PRINCIPAL COMPONENT ANALYSIS OF A SINGLE LEG SQUAT

INTRODUCTION Single leg squat (SLS) is a clinical test that is useful in assessing the biomechanical performance of the lower limb. In particular, SLS may provide indicators of muscle strength and balance of a patient [1]. In order to measure the movements present in the single leg squat, a motion capture system is typically used in conjunction with a set of reflective markers. This use of motion capture enables the study of the kinetics and kinematics of the squat for the chosen sample population. Since the use of motion capture in recording the SLS results in a continuous dataset, it is helpful to compress the data to compare the results. Principal component analysis (PCA) is a technique that may be used to reduce the dimensionality in a given dataset so fewer comparisons need to be made. In addition, by interpreting these principal components, any significant differences found can be related back to the biomechanics of the squat. Therefore, the main objective of this study is to run PCA on the SLS data, and look for kinematic differences between the injured and non-injured populations. A secondary objective is to interpret the principal components to find a meaning for any differences found. METHODS The SLS was performed by 50 subjects with intra-articular knee injuries and 50 healthy controls (age: 21.3±2.9, BMI: 24.4±3.7). Each subject performed 3 trials consisting of 5 squats to approximately 45° knee flexion each.  The data was collected using a motion capture system (Motion Analysis, Santa Rosa, CA) at a sampling frequency of 240 Hz. The 3D motion capture data was imported into MATLAB, and the knee FE angle was computed according to [2]. For each subject, the squat that reached closest to 45° knee flexion was chosen for the PCA. PCA was computed on the knee flexion angles of all subjects with respect to time. The resulting principal components (PCs) represent the variation between subjects accounted for by each time point. The first 3 principal components were selected for further analysis, in order to account for 95% of the total variance in the data. In order to determine the meaning of the selected PCs, patient waveforms corresponding to the highest and lowest 5% of PC scores were compared and interpreted, according to Deluzio [3]. The subject data was reduced to 3 dimensions by plotting the FE angle along the selected PC axes. To study the difference between the injured and non-injured subjects, a student t-test was performed on the PC scores for each of the three components, with p value at 0.05. RESULTS High values along the first PC axis were found to correspond to a leftward shift in the subject waveform. A high second PC was found to correspond to a more gradual FE curve, and a high third PC corresponded to a larger downward slope compared to the upward component. When performing a t-test, the PC 1 values for the non-injured group were found to be statically lower (p < 0.0164) than the injured group. No significant difference was noticed along the PC 2 or PC 3 axes. DISCUSSION AND CONCLUSIONS The interpretation of the first PC was taken to be the shift in time of the squat from the average pattern. Based on the results of the t-test, the non-injured group shows a lower PC 1. Based on the interpretation above, the non-injured group appears to spend a larger proportion of the total time in the downward portion of the squat when compared to the non-injured group. This may indicate a deficiency in controlling the flexion descent for subjects suffering from knee injury, which may in turn indicate a possibility for future therapeutic intervention

A computational method for estimating trunk muscle activations during gait using lower extremity muscle synergies

One of the surgical treatments for pelvic sarcoma is the restoration of hip function with a custom pelvic prosthesis after cancerous tumor removal. The orthopedic oncologist and orthopedic implant company must make numerous often subjective decisions regarding the design of the pelvic surgery and custom pelvic prosthesis. Using personalized musculoskeletal computer models to predict post-surgery walking function and custom pelvic prosthesis loading is an emerging method for making surgical and custom prosthesis design decisions in a more objective manner. Such predictions would necessitate the estimation of forces generated by muscles spanning the lower trunk and all joints of the lower extremities. However, estimating trunk and leg muscle forces simultaneously during walking based on electromyography (EMG) data remains challenging due to the limited number of EMG channels typically used for measurement of leg muscle activity. This study developed a computational method for estimating unmeasured trunk muscle activations during walking using lower extremity muscle synergies. To facilitate the calibration of an EMG-driven model and the estimation of leg muscle activations, EMG data were collected from each leg. Using non-negative matrix factorization, muscle synergies were extracted from activations of leg muscles. On the basis of previous studies, it was hypothesized that the time-varying synergy activations were shared between the trunk and leg muscles. The synergy weights required to reconstruct the trunk muscle activations were determined through optimization. The accuracy of the synergy-based method was dependent on the number of synergies and optimization formulation. With seven synergies and an increased level of activation minimization, the estimated activations of the erector spinae were strongly correlated with their measured activity. This study created a custom full-body model by combining two existing musculoskeletal models. The model was further modified and heavily personalized to represent various aspects of the pelvic sarcoma patient, all of which contributed to the estimation of trunk muscle activations. This proposed method can facilitate the prediction of post-surgery walking function and pelvic prosthesis loading, as well as provide objective evaluations for surgical and prosthesis design decisions

