Terry K. Koo

Posture effect on seating interface biomechanics: Comparison between two seating cushions☆☆☆

OBJECTIVE This study aimed at investigating the effects of sitting posture on ischial pressure and pelvic orientation for two types of cushions. DESIGN Two types of seating devices, Roho and Polyurethane (PU) Foam cushions, six predefined postures, and two subject groups, Normal and Paraplegic, were tested. Ischial pressure and pelvic orientation were monitored. The sequence in which the cushions were tested were randomized, and the postures were tested according to a preplanned sequence. SETTING The experiments were carried out in the laboratory of a rehabilitation engineering centre in Hong Kong. PARTICIPANTS Six paraplegic subjects (referred sample) and 8 normal volunteers were tested. At the time of study, no subjects showed any signs of pressure sore or other health problems. MAIN OUTCOME MEASURES It was hypothesized that sitting posture could significantly affect pelvic orientation as well as pressure distribution. It was also hypothesized that cushion selection was a critical factor in compensating the adverse effects of postures on pressure distribution. RESULTS With the paraplegic group, the maximum ischial pressure recorded on Roho for various postures ranged from 88 mmHg for the Trunk-Bent-Forward (PF) posture to 146 mmHg for the Trunk-Bent-Right (PR) posture, whereas on PU foam cushions, the values ranged from 106 mmHg for the PF posture to 221 mmHg for the PR posture. With the paraplegic subjects sitting with their trunks bent laterally, it was found that the mean pressure difference between the left and right ischial tuberosities was more prominent on the PU foam cushion than on the Roho. CONCLUSION Sitting posture can significantly affect pelvic orientation and ischial pressure. This study was also showed that the Roho cushion was significantly more efficient in compensating the adverse effects of sitting posture on pressure distribution.

Relationship between shear elastic modulus and passive muscle force: An ex-vivo study

As muscle is stretched, it reacts with increasing passive resistance. This passive force component is important for normal muscle function. Unfortunately, direct measurement of passive muscle force is still beyond the current state-of-the-art. This study aimed to investigate the feasibility of using Supersonic shear wave elastography (SSWE) to indirectly measure passive muscle force. Sixteen gastronomies pars externus and 16 tibialis anterior muscles were dissected from 10 fresh roaster chickens. For each muscle specimen, the proximal bone-tendon junction was kept intact with its tibia or femur clamped in a fixture. Calibration weights (0-400 g in 25 g per increment) were applied to the distal tendon via a pulley system and muscle elasticity was measured simultaneously using SSWE. The measurements were repeated for 3 cycles. The elasticity-load relationship of each tested muscle for each loading cycle was analyzed by fitting a least-squares regression line to the data. Test-retest reliability was evaluated using intraclass correlation coefficient (ICC). Results demonstrated that the relationships between SSWE elasticity and passive muscle force were highly linear for all the tested muscles with coefficients of determination ranging between 0.971 and 0.999. ICCs were 0.996 and 0.985, respectively, for the slope and y-intercept parameters of the regression lines, indicating excellent reliability. These findings indicate that SSWE, when carefully applied, can be a highly reliable technique for muscle elasticity measurements. The linear relationship between SSWE elasticity and passive muscle force identified in the present study demonstrated that SSWE may be used as an indirect measure of passive muscle force.

Quantifying the passive stretching response of human tibialis anterior muscle using shear wave elastography

