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Biomedical subjects

Michael E Hahn

Publications and source records attributed to Michael E Hahn.

15 recordsLinked to original sources
PubMed2006

Feasibility of estimating isokinetic knee torque using a neural network model.

Many studies have investigated the relationships between electromyography (EMG) and torque production. A few investigators have used adjusted learning algorithms and feed-forward artificial neural networks (ANNs) to estimate joint torque in the elbow. This study sought to estimate net isokinetic knee torque using ANN models. Isokinetic knee extensor and flexor torque data were measured simultaneously with agonist and antagonist EMG during concentric and eccentric contractions at joint velocities of 30 degrees /s and 60 degrees /s. Age, gender, height, body mass, agonist EMG, antagonist EMG, joint position and joint velocity were entered as predictive variables of net torque. A three-layer ANN model was developed and trained using an adjusted back-propagation algorithm. Accuracy results were compared against those of forward stepwise regression models. Stepwise regression models included body mass, body height and joint position as the most influential predictors, followed by agonist EMG for concentric and eccentric contractions. Estimation of eccentric torque included antagonist EMG following the agonist activation. ANN models resulted in more accurate torque estimation (R=0.96), compared to the stepwise regression models (R=0.71). ANN model accuracy increased greatly when the number of hidden units increased from 5 to 10, continuing to increase gradually with additional hidden units. The average number of training epochs necessary for solution convergence and the relative accuracy of the model indicate a strong ability for the ANN model to generalize these estimations to a broader sample. The ANN model appears to be a feasible technique for estimating joint torque in the knee.

Adult↗
PubMed2005

Increased muscular challenge in older adults during obstructed gait.

Skeletal muscle strength is known to decline with age. Although lower extremity (LE) muscle strength is critical to maintaining dynamic stability, few studies have investigated lower extremity muscle challenge during activities of daily living. The purpose of this study was to investigate the effects of age and obstructed gait on relative lower extremity muscular challenge, with respect to available joint strength. Fifteen healthy young and fifteen healthy older adults were asked to walk over level ground and step over obstacles. Pre-amplified surface electrodes were used to measure bilateral muscular activation of the gluteus medius (GM), vastus lateralis (VL), and gastrocnemius (GA). Muscle activation signals were normalized to peak magnitudes collected during maximal manual muscle testing (MMT). Normalized magnitudes were analyzed during the double-support phase for gluteus medius and vastus lateralis and during the single-support phase for gastrocnemius. A two-factor ANOVA was used to test for age group effect, with repeated measure of obstacle height. In general, older adults demonstrated greater relative activation levels compared to young adults. Gluteus medius activity was significantly greater in the elderly as compared to young during periods of double-support (weight transfer). Increased obstacle height resulted in greater relative activation in all muscles, confirming the increased challenge to the musculo-skeletal system. While healthy elderly adults were able to successfully negotiate obstacles of different heights during walking, their muscular strength capacity was significantly lower than young adults, resulting in relatively higher muscular demands. The resulting potential for muscular fatigue during locomotion may place individuals at higher risk for trips and/or falls.

Activities of Daily Living↗
PubMed2005

A model for detecting balance impairment and estimating falls risk in the elderly.

Traumatic falls are a prevalent and costly threat to elderly adults. Accurate risk assessment is necessary for reducing incidence of falls. The objective of this study was to test the feasibility of a balance impairment detection model using tasks of sample categorization and falls risk estimation. Model design included an artificial neural network and a statistical discrimination method. The first system produced an individual categorization value, which was then assessed in the second system for relative risk of falls, compared to a normative distribution of healthy elderly peers. Input data included leg muscle electromyographic amplitudes, temporal-distance measures of gait, and medio-lateral measures of whole body center of mass motion. These input data were compiled from a sample of healthy elderly adults (n = 19) and a sample with impaired balance (n = 10) to develop and test the model. Accuracy of sample categorization was assessed using a relative operating characteristic (ROC) value. For relative risk estimation, categorical delineation of risk level was adopted. Sample categorization results reached ROC values of 0.890. Relative risk was frequently assessed at high or very high risk for experiencing falls. Temporal-distance measures were most influential in categorization accuracy, producing the most consistent risk estimates. Combined inputs further improved model performance. This model shows potential for detecting balance impairment and estimating falls risk; thereby indicating need for referral for falls prevention intervention.

