Spine loading as a function of gender.

STUDY DESIGN In vivo laboratory studies were conducted to investigate the spine loads imposed on men and women during a series of lifting tasks that varied in the degree of lifting control required by the subject. OBJECTIVE To identify and understand differences in spine loading and musculoskeletal control strategies between men and women performing lifts of varying task complexity. SUMMARY OF BACKGROUND DATA Few studies have examined differences in spine loading as a function of individual factors such as subject gender. Furthermore, no biomechanical studies have attempted to quantify and understand how differences in anthropometry between genders might influence muscle recruitment and subsequent spine loads. Because the modern workplace seldom discriminates between genders in job assignments, it is important to understand how differences in spine loading and potential low back disorder risk might be associated with gender differences. METHODS For this study, 140 subjects participated in two separate experiments requiring different degrees of musculoskeletal motion control during sagittal plane lifting. The two experiments consisted of 35 men and 35 women performing lifts in which motion was isolated to the torso and 35 men and 35 women completing whole-body free-dynamic whole body lifts. An electromyography-assisted model was used to evaluate spine loading under these conditions. RESULTS Absolute spine compression generally was greater for the men. Under the highly controlled (isolated torso) conditions, most differences were attributed solely to differences in body mass. Under a whole-body free-dynamic condition, significant differences in muscle coactivations resulted in greater relative compression and anterior-posterior shear spine loading for the women. CONCLUSIONS Differences in spine loadings as a function of gender under the more controlled lifting conditions were primarily a function of different body masses. However, loading pattern differences existed between the genders under whole-body free-dynamic conditions as a result of kinematic compensations and increases in muscle cocontraction, with women generally experiencing greater relative loads. When spine tolerance differences are considered, one would expect that females would be at greater risk of musculoskeletal overload during lifting tasks.