Strength-limiting factors for the determination of frequent and infrequent lifting capacities

Abstract

The main objective of this study was to investigate different limiting factors in psychophysically determined maximum acceptable weight of lift (MAWL) for frequent and infrequent tasks. Seven isometric strengths were tested to obtain various muscular strength scores. They were: back lift (BL), leg lift (LL), and pulling backward strength (PB), arm lift (AL), shoulder lift (SL), and vertical pulling up strength (PU). Capacity-predicting models were developed based on the best single strength and anthropometric variables in task conditions of 3 lifting frequencies, 2 lifting ranges, and 2 sizes. To realize the effect of task conditions on strength recruitment, the lifting posture was recorded and the EMG activities of arm (biceps brachii), back (erector spinae), and leg (rectus femoris) of each strength test were also recorded. It was found that different strength-limiting factors do exist between the MAWL of frequent and infrequent lifting tasks. The conclusion was supported by the fact that different strength variables of PB and PU were selected as the best single predictor of the one time maximum MAWL and the MAWL of 4 lifts/min. Posture analysis indicated that more flexed elbow and shoulder joints were observed in the infrequent lifts. The advantage of the stiffened upper extremity is that it can shorten the moment of arms, particularly when the loads are heavy. IEMG data indicated that besides back strength, various muscle strengths (arm, back, and leg) do contribute differently in various lifting phases.