The Effect of Stochastic Resonance Stimulation on Proprioception and Postural Control in Anterior Cruciate Ligament Reconstructed Patients

The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments in the knee that frequently results in reconstruction surgery. Some degree of chronic proprioception and postural balance deficiency has been reported following ACL reconstruction (ACLR) surgery, which may be associated with a higher risk of ACL re-injury in these patients. Stochastic resonance (SR) has been shown to improve proprioception in various clinical populations with comparable postural and proprioceptive deficiencies as the ACLR population. In this dissertation, the existence of such deficiencies has been investigated in female ACLR participants and healthy controls. The effect of SR on improving the postural balance and knee proprioception in ACLR and healthy populations has also been studied. The ACLR participants were tested at three months (n = 19) and six months post-surgery (n = 15), while healthy participants were tested once (n = 28). The SR vibration was applied locally to the knee region. Proprioception was evaluated using movement threshold and movement repeatability tests. The effects of the following factors on proprioception were studied: SR (ON vs. OFF), movement direction (flexion vs. extension), and limb condition (ACLR vs. contralateral; ACLR vs. healthy dominant control). Postural balance during single leg standing (duration of 30 sec) was assessed with new measures including entropic half-life (EnHL) and surrogate entropy (ΔE_surr). These measures were developed in conjunction with this dissertation. The effects of the following factors on postural balance were studied: SR (ON vs. OFF), limb side (ACLR vs. contralateral; ACLR vs. healthy dominant control), and vision (eyes open vs. eyes closed). SR vibration successfully improved proprioception in the ACLR and healthy controls. These study results suggest that SR could potentially aid in pre/post-surgery proprioception rehabilitation. This study showed that a postural balance deficiency was present when the ACLR limb was compared to healthy dominant control limbs. When the ACLR limb was compared to the contralateral, the deficiency was only present when the eyes were closed. These findings may suggest that the postural balance deficiency is subtle. Therefore, more stringent or demanding experimental protocols may be necessary to test postural balance in functional groups with deficiencies such as the ACLR group.

ALTERED DYNAMIC TIBIOFEMORAL CONTACT PATH LENGTH IN ACL DEFICIENT KNEES

INTRODUCTIONOsteoarthritis (OA) is a degenerative joint disease characterised by the irreversible degradation of cartilage. Ligament injuries in the knee are a known risk factor for post-traumatic OA (PTOA),1 the aetiology of which may be due to a combination of altered mechanical and biological factors.In this study, we investigated how anterior cruciate ligament (ACL) tear affects the relative motion of the subchondral bone surfaces in the knee (i.e., “surface interactions”), which is abnormal in ACL-deficient animal models.2 Tibiofemoral contact path was calculated based on the relative surface motions. We hypothesised that contact path length and shape are altered in ACL-deficient subjects. METHODSTwo  ACL-deficient subjects and one healthy control subject  (male, ages 34-55) underwent magnetic resonance (MR) imaging scans (3T FIESTA sequence) of both knees, then performed walking trials on an instrumented treadmill. During the walking trials, ground reaction force data were collected and fluoroscopy images from two separate views of the knee were taken. Using Amira (VSG, Germany), 3D models of the tibia and femur were generated from segmented MR images, and the in vivo bone alignments were determined using AutoScoper (Brown University, RI).For each in vivo frame, tibiofemoral proximity was mapped in Matlab (version R2013a, Natick, MA). Weighted centroids were calculated for each of the four tibiofemoral surfaces, with closer proximities having a higher weighting. The weighting factor used was w = (15 mm – proximity)3, counting only proximities less than 15 mm. Contact path was defined as the path that the weighted centroid made across the frames analyzed. Differences in contact path between left and right knees were assessed qualitatively for each of the subjects.RESULTSFor the healthy control subject, contact paths were similar between knees, and were in a straight line, primarily in the anterior-posterior direction. In the two ACL-deficient subjects, the unaffected limb contact path displayed a shape similar to the control limbs. In contrast, the paths in the affected knees were shorter, were not consistently in the same location, and underwent greater mediolateral excursions.DISCUSSION AND CONCLUSIONSThe qualitative results from three subjects support the hypothesis that contact path location and direction may be altered in ACL-deficient individuals. The changes in contact path show similarities to past studies with animal models.2There exist limitations in the ability to measure surface interactions, as only a small portion of the gait cycle can be analyzed using the dual-fluoroscopy system. Despite this, the dual-fluoroscopy method of bone tracking is superior to traditional marker-based motion capture systems, as it is more accurate than marker-based methods.The results suggest that there may be a correlation between ACL status and contact path shape during level walking. Future studies will increase the number of subjects and explore means of comparing contact paths quantitatively. Identification of associations between contact path shape and severity of joint damage may provide new insight into the pathogenesis of osteoarthritis