Kyu-Jung Kim

Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera.

Background: Both static and dynamic factors are responsible for glenohumeral joint stability. We hypothesized that dynamic factors could potentially operate throughout the entire range of glenohumeral motion, although capsuloligamentous restraints (a static factor) have been thought to be primarily responsible for stability in the end-range of motion. The purpose of this study was to quantitatively compare the dynamic glenohumeral joint stability in the end-range of motion (the position of anterior instability) with that in the mid-range by investigating the force components generated by the rotator cuff muscles. Methods: Ten fresh-frozen shoulders from human cadavera were obtained, and all soft tissues except the rotator cuff were removed. The glenohumeral capsule was resected after the rotator cuff muscles had been released from the scapula. A specially designed frame positioned the humerus in 60 degrees of abduction and 45 degrees of extension with respect to the scapula. The compressive and shear components on the glenoid were measured before and after a constant force was applied individually to each muscle with the humerus in five different positions (from neutral to 90 degrees of external rotation). The dynamic stability index, a new biomechanical parameter reflecting these force components and the concavity-compression mechanism, was calculated. The higher the dynamic stability index, the greater the dynamic glenohumeral stability. Results: In the mid-range of motion, the supraspinatus and subscapularis provided higher dynamic stability indices than did the other muscles (p < 0.05). On the other hand, when the position of anterior instability was simulated in the end-range of motion, the subscapularis, infraspinatus, and teres minor provided significantly higher dynamic stability indices than did the supraspinatus (p < 0.005). Conclusions: The rotator cuff provided substantial anterior dynamic stability to the glenohumeral joint in the end-range of motion as well as in the mid-range. Clinical Relevance: A glenohumeral joint with a lax capsule and ligaments might be stabilized dynamically in the end-range of motion if the glenoid concavity is maintained and the function of the external and internal rotators, which are efficient stabilizers in this position, is enhanced.

Biomechanics of fall arrest using the upper extremity: age differences.

OBJECTIVE This study tried to isolate critical biomechanical factors in fall arrests using the upper extremity during simulated forward falls. This study also attempted to find the differences in those factors between young and old age groups. BACKGROUND The role of the upper extremity is not well defined despite its primary usage as a local shock absorber during fall impact. DESIGN Comparative study in which two age groups underwent motion analysis.Methods. Ten healthy older males (mean age, 66.4 years) and 10 young males (mean age, 24.1 years) volunteered to perform self-initiated and cable-released falls at selected falling distances, while the joint motion and impact forces at the hand were recorded. RESULTS Significant age differences were demonstrated in joint kinematics and impact force parameters at close distances. Excessive reflexive responses of the upper extremity in cable-released falls for the older adults resulted in 10-15 times higher peak impact forces and 2-3 times shorter body braking time than in self-initiated falls. CONCLUSIONS Pre-impact activities of the upper extremity predispose the post-impact response during fall arrests. Suppressing excessive pre-impact reflexive activation of the arms could efficiently decrease the risk of fall-related injuries, which calls for securing sufficient arm movement time. Any fall prevention strategy that can increase arm movement time would be effective against injuries of the upper extremity during falling in the older adults. RELEVANCE The findings will help to understand underlying mechanisms of fall arrest using the upper extremity for prevention of fall-related fractures.

An instrumented wheel for kinetic analysis of wheelchair propulsion

An instrumented wheel system for three-dimensional kinetic analysis of upper extremity during wheelchair propulsion has been designed and validated. This system allows the direct measurements of three-dimensional dynamic forces and moments on the handrim during wheelchair propulsion in a laboratory setting as well as in the field. Static loading tests showed a high linearity and little drift (coefficient of determination, r2 > 0.999). Under dynamic loading, the instrumented wheel provided the well-matched measurement forces and moments with the predicted values from the inverse dynamic method using video-based kinematic data (correlation coefficient, p > 0.97). The three-dimensional handrim forces and moments during wheelchair propulsion by a non-disabled subject were demonstrated.

