Wearable Technology can Assist Physical Therapists Conducting Biomechanical Assessments

Purpose of the Study

Physical therapists have limited tools for measuring their patients’ physical movements during a functional task. Goniometers are commonly used to measure single joint range of motion, however measuring multiple joints during a functional task is often a challenge. By overlaying the goniometer on certain body parts, like the knee joint, clinicians can measure the single-joint range of motion to assist with the diagnosis and assess a patient’s progress as they undergo rehabilitative therapy.

While a patient’s more complex body movements can be assessed using an optical motion capture system, typically found in biomechanics research laboratories, they are often too expensive and labor intensive for a clinic setting. Physical therapists would require specific training and access to an optical motion system, which takes a considerable amount of time for patient set up and post-collection data processing, making them costly and impractical tools. Due to these limitations, most clinicians diagnose and monitor patients’ more complex movements by sight. Even the most experienced clinicians may sometimes misinterpret movement patterns, especially for dynamic tasks involving multiple body parts and multi-planar movements.

Moveo Explorer by APDM Wearable Technologies enhances clinician observation by providing a portable motion capture system for objective measurement of a patient’s joint range of motion. A 2017 study in the Journal of Applied Biomechanics has taken a look at the Opal movement sensor’s potential as a clinical tool by comparing its measurements to those of an optical motion capture analysis system.


Tulipani and colleagues predicted that the Opal inertial sensors would be as accurate as the Qualisys motion capture system at measuring peak angular displacement for the femur, tibia and lumbar spine segments of the human body when the participants performed functional movement tasks. Accuracy was defined as any measurement within 5 degrees of the other system.¹

Angular displacement – The difference in angle observed when a body part moves in a circular motion from a joint or around a segment’s axis.

Segmentation – The division of the human body into geometrical “segments,” or parts, according to a set of agreed-upon parameters so the movements of the human body can be more easily measured, tracked and analyzed.

Functional movement – A term frequently used in biomechanics and physical therapy that refers to the basic movements a person performs in real-life situations, especially those involving the core and spine.


For the present study, 10 students – five men and five women – from the Auckland University of Technology were selected as participants. While this is a relatively small sample size, it should be noted that prior research has been based on even fewer participants.

The 10 participants were outfitted with 41 retro-reflective markers, which were attached with tape to the surface of their skin, and Opal sensors, which were held in place by elastic bands. Physical therapists and biomechanists were in charge of marker and sensor placement to improve accuracy of the results. A series of nine motion capture cameras taped participants’ movements for later analysis.

In order to test the validity of the Opal wearable sensors compared to the Qualisys motion capture system, participants were required to complete six functional tasks over the course of three trials. These tasks were determined according to the Movement Competency Screen (MCS):

  • DLS – double leg squat
  • LTL – lunge and twist to the left
  • LTR – lunge and twist to the right
  • BAP – bend and pull
  • SLL – single leg squat on left leg
  • SLR – single leg squat on right leg

Participants could complete these movements naturally within a 10-second timeframe. They were also asked to stand still for five seconds prior to their movements so the systems could get accurate baseline measurements.

Computing Results

Both the Opal sensors and Qualisys system captured three-dimensional movement data across all the major planes of motion. These planes include:

  • Sagittal plane – runs from the top of the head to the ground, bisecting the body into left and right halves
  • Coronal plane – runs from the top of the head to the ground, bisecting the body into front and back halves
  • Transverse plane – cuts horizontally across the midsection of the body, as if to separate the head from the feet

The Opal sensors measured angular velocity movements, then input them into an algorithm and computed angular displacement. By contrast, the Qualisys system captured data that was then transferred into Visual3D software, which constructed a geometric approximation of the human body consisting of nine different segments: thorax, lumbar spine, pelvis, right thigh, left thigh, right shank, left shank, right foot and left foot. Because the Opal sensors and Qualisys system operate at different frequencies, their measurements were then aligned using MATLAB software.

Once the data sets were made comparable, they were analyzed using:

  • Root mean square error (RMSE)
  • Difference in peak angular displacement between the Opal sensors and Qualisys system
  • Bland and Altman plots

Relationships between data were determined to be statistically significant if the p-value was less than or equal to 0.05.


Overall, the study suggests that the Opal wearable sensors are capable of measuring angular displacements within 5° of a  motion capture analysis system, especially for movements of the femur, tibia and pelvis segments on the sagittal plane. The RMSE calculations indicated an average angular displacement difference of just 3.7 degrees across all tasks, lower than the 5 degrees allowed. However, there were a few points where the Opal sensors did not align with the Qualisys system:

  • Lumber flexion was overestimated
  • Lumbar and pelvic rotation was underestimated during LTL and LTR
  • Sagittal tibial rotation was slightly overestimated

The researchers felt that these differences and the large variability in the data could be explained. Three of the participants had data that varied greatly from the other six participants. Perhaps the complexity of the functional movements, the amount of body segments tested, sensor and marker placement or individual differences in movement coordination introduced this variability. Additionally, the way lumbar measurements were taken differed between systems. Only one lumbar level was tested by the Opal sensors, while the lumbar spine was treated like an entire rigid segment by Qualisys.

The researchers also believe skin artifact (distortion of measurements) and soft tissue movement could disrupt data collection and called for even more accurate placement of sensors and markers in future studies.

Moveo Explorer Provides Objective Outcome Measures that Support Your Patient Assessments

APDM Wearable Technologies is a digital health company focused on delivering reliable and objective outcome measures for both researchers and healthcare professionals. Our Opal wearable sensors have an impressive 8-hour data streaming battery life, 16-hour data logging battery life and 720-hour data storage that allow you to collect real-life movement data. The Moveo Explorer system integrates the Opal technology to collect full-body kinematic data and provides easily decipherable reports for quick analysis.

If you are looking for a portable, sensitive and reliable system to test your patients’ mobility, please contact us today. Also make sure to stop by booth 1430 if you will be attending the upcoming APTA Combined Sections Meeting in New Orleans this February 21 – 24, 2018.

  1. https://www.ncbi.nlm.nih.gov/pubmed/28787248
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