ELECTROMYOGRAPHIC AND SUBJECTIVE EVALUATION OF HAND TOOLS - THE EXAMPLE OF MASONS TROWELS

The objective of this study was to evaluate an ergonomically designed handle of a mason's trowel in comparison with two standard types, both with a round cross-section of the handle and either a straight neck o r a swan's neck. All the tools were equipped with the same blade. Job- specific dynamic and static working elements were performed by 10 subj ects in a laboratory. Under well-controlled conditions, physiological cost associated with mixing and throwing of mortar onto a vertical wal l, translatory carrying and depositing of sand on a horizontal wall, r otatory scooping movements (supination and pronation of the forearm) w ith and without an external load of the trowel, and static holding of the tool in different working postures were measured. Electromyographi c activity (EA) of the biceps brachii, pronator teres, flexor digitoru m, and extensor carpi ulnaris was registered continuously and summed u p during all of the test sessions lasting 30 or 45 s, each. All data w ere standardized by means of maximum EA resulting from preceding job-s pecific maximum voluntary contractions. Before and after the working s essions, which lasted about 4 h for each subject, the ergonomic qualit y of the handles had to be rated by means of a questionnaire with 9 it ems on a bipolar 4-step scale. In accordance with the hypothesis that the ergonomically designed handle should enable a specific relief of t he strain in the grip musculature and the ulnar deviation muscles, sig nificantly lower EA values were measured with this model during most o f the test phases. But the effect was much less in scale than was expe cted from the subjective assessment before the tests. Also, subjective rating data after the working sessions differed clearly between the 3 handles mostly corresponding with the pretest assessment.