INFLUENCES OF MUSCLE FIBER COMPOSITION AND STRENGTH ON EMG, SPINAL MOTION, AND LOAD ACCELERATION DURING A REPETITIVE LIFTING TASK

Although back injuries, often related to lifting, are a serious and widespread problem, many factors regarding their etiology remain unclear. Adequate muscle function would seem to be an important element in preventing disabilities of the low back and this investigation was directed principally at examining the role of muscular fatigue as a possible causative factor in overexertion injuries of the low back during repetitive lifting.;Intercepts and slopes of the trend measures were found to be highly variable across the subjects studied. Several anticipated correlations were found between different measures of EMG amplitude and EMG center frequency but the EMG measures showed no significant correlations with any other measures although some weak, non-significant tendencies appeared to be present. Lifting strength displayed a significant relationship to trunk range of motion intercept, height, and weight. Muscle fiber composition showed a significant correlation with maximum vertical acceleration intercept. Multiple regression analyses revealed no additional relationships.;In addition to these findings, this study provides the basis for formulating additional questions as to why different individuals adopt a particular strategy to cope with a repetitive, fatiguing lifting task.;Twenty-four young, healthy, adult males were tested for lifting strength and then undertook a symmetrical, repetitive, strenuous lifting task for one and a half hours. During this time lumbar paraspinal EMG, trunk motion, and load acceleration were monitored. Muscle biopsies were later taken on twelve of the subjects from the lumbar erector spinae musculature. Data from each of 180 lifts were analyzed and trend measures were derived to describe overall performance for each subject. These trend measures were used to describe any changes, over time, which occurred in EMG, trunk motion, and load acceleration, and to determine the influences of muscle fiber composition and lifting strength on these changes.