BACKGROUND Quantifying passive stretching responses of individual muscles helps the diagnosis of muscle disorders and aids the evaluation of surgical/rehabilitation treatments. Utilizing an animal model, we demonstrated that shear elastic modulus measured by supersonic shear wave elastography increases linearly with passive muscle force. This study aimed to use this state-of-the-art technology to study the relationship between shear elastic modulus and ankle dorsi-plantarflexion angle of resting tibialis anterior muscles and extract physiologically meaningful parameters from the elasticity-angle curve to better quantify passive stretching responses. METHODS Elasticity measurements were made at resting tibialis anterior of 20 healthy subjects with the ankle positioned from 50° plantarflexion to up to 15° dorsiflexion at every 5° for two cycles. Elasticity-angle data was curve-fitted by optimizing slack angle, slack elasticity, and rate of increase in elasticity within a piecewise exponential model. FINDINGS Elasticity-angle data of all subjects were well fitted by the piecewise exponential model with coefficients of determination ranging between 0.973 and 0.995. Mean (SD) of slack angle, slack elasticity, and rate of increase in elasticity were 10.9° (6.3°), 5.8 (1.9) kPa, and 0.0347 (0.0082) respectively. Intraclass correlation coefficients of each parameter were 0.852, 0.942, and 0.936 respectively, indicating excellent test-retest reliability. INTERPRETATION This study demonstrated the feasibility of using supersonic shear wave elastography to quantify passive stretching characteristics of individual muscle and provided preliminary normative values of slack angle, slack elasticity, and rate of increase in elasticity for human tibialis anterior muscles. Future studies will investigate diagnostic values of these parameters in clinical applications.

In vivo determination of subject-specific musculotendon parameters: applications to the prime elbow flexors in normal and hemiparetic subjects

OBJECTIVE This study aimed at estimating the musculotendon parameters of the prime elbow flexors in vivo for both normal and hemiparetic subjects. DESIGN A neuromusculoskeletal model of the elbow joint was developed incorporating detailed musculotendon modeling and geometrical modeling. BACKGROUND Neuromusculoskeletal modeling is a valuable tool in orthopedic biomechanics and motor control research. However, its reliability depends on reasonable estimation of the musculotendon parameters. Parameter estimation is one of the most challenging aspects of neuromusculoskeletal modeling. METHODS Five normal and five hemiparetic subjects performed maximum isometric voluntary flexion at nine elbow positions (0 degrees -120 degrees of flexion with an increment of 15 degrees ). Maximum flexion torques were measured at each position. Computational optimization was used to search for the musculotendon parameters of four prime elbow flexors by minimizing the root mean square difference between the predicted and the experimentally measured torque-angle curves. RESULTS The normal group seemed to have larger maximum muscle stress values as compared to the hemiparetic group. Although the functional ranges of each selected muscle were different, they were all located at the ascending limb of the force-length relationship. The muscle optimal lengths and tendon slack lengths found in this study were comparable to other cadaver studies reported in the literature. CONCLUSION Subject-specific musculotendon parameters could be properly estimated in vivo. RELEVANCE Estimation of subject-specific musculotendon parameters for both normal and hemiparetic subjects would help clinicians better understand some of the effects of this pathological condition on the musculoskeletal system.

Joint Position Dependence of Weakness During Maximum Isometric Voluntary Contractions in Subjects With Hemiparesis

OBJECTIVE To determine the distribution of weakness across elbow range of motion (ROM) in subjects with hemiparesis. DESIGN A detailed analysis of elbow torque and associated electromyographic signals of 5 prime elbow muscles generated during maximum isometric voluntary flexion (MIVF) and extension (MIVE) at 8 different elbow positions. SETTING Rehabilitation center research laboratory. PARTICIPANTS Convenience samples of 5 controls and 10 subjects with hemiparesis with sufficient passive (>90 degrees ) and active (>60 degrees ) ROM on their paretic side. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Measured and normalized MIVF and MIVE torques and normalized moving average electromyographic signals of each muscle at each testing position. RESULTS Measured MIVF and MIVE torques generated by the hemiparetic group were marginally and significantly smaller than those of the control group (2-factor repeated-measures analysis of variance [ANOVA]: P=.053 for MIVF, P=.011 for MIVE). Distribution of weakness was nonuniform across elbow positions, as shown by normalized torque-position curves. Normalized MIVE torque of the hemiparetic group was significantly and marginally smaller than that of the control group at 15 degrees and 30 degrees (Student t test: P<.0001, P=.054), respectively. Although statistically not significant, the normalized MIVF torque of the hemiparetic group was slightly larger than that of the control group but became smaller than the control groups as the elbow flexed beyond 90 degrees. Our electromyographic recordings supported the normalized MIVF torque findings, showing a significant increase in brachioradialis activation in the control group at flexed positions during MIVF (1-factor repeated-measure ANOVA, P=.003), but not in the hemiparetic group (P=.392). CONCLUSION Our findings suggest that measuring the strength in multiple joint positions is useful for characterizing the basic changes in muscle activation strategies and properties and provides a relevant measure of elbow weakness from a clinical and functional perspective. Various mechanisms of action are discussed to better understand the relation between joint position and weakness.

