**4. Extrinsic risk factors**

Extrinsic risk factors are the environmental factors, which predispose a person to ACL injury [75–79]. Firstly, weather plays a role wet and rainy days have an increased risk of Anterior cruciate ligament injury as they decrease friction in-between shoes and field. As a result, there is a greater risk of ACL injury. In addition, there is more risk of ACL injury in hot weather as heat increases ligament extensibility predisposing the patent to ACL injury. Orchard et al. prospective study showed increased risk with hot climate for ACL injury [76–79].

Secondly, the type of Cleats on shoes play a role in ACL injury. Cleats that have higher torsion such as edge cleats have higher resistance and will increase the risk of Anterior cruciate injury. Lampson et al. prospective study showed that cleats with higher torsional resistance have a significant risk of ACL injury. Their study was conducted on 3119 athletes which showed that 42 Athletes with edge cleats had ACL injuries which showed a higher predisposition to ACL injury with Edge cleats. Cleats design such as flat cleats, small cleats and fewer cleats had a fewer ACL injury [75].

Another factor that can contribute to the risk of ACL injury is the surface of the field. The field which has Bermuda grass have a higher risk of ACL injury compared to the field with Ryegrass due to cleats getting trapped in Bermuda grass. Orchard et al. prospective study show an increased risk of trapping in Bermuda grass compared to the field with Ryegrass, but this study did not take into account the different cleats used by the players [76–79].

Another study by Olsen Et al, compared ACL injuries in handball players in synthetic and wooden courts. They have found that the incidence of ACL injuries was 2.35 times higher in synthetic courts compared to the wooden courts [49]. There is usually an interplay between the foot ware and surface of which indoor games take place which increases the ACL injury risk and similar interplay could play in outdoor activities as well in combination with the weather conditions.
