Lifting tasks and forward flexion movements are often related to low-back problems. It is possible that those who suffer back problems have a different muscle activity pattern during these types of movements than those with healthy backs. To investigate this theory, researchers examined the electromyographic (EMG) activity of the gluteus maximus (GM), biceps femoris (BF), and paraspinal (PS) muscles during trunk flexion and extension, comparing the EMG patterns of 19 women with low-back symptoms with those of 19 healthy controls.

Each subject performed a controlled trunk flexion forward in the standing position lasting five seconds, then a trunk extension back to upright, also lasting five seconds. The EMG activity of the GM, BF and PS muscles was recorded during the whole of the movement. The muscle activity levels and the time each of the muscles was active and relaxed during three movements was noted.

The researchers discovered the following general pattern: during the early phase of forward flexion the PS and BF muscles were activated together. By the end of flexion and during extension, all three muscles were active. However, the key difference between back patients and controls was the amount of GM activity. During flexion GM activity in back patients was significantly shorter than in the controls, while during extension the GM activity ended earlier. In other words, the GM of the back patients was switching on later and turning off sooner during the flexion extension movement.

The results of this study add weight to the idea that the gluteal muscles in back patients are subject to deconditioning and need to be trained to function normally. Back patients need to re-educate their gluteal muscles to be more active during trunk flexion extension movements. More specifically, they should train the gluteals to be active for longer in order to provide more stability for the lumbar area.

2000. Arch Phys Med Rehab, 81, pp 32-37