# Velocity

Definition

The distance moved by the center, nose or tail-base point of the subject per unit time.

Velocity is obtained by dividing Distance moved by the time difference between a sample and the previous one:

where Vk = velocity at sample k (expressed in the unit you have defined in the Experiment Settings) and DMk = Distance moved at sample k.

How to specify Velocity

1.Click the Add button next to Velocity.

2.Under Outlier filter, specify the Averaging interval. Leave 1 if you want to keep the raw data; enter a value to smooth the values using a running average method. See Averaging interval

3.Complete the procedure to add the variable. See Calculate statistics: procedure.

Notes

▪If your experiment is set to Center-point, nose-point and tail-base detection, click the Body points tab and select the body points for which you want to calculate the velocity.

▪Sample rate influences the calculation of Distance moved, and therefore Velocity. When tracking at too low a sample rate, parts of the actual path are cut off, resulting in an underestimation of per-sample Velocity. If, on the other hand, the sample rate is too high, EthoVision XT catches all random movements and wobbling of the body point of the walking subject, causing extra apparent displacement, therefore an overestimation of Velocity.

See Track Smoothing for a description of how to filter out small movements.

▪Because it is based on change in distance moved, Velocity needs two valid (non-missing) samples. A missing sample in your track results in missing Velocity values in that and in the next sample. Make sure that the proportion of missing samples is low (less than 1%).

You can view the proportion of missing samples as one of the System Variables in the Trial list.

Application

▪Apart from the obvious applications of this dependent variable, the mean velocity is sometimes used as a measure of general activity (for example, Nilsson et al. 1993, J. Exp. Biol. 180, 153-162; Winberg et al. 1993, J. Exp. Biol. 179, 213-232).

▪tip Define Velocity and Movement in the same Analysis profile to find out which averaging interval results in the best match between movement bouts and video.

▪In rodent models of aging, the average velocity is used to calculate the frailty index. See Parks et al. (2012) J. Gerontol. A 67(3): 217-227.