RESUMEN
The main purpose of this study was to determine the height, angle and velocity of conveyors for baggage handling at Schiphol Airport. These variables have a great influence on the postures and workload of baggage handlers. One hundred and seven healthy male baggage handlers participated in a test, during which a regular baggage handling task was simulated with a mock-up. Eighteen combinations of heights, angles and velocities were tested. Data on postures were collected with the aid of task recording and analysis on computer (TRAC). The minimal volume of open space under the conveyor was determined with light cells that scored the positions of legs and feet of the subjects. In addition, opinions from the baggage handlers were gathered with a questionnaire, immediately after the test. Finally the number of selection errors made and the number of bags that fell off the conveyor were noted, to make sure that ergonomics improvements did not reduce the quality of baggage handling. On the basis of these three information sources, alternative conveyor dimensions were recommended, which are expected to improve the working conditions of the baggage handlers, without any reduction in the quality of work.
RESUMEN
Net joint moments are often used to quantify the loading of structures (e.g. the intervertebral disc at L5S1) during lifting. This quantification method is also used to evaluate the loading of the knee, for instance, to determine the effect of backlifting as opposed to leglifting. However, the true loading of the joint as derived from net joint moments can be obscured by a possible co-contraction of antagonists. To unravel the mechanisms that determine the net joint moments in the knee, the leglift was compared to the backlift. Although a completely different net knee moment curve was found when comparing the two lifting techniques, it appeared to be closely related to the ground reaction force vector and its orientation with respect to the joint centre of rotation (R > 0.995). This close relation was established by co-contraction of both flexors and extensors of the knee. Furthermore, a close relation appeared to exist between the joint moment difference between hip and knee and the activity difference between rectus femoris muscle and hamstring (R = 0.72 and 0.83 in leglift and backlift, respectively). The knee-ankle joint moment difference and the activity of the gastrocnemius showed a close relation as well (R = -0.89 and 0.96 in leglift and backlift, respectively). These relations can be interpreted as a mechanism to distribute net moments across joints. It is concluded that during lifting tasks the intermuscular coordination is aimed at coupling of joint moments, such that the ground reaction force points in a direction that provides balance during the movement. The use of net joint moments as direct indicators for joint loading (e.g. knee) seems, therefore, questionable.