An in vitro study of individual ankle muscle actions on the center of pressure

The objective of this study was to correlate the effects of muscle force on the movement of the center of pressure (COP) for increased clinical utility of the COP measurement. Five fresh frozen cadaveric specimens were used to apply a 49 N sinusoidal muscle force to isolated or grouped extrinsic ankle muscles, and a constant ankle joint reaction force at different tibial positions. The muscle force and the vertical ground reaction force (GRF) play the role of a mechanical lever system so that the differential COP movement can be interpreted as a moment arm for the vertical GRF.

IN VITRO SIMULATION OF THE STANCE PHASE IN HUMAN GAIT

The purpose of this study is to develop an electromechanical system for dynamic simulation of the stance phase of a human gait using cadaveric foot specimens. The system can be used for quantification of foot and ankle pathomechanics and design of foot and ankle reconstructive surgeries. Servo-pneumatic systems were used for application of the tibial weight loading and muscle loadings. A four-bar mechanism was constructed to provide the progressive motion of a tibia during the simulation while the external loadings were simultaneously applied. Muscle loadings were estimated based on the physiological cross-sectional area and normal electromyography (EMG) data with the assumption of the linear EMG–force relationship. Ad hoc tuning of the unknown muscle gains was conducted until a reasonable match with the normal vertical ground reaction force profile, center of pressure advancement, and characteristic foot motion events (heel strike, foot flat, heel rise and toe-off) could be made. Three cadaver feet and an artificial foot were tested with five repeated trials. The simulator reproduced the stance phase of a human gait in the sagittal plane with reasonable accuracy and consistency without compromising either kinematics or kinetics of the foot and ankle complex.

Shock Attenuation of Various Protective Devices for Prevention of Fall-Related Injuries of the Forearm/Hand Complex

Background Attenuation of the peak impact force is essential in any protective devices for prevention of fall-related injuries. Hypothesis Common wrist guards have limited effectiveness because of the multifaceted nature of wrist injury mechanisms, and other modalities may provide enhanced shock-absorbing functions. Study Design Controlled laboratory study. Methods A free-fall device was constructed using a mechanical surrogate to simulate falling impact. At 4 different falling heights, 5 different hand conditions were tested: bare hand, a generic-brand wrist guard, a Sorbothane glove, an air cell, and an air bladder condition. The impact force from the ground and the transmitted impact force to the forearm/hand complex were simultaneously measured. Results The falling height and hand condition significantly modulated the impact responses. The padded conditions always had significantly smaller peak impact forces compared with the bare-hand condition. The wrist guard became ineffective in impact force attenuation beyond the falling height of 51 cm. On the other hand, the air bladder condition maintained less than 45% of the peak impact force of the bare-hand condition and remained below the critical value, whereas other conditions were all ineffective. Conclusion It was reconfirmed that common wrist guard design could provide limited impact force attenuation, whereas damped pneumatic springs would provide substantially enhanced shock-absorbing functions. Clinical Relevance A wrist guard incorporating volar padding with the pneumatic spring design principle might be more effective at preventing injuries than are currently available designs.

Segmental dynamics of forward fall arrests: A system identification approach

BACKGROUND Fall-related injuries are multifaceted problems. One approach to identify the critical biomechanical factors is biodynamic simulation. METHODS A 2-degree-of-freedom discrete impact model was constructed through system identification and validated using experimental data in order to understand the dynamic interactions of various biomechanical parameters in bimanual forward fall arrests. FINDINGS The bimodal reaction force responses from the identified models had very small identification errors (<3.5%) and high coherence (R(2)=0.95) between the measured and identified model responses. Model validation with separate experimental data also demonstrated excellent validation accuracy and coherence, less than 7% errors and R(2)=0.87, respectively. The first force peak was usually greater than the second force peak and strongly correlated with the impact velocity of the upper extremity, while the second force peak was associated with the impact velocity of the body. The impact velocity of the upper extremity relative to the body could be a major risk factor to fall-related injuries as observed from model simulations that a 75% faster arm movement relative to the falling speed of the body alone could double the first force peak from that of a soft landing, thereby readily exceeding the fracture strength of the distal radius. INTERPRETATION Despite the time-critical nature of falling often calling for a rapid arm movements, the safe use of the upper extremity in forward fall arrests requires adequate reaction times and coordinated protective motions of the upper extremity.

