THE EFFECT OF HIP FLEXION ON LUMBAR HYPEREXTENSION DURING THE BENCH PRESS

Abstract

INTRODUCTION A common belief among exercise professionals is that arching of the low back while performing the bench press is potentially injurious. It has been recommended for many years that increasing hip flexion during the bench press can reduce the risk of low back injuries that may be incu~~edudr ing this exercise. To date, we are unaware of any direct experimental evidence which validates or refutes this claim. One study has addressed the effects of altering hip flexion on the compressive and shear forces in abdominal training (Johnson and Reid, 1991). Their fmdings suggested that, for abdominal training, increased hip flexion reduced potentially harmful vertebral compressive and shear forces. It has been suggested that maximizing hip flexion during the performance of the bench press will decrease lumbar hyperextension and thus reduce the potential for injury to the lumbar intervertebral discs. To address this issue, the present investigation sought to determine the effects of varying hip flexion (0". 45". and 90") on electrical activity of lumbar spinal extensors and the extent of lumbar spinal hyperextension during the bench press. Specifically, we tested the hypothesis that increased hip flexion would reduce erector spinae activation (iEMG) and vertical lumbar displacement. METHODS Pre-amplified electrodes were used to measure bilaterally the electromyographic responses of the erector spinae at the levels of L1 and L5. Lumbar spinal extension, expressed as vertical displacement of the lumbar spine, was measured by videotaping subjects from a lateral view while performing their 5 repetitions maximum (RM) in the bench press. Reflective reference markers placed along the mid-axillary line at the vertebral levels of L1, L3 & L5 were used to quantify vertical displacement. ANOVA and least squares linear regression were used to determine treatment effects. RESULTS AND DISCUSSION No differences among electromyographic responses of the erector spinae among the three hip flexion conditions were found. Surprisingly, vertical displacement of the lumbar spine actually increased at the level of L1 and L3 when the hip was flexed 45" from the fully extended position. Because the erector spinae is a major extensor of the back, and can promote anterior pelvic tilt during activation, it was hypothesized that iEMG of the erector spinae would reduce as hip flexion was maximized. This hypothesis was not supported by the findings of this study. We conclude that augmented hip flexion does not reduce erector spinae muscle activation nor lumbar hyperextension during the bench press. REFERENCES Johnson, C. & Reid, J.G. (1991). Lumbar compressive and shear forces during various trunk curl-up exercises. Clin. Biol., 6.97-104.