Accidental Falls↗
PubMed2005

Neural network estimation of balance control during locomotion.

Gait patterns of the elderly are often adjusted to accommodate for reduced function in the balance control system and a general reduction in skeletal muscle strength. Recent studies have demonstrated that measures related to motion of whole body center of mass (COM) can distinguish elderly individuals with balance impairment from healthy peers. Accurate COM estimation requires a multiple-segment anthropometric model, which may restrict its broad application in assessment of dynamic instability. Although temporal-distance measures and electromyography have been used in evaluation of overall gait function and determination of gait dysfunction, no studies have examined the use of gait measurements in predicting COM motion during gait. The purpose of this study was to demonstrate the effectiveness of an artificial neural network (ANN) model in mapping gait measurements onto COM motion in the frontal plane. Data from 40 subjects of varied age and balance impairment were entered into a 3-layer feed-forward model with back-propagated error correction. Bootstrap re-sampling was used to enhance the generalization accuracy of the model, using 20 re-sampling trials. The ANN model required minimal processing time (5 epochs, with 20 hidden units) and accurately mapped COM motion (R-values up to 0.89). As training proportion and number of hidden units increased, so did model accuracy. Overall, this model appears to be effective as a mapping tool for estimating balance control during locomotion. With easily obtained gait measures as input and a simple, computationally efficient architecture, the model may prove useful in clinical scenarios where electromyography equipment exists.

Age Factors↗
PubMed2005

Manipulating proteins with chemistry: a cross-section of chemical biology.

Chemistry-driven strategies for modifying, controlling and monitoring protein function in vitro and in vivo have attracted widespread interest among chemists in recent years. Several strategies have now emerged that complement standard genetics-based approaches, and they are being increasingly adopted by biologists to address issues in relevant contexts from cells to animals. With the development of these chemical biology tools, we might be approaching a time when detailed quantitative analysis of protein function, to a degree previously available only in reconstituted systems, is attainable in an in vivo setting.

Animals↗
PubMed2004

Simultaneous triggering of protein activity and fluorescence.

Many areas of biology can benefit greatly from methods to spatially and temporally control protein activity. Here, we describe an approach that allows the simultaneous photo-triggering of the activity and the fluorescence of a protein. Smad2, a protein central to the transforming growth factor-beta (TGF-beta) signal transduction pathway, was modified with a fluorophore and a photocleavable moiety that acted as both a caging and a fluorescence quenching group. In its caged state, the protein formed a non-fluorescent heterodimer with the protein SARA. Irradiation with UV light and photocleavage of the caging group produced a fluorescent homotrimer. These in vitro experiments demonstrated that a photochemical trigger mimicking the critical biochemical event of serine phosphorylation involved in the TGF-beta signaling pathway could be obtained and that fluorescence could be used as a read-out of protein activity. This approach should prove particularly useful for the monitoring of a protein's activity and location inside of living cells.

Chromatography, Gel↗
PubMed2004

Age-related reduction in sagittal plane center of mass motion during obstacle crossing.

