Why use Butterworth Zero-Lag filter?

[lucy smith]

Hi All,

I just wanted to know if anyone could explain why a Butterworth zero-lag filter is so popular in Biomech research? I notice it is by far the most popular filter used but I wondered if anyone could explain this (in relatively lay terms) why this is? My engineering side of things is not my strongest, from what I understand it fits a fourth order polynomial (unsure why this is and not another order) and that zero-lag means that it has been filtered and reverse filtered (I THINK... confirmation would be great) so that there is no lag? No lag is particularly important when examining RFD etc. So that would make sense.

Any help would be appreciated!

Thank you in advance,

Lucy

[Brian Davis]

I suspect the popularity has much to do with the enormously popular books that David Winter published on gait analysis. He was trained initially as an electrical engineer, but became a pioneer in biomechanics. Also, this filter is also recursive, in that it uses previous filtered and raw data points when computing a new filtered value.

[Schwartz Cedric]

Indeed, zero-lag filters are useful when timing of events are important in your analysis. As you said zero-lag filters imply to filter both ways. There is two consequences :
- they cannot be used in real-time (which is not often an issue in biomech),
- as the filtering is applied two times, the order of your "based" filter should be half the final order.

An interesting paper (including the adjustment of the cutoff frequencies) is the one of Gordon et al. 2003.