THE EFFECT OF FATIGUE ON MULTIJOINT KINEMATICS AND LOAD SHARING DURING A REPETITIVE LIFTING TEST

Study Design. A repetitive lifting test in the sagittal plane was perf ormed with a submaximal load at a maximal lifting rate to understand t he effects of fatigue on kinematic and kinetic measures of performance . Objectives. To quantify the effect of fatigue during a highly repeti tive lifting task, in terms of lifting force transmitted to the load, joint motion patterns, and internal joint load sharing. Summary of Bac kground Data. Industrial surveillance and epidemiologic data suggest t he repetitive lifting is a risk factor for low back pain. Previous stu dies examining the effect of fatigue have either been constrained to i solated trunk movement, or have not explored the internal load distrib ution and potential alteration in the loading pattern. Methods. Sixtee n healthy male subjects performed repetitive lifting in the sagittal p lane with a load equal to 25% of their maximal lifting capacity, at a maximal lifting rate. Changes in lifting performance were determined f rom the power transferred to the box, joint kinematics, and joint kine tics. Data from three cycles a the start and end of the exercise were tested for the effect of fatigue using repeated-measures analysis of v ariance. Results. Fatigue was documented by a reduction in average lif ting force and hip and spine torque generation, whereas internal joint load sharing was relatively unchanged. The fatigue was associated wit h decreased knee and hip motion, and increased lumbar flexion. Decreas ed postural stability also was evident. Conclusions. The significant d ecreases in postural stability and force generation capability because of the repetitive and force generation capability because of the repe titive lifting task indicated a higher risk of injury in the presence of unexpected perturbation. Multijoint coordinated lifting tasks provi de a more realistic protocol the study neuromuscular fatigue.