Clinical studiesGender influences on spine loads during complex lifting☆
Introduction
Over the past several decades, women have increasingly undertaken physically demanding jobs that have traditionally been performed primarily by men [1]. Although job requirements do not discriminate between genders, there is reason to expect that the biomechanical effect on the worker may differ as a function of gender. For example, female lifting strength ranges between 40% and 73% of male strength 2, 3, 4, 5, 6, 7, 8, 9. Women also have lower lateral bending and axial twisting strength 2, 7, 10, 11, 12. Such factors as anthropometry may contribute to strength differences between genders, and it is important that ergonomists, biomechanists and occupational medical personnel also consider inherent gender biomechanical differences when workers perform material handling tasks.
Differences in strength and anthropometry between men and women may influence the trunk motions, muscle activities and subsequent spine loads. For example, more extreme postures (forward flexed or asymmetric) result in greater external trunk moments as well as alter the muscle mechanics of the extensor muscles 13, 14. Furthermore, recruitment of nonprimary extensor muscles, such as the internal oblique and latissimus dorsi muscles, will complicate the loading pattern, because these muscles contribute significantly to shear as well as contribute a mechanical advantage to offset the external lifting moment. This also results in increased antagonistic concontraction [15]. Thus, the combination of muscle coactivity patterns and trunk kinematics produce spinal loads during lifting that may be dependent on an interaction between the genders and the workplace factors.
To date, few studies have investigated whether differences in spine loading exist between men and women when performing a common lifting task. Until recently, only four studies 13, 16, 17, 18 have considered the influence of gender on the resulting spine loads. In two of the studies 13, 18, a simple two-dimensional static model was used to estimate the loads and consider body mass differences but not differences in muscle actions. Two other studies 16, 17 evaluated the extensor moments generated during lifting. Recently, Marras et al. [19] found that relative loading differences between the genders depended on the degree of musculoskeletal control required during the exertion. When sagittally symmetric lifting motions were restricted to the torso (eg, subject were not able to move their hips), spine loading differences were directly attributable to body mass differences. However, when more kinematic freedom was allowed, the relationship between external moment (or body mass) and spine loading became much more complicated, with female spine loading increasing compared with that of men. This study will extend these investigations to three-dimensional loading conditions as would be expected in industrial tasks.
Hence, the objectives of the current study are threefold: 1) to assess the magnitude of the differences in spine loads that exist for women and men; 2) to determine how the impact of gender might be altered by workplace adjustments and 3) to determine if any differences exist that may be attributed to biomechanical compensations or adjustments (eg, trunk kinematics and muscle coactivity).
Section snippets
Approach
Subjects performed a series of sagittally symmetric and asymmetric lifts from set and relative (to body size) origins to set and relative destinations while their feet remained stationary on a forceplate. In this manner it was possible to assess the impact of workspace adjustment on spine load as a function of gender.
Participants
Twenty men and 20 women, asymptomatic for low back pain, participated in the study. The mean (SD) age, weight and height for the men were 23.5 (3.7) years, 177.0 (9.1) cm and 74.0
Results
A summary of the statistically significant differences for the three-dimensional spine loads as a function of gender and the experimental factors is shown in Table 1. The statistical analyses indicated that each of the experimental factors (independent of gender) significantly impacted the three-dimensional loads. As expected, increases in box weight, lower origin heights, greater task asymmetry and, to a lesser extent, higher destination heights resulted in greater three-dimensional spine
Discussion
Previous work evaluating how men and women respond biomechanically to similar lifting demands provided the first evidence that women are not simply proportionally scaled down versions of men [19]. In other words, the differences in spine loading are not just a function of size. In that study, subjects performed controlled sagittally symmetric lifts. When the lifting was isolated to the torso, the spine load differences were directly related to the body mass differences. However, when more
Conclusion
The results of the current study indicate that women are not just scaled down versions of men. Rather, kinematic trade-offs and muscle coactivity differences in combination with unequal body masses resulted in the gender differences in spine loading. Women relied on more hip motion as well as required more extensor muscle activity in response to the same workplace demands as compared to the men. Most importantly, men had higher spine loads than women under more demanding conditions (eg, greater
Acknowledgements
The authors would like to thank Duprane Young for her contributions to the study.
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FDA device/drug status: not applicable. Nothing of value received from a commercial entity related to this research. This study was partially funded by the U.S. Army Medical Research and Materiel Command.