Use of force continuum

A use of force continuum is a standard that provides law enforcement officers and civilians with guidelines as to how much force may be used against a resisting subject in a given situation. In some ways, it is similar to the U.S. military's escalation of force (EOF). The purpose of these models is to clarify, both for law enforcement officers and civilians, the complex subject of use of force. They are often central parts of law enforcement agencies' use of force policies. Various criminal justice agencies have developed different models of the continuum, and there is no universal or standard model. Generally, each different agency will have their own use of force policy. Some agencies may separate some of the hand-to-hand based use of force. For example, take-downs and pressure point techniques may be one step before actual strikes and kicks. Also, for some agencies the use of aerosol pepper spray and electronic control devices (TASER) may fall into the same category as take-downs, or the actual strikes.
The first examples of use of force continuum were developed in the 1890s and early 1900s. Early models were depicted in various formats, including graphs, semicircular "gauges", and linear progressions. Most often the models are presented in "stair step" fashion, with each level of force matched by a corresponding level of subject resistance, although it is generally noted that an officer need not progress through each level before reaching the final level of force. These progressions rest on the premise that officers should escalate and de-escalate their level of force in response to the subject's actions.
Although the use of force continuum is used primarily as a training tool for law enforcement officers, it is also valuable with civilians, such as in criminal trials or hearings by police review boards. In particular, a graphical representation of a use of force continuum is useful to a jury when deciding whether an officer's use of force was reasonable.
Example model.
While the specific progression of force varies considerably (especially the wide gap between empty hand control and deadly force) among different agencies and jurisdictions, one example of a general use of force continuum model cited in a U.S. government publication on use of force is shown below.
The U.S. Navy teaches a six-step model: Officer presence, Verbal commands, Soft controls, Hard controls, Intermediate Weapons, and Lethal force. Hard controls includes the use of tools such as hand-cuffs, while soft controls equates to empty hand above, describing techniques where the officer may engage a resisting detainee. When escalating, voluntary submission to cuffs is a viable way to prevent the need for empty hand submission techniques which place the officer and the detainee at physical risk. When de-escalating, hard controls (i.e.: cuffs and isolation in the rear seat of a cruiser) gives officers a reasonable and achievable goal after altercation with a detainee during which higher levels of force may have been required.
Subject classifications.
In all use of force continuum models, the actions of the subject is classified in order for the officer to quickly determine what level of force is authorized and may be necessary to apprehend or compel compliance from the individual. Listed below are examples of how subjects are classified.
Generally, the passive subjects and active resistors fall under levels 1–3 of the use of force continuum, while active aggressors fall under levels 4–6. The officers are trained to apply the proper measure of force within the continuum based on the actions and classification of the subject.
Reasonableness standard.
The United States Supreme Court, in the case of "Graham v. Connor", (1989) ruled that excessive use of force claims must be evaluated under the "objectively reasonable" standard of the Fourth Amendment. Therefore, the "reasonableness" factor of a use of force incident must be judged from the perspective of a reasonable officer on the scene, and judged with the understanding that police officers are often forced to make split-second decisions about the amount of force necessary in a particular situation. 
Broadly speaking, the use of force by an officers becomes necessary and is permitted under specific circumstances, such as in self-defense or in defense of another individual or group. However, there is no all encompassing consensus about when an officer would always need to use force, nor is there any agreed upon method that can efficiently measure or predict specific types of force actions that one would deem reasonable before the time comes. 
The International Association of Chiefs of Police, has described use of force as the "amount of effort required by police to compel compliance by an unwilling subject".
When Force is Observed.
Garner and Maxwell (1996) found that when force was necessary, in 80 percent of the encounters, police opted to use weaponless force such as grabbing or shoving. Alpert and Dunham (1999) show that police use of force is reactionary, initiated by suspect resisting arrest. Force is more likely to be employed if suspect is disrespectful, intoxicated, and/or wielding a weapon. Research has also found that special division officers are more likely to use deadly force on suspects. 
Studies examining gender influences on the use of force are still inconclusive. Some findings suggest that male suspects are more likely to have force used against them, whereas others show insignificant differences. However, research examining male-female patrol teams show that these pairings are less likely to use force compared to male-male pairings. Conclusions suggest that female officers may be more effective at diffusing tense situations 
References.
Marine Corps