Effect of Lifting Height and Weight Magnitude on Biomechanical Loading During Manual Lifting

Abstract

In India, manual material handling (MMH) activities in asymmetrical postures are very common in the manufacturing industry and building construction sites. MMH involves any tasks that require a person to lift, lower, push, push, hold, or carry objects, vertically or horizontally, from one location to another with hands. These physical manual activities overtax the human musculoskeletal system, which may exceed workers’ physical limitations. The measurement of ground reaction forces (GRFs) shows a good measure of the musculoskeletal stresses being exerted on the workers performing such tasks. The research studies on the effect of vertical GRF and loading rate (LR) on the human body during running, walking, and jumping were well documented, but rarely have been used to evaluate lifting tasks. There is thus a need for a scientific study to investigate the effect of lifting task parameters and their interactions on the GRF and LR of workers in dynamic lifting human activities. This experimental study investigated the effect of lifting task parameters; lifting magnitude and lifting height based on subjective and biomechanical loading estimates. The subjective estimate was obtained using workload assessment by body discomfort chart. The biomechanical loading (loading rate) was calculated from GRF data, obtained using a force plate. Twelve male subjects in the age group of 20–25 years lifted containers of five different weights between 10 and 20 kg, from below knee height (origin) to various destination heights (below knee, knee, waist, shoulder, and ear level). In the experiment, the subjects were instructed to lift the weight in a 90° asymmetric posture using free-style lifting techniques. Both the task parameters, lifting weight and lifting heights, were observed to be significant. The results showed that vertical reaction forces increase when subjects lift the weight from the floor to ear height. It was also observed that the instantaneous loading rate increases with an increase in the level of lifting magnitude and lifting height; a significant extra loading rate is required to change the lower level of load and destination height to higher levels. Safe limits for significant factors are proposed to result in the optimal performance of the manual lifting task.