Spinal compression at peak isometric and isokinetic exertions in simulated lifting in symmetric and asymmetric planes

OBJECTIVE: To determine if the spinal compression can be used as a criterion for the safety of the back. DESIGN: Various lifting strength activities were analysed using a 3-D biomechanical model to determine spinal compression forces. BACKGROUND: Despite many standards being set and the use of varied strategies, low-back injuries continue to be common in our society. Since the injuries occur to the tissues in the body, it may be more appropriate to investigate the effects of external loads on the tissues. Therefore it was decided to determine the spinal compression in standardized lifting tasks. METHODS: Twenty young adults (12 males and 8 females) performed maximal stoop and squat lifts in sagittal, 30 degrees lateral, and 60 degrees lateral planes at half, three-quarters, and full horizontal reach distances. The stoop lifts were performed in isokinetic and isometric modes; the isometric mode was performed with the hip at 60 degrees and 90 degrees. The squat lifts were also performed in isokinetic and isometric modes; the isometric mode consisted of postures with knee at 90 degrees and 135 degrees of flexion. In addition, the subjects also performed isometric lifts in stoop and squat postures at a self-selected optimum posture. During these activities the strength was measured using a static dynamic strength tester employing a load cell and force monitor. Three-dimensional postural recording was made using a 3-D Peak Performance Technologies imaging system. Using the postural and force data as input to a 3-D biomechanical model, the lumbosacral spinal compression was calculated. The values of strength and spinal compression were analysed and compared. RESULTS: The strength was significantly affected by the gender, the type of lift, plane of lift and reach of lift (P < 0.01); whereas the spinal compression was not affected by the reach at all. Two-way and three-way interactions were significant for strength (P < 0.02) but not for spinal compression. CONCLUSION: It was concluded that the spinal compression was the ultimate cost function in the safety of the back and as such preventative strategies should be based on this. RELEVANCE: The use of an appropriate cost function in a manner in which it can maximize the safety of the back will be of considerable value. Since injuries are biomechanical perturbation of the tissues, it is argued that the cost function be used in a manner where tissue load be considered as the primary factor.