Sangeun Jin

Describing the active region boundary of EMG-assisted biomechanical models of the low back.

BACKGROUND Electromyography-assisted (EMG-assisted) biomechanical models are used to characterize the muscle and joint reaction forces in the lumbar region. However, during a full-range trunk flexion, there is a transition of extension moment from the trunk extensor muscles to the passive tissues of the low back, indicating that the empirical EMG data used to drive these EMG-assisted models becomes less correlated with the extensor moment. The objectives of this study were to establish the trunk flexion angles at which the passive tissues generate substantial trunk extension moment and to document how these angles change with asymmetry. METHODS Participants performed controlled trunk flexion-extension motions in three asymmetric postures. The trunk kinematics data and the electromyographic activity from L3- and L4-level paraspinals and rectus abdominis were captured. The time-dependent net internal active moment (from an EMG-assisted model) and the net external moment were calculated. The trunk and lumbar angles at which the net internal active moment was less than 70% of the external moment were found. FINDINGS The trunk flexion angle at which the net internal moment reaches the stated criteria varied as a function of asymmetry of trunk flexion motion with the sagittally symmetric case providing the deepest flexion angle of 38° (asymmetry 15°: 33°; asymmetry 30°: 26°). INTERPRETATION These results indicate that EMG-assisted biomechanical models need to consider the role of passive tissues at trunk flexion angles significantly less than previously thought and these flexion angles vary as a function of the asymmetry and direction of motion.

An evaluation of arborist handsaws

A review of the scientific literature reveals little research on the ergonomics of handsaws and no literature on the specific challenges of arborist saws (saws for cutting and pruning living trees). This study was designed to provide some insight into the effects of saw design and height of sawing activity on the biomechanical response of the upper extremity. Eighteen participants performed a simple sawing task at three different heights using six different arborist handsaws. As they performed this task, the electromyographic activity of several muscle groups of the forearm (flexor and extensor digitorum), arm (biceps brachii long and short heads) and shoulder girdle (posterior deltoid, infraspinatus and latissimus dorsi) were sampled. Also gathered were the wrist postures in the radial/ulnar plane at the beginning and ending of the sawing stroke, the time to complete the sawing task and a subjective ranking of the six different saws. The results show an interesting mix of biomechanical and subjective responses that provide insight into handsaw design. First, there were tradeoffs among muscle groups as a function of work height. As work height increased the biceps muscles increased their activation levels (approximately 19%) while the posterior deltoid activity decreased (approximately 17%) with the higher location. The results also showed the benefits of a bent handle design (average 21% reduction in ulnar deviation). The subjective responses of the participants generally supported the productivity data, with the saws demonstrating the shortest task completion time also being the ones most highly ranked. RELEVANCE TO INDUSTRY: Understanding the stresses placed on the upper extremity during sawing activities, and design features that can reduce these stresses, may help saw designers to create products that reduce the risk of injury in workers who use handsaws.

The effects of horizontal load speed and lifting frequency on lifting technique and biomechanics

Lifting loads that have a horizontal velocity (e.g. lifting from a conveyor) is often seen in industry and it was hypothesised that the inertial characteristics of these loads may influence lifting technique and low back stress. Seventeen male participants were asked to perform lifting tasks under conditions of four horizontal load speeds (0 m/s, 0.7 m/s, 1.3 m/s and 2.4 m/s) and two lifting frequencies (10 and 20 lifts/min) while trunk motions and trunk muscle activation levels were monitored. Results revealed that increasing horizontal load speed from 0 m/s to 2.4 m/s resulted in an increase in peak sagittal angle (73° vs. 81°) but lower levels of peak sagittal plane angular acceleration (480°/s2 vs. 4°/s2) and peak transverse plane angular acceleration (200°/s per s vs. 140°/s per s) and a consistent increase in trunk muscle co-activation. Participants used the inertia of the load to reduce the peak dynamics of the lifting motion at a cost of increased trunk flexion and higher muscle activity. Statement of Relevance: Conveyors are ubiquitous in industry and understanding the effects of horizontal load speed on the lifting motions performed by workers lifting items from these conveyors may provide some insight into low back injury risk posed by these tasks.

