Biomedical subjects

Yasin Y Dhaher

Publications and source records attributed to Yasin Y Dhaher.

6 recordsLinked to original sources

PubMed2006

Determination of the abduction-adduction axis of rotation at the human knee: helical axis representation.

This study used a finite helical axes representation to derive the axis of rotation of the human knee in the frontal plane for the neutral flexion/extension posture during resting and no load-bearing conditions. The three-dimensional finite helical axis pathway of the tibia relative to the femur was computed by passively adducting/abducting the lower limb via a servomotor system. Knee joint movements as a result of the positional perturbations were captured with an active marker kinematic tracking system. Contrary to traditional assumptions used in studies conducted under a similar experimental paradigm, our results indicated that the knee joint center, defined as the intercept point between the finite helical axis and the mid-coronal plane of the distal femur, was located within the femoral notch for a wide range of abduction and adduction angles (6 degrees abduction to 6 degrees adduction angles). Our data also indicated that at the neutral posture of the knee, the helical axes directions change as a function of the abduction/adduction perturbation angle. These findings are not only essential to error minimization during joint moment calculations, but can also facilitate new biomechanical interpretations of, for example, the functional role of the quadriceps and patellofemoral joint mechanics to overall knee stability in the medial-lateral direction.

Adult↗

PubMed2006

Hip joint position modulates volitional knee extensor muscle activity after stroke.

Evidence from animal and human models has demonstrated the importance of hip proprioceptors and vestibular inputs in modulating lower-extremity muscle activity through reflex pathways. Comprehension of the role of these sensory inputs following stroke may be important in understanding pathological muscle activity during functional activities. We therefore examined the influence of both hip and head/trunk position on volitional quadriceps activity in chronic stroke and control subjects. With the knee held at 60 degrees, maximal voluntary isometric quadriceps contractions were elicited with trunk orientation (head position) and hip angle systematically positioned at 0 degrees, 45 degrees, and 90 degrees. Integrated electromyographic activity from the quadriceps was compared between groups and conditions. Vasti activity in the stroke group was greater in a seated upright posture (hip flexed) than supine (hip neutral). Controlling for vestibular input, the stroke group demonstrated greater quadriceps activity (VL and RF) with a neutral hip compared to flexion. Such findings may have implications for understanding inappropriate muscle activity during walking after stroke, as hip extension occurs immediately prior to toe off, when inappropriate quadriceps activity is commonly observed.

Adult↗

PubMed2004

Joint-afferent-mediated muscle activations yield a near-maximum torque response of the quadriceps.

Previous work from our laboratory has shown that reflex activity is systematically evoked in a number of major knee muscles by large (>5 degrees ) (non-physiological) abduction angular perturbations of the human knee. This reflex action was shown to originate from periarticular tissue afferents. Furthermore, it was demonstrated that specific muscle activation patterns exist in knee muscles with preferential activation in medial muscles in response to the lateral perturbations. This study examines the hypothesis that in response to the mechanical stimulus, the sensory information mediated by these afferents results in activation patterns that provide the largest resisting moment by the knee muscles. It is further hypothesized that this near maximum resistance cannot be achieved by the selective activation of medial muscles alone. To examine this, the previously reported mechanically induced reflex EMG activation patterns, a stochastic 3D musculoskeletal patello-femoral joint model, and new data from selective electrical stimulation experiments were used. Using the model, the knee adduction-abduction moment in response to an applied abduction load at the knee joint for every possible random set of quadriceps activity was computed. These adduction moments were then compared to the adduction moment computed by the model when the mechanically induced muscle activation patterns were used. The data presented here illustrated that selective activation of a medial muscle alone would result in an abduction moment, regardless of the knee flexion angle. Furthermore, the findings of this study revealed that the recorded combinations of muscle activity provide a near maximum capability of the quadriceps muscles to resist externally applied abducting stimuli. It was concluded that stabilization in the abduction direction could only be achieved by a control strategy that involves activation of both medial and lateral muscles at the knee. It was also concluded that this control strategy was near optimal when mediated by joint afferents.

Adult↗

PubMed2002

The effect of vastus medialis forces on patello-femoral contact: a model-based study.

A mathematical model of the patello-femoral joint was introduced to investigate the impact of the vastus medialis (longus, obliquus) forces on the lateral contact force levels. In the model, the quadriceps were represented as five separate forces: vastus lateralis, vastus intermedius, rectus femoris, vastus medialis longus (VML), and obliquus (VMO). By varying the relative force generation ratios of the quadriceps heads, the patello-femoral contact forces were estimated. We sought to analytically determine the range of forces in the VMO and VML that cause a reduction or an increase of lateral contact forces, often the cause of patello-femoral pain. Our results indicated that increased contact forces are more dependent on combinations of muscle forces than solely VMO weakness. Moreover, our simulation data showed that the contact force levels are also highly dependent on the knee flexion angle. These findings suggest that training targeted to reduce contact forces through certain joint angles could actually result in a significant increase of the contact forces through other joint angles.

Computer Simulation↗

PubMedDate not supplied

Estimation of active cortical current source regions using a vector representation scanning approach.

The objective of this article is to present a framework for cortical current source reconstruction that extracts a center and magnitude of electrical brain activity from EEG signals. High-resolution EEG recordings, a subject-specific MRI-based electromagnetic boundary element method (BEM) model, and a channel reduction technique are used. This new geometric measure combines the magnitude and spatial location of electrical brain activity of each of the identified subsets of channels into a three-dimensional resultant vector. The combination of the two approaches constitutes a source reconstruction scanning technique that provides a real-time estimation of cortical centers that can be tracked over time. Simulations demonstrate that the ability of this method to find the best-fit cortical location is more robust both in terms of accuracy and precision than traditional approaches for single-source conditions. Experimental validation demonstrates its ability to localize and separate cortical activity in plausible sites for two different motor tasks. Finally, this method provides a statistical measure to compare electrical brain activity associated with different motor tasks.

Adult↗

PubMedDate not supplied

Electrical cortical activity associated with joint torque direction in the human arm.

The objective of this study is to determine whether electrical brain activity differs for static joint torques generated in the elbow flexion/extension and shoulder abduction/adduction directions in humans. Electrical brain activity was quantified using a technique that incorporates a realistic, subject-specific electromagnetic head model to create a three-dimensional spatial resultant vector representation of the cortical region of activation. The findings demonstrate that generation of torque in each of the four directions produced significantly different locations of centers of cortical activation. These differences in location were maintained from preparatory to the early execution phases of the task. The organization of the centers of cortical activity during the generation of elbow/shoulder torques resulted in centers associated with the generation of elbow torques that were more lateral than centers accompanying shoulder torques in all five subjects tested. The authors conclude that electrical brain activity is spatially organized during the generation of joint torques in opposing directions at the elbow and shoulder joints. In addition, the results indicate that the center of the electrical brain activity associated with these static tasks is localized over the primary motor cortex as opposed to secondary sensorimotor cortices.

Adult↗