The Thing About Error Correction

How movement errors can be defined, detected, and corrected using technology

Posted by Articul8 on January 20, 2019

The Problem

Defining errors in movement poses a very difficult problem for engineers. This is because no quantitative system detailing the correct way for people to move exists, nor should it. Everyone's bodies are differently shaped, everyone has different requirements put on their bodies, and so everyone moves in different ways. For that reason, it is usually the job of a physiotherapist to determine what a healthy movement is for their unique patient. This highly trained therapist will take into account a patient's past injuries, body type, personal goals, and a variety of other factors to design programs to help them rehabilitate from disability or generally improve their physical health.

Rather than trying to automate the vastly difficult and sensitive task of determining correct movements for patients, we leverage the expertise of physiotherapists to do this, and then assist them in teaching good movements to their patients once they've been defined.

The Method

The way that we can leverage the expertise of a physiotherapist is by having them design a movement for their patient, and then use a wearable motion tracking device to record the patient themselves doing this movement in the physiotherapist's office. This recording gives us the quantitative definition of a good measurement for us to help the user try to achieve on their own. Once recordings of good movements have been taken, the patient can go home and perform the movements, with the Articul8 motion tracking device helping them do the exercises as prescribed.

When the patient is performing the exercises without the supervision of the physio, the Articul8 system will track their movement, compare it to the pre-recorded baseline, and vibrate to guide the user away from errors. The system can quantitatively measure the difference between their current movement and the baseline, and vibrate in a way that ensures they do the movements correctly. The system can then track the user's progression over time, which can be summarized at the patient's next meeting with the physiotherapist.

The Technology

To track the movement of the patient, accelerometer and gyroscope motion sensors are sewn into comfortable garments such as bands or leg and arm sleeves. This sensor data can be fed into a kinematic model of the patient's body to allow for good estimation of all of the patient's joint angles. These joint angles completely define the person's position in a movement, and the difference between their current joint angles and the joint angles in the baseline movement defines their error.

Defining error as a set of errors in the patient's joint angles gives us a basis for correcting their movement and guiding them back towards the baseline. For example, if a user is performing a bicep curl and their arm is not opening up enough, this error would be represented as an insufficient elbow angle. The user would then be signaled to open their arm up until they reached the elbow angle in the baseline, at which point they would be signaled to close their arm.

To signal the user to adjust their joint angles, eight vibrating motors are placed in the garment around every body segment involved in the movement. In the example of the bicep curl, an arm sleeve would be slipped on to the arm before performing the movement, and it would have eight vibrating motors around the forearm and eight around the upper arm. To signal the user to open their arm, motors on the inside of the arm would vibrate, and to signal them to close their arm, motors on the back of the arm would vibrate. The Articul8 system can be used for more complex movements, such as squats, where a leg sleeve is worn and can help correct for errors such as insufficient depth or heels coming off of the ground.


In summary, Articul8 avoids the fundamentally ill-posed problem of trying to determine an objectively correct movement. Instead, we allow physiotherapists to do what they do best: prescribe exercises to help people recover. We then assist physiotherapists where they most need it, by helping patients do their prescribed exercises on their own.