Biomechanical efficiency of wrist guards as a shock isolator.

Despite the use of wrist guards during skate- and snowboard activities, fractures still occur at the wrist or at further proximal locations of the forearm. The main objectives of this study were to conduct a human subject testing under simulated falling conditions for measurement of the impact force on the hand, to model wrist guards as a shock isolator, to construct a linear mass-spring-damper model for quantification of the impact force attenuation (Q-ratio) and energy absorption (S-ratio), and to determine whether wrist guards play a role of an efficient shock isolator. While the falling direction (forward and backward) significantly influenced the impact responses, use of wrist guards provided minimal improvements in the Q- and S-ratios. It was suggested based on the results under the submaximal loading conditions that protective functions of the common wrist guard design could be enhanced with substantial increase in the damping ratio so as to maximize the energy absorption. This would bring forth minor deterioration in the impact force attenuation but significant increase in the energy absorption by 19%, which would help better protection against fall-related injuries of the upper extremity.

Mechanisms of female urinary continence under stress: frequency spectrum analysis

Intravesical and urethral pressure signals during cough and Valsalva maneuvers for 15 continent women were analyzed with frequency spectrum analysis. Clear modulation of the urethral pressure changes by the intravesical pressure rise during stress maneuvers was demonstrated in the frequency bands of 14 and 7 Hz for cough and Valsalva, respectively. The linearity between the urethral and intravesical pressure signals was strong for cough, but relatively weaker for Valsalva. The observed linearity lead to the formulation of a modified continence equation to mathematically quantify stress leak point pressure (sLPP): sLPP=MUCP/(1-alpha1)+RBP. This algebraic equation demonstrated that sLPP depends on pressure transmission, resting bladder pressure, and maximum urethral closure pressure. The equation was validated with excellent theoretical predictions for the 15 continent subjects (R(2)=0.98 and 0.97 for cough and Valsalva leak point pressure, respectively) and good but somewhat weaker predictions for 46 stress incontinent women (R(2)=0.79 and 0.48, respectively). It has been shown that pressure transmission plays the most important role in female continence function, while it may be attributable to passive structural origin as evidenced by the minimal time delay between the two pressure signals, in the order of a few milliseconds. It can be concluded that coughing seems to have a more mechanical, rather than neuromuscular basis for its signal dynamics. This study suggests that a complete assessment of female stress continence function requires comprehensive urodynamic information in terms of pressure transmission, maximum urethral closure pressure, and resting bladder pressure.

Development of a quantitative reflex hammer for measurement of tendon stretch reflex

Quantification of tendon stretch reflex requires precise measurement of the tapping force of a reflex hammer. A quantitative reflex (QR) hammer consisting of two cut rubber pieces from a generic rubber reflex hammer and a uniaxial force transducer was constructed. Finite element stress analyses were conducted to estimate the natural frequency characteristics of the hammer and to find the stress distributions during the impact. Pendulum impact testing was conducted at four different heights to assess the calibration linearity and repeatability of the measurement The QR hammer had a fundamental natural frequency of 515 Hz and showed minimal displacement and stress at the tip from the finite element simulation of the impact. The QR hammer also provided reliable and repeatable measurements as demonstrated with high coefficients of determination, exceeding 0.994 and small coefficients of variations, less than 4%. The calibration linearity was 0.64% compared with the reference force platform measurement. The QR hammer demonstrated sufficient accuracy and reliability for precise clinical assessment of tendon stretch reflexes.