Human capabilities in repetitive lifting

In emergencies, it may be necessary for men to manually handle very large quantities of materials. Although data are available for loads carried on the back and there are specific civilian and military recommendations concerning safe maximum loads and acceptable workloads for repetitive lifting, data quantifying maximum capabilities for repetitive lifting tasks are not available. We have therefore undertaken a preliminary study to determine the relationship between lifting rate and time to exhaustion using three standard loads. Eight healthy, fit, well-trained young soldiers bimanually lifted three loads (25, 50 and 75% of their maximal lifting capacity (MLC) for a single lift to waist height) from the floor to a platform at waist height (40% stature) situated directly in front of them at lifting rates of 2, 3 and 4 lifts min −1 (75% MLC), 4, 6 and 8 lifts min −1 (50% MLC) and at 8, 10 and 12 lifts min −1 (25% MLC) for 1 hour or until they become exhausted or were unwilling or unable to continue or maintain the prescribed lifting rate. The observed lifting duration (LD obs) was noted, or if the subject was not exhausted after 1 hour of lifting, he estimated his lifting duration (LD est). Oxygen consumption (VO 2 ) and heart rate (HR) were measured for 1 min each 10 min. The mean (S.D.) LD obs and LD est at 25% MLC were 156(57), 99(61) and 65 (34) min for 8, 10 and 12 lifts min −1 , respectively. At 50% MLC they were 111(65), 72(45) and 35(18) min for 4, 6 and 8 lifts min −1 1 respectively. At 75% MLC they were 93(69), 61(35) and 30(23)min for 2, 3 and 4 lifts min −1 , respectively. The mean relationships between load and lifting rate at 50, 35 and 23% V˙O 2 max (determined for uphill treadmill running), and for an HR of 110 beat min −1 were constructed. These results provide a preliminary quantification of maximum human capabilities in repetitive lifting and an indication of the upper limits of tolerance for industrial workers undertaking repetitive lifting over an 8 hour workday. Further work is necessary to evaluate interpersonal and ergonomic factors in repetitive lifting tasks.