Science for Society | How a suspect gets caught

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We are usually able to recognize people by their gait, even when it is dark. However, we often do not know how we are able to tell. Emeritus Professor of Neuromechanics Bert Otten and Mickey Wiedemeijer, MSc, developed a method to analyse people’s gaits. Standard movement science, right? Not at all: the method is so sophisticated and unique that it is used to solve crimes. A person suspected of having committed arson in Harlingen could be arrested based on a gait analysis, after which more evidence was found and the suspect confessed.

What exactly do they do?

Otten and Wiedemeijer compare video recordings of a crime scene with recordings of a suspect. They analyse the gait pattern of both sets of footage and subsequently calculate the chance of it being the same person, using Bayesian statistics. In short, that is the statistic probability that they find a gait similarity, assuming it is the same person, divided by the chance of a similarity if assuming it is not the same person. ‘You have to word it exactly like that, or you’ll be nowhere in court,’ says Otten with a grin.

The gait pattern is analysed based on 20 different traits, such as the position of the knees and feet. These traits are then compared to a large database based on video recordings of people in the street to see how ‘remarkable’ a particular gait is. They also specifically look at the combination of those traits: knock-knees are reasonably common and so are feet that point outward, but those two combined in one gait pattern; that is rare. When a gait is very remarkable, chances increase that it is indeed the same person.

The analysis is done with the naked eye: Otten and Wiedemeijer look at the footage and the traits themselves and score them, without relying on AI. ‘At first, we wanted to use Computer Vision but time after time we came to the conclusion that it doesn’t work,’ says Otten. ‘Perhaps some day it will, but at the moment, us gait experts are better at it than computers.’

That gait, can you take a look at it?

How did this come about? ‘The police came to us with the question,’ says Wiedemeijer. ‘They had a suspect of a robbery who had a “weird gait”. If we could take a look at it.’ At the time, Wiedemeijer was still doing her Master’s in Movement Sciences and was immediately intrigued. ‘I want to do something with that!’ That is how her Master’s thesis became the foundation for the method they now work with.

It has to be very precise

Based on this method, people are arrested, convicted, or acquitted. It is extremely important that the result is accurate and that its significance is not overestimated. Otten and Wiedemeijer explain—sometimes incessantly—what the result means. ‘If we give a percentage of 99.2, people think: they must have the right person! But there is still a chance that it is not the same person.’ So they started using odds: ‘I explain that with 4000:1 odds, there are still a couple of hundred people in the city who also have that gait. This helps enormously to prevent the result from being overestimated and the person from being convicted based on their gait alone,’ says Otten.

Otten and Wiedemeijer also include their margin of error in the statistics, and they test themselves extensively. They are also being checked by statistics experts of the Netherlands Forensic Institute. ‘That creates trust and is the reason why we have now worked on 70 cases in which we could be of added value,’ adds Wiedemeijer.

A lot of potential

Wiedemeijer and Otten have by now founded a company and are frequently asked to act as experts by the detective force, public prosecutor, and lawyers. Movement sciences appears to be a wonderful addition within the forensic field, which is why public prosecutors, lawyers, and the police force already regularly contact them—also for other movement analyses, such as recognizing cycling people, arms transfers, and falling accidents. ‘For 30 years, I have worked on knowledge of which I was unaware how well usable it was in forensic work,’ Otten says. ‘It has a lot of potential. In fact, forensic movement science could be a specialization!’

More information

• More about Bert Otten
• Profile Bert Otten