Test-Retest Reliability, Repeatability, and Sensitivity of an Automated Deformation-Controlled Indentation on Pressure Pain Threshold Measurement

OBJECTIVE The purpose of this study was to construct a computerized deformation-controlled indentation system and compare its test-retest reliability, repeatability, and sensitivity with a manual algometer for pressure pain threshold (PPT) measurements. METHODS Pressure pain threshold measurements were made on 16 healthy subjects for 2 sessions on bilateral erector spinae muscles at L1, L3, and L5 spinal levels, consisting of 5 repeated trials each using computerized algometry on one side and manual algometry on the other side. Mean, SD, coefficient of variation, standard error of measurement, minimal detectable change, and intraclass correlation coefficient were calculated for both manual and computerized PPT measurements. Effects of session, level, method, and side on PPT measurements were evaluated using analysis of variance. RESULTS Manual PPT measurements were significantly larger than computerized PPT measurements (P = .017), and session 2 was significantly larger than session 1 (P = .021). Coefficient of variation, intraclass correlation coefficient, standard error of measurement, and minimal detectable change of the manual and computerized PPT measurements were 10.3%, 0.91, 0.19 kg/cm(2), and 0.54 kg/cm(2) and 15.6%, 0.87, 0.26 kg/cm(2), and 0.73 kg/cm(2), respectively. CONCLUSIONS Although computerized algometry offers the benefits of eliminating the effects of operator reaction time, operator anticipation, alignment error, and variation in indentation rate on PPT measurements, these results indicate that manual algometry using load-controlled strategy may be better than computerized deformation-controlled algometry in terms of test-retest reliability, repeatability, and sensitivity. Constant load-controlled indentation protocol may be more favorable for PPT measurements. Future computerized instrumentation for PPT measurements should adopt a load-controlled mechanism.

Evaluation of an Active Seating System for Pressure Relief

In the first part of this study, the inflation-pressure and interface-pressure profiles of an active cushion system, the Talley active air bellows cushion, were examined continuously for one complete working cycle using the dynamic pressure monitor. The relationship between the inflation pressure and the interface pressure was explored. A well-defined relationship was found in the areas directly over the air bellows. In the second part of this study, the pressure-relieving characteristics of the active cushion were assessed quantitatively and compared to two types of passive cushions--the Roho high-profile air floatation cushion and the polyurethane (PU) foam cushion. Eight non-disabled subjects were positioned on the active cushion at two inflation-pressure levels--30 mmHg and 60 mmHg, or on the Roho or the PU foam cushions. Interface pressures were recorded using the Oxford pressure monitor. For the active cushion it was shown that the higher the inflation pressure was, the better the pressure-relieving characteristics seemed to be. In general, the pressure-relieving characteristics of the active cushion were not as good as those of the passive cushions being tested. The active cushion could alter the pressures over the ischial tuberosities cyclically but the amount of pressure alternation depended on the relative position of the ischial tuberosities and the air bellows.

Reliability of sonomyography for pectoralis major thickness measurement.

