Lifting in four restricted lifting conditions: Psychophysical, physiological and biomechanical effects of lifting in stooped and kneeling postures

Abstract

Many underground coal mines are less than 1·2 m in height, and require that manual lifting tasks be performed in restricted postures (usually stooped or kneeling). Unfortunately, little is known about how these postures affect the underground coal miner's capabilities to perform such work. A previous Bureau of Mines study indicated that lifting capacity is greater when stooped than kneeling when lifting under a 1·2 m roof height (Gallagher et al, 1988). However, many low-seam coal mines present even more restricted headroom than 1·2 m. Therefore, a study was conducted to: (1) examine the psychophysical lifting capacity of low-seam coal miners under four restricted lifting conditions, (2) investigate the associated metabolic costs, and (3) analyse the electromyography (EMG) of trunk muscles when lifting in these restricted positions. Subjects were 12 coal miners accustomed to handling materials in restricted postures ( mean age = 35·9 yrs ± 6·4 SD ). Results of this study indicated that lifting capacity was greater when the subjects could assume a stooped posture than when kneeling ( p < 0·01). Furthermore, the metabolic cost was greater in the kneeling posture for heart rate ( p < 0·001), oxygen consumption ( p < 0·001), minute ventilation ( p < 0·05), and respiratory exchange ratio ( p < 0·05), despite the fact that miners lifted less weight in this posture. Analysis of the EMG data indicated increased activity of the electores spinae when kneeling ( p < 0·001), but higher latissimus dorsi activity when stopped ( p < 0·001). The findings of this study indicate that the weight of supply items should be reduced approximately 14–18% when the kneeling posture must be used for lifting.