The response of the flexion-relaxation phenomenon in the low back to loading.

Fick hypothesized in 1911 that the erector spinae muscles are not active when the trunk is in the fully flexed position. This effect was later called the flexion-relaxation phenomenon (FRP) and is believed to be the result of the ligaments and other passive elements of the spine taking over the load of the muscles. This study examined the effect of loading on the EMG activity of five males and five females during postures of standing at 45 degrees, 90 degrees, and full flexion. The results showed major differences in the relationship between the electromyographic signal (EMG) of the erector spinae and loading for the four postures. The erector spinae muscles did not activate in positions of full flexion (or even 90 degrees for some subjects) for loading as high as 50% of their maximum voluntary contraction, suggesting that alternative muscles are being activated and that the passive tissues may be put under higher loads than originally thought in these positions. The results suggested that the FRP could be used as a biofeedback tool to illustrate to workers that their muscles are not turning on in the fully flexed positions, and therefore, these positions should be avoided.