Evaluation of the maximum voluntary isometric lumbar extension in three-dimensional positions

<i>Objective</i>. Asymmetrical trunk positions to perform daily tasks are known as risk factors for back injuries. This study aimed to investigate the lumbar extension (main activity for lifting) in symmetrical, asymmetrical and combined positions. <i>Methods</i>. Sixteen lumbar and abdominal muscle activities during maximum voluntary contraction in six main directions and the isometric extension in 24 different positions were recorded for 14 healthy men. The co-contraction index was calculated between all the muscle pairs for each trial (120 pairs per trial). The relationship between uniaxial and combined positions was investigated using Pearson’s correlation and the response surface methodology model. <i>Results</i>. Pearson’s correlation between the combined positions and extension was 0.482–0.588 and for the other five main directions was &lt;0.3. According to the models, a significant relationship was observed for extension. <i>Conclusions</i>. Secondary activities due to asymmetrical position reduce the subject ability to apply maximum extension. The optimal asymmetrical lifting mode is when it is accompanied by flexion and avoids side bending as much as possible. To reduce back injuries, and increase ergonomics and occupational safety, the work environment should be designed in such a way that the spine is not exposed to excessive loads.