Police are warning anyone who uses a stretch of woodland of wires tied deliberately tied between trees.
One cyclist was injured slightly when riding his bike in Ransom Wood off the Rainworth bypass in Nottinghamshire.
Officers are also concerned that if a motorcyclist hit the wire they could be seriously, if not fatally, injured.
Pc John Albanese said: "If they hit the wire at anything faster than 10mph they would be seriously injured, perhaps even decapitated."
He added: "It could be the work of a prankster, but it is more likely a crude attempt to stop illegal off-road bikers from using the path.
"It may seem a justified course of action to whoever did this, but their method is very dangerous and could cost someone their life."
The 37-year-old cyclist was saved from severe injury by his helmet, which took the force of the impact with the head-height wire.