The effect of a lower extremity kinematic constraint on lifting biomechanics

Leaning against a stationary barrier during manual materials handling tasks is observed in many industrial environments, but the effects of this kinematic constraint on low back mechanics are unknown. Thirteen participants performed two-handed lifting tasks using both a leaning posture and no leaning posture while trunk kinematics, muscle activity and ground reaction force were monitored. Results revealed that lifting with the leaning posture required significantly less activity in erector spinae (26% vs. 36% MVC) and latissimus dorsi (8% vs. 14% MVC), and less passive tissue moment compared with the no leaning posture. Peak sagittal accelerations were lower when leaning, but the leaning posture also had significantly higher slip potential as measured by required coefficient of friction (0.05 vs. 0.36). The results suggested that the leaning lifting strategy provides reduced low back stress, but does so at the cost of increased slip potential.

The Effect of the Lower Extremity Posture on Trunk While Sitting

The goal of this study was to investigate the interactions between upper extremity and lower extremity in sitting postures. Ten healthy participants were recruited from the university population, and were asked to sit on a chair with six different lower extremity postures (2 trunk-thigh angles, 3 knee angles). The head, trunk and lower extremity postures were captured by using fourteen motion trackers, and used to calculate the head flexion angle, thoracic flexion angle, lumbar flexion angle, pelvic flexion angle, and shoulder angle. Results showed the effects of changes in the trunk-thigh angle and the knee angle on all dependent measures. First, the bigger trunk-thigh angle (135°) showed better lumbar lordosis, smaller head flexion angle, and less rounded shoulder as compared to the 90° trunk-thigh angle. In addition, the current study revealed that the bigger the trunk-thigh angle, the better the whole trunk postures including the better lumbar and thoracic lordosis, less flexed head, and less rounded shoulder. Second, the bigger knee flexion angle negatively influenced on all trunk posture measures, suggesting that the extended knee joint tightens hamstring muscles and pulls the pelvis backward, and finally results in the bad trunk posture such as less lordotic lumbar and thoracic postures, more head flexion angle and more rounded shoulder angle. On this basis, the office chair should be designed under the consideration of the lower extremity postures such as the position of foots, the knee supporter, and the seat inclination angle.

Combined effect of low back muscle fatigue and passive tissue elongation on the flexion-relaxation response

Previous literature has documented the alterations in the flexion-relaxation response of the lumbar extensor musculature to passive tissue elongation (PTE) and muscle fatigue (MF). There is no study, however, that has explored this response as a function of the combined effect of both PTE and MF, which is often seen in occupational settings. Twelve participants performed three experimental protocols on three different days to achieve (1) PTE, (2) MF and (3) PTE&MF (combined). Trunk kinematics and muscle activities were monitored to assess the effects of these protocols on the peak lumbar flexion angle and the lumbar angle of the flexion-relaxation of the trunk extensor muscles. Results showed responses to the uni-dimensional stresses (PTE and MF) consistent with those seen in the previous literature, while the combined protocol elicited responses that more closely matched the PTE protocol.

Lifting from a Conveyor

This study aimed to achieve a better understanding of the effect of the load speed on lifting kinematics and kinetics while lifting from a conveyor. The inertial characteristics of a load moving on a conveyor have the potential to influence lifting technique and low back stress. Participants in this study lifted loads from a conveyor with four load speeds (0 m/s, 0.7 m/s, 1.3 m/s, and 2.4 m/s) while trunk motions and ground reaction forces were monitored. Results revealed that increasing horizontal load speed generated greater sagittal angle and greater lateral ground reaction forces but significantly lower trunk accelerations. The results suggested that the participants maintained a more flexed trunk posture under the higher load speed conditions and utilized the inertia of the load to reduce the peak accelerations of the torso.