Reliability of repetitive dynamic strengths as a screening tool for manual lifting tasks

Recent studies have shown that maximal dynamic strengths (MDSs) of individuals are superior and more reliable predictors of their maximum safe lifting capacity compared to maximal static strengths. The evidence, however, is based on studies that have investigated only infrequent lifting activities, i.e. lifting only a few times a day and with complete recovery from fatigue between successive lifts. It remains to be seen if MDSs are also as highly correlated to lifting capacity for frequent lifting tasks i.e. tasks that do not allow complete recovery from fatigue between successive lifts. The study reported here was conducted to determine the degree of affinity between MDSs and lifting capacities of individuals for frequent and infrequent lifting tasks. In addition, the hypothesis that lifting capacity for frequent manual lifting tasks will be more highly correlated to dynamic strength values which take into account the effect of repetition related fatigue, instead of MDSs, was tested. The results overwhelmingly supported the hypothesis. We therefore concluded that repetitive dynamic strength (RDS) is a more accurate measure of an individual's lifting capacity for frequently performed tasks, than maximal static or dynamic strengths, and deserves recognition as a reliable pre-employment screening tool for frequently performed manual lifting tasks. The experimental evidence also indicated that manual lifting tasks performed once every minute or less frequently are strength oriented while those per formed three times a minute or more frequently are not.