Evaluation of an electromagnetic position tracking device for measuring in vivo, dynamic joint kinematics

Implementation of Sequence-Based Classification Methods for Motion Assessment and Recognition in a Traditional Chinese Sport (Baduanjin)

This study aimed to assess the motion accuracy of Baduanjin and recognise the motions of Baduanjin based on sequence-based methods. Motion data of Baduanjin were measured by the inertial sensor measurement system (IMU). Fifty-four participants were recruited to capture motion data. Based on the motion data, various sequence-based methods, namely dynamic time warping (DTW) combined with classifiers, hidden Markov model (HMM), and recurrent neural networks (RNNs), were applied to assess motion accuracy and recognise the motions of Baduanjin. To assess motion accuracy, the scores for motion accuracies from teachers were used as the standard to train the models on the different sequence-based methods. The effectiveness of Baduanjin motion recognition with different sequence-based methods was verified. Among the methods, DTW + k-NN had the highest average accuracy (83.03%) and shortest average processing time (3.810 s) during assessing. In terms of motion reorganisation, three methods (DTW + k-NN, DTW + SVM, and HMM) had the highest accuracies (over 99%), which were not significantly different from each other. However, the processing time of DTW + k-NN was the shortest (3.823 s) compared to the other two methods. The results show that the motions of Baduanjin could be recognised, and the accuracy can be assessed through an appropriate sequence-based method with the motion data captured by IMU.

Monitoring Flexions and Torsions of the Trunk via Gyroscope-Calibrated Capacitive Elastomeric Wearable Sensors

Reliable, easy-to-use, and cost-effective wearable sensors are desirable for continuous measurements of flexions and torsions of the trunk, in order to assess risks and prevent injuries related to body movements in various contexts. Piezo-capacitive stretch sensors, made of dielectric elastomer membranes coated with compliant electrodes, have recently been described as a wearable, lightweight and low-cost technology to monitor body kinematics. An increase of their capacitance upon stretching can be used to sense angular movements. Here, we report on a wearable wireless system that, using two sensing stripes arranged on shoulder straps, can detect flexions and torsions of the trunk, following a simple and fast calibration with a conventional tri-axial gyroscope on board. The piezo-capacitive sensors avoid the errors that would be introduced by continuous sensing with a gyroscope, due to its typical drift. Relative to stereophotogrammetry (non-wearable standard system for motion capture), pure flexions and pure torsions could be detected by the piezo-capacitive sensors with a root mean square error of ~8° and ~12°, respectively, whilst for flexion and torsion components in compound movements, the error was ~13° and ~15°, respectively.

Wearable Detection of Trunk Flexions: Capacitive Elastomeric Sensors Compared to Inertial Sensors

Continuous monitoring of flexions of the trunk via wearable sensors could help various types of workers to reduce risks associated with incorrect postures and movements. Stretchable piezo-capacitive elastomeric sensors based on dielectric elastomers have recently been described as a wearable, lightweight and cost-effective technology to monitor human kinematics. Their stretching causes an increase of capacitance, which can be related to angular movements. Here, we describe a wearable wireless system to detect flexions of the trunk, based on such sensors. In particular, we present: (i) a comparison of different calibration strategies for the capacitive sensors, using either an accelerometer or a gyroscope as an inclinometer; (ii) a comparison of the capacitive sensors’ performance with those of the accelerometer and gyroscope; to that aim, the three types of sensors were evaluated relative to stereophotogrammetry. Compared to the gyroscope, the capacitive sensors showed a higher accuracy. Compared to the accelerometer, their performance was lower when used as quasi-static inclinometers but also higher in case of highly dynamic accelerations. This makes the capacitive sensors attractive as a complementary, rather than alternative, technology to inertial sensors.

Differences in Motion Accuracy of Baduanjin between Novice and Senior Students on Inertial Sensor Measurement Systems

This study aimed to evaluate the motion accuracy of novice and senior students in Baduanjin (a traditional Chinese sport) using an inertial sensor measurement system (IMU). Study participants were nine novice students, 11 senior students, and a teacher. The motion data of all participants were measured three times with the IMU. Using the motions of the teacher as the standard motions, we used dynamic time warping to calculate the distances between the motion data of the students and the teacher to evaluate the motion accuracy of the students. The distances between the motion data of the novice students and the teacher were higher than that between senior students and the teacher (p < 0.05 or p < 0.01). These initial results showed that the IMU and the corresponding mathematical methods could effectively distinguish the differences in motion accuracy between novice and senior students of Baduanjin.

