Loud, startling noises may increase the risk of knee injury, says a study published online in the Scandinavian Journal of Medicine & Science in Sports.

Honking horns, sirens and other sudden noises can disrupt circuits in the brain controlling muscles and ligaments that stabilize the knee, causing people to trip and fall, researchers suggest. Buzzers and shouting during sports competitions could affect athletes who normally have good balance and muscular control, they said.

An estimated 250,000 sprains and tears to the anterior cruciate ligament, which is essential for normal knee function, are reported every year in the U.S., according to the Centers for Disease Control and Prevention. Researchers suggest some ACL injuries are likely due to a noise-induced startle response in the central nervous system that results in coordination errors.

The study involved 18 men and 18 women in their early 20s from the University of Delaware. The subjects were seated in a motorized chair designed to move either leg at a controlled speed. Their dominant leg was strapped at the ankle while a special arm attached to the chair raised the knee at a 30-degree angle. Electrodes recorded thigh-muscle activity as the arm slowly forced the knee to bend a further 40 degrees during six trials. Participants were instructed to resist knee bending as fast and as hard as possible in all six trials. During three trials, subjects heard a brief, high-pitched beep, about as loud as a motorcycle, through earphones just before the knee began to bend.

The beep induced a startle response that significantly increased muscle stiffness during the first four degrees of bending compared with no-beep trials. As the startle response subsided, muscle activity and joint stiffness were significantly reduced as the knee continued to bend to the full 70 degrees. The higher initial stiffness followed by markedly reduced stiffness likely reflects a disturbance in neuromuscular control that can lead to abnormal stresses on the joint and unintentional injury, the researchers said.

Caveat: The beep used in the study may not compare with loud sounds heard during athletic competitions, researchers said. Fluctuating estrogen levels may affect joint looseness and neuromuscular control in women, they said.

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