A COMPARISON OF ISOKINETIC LIFTING STRENGTH WITH STATIC STRENGTH AND MAXIMUM ACCEPTABLE WEIGHT WITH SPECIAL REFERENCE TO SPEED OF LIFTING

A laboratory study was conducted to deter-mine the effects of the spee d of lifting and box size on isokinetic strength and to compare isokin etic lifting strengths with static lifting strengths and psychophysica lly determined maximum acceptable weights. Nine male college students lifted three different boxes (250, 380 and 510 mm wide) from the floor to a bench height of 0.8 m using a free-style lifting technique at a rate of 0.2 lifts min-1. For each lifting task static strength was mea sured at the origin of lift. Isokinetic lifting strength was measured at 0.41, 0.51 and 0.6 ms-1 using a Biokinetic ergometer and attaching boxes to the load cell. Ratings of perceived exertion were recorded fo r the low back. There was a progressive decrease in mean and peak isok inetic lifting strengths both with an increase in lifting speed and wi th an increase in box width (p < 0.01). The lifting speed had a much g reater effect (29% and 27%) than the box width (18% and 15%) on mean a nd peak isokinetic lifting strengths. However, high speed lifting was perceived subjectively to be less stressful (RPE = 10.7) than slow spe ed lifting (RPE = 12.7). Static strength and maximum acceptable weight had higher correlations with mean isokinetic strength (r = 0.65 and 0 .82) than with peak isokinetic strength (r = 0.52 and 0.73). At 0.41 m s-1, mean isokinetic strength was 6% greater than the mean static stre ngth (p greater-than-or-equal-to 0.05). Extrapolation of mean isokinet ic strength data showed that at 0.73 ms-1 the estimated mean isokineti c strengths were within 6% of maximum acceptable weights. It is conclu ded that isokinetic strength is highly dependent upon the speed of lif ting. At a slow speed (0.41 ms-1), mean isokinetic strength is equal t o mean static strength; and, at a high speed (0.73 ms-1), it appears t o be equal to the maximum acceptable weight. It is recommended that bo th speed of lifting and box width should be controlled carefully to si mulate job-specific isokinetic lifting strength.