The 21-foot principle: Effects of age and sex on knife attack characteristics

Knives are commonly-used weapons in criminal activities and interpersonal assaults worldwide. Injury reports have identified the upper body as the most frequent location of knife injuries, and that stabbing attacks are more fatal than slashing attacks. The first two aims of the study explore whether the type of attack and attack location could be predicted from age and sex group. The following aims compared attack times between sex and age groups and evaluated the influence of subject characteristics on attack duration. The exploratory aim surveyed subjects on attack location during the scenario if the target was not wearing body armor.

A total of 74 subjects (male: n = 40; female: n = 34) participated. Subjects were video recorded during a 21-foot (ft) attack and participated in follow-up questions and completed a push-up test to determine physical strength. Attack time (AT), concealment location, attack type, and demographic information were correlated to age group and sex.

Mean AT from 21 ft was 2.40 ± 0.47 s, with the fastest of 1.75 s. The most commonly observed concealment location, attack type, and attack location on the target included the right hip (n = 62), stab (n = 62), and the chest (n = 28), respectively. After controlling for push-up ability (p < 0.05), no differences were reported between any combination of sex and age group in AT from 21 ft. Push-up ability was the sole significant predictor of AT from 21 ft (p < 0.05). Twenty-eight subjects reported they purposely avoided the body armor vest during the 21 ft attack.

Law enforcement officers are encouraged to train for quick defensive or offensive responses, such as hand-to-knife combat or rapid firearm skills that can be performed in under 1.75 s. Stronger people were predicted to complete the 21 ft attack faster.