Ratings of perceived thigh and back exertion in forest workers during repetitive lifting using squat and stoop techniques.

Abstract

Local thigh and low back perceived exertion (RPE), sagittal peak load moments, and leg and trunk muscular activity during repetitive submaximal lifting, with squat and stoop technique, were investigated. This study analyzed changes in kinetic variables caused by changes in body movements during the lifting bouts, and the contribution of the biomechanical and physiologic variables to the variability in the local RPE responses. Despite instructions that emphasize the "correct" lifting technique as the squat technique, the stoop technique is reported as more commonly used in practice. Few studies have investigated the effect of lifting technique on differentiated perceptual responses in repetitive lifting. Ten experienced forest workers performed submaximal repetitive lifting bouts until steady-state VO2 was reached, using five different weight and frequency combinations with both squat and stoop techniques. Borg's scale was used for RPE measurements. Muscular activity in lumbar, hip, knee extensors, and knee flexors was recorded with surface electrodes. Kinematic data was obtained from electronic liquid-level sensors, and vertical ground reaction forces from a two-dimensional force plate. Low back RPE was higher for stoop than for squat, whereas the opposite was true for thigh RPE. The total accountable variance (R2) for the biomechanical and physiologic variables to the RPE responses ranged from 0.25 (low back RPE in squat lifting) to 0.61-0.76 for the other assessments. During the time course in squat lifting at the highest frequency, the knee load moment decreased and the vertical ground reaction forces increased. The study indicates that "movement strategies" to reduce the demand on the knee-extensor muscles were used during the squat lifting bouts at the highest frequencies, which combined with the relatively high assessed thigh exertions, leads to the hypothesis that quadriceps muscle exertion is the "weak link" for the squat technique. The study also indicates a discrepancy between the measured and perceived low back stress in squat repetitive lifting.