Prediction of Ratings of Lifting Difficulty from Individual and Task Variables

Abstract

Subjects (6 male, 6 female) lifted and lowered boxes at eight variations of location and direction. For each of the eight, there were three weights (5, 10, and 15 kg for males, 5, 7.5, and 10 for females), three volumes (150, 225, and 300 mm cubes), and two techniques (bend and squat); 144 conditions in all. Subjects rated difficulty using a 7-point scale (1 = prepared to do continuously, 3 = 1110 min for 8 hr, 5 = 1/120 min for 8 hr, 7 = refuse to do). For the same weight, females rated tasks more difficult than males. In addition, for each additional kg of weight, the female rating increased 0.44 versus 0.14 for males. The effect of volume was significant for females but not males. Lowering was 3% easier than lifting. Bending was 2% easier than squatting. Isometric back muscle strength is a good index of the maximum weight a person will lift (r = 0.84; rating standard error = 0.4).