Focal muscle vibration set at 60 cps with 0.4 mm displacement produces presynaptic inhibition of muscle spindles. Research has shown that observing postural sway when these motors are strapped to the calves allows you to distinguish between low back and non-low back pain patients. Individuals who sway posteriorly during this brief test are nearly 4 times more likely to suffer low back injury within the next 2 years (see article for references).