Trunk Exertions Research Articles

Estimation of trunk muscle forces and spinal loads during fatiguing repetitive trunk exertions.

The effects of human trunk extensor muscle fatigue on the estimated trunk muscle forces and spinal loading were investigated during the performance of repetitive dynamic trunk extension. To evaluate if alterations in the trunk muscle recruitment patterns resulted in a greater estimated active loading of the spine and, in turn, an increased risk of injury. Epidemiologic studies highlight the increased risk of low back injury during repetitive lifting, implicating fatigue of muscles and/or passive tissues as causes of such injury. Increased trunk muscle activity or altered recruitment patterns resulting from fatigue in the primary trunk extensor muscles may indicate an increase in the active loading of the spine, which could contribute to an increased risk of injury. Sixteen healthy study participants performed repetitive isokinetic trunk extension endurance tests at two load levels and two repetition rates, while their net muscular torque output and trunk muscular activity were measured. During each exertion, trunk torque, position, and velocity were controlled, so that any change in muscle activity could be attributed to fatigue. An electromyography-assisted model, adapted to accommodate the decline in maximum muscular tension generation resulting from fatigue, was used to estimate the 10 trunk muscle forces and spinal loading. Linear regression was used to quantify the rate of change in muscle force and spinal loading resulting from fatigue, while analysis of variance was used to determine if the rate of change was dependent on the task conditions (load and repetition rate). Significant elevations were estimated for the latissimus dorsi and external oblique muscle forces in more than 70% of the endurance tests, whereas significant reductions in the erector spinae muscle force were predicted in 75% of the trials. The magnitude of the range of change of the erector spinae and latissimus dorsi muscle forces was dependent on the load level and repetition rate. The reduction in erector spinae forces offset the augmented force in the other muscles, because the net changes in compression and lateral shear forces on the spine were not significant, and the anteroposterior shear was reduced. The results of the study do not suggest that an increase in the muscular loading of the spine occurs as a result of changing trunk muscular recruitment patterns. Therefore, future studies should focus on injury mechanisms that may occur as a result of a change in the viscoelastic passive tissue responses, muscular insufficiency, or a decline in neuromuscular control and coordination.

Changes in trunk dynamics and spine loading during repeated trunk exertions.

Trunk and hip kinematic and spinal loading changes were documented as experienced subjects performed a set of standard free-dynamic lifting tasks at various intervals throughout a 5-hour repetitive manual materials handling session. To document how spine loading changes as a function of repetitive lifting during an extended period of lifting cycles. Many studies have evaluated spine loading as a function of a specific lifting task, but no studies could be found in the literature that evaluated how spine loading may change with repeated exposure to a lifting task. Ten experienced material handlers were recruited. Each was required to transfer 11 pallets of 23 kg boxes at a rate of 125 lifts per hour. Before and after unloading each pallet, subjects were asked to perform a set of five standard free-dynamic lifting tasks. Electromyographic activities of 10 trunk muscles were recorded along with kinematic and kinetic data. An electromyography-assisted model was used to evaluate spine loading during the standard lifting tasks. Subjects significantly changed their motion patterns throughout the lifting session. Trunk range of motion, velocity, and acceleration decreased in the sagittal plane, whereas these same measures increased for the hip. Trunk moment also decreased by 7% during the standard lifts. These changes were accompanied by a redistribution of muscle recruitment patterns, resulting in a decrease in spine compression and an increase in anterior/posterior shear. This study has shown that spine loading patterns do indeed change with repetition, suggesting that one needs to monitor these changes throughout repetitive lifting tasks if low back disorder risk is to be minimized.

Simulift: a simulation model of human trunk motion.

In this paper, the authors present a deterministic simulation model, which they call Simulift, of trunk-muscle activity and intra-abdominal pressure during a sagittally symmetric trunk exertion. Simulift is a descriptive model that quantifies the time-varying loading of the spine based on observed internal forces. Recent findings about the time sequence of events during trunk motion and established equilibrium formulas provide the theoretical bases for the simulation. A profile of electromyographic activity in ten trunk muscles and intra-abdominal pressure is updated as simulated time passes, or as the trunk motion is simulated. Input to the model includes a list of motion-event times and a set of profile-component-behavior data. Simulift is an "impulse" model that computes instantaneous and time-integrated statistics on individual muscle activity, intra-abdominal pressure, compression, lateral shear, and anterior shear as the profile components of the simulated subject change. Computer results for the simulation model are presented.

Trunk motion during lifting: The relative cost

The cost of lifting motion to back loading has been investigated traditionally by monitoring the electromyographic (EMG) recordings of trunk muscle during a controlled lift. When subjects are tested on an isokinetic dynamometer, the EMG activity decreases as the velocity increases and trunk torque production also decreases with added velocity. However, during an actual lift, the necessary torque needed to handle the load remains constant regardless of the lift speed. This research has investigated the muscle force per unit torque which is needed to support a load under various trunk velocities. Forty-five subjects were tested for maximum torque production under various velocity and angle conditions. The relative trunk loading cost of velocity was evaluated and described in an equation for slow velocity (0–30 deg/s) trunk exertions. These results were used to discuss how static lifting models might be adjusted to account for the added trunk load due to velocity.