Accidental falls are a leading cause of injury and death in the growing elderly population. Traumatic falls are frequent, costly, and debilitating. Control of balance during locomotion is critical for safe ambulation, but relatively little is known about the natural effect of aging on dynamic balance control. Samples of healthy young (n = 13) and elderly (n = 13) subjects were compared in the interactive measures of center of mass (COM) and center of pressure (COP) during level walking and obstacle crossing conditions. Obstacle heights were normalized to individual body height (2.5%, 5%, 10%, and 15%). Temporal-distance (T-D) variables of gait were also compared. Statistical analyses were conducted using a two-way ANOVA for subject group and obstacle height. T-D parameters were not significantly different between groups; nor were frontal plane COM and COP parameters. Significant age differences did exist for antero-posterior (A/P) motion of the COM (decreased motion in the elderly), and its relationship with the COP (reduced separation between the two variables in the elderly). Anterior COM velocities were also significantly lower in the elderly group. The results confirm the ability of healthy elderly adults to maintain dynamic balance control in the frontal plane during locomotion. Reduced A/P distances between the COM and COP indicate a conservative reduction of the mechanical load on joints of the supporting limb. This conservative strategy may be related to a reduction in muscle strength as it occurs in the natural aging process.

Accidental Falls↗
PubMed2003

Can motion of individual body segments identify dynamic instability in the elderly?

OBJECTIVE: To determine if medio-lateral motion of the head, trunk, or pelvis demonstrates dynamic stability as well as whole-body center of mass during obstructed walking. DESIGN: Group comparison of two elderly populations using whole-body motion analysis. BACKGROUND: Detection of imbalance through analysis of center of mass motion is commonly adopted, requiring three-dimensional reconstruction of a multi-link biomechanical model. It would be advantageous clinically if similar detection could be made by analyzing segmental displacements of the pelvis, trunk, or head. METHODS: Healthy elderly adults and elderly patients with balance disorders walked over level ground and crossed obstacles of height ranging from 2.5% to 15% of body height. Whole-body center of mass was calculated as the weighted sum of segmental centers of mass. Group differences in medio-lateral displacements and peak velocities of head, trunk, pelvis, and the center of mass were analyzed using a two-way ANOVA with repeated measures for obstacle height. RESULTS: Elderly patients with balance disorders exhibited greater medio-lateral displacement and peak velocities of all segments. However, significant group differences were only detected in the center of mass displacement and peak velocity. CONCLUSION: Whole-body center of mass motion distinguishes elderly patients with balance disorders from healthy peers more consistently than markers representing the head, trunk, or pelvis. Large variation of individual segment motion makes dynamic stability difficult to assess. This study demonstrates that center of mass motion allows more sensitive detection of dynamic instability. RELEVANCE: Detection of dynamic instability in at-risk individuals before falls occur will allow preventative interventions, preserving quality of life in the elderly population.

Aged↗
PubMed2003

Medio-lateral motion of the center of mass during obstacle crossing distinguishes elderly individuals with imbalance.

This study was performed to investigate whether elderly patients with imbalance can be distinguished from healthy elderly subjects by comparing their whole body center of mass (COM) motion in the medio-lateral (M-L) direction during obstacle crossing. Nine healthy elderly adults and six elderly patients having complaints of 'dizziness' or 'unsteadiness' during walking (three with bilateral/unilateral vestibular weakness and three with unclear diagnosis) were recruited to perform unobstructed level walking and crossing of obstacles set to 2.5, 5, 10 and 15% of each subject's height. Kinematics of the COM was calculated using a weighted sum average of a 13-segment biomechanical model. There were no significant group differences for the temporal-distance gait parameters during all testing conditions. However, elderly patients with balance disorders demonstrated significantly greater and faster lateral motion of the COM when crossing over obstacles. These measurements distinguish elderly patients with imbalance from healthy elderly subjects. Furthermore, the increased M-L motion of the COM during obstacle crossing showed a positive correlation with an increased M-L range of motion of the swing foot trajectory. This increase in M-L motion indicates a compensatory adjustment in the swing foot trajectory to land the swing foot at an appropriate location that would establish a new base of support to counter the balance disturbance in the frontal plane.

Aged↗
PubMed2003

Radioulnar convergence after distal ulnar resection: mechanical performance of two commonly used soft tissue stabilizing procedures.