Mapping Stresses on the Tibial Plateau Cartilage in an Ovine Model Using In-Vivo Gait Kinematics

Understanding stresses within the knee joint is central to understanding knee function, and the etiology and progression of degenerative joint diseases such as post-traumatic osteoarthritis. In this study, in vivo gait kinematics of four ovine subjects were recorded using a highly accurate Instrumented Spatial Linkage (ISL) as each subject walked on a standard treadmill. The subjects were then sacrificed, and the right hind limbs removed. Ten purpose-built Fibre Bragg Grating (FBG) sensors were positioned within each stifle joint and used to measured contact stresses on the articulating surface of the tibial plateau as the recorded gait was replicated using a 6-degrees-of-freedom parallel robotic system. This study provides the first accurate, direct measurement of stress in a joint during in vivo gait replication. It was hypothesized that the results would indicate a direct link between gait kinematics and measured stress values. Contrary to this expectation no direct link was found between individualistic differences in kinematics and differences in stress magnitudes. This finding highlights the complex multifactorial nature of stress magnitudes and distribution patterns across articular joints. The results also indicate that stress magnitudes within the knee joint are highly position dependent with magnitudes varying substantially between points only a few mm apart.

Relative Surface Velocity of the Tibiofemoral Joint and Its Relation to the Development of Osteoarthritis After Joint Injury

The relative velocity of the tibiofemoral surfaces during gait before and after partial-ACL and full MCL transection (p-ACL/MCL Tx) was examined in an ovine model (N = 5) and the relation between the variation in the relative sliding velocity component and gross morphological damage was investigated. We defined the in vivo kinematics of the tibiofemoral joints by using an instrumented spatial linkage and then determining the relative velocity components on the reconstructed femoral condyle surfaces. One major finding was that the magnitude of the relative velocity components was relatively high during the initial stance period of the gait and oscillated with a decaying envelope. Interestingly, for most subjects, the highest value of relative sliding velocity occurred during the stance phase, and not swing. The magnitude of the relative velocity components was increased in 3/5 subjects during stance after an injury. For the lateral compartment, there was a significant correlation (p value = 0.005) between the joint gross morphological damage and the increase in the maximum relative sliding velocity during stance. For the medial compartment, there was a trend (p value < 0.1) between the joint gross morphological score and the increase in the maximum relative sliding velocity during stance, 20 weeks after injury. In conclusion, a connection between an increase in the relative surface velocity and gross morphological damage might be due to an increase in the normal stress and the plowing friction between the surfaces.

Accuracy of human motion capture systems for sport applications; state-of-the-art review

OBJECTIVE

Sport research often requires human motion capture of an athlete. It can, however, be labour-intensive and difficult to select the right system, while manufacturers report on specifications which are determined in set-ups that largely differ from sport research in terms of volume, environment and motion. The aim of this review is to assist researchers in the selection of a suitable motion capture system for their experimental set-up for sport applications. An open online platform is initiated, to support (sport)researchers in the selection of a system and to enable them to contribute and update the overview.

DESIGN

systematic review; Method: Electronic searches in Scopus, Web of Science and Google Scholar were performed, and the reference lists of the screened articles were scrutinised to determine human motion capture systems used in academically published studies on sport analysis.

RESULTS

An overview of 17 human motion capture systems is provided, reporting the general specifications given by the manufacturer (weight and size of the sensors, maximum capture volume, environmental feasibilities), and calibration specifications as determined in peer-reviewed studies. The accuracy of each system is plotted against the measurement range.

CONCLUSION

The overview and chart can assist researchers in the selection of a suitable measurement system. To increase the robustness of the database and to keep up with technological developments, we encourage researchers to perform an accuracy test prior to their experiment and to add to the chart and the system overview (online, open access).

An instrumented spatial linkage for measuring knee joint kinematics

BACKGROUND

In this study, the design and development of a highly accurate instrumented spatial linkage (ISL) for kinematic analysis of the ovine stifle joint is described. The ovine knee is a promising biomechanical model of the human knee joint.

METHODS

The ISL consists of six digital rotational encoders providing six degrees of freedom (6-DOF) to its motion. The ISL makes use of the complete and parametrically continuous (CPC) kinematic modeling method to describe the kinematic relationship between encoder readings and the relative positions and orientation of its two ends. The CPC method is useful when calibrating the ISL, because a small change in parameters corresponds to a small change in calculated positions and orientations and thus a smaller optimization error, compared to other kinematic models. The ISL is attached rigidly to the femur and the tibia for motion capture, and the CPC kinematic model is then employed to transform the angle sensor readings to relative motion of the two ends of the linkage, and thereby, the stifle joint motion.

RESULTS

The positional accuracy for ISL after calibration and optimization was 0.3±0.2mm (mean +/- standard deviation). The ISL was also evaluated dynamically to ensure that accurate results were maintained, and achieved an accuracy of 0.1mm.

CONCLUSIONS

Compared to the traditional motion capture methods, this system provides increased accuracy, reduced processing time, and ease of use. Future work will be on the application of the ISL to the ovine gait and determination of in vivo joint motions and tissue loads.

