Biomedical subjects

G A Mirka

Publications and source records attributed to G A Mirka.

6 recordsLinked to original sources

PubMed1992

A comprehensive evaluation of trunk response to asymmetric trunk motion.

An experiment was performed to determine the reaction of the trunk muscles, using electromyography, and intra-abdominal pressure to components of trunk loading commonly seen in the workplace during manual materials handling. These components included angular trunk velocity, trunk position in three-dimensional space and trunk torque exertion level. The experiment was performed using 44 subjects. Subjects produced constant trunk extension torque about the lumbosacral junction while moving the trunk under constant angular velocity (isokinetic) conditions. Significant reactions to trunk angular velocity, trunk torque level, and unique combinations of trunk position and velocity were seen in all muscles of the trunk. The other components affected the muscles selectively according to function. Intra-abdominal pressure only reacted significantly to trunk angle and some unique trunk angle-asymmetry positions. The biomechanical implications of these findings are discussed. The reactions of the muscles to the various workplace components also were described quantitatively through equations that predict muscle activity levels.

Abdomen↗

PubMed1991

The quantification of EMG normalization error.

Electromyography (EMG) and normalized EMG have been accepted as methods of quantifying the activity level of a muscle. Normalized EMG, in conjunction with the EMG/force relationship and muscle cross-sectional area data, allows researchers to estimate the amount of muscle force exerted across a joint. An accurate description of this muscle force is a critical input to models designed to describe the risk of injury of a task. In order to be able to make statements about the relative intensity of an EMG signal, researchers who use normalization procedures take a given EMG activity level, at a known joint angle, and relate it to some reference activity level obtained at that particular joint angle. However, there have been studies where the EMG activity of an unrestricted dynamic task, such as walking, cycling, performing an occupational task, etc., has been normalized with respect to an EMG value taken during a single maximum voluntary contraction performed at one reference joint angle. This practice will render inaccurate results because at different joint angles there are changes in the portion of the muscle within the viewing area of the electrode, as well as changes in the length/tension relationship of the muscle which would cause changes in the maximum EMG value. The present study was an attempt to quantify the errors associated with normalization relative to a reference EMG value collected at an arbitrary joint position. Four subjects performed a series of controlled trunk extension exertions. As they performed these exertions the EMG activities were collected for eight trunk muscles. The task EMG values that resulted were then: (1) all normalized with respect to the maximum EMG at a single arbitrary trunk angle and (2) each normalized with respect to that specific trunk angle's maximum EMG. The results show that for the primary trunk extensors (erector spinae) large errors (greater than 75%) resulted from normalization using a single reference point and the magnitude of these errors followed consistent patterns within subjects as a function of trunk angle.

Back Pain↗

PubMed1990

Muscle activities during asymmetric trunk angular accelerations.

The objective of this study was to characterize trunk muscle and intra-abdominal pressure behavior during extensions of the trunk when angular trunk acceleration levels and trunk twist were varied during lifting exertions. Since force is related to acceleration, it was believed that changes in trunk acceleration would cause activity changes in the muscles and abdominal cavity pressurization mechanics that load the spine during manual materials handling tasks. The electromyographic activity of 10 trunk muscles and intra-abdominal pressure were studied in 39 subjects as they moved their trunks under high, medium, and low constant angular acceleration conditions. The results indicated that almost all the muscles were affected by acceleration and asymmetry. Muscle activities of up to 50% of maximum were observed even though a minimal amount of torque was being produced by the back. Coactivation of muscles was also apparent. Muscles located at the greatest distances from the spine, such as the latissimus dorsi and oblique groups, increased their activities the most as trunk acceleration increased. Muscles located farthest from the spine also played an important role as the trunk became more asymmetric. Intra-abdominal pressure changed minimally over the test conditions. The nature of these responses and their impact on spine loading are discussed.

Acceleration↗

PubMed1990

Lumbar motion response to a constant load velocity lift.

An experiment was performed to evaluate the motions of the lumbar spine during a constant load velocity lift. For the purposes of this study, a constant load velocity refers to the linear vertical velocity of the load. This vertical load velocity was controlled using a modified angular isokinetic dynamometer, which produced linear isokinetic motion during a lift. A lumbar monitor was used to observe the position, velocity, and acceleration changes that occurred in the lumbar spine during the lifting task. The results indicate that under constant load velocity conditions, significant angular accelerations occur at the lumbar level. The nature of these accelerations was found to depend on several variables associated with a lifting task, such as the load velocity and the asymmetry of the lift. The physical significance of these results would be increased spinal loading above that which would be predicted using a static model.

Biomechanical Phenomena↗

PubMed1989

The effects of preview and task symmetry on trunk muscle response to sudden loading.

The effect of warning time (preview) and task symmetry on the trunk muscular response to sudden loading conditions was investigated. Eleven subjects were asked to catch falling weights with four levels of preview (0, 100, 200, and 400 ms) in saggitally symmetric posture and asymmetric posture. For each of the eight muscles sampled with surface electrodes, the integrated electromyographic (EMG) signal was interpreted in terms of its peak value, mean value, onset rate, and lead/lag time with reference to the weight drop. Results show linear relationships between preview times and peak EMG, preview times and mean EMG, and preview times and lead times. The results show significant change when going from symmetric to asymmetric conditions across most dependent measures. Analysis of peak changes in compression were performed across all conditions but yielded unexpected results.

Adult↗

PubMed1989

Trunk strength during asymmetric trunk motion.

It is important to understand how trunk strength varies as a function of workplace factors so that the work environment can be designed to minimize the risk of low back injury. In this study maximal trunk torque production around the lumbosacral junction was measured in 44 subjects as trunk concentric and eccentric isokinetic velocity and trunk asymmetric line of action were varied. Trunk torque decreased by approximately 8.5% of maximum for every 15 deg of asymmetric trunk angle. Increases in concentric velocity decreased trunk strength, whereas increases in eccentric trunk velocity increased strength. Significant interactions were also found, and it was determined that the common finding that eccentric strength exceeds concentric strength is true only for forward trunk angles at all asymmetric angles. These results should have significant implications for the design of manual materials handling tasks.

Adolescent↗