Tool design, user characteristics and performance: a case study on plate-shears

Performance, grip forces and fatigue were studied in six male and six female subjects while cutting with plate-shears. Three types of plate-shears were used, one standard and two modified, either with a spring grip, or with a spring grip in combination with a reduced grip span. In addition, three types of plate - easy, moderately difficult and difficult to cut - were used. Male subjects used around 40% of their maximal grip force and female subjects around 60% with the moderately difficult plate; the male subjects produced more than twice as long a cutting distance as the females. Neither EMG analysis (frequency shifts) nor subjective exertion or reduction of handgrip MVC indicated a more pronounced fatigue in women than in men, probably because the female subjects used about a 50% lower cutting rate than the men. Productivity (in cm cut per min) was strongly related to measures of hand size and to the relative grip force used. Thus in a multiple regression analysis using metacarpal hand circumference and relative grip force as independent variables, R2 was 0.77. The two types of modified plate-shears were preferred by all and gave a roughly 30% higher productivity in the male subjects but did not improve productivity in the females. However, with a spring grip and reduced grip span, the female subjects reduced the relative grip forced used from around 65 to 50%. The total work (force-time integral) per cm cut was not influenced by type of plate-shear.