CLINICAL RELEVANCE

Accurate measurement of knee joint kinematics is essential in understanding injury mechanisms and development of potential preventive or treatment strategies.

Evolving strategies in mechanobiology to more effectively treat damaged musculoskeletal tissues

In this paper, we had four primary objectives. (1) We reviewed a brief history of the Lissner award and the individual for whom it is named, H.R. Lissner. We examined the type (musculoskeletal, cardiovascular, and other) and scale (organism to molecular) of research performed by prior Lissner awardees using a hierarchical paradigm adopted at the 2007 Biomechanics Summit of the US National Committee on Biomechanics. (2) We compared the research conducted by the Lissner award winners working in the musculoskeletal (MS) field with the evolution of our MS research and showed similar trends in scale over the past 35 years. (3) We discussed our evolving mechanobiology strategies for treating musculoskeletal injuries by accounting for clinical, biomechanical, and biological considerations. These strategies included studies to determine the function of the anterior cruciate ligament and its graft replacements as well as novel methods to enhance soft tissue healing using tissue engineering, functional tissue engineering, and, more recently, fundamental tissue engineering approaches. (4) We concluded with thoughts about future directions, suggesting grand challenges still facing bioengineers as well as the immense opportunities for young investigators working in musculoskeletal research. Hopefully, these retrospective and prospective analyses will be useful as the ASME Bioengineering Division charts future research directions.

Electromagnetic tracking system with reduced distortion using quadratic excitation

PURPOSE

Electromagnetic tracking systems, frequently used in minimally invasive surgery, are affected by conductive distorters. The influence of conductive distorters on electromagnetic tracking system accuracy can be reduced through magnetic field modifications. This approach was developed and tested.

METHODS

The voltage induced directly by the emitting coil in the sensing coil without additional influence by the conductive distorter depends on the first derivative of the voltage on the emitting coil. The voltage which is induced indirectly by the emitting coil across the conductive distorter in the sensing coil, however, depends on the second derivative of the voltage on the emitting coil. The electromagnetic tracking system takes advantage of this difference by supplying the emitting coil with a quadratic excitation voltage. The method is adaptive relative to the amount of distortion cause by the conductive distorters. This approach is evaluated with an experimental setup of the electromagnetic tracking system.

RESULTS

In vitro testing showed that the maximal error decreased from 10.9 to 3.8 mm when the quadratic voltage was used to excite the emitting coil instead of the sinusoidal voltage. Furthermore, the root mean square error in the proximity of the aluminum disk used as a conductive distorter was reduced from 3.5 to 1.6 mm when the electromagnetic tracking system used the quadratic instead of sinusoidal excitation.

CONCLUSIONS

Electromagnetic tracking with quadratic excitation is immune to the effects of a conductive distorter, especially compared with sinusoidal excitation of the emitting coil. Quadratic excitation of electromagnetic tracking for computer-assisted surgery is promising for clinical applications.

A specialized motion capture system for real-time analysis of mandibular movements using infrared cameras

BACKGROUND

In the last years, several methods and devices have been proposed to record the human mandibular movements, since they provide quantitative parameters that support the diagnosis and treatment of temporomandibular disorders. The techniques currently employed suffer from a number of drawbacks including high price, unnatural to use, lack of support for real-time analysis and mandibular movements recording as a pure rotation. In this paper, we propose a specialized optical motion capture system, which causes a minimum obstruction and can support 3D mandibular movement analysis in real-time.

METHODS

We used three infrared cameras together with nine reflective markers that were placed at key points of the face. Some classical techniques are suggested to conduct the camera calibration and three-dimensional reconstruction and we propose some specialized algorithms to automatically recognize our set of markers and track them along a motion capture session.

RESULTS

To test the system, we developed a prototype software and performed a clinical experiment in a group of 22 subjects. They were instructed to execute several movements for the functional evaluation of the mandible while the system was employed to record them. The acquired parameters and the reconstructed trajectories were used to confirm the typical function of temporomandibular joint in some subjects and to highlight its abnormal behavior in others.

CONCLUSIONS

The proposed system is an alternative to the existing optical, mechanical, electromagnetic and ultrasonic-based methods, and intends to address some drawbacks of currently available solutions. Its main goal is to assist specialists in diagnostic and treatment of temporomandibular disorders, since simple visual inspection may not be sufficient for a precise assessment of temporomandibular joint and associated muscles.

Extraction of bowing parameters from violin performance combining motion capture and sensors

A method is described for measurement of a complete set of bowing parameters in violin performance. Optical motion capture was combined with sensors for accurate measurement of the main bowing parameters (bow position, bow velocity, bow acceleration, bow-bridge distance, and bow force) as well as secondary control parameters (skewness, inclination, and tilt of the bow). In addition, other performance features (moments of on/off in bow-string contact, string played, and bowing direction) were extracted. Detailed descriptions of the calculations of the bowing parameters, features, and calibrations are given. The described system is capable of measuring all bowing parameters without disturbing the player, allowing for detailed studies of musically relevant aspects of bow control and coordination of bowing parameters in bowed-string instrument performance.