Resection of the distal ulna (Darrach operation) is a common method for salvaging the arthrotic distal radioulnar joint (DRUJ). However, problems have been reported with this procedure due to residual instability and radioulnar convergence. As a result, several methods of soft tissue stabilization for the unstable distal ulna have been developed. Although their clinical efficacy has been reported, biomechanical investigations of these procedures have not been reported. The purpose of our study was to evaluate the dynamic effects on radioulnar convergence and dorsal-palmar displacement of three procedures: the Darrach procedure, a pronator quadratus interposition flap and an extensor and flexor carpi ulnaris tenodesis. We tested 7 fresh-frozen cadaver upper extremities using a dynamic computer-controlled device that generated forearm rotation with physiologic loading of relevant muscles. Displacement data concerning the ulna relative to the radius through the range of forearm rotation was collected for 4 experimental conditions: intact, distal ulna resection alone, distal ulna resection with pronator quadratus interposition and distal ulna resection with extensor and flexor carpi ulnaris tenodesis. Distal ulna resection altered the kinematics, most predictably creating a convergence of the radius towards the ulna. Anteroposterior translations in each loading condition could be detected as well. The interposition of the pronator quadratus muscle or tenodesis with the extensor and flexor carpi ulnaris tendons did not reduce the radioulnar convergence created by resection of the distal ulna.

Aged↗
PubMed2002

Analysis of dynamic distal radioulnar convergence after ulnar head resection and endoprosthesis implantation.

The most common method to treat the arthralgic distal radioulnar joint is resection of the entire ulnar head (Darrach procedure). Pain and weak grip strength usually manifest complications related to instability of the distal forearm. In an attempt to mechanically stabilize the distal forearm after ulnar head resection, an endoprosthesis was developed to replace the ulnar head after Darrach resection. The goals of this study were to evaluate the dynamic effects of the Darrach procedure on radioulnar convergence and the mechanical efficacy of implantation of an ulnar head endoprosthesis after Darrach resection. Cadaver forearms were rotated actively and passively and relevant muscles were loaded simultaneously with a dynamic PC-controlled forearm simulator. Resultant total forearm torque and 3-dimensional kinematics of the ulna, radius, and third metacarpal were recorded simultaneously in 7 fresh-frozen cadaver upper extremities. Comparisons were made between the intact state, after Darrach resection, and after implantation of ulnar head endoprosthesis. The Darrach resection created substantial forearm instability with movement of the radius ulnarly (0.92-0.38 cm vs intact state) and anteroposterior translation in each loading condition. Implantation of the ulnar head endoprosthesis effectively restored distal radioulnar joint stability by simulating the geometry of the ulnar head, further stabilized by attaching the triangular fibrocartilage complex. These laboratory data provide validity to implanting an ulnar head endoprosthesis to stabilize the distal forearm after Darrach resection.

Adult↗
PubMed2002

The effects of dorsally angulated distal radius fractures on carpal kinematics.

A cadaver model was used in a biomechanical study of dorsally angulated distal radius fractures to evaluate alterations in carpal kinematics. Distal radius fractures were simulated by dorsal closing-wedge osteotomy and fixed with a custom-designed external fixator. A magnetic tracking device measured the carpal bone motions in several positions of dorsal angulation from neutral tilt to 30 degrees dorsal tilt. Changes in carpal alignment showed different patterns between each specimen consisting of a spectrum from dorsal subluxation of the entire carpus to adaptive dorsal carpal instability (DISI deformity). Components of carpal bone motion were altered markedly at all positions of dorsal angulation of the distal radius. The severity of the DISI deformity and related carpal instability correlated well with the alterations of carpal kinematics during wrist flexion and extension, whereas dorsal subluxation alone had a poor relationship with changes in carpal kinematics. The amount of DISI deformity and the degree of dorsal angulation of the radius may be prognostic factors when considering whether to perform a corrective osteotomy of the distal radius.

Aged↗