Wearable Sensors Used in Underwater Kinematic Analysis

APDM Opal sensors provided interesting data in a recent study of underwater gait kinematics.

Water therapy can be a useful tool for rehabilitation of patients with impaired posture or movement, as well as athletes, because it eases the gravitational load on the joints. Additionally, the resistance that water provides is useful for muscle and motor task training.


Researchers Silvia Fantozzi, Andrea Giovanardi, Davide Borra, and Giorgio Gatta of the University of Bologna, Italy, aimed to estimate the 3D joint kinematics of the lower extremities and thorax-pelvis joints in the sagittal and frontal planes during underwater walking. The team started looking into wearable sensors for underwater gait collection in 2012. Previous studies reported analysis for only the sagittal plane and lower limbs, and did not take into account the frontal and transverse planes or thorax-pelvis kinematics. These studies also used underwater cameras, which have a limited field of view, only allowing the measure of a few steps at a time. They also have a larger margin of error due to things like water refraction, and they are much more time consuming to set up.

Using Opals, the researchers were able to avoid these complications. They commented on the fast sensor setup and immediate data capture, which can provide accurate analysis to a therapist running an aquatic therapy session.The researchers placed 8 waterproofed Opals on 11 healthy adults and studied their gait cycles in shallow water and dry land conditions. 704 gait cycles were recorded, then normalized in time and averaged to find the mean cycle of each joint measured.

Researcher Andrea Giovanardi comments “Among the wearable technology companies, APDM was the only one to match all our requirements: simultaneous collection of both orientation and calibrated raw data, wireless streaming, large internal memory (8Gb), small and lightweight sensors, and a sampling frequency of 128 Hz. After a few years, we can say that we made the right choice – the APDM Opal system is reliable, easy to use, and non-invasive for the athlete performing the test. The robust synchronized streaming works very well, allowing us to start the test with the Opals out of the water, and stop it while the Opals are still in the water.”



Results: The group found that the mean speed in shallow water conditions was 40% lower than the dry land condition. Additionally, underwater walking showed longer stride duration and shorter stride distance. The knees of participants were more flexed in the sagittal plane (23º), the ankles more dorsiflexed (9º) at heel strike, and the hips were more flexed at toe-off (13º) than on land. The ankles were more inversed at toe-off (7º) and smoother joint angle patterns were observed in the frontal plane for thorax-pelvis and hip areas.

A paper on the study states “The results were mainly explained by the effect of the speed in the water as supported by the linear mixed models analysis performed. Thus, it seemed that the combination of speed and environment triggered modifications in the joint angles in underwater gait more than these two factors considered separately.”

An article on the study is available for download here – “Gait Kinematic Analysis in Water Using Wearable Inertial Magnetic Sensors.”

A similar study, done by many of the same researchers, was conducted to look at upper limb kinematics during simulated swimming.

Assessment of Three-Dimensional Joint Kinematics of the Upper Limb During Simulated Swimming Using Wearable Inertial-magnetic Measurement Units was carried out by Silvia Fantozzi, Andrea Giovanardi, Fabrício Anício Magalhães, Rocco Di Michele, Matteo Coresi, and Giorgio Gatta.

Contrary to traditional video analysis, Opals could provide data for both underwater and aerial phases of swimming strokes over the length of a swimming pool. Additionally, researchers were provided fast data collection in a short amount of time – a potential advantage for coaches and athletes. Mesh synchronization, continuous recording, and long battery life allowed researchers to gather data over long periods of time, translating to the potential for a coach to be able to accurately record an athlete’s full workout.

This study provides a better look at the use of 3D joint kinematics for more accurate estimation of swimming performance indicators. In addition, the overall movement patterns of a swimmer and their relationship to injuries can potentially be examined with greater precision.


The findings from both studies are an important addition to the practical use of Opals for activities and rehabilitation outside of a clinical setting.

For more information on Opal Wearable Sensors, click here. 

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