Magnetic resonance imaging was used to study the effect of different curvatures in the lumbar spine on lever arm lengths of the erector spinae musculature. Eleven subjects were instructed to simulate static lifts while lying supine in a magnetic resonance camera with the lumbar spine either in kyphosis or lordosis. A sagittal image of the spine was obtained to analyze the lumbosacral angle and to guide the imaging of transverse sections through each disc (L1/L2 to L5/S1). Images were analyzed for lever arm lengths of the erector spinae muscle (ES) and the erector spinae aponeurosis (ESA), the latter functioning as a tendon for superiorly positioned ES muscle portions. The lumbosacral angle (between superior surfaces of S1 and L4) averaged 44° in the lordosed, 26° in the kyphosed and 41° in a neutral supine position. In lordosis, the lever arm lengths were significantly longer than in kyphosis for all levels, averaging 60–63 mm (ES) and 82–86 mm (ESA). The corresponding values for kyphosis were 49–57 mm (ES) and 67–77 mm (ESA), respectively. Thus, there was a considerable effect (10–24%) of lumbar curvature on lever arm lengths for the back extensor muscles. The change in leverage will affect the need for extensor muscle force and thus the magnitude of compression in the lumbar spine in loading situations such as lifting.