Velocity represents the average speed of the Center Of Pressure (COP) during the assessment. It is calculated by dividing the total distance traveled by the COP by the duration of the test. Despite being a bit more complex to figure out, mean velocity is preferred over total distance because it allows us to compare assessments of different lengths or those performed in research.
For example, the researcher Daniel Goble has done large studies in thousands of people across different ages using a Quet Stand - Eyes Closed test across a 20-second test duration revealing insightful data. Because COP Velocity is a standardized metric it allows it to be comparable across different periods.
Some research such as Mann et al 2010 have suggested that COP velocity may increase significantly in those with presentations such as lower back pain in an eyes-closed state due to the removal of visual sensory input and potential dysfunction to afferent sensory information to pick up the slack.
However, Salavati et al 2009 also compared COP results in quiet standing across individuals with lower back pain and health controls in eyes open, eyes closed, and a foam surface with little change to healthy controls in the following findings.
Research by Trajković et al 2021 looked at the results of 655 young (13-33 years old) national level athletes across 9 different disciplines performing a 30 second single leg balance test with eyes open finding the following results.
A low-velocity value is generally seen as a sign that someone has good control over their posture and body position on a balanced assessment, while a high velocity may suggest that an individual has poor balance control. It is advised to use this information in conjunction with other assessments as some individuals may have good balance control, but do so with high velocity as they respond to small movements quickly in keeping themselves upright.
References:
Mann L, Kleinpaul JF, Pereira Moro AR, Mota CB, Carpes FP. Effect of low back pain on postural stability in younger women: influence of visual deprivation. J Bodyw Mov Ther. 2010 Oct;14(4):361-6. doi: 10.1016/j.jbmt.2009.06.007. Epub 2009 Jul 25. PMID: 20850043.
Salavati M, Hadian MR, Mazaheri M, Negahban H, Ebrahimi I, Talebian S, Jafari AH, Sanjari MA, Sohani SM, Parnianpour M. Test-retest reliability [corrected] of center of pressure measures of postural stability during quiet standing in a group with musculoskeletal disorders consisting of low back pain, anterior cruciate ligament injury and functional ankle instability. Gait Posture. 2009 Apr;29(3):460-4. doi: 10.1016/j.gaitpost.2008.11.016. Epub 2009 Jan 23. Erratum in: Gait Posture. 2009 Jul;30(1):126. PMID: 19167891.
Trajković, N., Kozinc, Ž., Smajla, D. et al. Relationship between ankle strength and range of motion and postural stability during single-leg quiet stance in trained athletes. Sci Rep 11, 11749 (2021). https://doi.org/10.1038/s41598-021-91337-6