In vivo noninvasive evaluation of abnormal patellar tracking during squatting in patients with patellofemoral pain

BACKGROUND

Patellofemoral pain syndrome is one of the most common knee problems and may be related to abnormal patellar tracking. Our purpose was to compare, in vivo and noninvasively, the patellar tracking patterns in symptomatic patients with patellofemoral pain and those in healthy subjects during squatting. We tested the hypothesis that patients with patellofemoral pain exhibit characteristic patterns of patellar tracking that are different from those of healthy subjects.

METHODS

Three-dimensional patellar kinematics were recorded in vivo with use of a custom-molded patellar clamp and an optoelectronic motion capture system in ten healthy subjects and nine subjects with patellofemoral pain. The position of osseous knee landmarks was digitized while subjects stood upright, and then patellofemoral kinematics were recorded during squatting. The tracking technique was validated with use of both in vitro and in vivo methodologies, and the average absolute error was <1.2 degrees and <1.1 mm.

RESULTS

At 90 degrees of knee flexion, the patella showed lateral spin (the distal pole of the patella rotated laterally) in subjects with patellofemoral pain (mean and standard deviation, -10.13 degrees +/- 2.24 degrees) and medial spin in healthy subjects (mean, 4.71 degrees +/- 1.17 degrees) (p < 0.001). At 90 degrees of knee flexion, the patella demonstrated significantly more lateral translation in subjects with patellofemoral pain (mean, 5.05 +/- 3.73 mm) than in healthy subjects (mean, -4.93 +/- 3.93 mm) (p < 0.001).

CONCLUSIONS

Kinematic differences between healthy subjects and subjects with patellofemoral pain were demonstrated through a large, dynamic range of knee flexion angles. Increased lateral patellar translation and lateral patellar spin in subjects with patellofemoral pain suggest that the patella is not adequately balanced during functional activities in this group. Prospective studies are needed to identify when patellofemoral pain-related changes begin to occur and to determine the risk for the development of patellofemoral pain in individuals with abnormal kinematics.

Accuracy of an optical active-marker system to track the relative motion of rigid bodies

The measurement of relative motion between two moving bones is commonly accomplished for in vitro studies by attaching to each bone a series of either passive or active markers in a fixed orientation to create a rigid body (RB). This work determined the accuracy of motion between two RBs using an Optotrak optical motion capture system with active infrared LEDs. The stationary noise in the system was quantified by recording the apparent change in position with the RBs stationary and found to be 0.04 degrees and 0.03 mm. Incremental 10 degrees rotations and 10-mm translations were made using a more precise tool than the Optotrak. Increasing camera distance decreased the precision or increased the range of values observed for a set motion and increased the error in rotation or bias between the measured and actual rotation. The relative positions of the RBs with respect to the camera-viewing plane had a minimal effect on the kinematics and, therefore, for a given distance in the volume less than or close to the precalibrated camera distance, any motion was similarly reliable. For a typical operating set-up, a 10 degrees rotation showed a bias of 0.05 degrees and a 95% repeatability limit of 0.67 degrees. A 10-mm translation showed a bias of 0.03 mm and a 95% repeatability limit of 0.29 mm. To achieve a high level of accuracy it is important to keep the distance between the cameras and the markers near the distance the cameras are focused to during calibration.

The effects of orientation, temperature, and displacement magnitude changes on the sonometrics system accuracy

The accuracy and reliability of a sonomicrometry system (Sonometrics Corporation, Ontario, Canada) was evaluated for its potential use in measuring 3-D in vivo joint kinematics. Distances between different sets of piezoelectric crystals were measured through a salt solution using ultrasound technology. We evaluated crystal-to-crystal distance under simulated in vivo conditions of changing crystal orientation and displacement magnitude. Crystal-to-crystal distance was also evaluated under changing solution temperature, since the crystals may be used at different temperatures. The 2 mm round and peg crystals were accurate to within 0.5mm for 0 through 180 degrees rotations, but the 2mm round suture loop crystals were only reliable at 0 degrees rotation. The speed of sound through a salt solution (and hence the distance between crystals) versus temperature was fit using a second order polynomial, C=1421.1+3.9808T-3.09x10(-2)T2, with an R2 value of 0.9998. The translational error was less than 0.072 mm for crystal displacements of 0.012, 0.2, 1.0, and 5.0 mm. The system was also accurate under dynamic conditions with translational errors that were less than 0.045 mm under 0.65 Hz motion. These results suggest that the Sonometrics crystals possess attributes (translational accuracy and rotational independence) that could provide the basis for a system capable of measuring joint kinematics.