Zebris FDM pressure integrated to
gaitway-3D

25 APRIL 2025

Introduction

2025 is taking gaitway® 3D another step forward in its development! The gaitway-3D software now integrates the Zebris® pressure sensitive deck as an option to record the pressure distribution under each foot in the same software application as the Arsalis® 3D-force sensors. Using the P-Deck option, gaitway® 3D combines an infinite locomotion track implemented by a h/p/cosmos® treadmill, a ground reaction force measurement with Arsalis 3D-force sensors and the location of the center of pressure (CoP) under each foot with a Zebris pressure distribution sensor. This unique combination opens cutting edge biomechanical analyses of locomotion including the localization of the CoP under each foot during the double stance phase of walking, the assessment of running style and more.
Read further to understand the benefits of the new P-Deck option for gaitway® 3D.

How it works ?

The gaitway® 3D is a single-belt treadmill seated on 4 force transducers. Therefore, gaitway® 3D measures the resultant of all forces applied to the treadmill, i.e. the sum of the forces applied by the right and left legs during walking or running.

In running, only one foot is loading the treadmill at any time. Hence, the resultant applied force can be instantly attributed either to the left or to the right leg. In walking, while both legs are loading the treadmill during the double stance phase, gaitway® 3D uses a force decomposition algorithm [1,2,3] to determine the vertical, fore-aft and lateral force components under each foot. During the double stance phase, the decomposition algorithm relies on (1) the moment of foot contact and toe-off for each foot on the treadmill and (2) the location of the CoP under each foot.
Using an instrumented treadmill equipped with force transducers only, the instants of the foot contact and toe off can be determined based on the change of trajectory of the whole-body CoP; while the algorithm assumes that both feet follow the treadmill belt with no medio-lateral foot roll to determine the CoP under each foot. By combining the force and pressure signals using the P-Deck option for gaitway-3D, (1) the measurement of the foot contact timing is improved, especially for the detection of the toe off which generates only very slight changes in the applied force and (2) any assumption about the CoP under each foot is simply eliminated. Indeed, by sensing a clear pressure signal, the gaitway® 3D with the P-Deck option measures the foot contact time with a higher accuracy while the CoP is measured directly under each foot.

In practice, the Zebris pressure sensitive deck is mounted inside the treadmill and replaces the conventional deck supporting the treadmill belt. The pressure sensors are powered and sampled by the Zebris amplifier mounted laterally underneath the treadmill frame. The Zebris amplifier is powered by the treadmill itself and connects to the PC running the gaitway® 3D software via USB and to the force amplifier via a sync cable to synchronize the force and pressure signals.
Starting with version 2.0.5, the P-Deck option of the gaitway® 3D software reads the pressure distribution under each foot in addition to the readily available ground reaction forces, treadmill speed and video. Zeroing of the pressure distribution and force sensors is done at the same time. Records are started on all sensors with a single “one click” operation or via an external trigger. Digital data streaming to other biomechanics software (e.g. Qualisys QTM, Vicon Nexus, Noraxon MR4) includes the single foot CoP enhanced with the P-Deck option.

What are the benefits ?

Combinaison of the force and pressure measurement on a single-belt instrumented treadmill, as with the gaitway® 3D P-Deck option, offers several benefits to improve the biomechanical analysis of locomotion.

First, the direct measurement of the location of the CoP under each foot allows any single leg analysis to be more accurate. For instance, more accurate lever arms are taken into account for the computation of net joint torques and joint constraints using inverse dynamics thanks to the direct measure of the CoP under each foot.

Second, the location of the CoP under each foot is more accurate than with multiple-belt instrumented treadmills. Indeed, when walking on a split-belt (side-by-side belts) or a tandem belt (front-to-back belts) the CoP under each foot is measured as the CoP under each belt. During the aerial phase of a given foot the walking subject is supported by a single belt, hence the force applied to the other belt is null. When the aerial foot comes in contact with the treadmill belt, the CoP localization can be computed only approximately as it results from a ratio and, when the denominator (the total force applied to a belt) is close to zero, the ratio becomes close to infinity, leading to large errors in the location of the CoP under each foot at low loads. While this issue is commonplace in CoP measurements with force sensors it does not apply to CoP measurements with a pressure sensor. Therefore, the P-Deck option of gaitway3D improves the accuracy of the CoP localization, especially at low loads under a single leg during the double stance phase of walking.

Third, by making the most of the combined force and pressure measurements, gaitway ® 3D offers an improved detection of the foot-contact and toe-off timing, particularly at the slowest speeds of walking when these events are more difficult to detect and improves the accuracy of 3D force decomposition, especially in the vertical direction (2).

Altogether, the combination of force and pressure measurement, using the gaitway® 3D P-Deck option, offers a unique accuracy in the measurement of the kinetics of locomotion allowing an accurate gait analysis on a single-belt instrumented treadmill in the lab and in the clinic.

1. Meurisse GM, Bastien GJ
   Determination of vertical ground reaction forces under each foot during walking.
   Comput Methods Biomech Biomed Engin. 2014;17 Suppl 1:110-1. doi: 10.1080/10255842.2014.931483. PMID: 25074189.
2. Meurisse GM, Dierick F, Schepens B, Bastien GJ.
   Determination of the vertical ground reaction forces acting upon individual limbs during healthy and clinical gait.
   Gait Posture. 2016 Jan;43:245-50. doi: 10.1016/j.gaitpost.2015.10.005. Epub 2015 Oct 17. PMID: 26549482.
3. Bastien GJ, Gosseye TP, Penta M.
   A robust machine learning enabled decomposition of shear ground reaction forces during the double contact phase of walking.
   Gait Posture. 2019 Sep;73:221-227. doi: 10.1016/j.gaitpost.2019.07.190. Epub 2019 Jul 9. PMID: 31374439.