OBJECTIVE Muscle thickness is a widely used parameter for quantifying muscle function in ultrasound imaging. However, current measurement techniques generally rely on manual digitization, which is subjective, time consuming, and prone to error. The primary purposes of this study were to develop an automated muscle boundary tracking algorithm to overcome these limitations and to report its intraexaminer reliability on pectoralis major muscle. METHODS Real-time B-mode ultrasound images of the pectoralis major muscles were acquired by an integrated data acquisition system. A transducer placement protocol was developed to facilitate better repositioning of an ultrasound transducer. Intraexaminer reliability of the tracking algorithm for static measurements was studied using intraclass correlation coefficient based on the thickness data from 11 healthy subjects recruited from a chiropractic college measured at 3 independent sessions. Standard error of measurement and minimal detectable change were calculated. Feasibility of using the tracking algorithm for dynamic measurements was also evaluated. RESULTS All calculated intraclass correlation coefficients were larger than 0.96, indicating excellent reliability of the sonomyographic measurements. Minimal detectable changes were 9.7%, 6.7%, and 6.8% of the muscle thickness at the lateral, central, and medial aspects, respectively. For a 400-frame image stack with 3 pairs of 40 x 40 pixels tracking windows, the tracking took about 80 seconds to complete. CONCLUSIONS The tracking algorithm offers precise and reliable measurements of muscle thickness changes in clinical settings with potential to quantify the effects of a wide variety of chiropractic techniques on muscle function.

A Mechano-Acoustic Indentor System for In Vivo Measurement of Nonlinear Elastic Properties of Soft Tissue

OBJECTIVES Soft tissue exhibits nonlinear stress-strain behavior under compression. Characterizing its nonlinear elasticity may aid detection, diagnosis, and treatment of soft tissue abnormality. The purposes of this study were to develop a rate-controlled Mechano-Acoustic Indentor System and a corresponding finite element optimization method to extract nonlinear elastic parameters of soft tissue and evaluate its test-retest reliability. METHODS An indentor system using a linear actuator to drive a force-sensitive probe with a tip-mounted ultrasound transducer was developed. Twenty independent sites at the upper lateral quadrant of the buttock from 11 asymptomatic subjects (7 men and 4 women from a chiropractic college) were indented at 6% per second for 3 sessions, each consisting of 5 trials. Tissue thickness, force at 25% deformation, and area under the load-deformation curve from 0% to 25% deformation were calculated. Optimized hyperelastic parameters of the soft tissue were calculated with a finite element model using a first-order Ogden material model. Load-deformation response on a standardized block was then simulated, and the corresponding area and force parameters were calculated. Between-trials repeatability and test-retest reliability of each parameter were evaluated using coefficients of variation and intraclass correlation coefficients, respectively. RESULTS Load-deformation responses were highly reproducible under repeated measurements. Coefficients of variation of tissue thickness, area under the load-deformation curve from 0% to 25% deformation, and force at 25% deformation averaged 0.51%, 2.31%, and 2.23%, respectively. Intraclass correlation coefficients ranged between 0.959 and 0.999, indicating excellent test-retest reliability. CONCLUSIONS The automated Mechano-Acoustic Indentor System and its corresponding optimization technique offers a viable technology to make in vivo measurement of the nonlinear elastic properties of soft tissue. This technology showed excellent between-trials repeatability and test-retest reliability with potential to quantify the effects of a wide variety of manual therapy techniques on the soft tissue elastic properties.

A SURFACE EMG DRIVEN MUSCULOSKELETAL MODEL OF THE ELBOW FLEXION-EXTENSION MOVEMENT IN NORMAL SUBJECTS AND IN SUBJECTS WITH SPASTICITY

Spasticity often interferes with function, limits independence and may cause considerable disability. Elbow joint movement is involved in many daily living activities. A surface EMG driven musculoskeletal model was developed to predict joint trajectory and to compare the differences in the model parameters between the normal and spastic subjects. Three musculotendon actuators whose EMG could be assessed by surface electrodes (biceps, brachioradialis and triceps) were included in this musculoskeletal model. The proposed model took several sets of parameters (anthropometric parameters of the skeleton and muscle parameters) as inputs. Surface EMG signals of the three muscle groups were rectified, moving-averaged, scaled and converted to active states. These active states together with the initial angular position and velocity of the joint were also used as inputs for the model. The outputs were muscle forces and the trajectory of the elbow joint. Two groups of parameters, namely, maximal isometric muscle stress and electromechanical delay were estimated using the trajectory fitting algorithm. Results indicated that the model was successful in using the surface EMG as input signals in the prediction of elbow joint trajectory. The spastic subjects showed a lower maximum isometric muscle stress and longer electromechanical delay.