Parameters

During the measurements, you can analyze various parameters: the total duration measured by the wearables, dynamic movements, and static postures. The movements and postures depend on the set threshold values for alpha and beta. Alpha is the degree of forward or backward bending with the torso relative to the neutral position. Beta is the degree of folding in the back relative to the neutral position. EN1005-4 'Postures' defines static postures as a posture held for longer than four seconds. If movement occurs out of the respective zone within four seconds, it is considered a dynamic movement.

Depending on whether a risk movement or a risk posture is recorded, the user receives a different type of vibration signal: a risk movement for bending or overextension triggers a strong, short vibration signal, while a forward-bent risk posture causes a subtle but sustained vibration signal (this vibration signal only stops when the person returns to the green zone).

Dynamic movements

It's important to mention that the wearables operate on the principle of 'hysteresis' for dynamic movements. This means that when the angle exceeds the trigger threshold, a movement is detected. A new movement is only registered if the angle falls below the reset threshold and then rises above the trigger threshold again. For example, in the red zone, a movement is registered if the alpha angle exceeds 60°. A new movement is only registered in the red zone if the alpha angle has been below 50° and then rises above 60° again. If the alpha angle does NOT fall below 50° after the first rise above 60°, it is considered as one movement.

The dynamic parameters measured are:

• The number of overextension movements in the back: with standard settings, this is the number of movements beyond -20° alpha and -20° beta.
• The number of bending movements in the back: with standard settings, this is the number of movements beyond 60° alpha and 45° beta.
• The number of torsion movements.
• The number of squat movements and the percentage of squat movements compared to all (red) bending movements.
• The timing of each movement.

By default, the total movements are displayed, but you can also choose to view the movements per 8 hours.

Forward bending with the back

According to ergonomic guideline EN1005-4 ('Evaluation of working postures and movements in machine operations'), the degree of torso tilt is divided into three zones: 0°-20° (green zone), 20°-60° (orange zone), and >60° (red zone, see above figure). Because this guideline does not take into account the degree of back folding, an additional parameter is added. A bending movement is only recorded as a risk movement once the back is bent forward more than 60° (alpha) AND folded more than 45° compared to the neutral position (beta). Forward bending with a straight back, such as in squat or semi-squat techniques (alpha larger than 60° and beta smaller than 45°), or kneeling, does not trigger a registration (or vibration signal) for forward bending.

Overextension of the back

Overextension is defined by this guideline as occurring when the torso tilt is lower than 0° compared to the neutral position. In reality, this threshold value appears to be too strict: if we adhere to this threshold, even simple movements such as walking will register as overextension movements. Therefore, we set this threshold at <-20°. Additionally, an extra parameter is added here to take into account the degree of back folding. An overextension movement in the back is only recorded as a risk movement once the back is tilted back further than -20° (alpha) AND is more than 20° hollow compared to the neutral position (beta).

Torsion

A torsion movement is defined by a minimum 25° difference in the rotation angle between the neck and the belt, based on the neutral position. This movement involves the neck rotating away from and then returning to the neutral position, achieving at least this 25° angular displacement.

Squat

Our squat detection algorithm is based on a confidential model. Please let us know if you encounter situations where this model performs suboptimally. Crouching is no longer predicted separately from squats.

Static movements

For static movements, hysteresis is not taken into account.

The static postures that are recorded are:

• The number of movements in the red zone determined by the set threshold values.
• The number of movements in the orange zone determined by the set threshold values.
• The number of static overextension postures.
• The time spent in the different zones represented as a percentage of the total measured time of the wearable.

To define a forward-bent risk posture, the threshold values of ergonomic standard ISO 11226 ('Evaluation of static working postures') are used. According to ISO 11226, a forward-bent posture of 20° can be maintained for four minutes before it becomes risky, at 60° it's only one minute. Between these thresholds, a sliding score is calculated: the deeper the person leans forward, the shorter the allowed time before it is registered as a risk posture. Remaining beyond 60° for more than four seconds is always considered a risk posture. Once the risk posture is registered, a recovery time of one minute in the green zone is required before the sliding score returns to zero.

The zones are determined by the figure below.

To ensure comparability with ISO 11226, the percentage of time displayed in the zones is always determined based on the alpha angle, even if beta is checked in the configuration screen.

Timeline

The timeline shows when each movement occurred.

Small fluctuations around the movement threshold will not always trigger a movement registration.

The last 5 seconds of usage are cut from the data set.