 0.086ms-1 and 0.047ms-1 for the obese group and normal group respectively. In contrast, model 3 achieved RMSE values of 0.064 ms-1 and 0.075 ms-1 for the obese group and normal group respectively for overground walking. The differing accuracies between BMI groups according to estimation model (yet for the same walking mode) suggest that there is a difference between BMI groups that is accounted for by one model and not be the other. Because the data was divided into speed bands, these differences between BMI groups are less likely to be influenced by the speeds performed by each BMI group.