The term "excessive force" is a legal one and is fairly precisely defined. Police are trained in what constitutes excessive force and how to avoid its use. They are legally liable not only for using it but also for failing to prevent other police from using it.
Of course, making a quick judgment about what degree of force is needed during an incident that is unfolding rapidly and during which lives may be at stake is difficult, but we can look at some examples.
Imagine that an officer sees a six-year old child pocketing a candy bar at a supermarket checkout. It is true that the child is a "criminal" but any use of force would be excessive. Instead, the officer should just politely alert the parent to the child's activity and let the parent persuade the child to return the candy and explain to the child why stealing is wrong.
At the other end of the spectrum, if the same officer were part of a team arresting heavily armed drug dealers who were resisting arrest, even a lethal level of force might be justified.
The amount of force required will obviously be different depending on the size and strength of the person being restrained and whether the person is armed. The amount of force used to subdue a strong, young man might badly injure or even kill an elderly woman with dementia.
The most difficult situations are those involving the mentally ill or the elderly with dementia. These are often very difficult situations for the police and police training is increasingly looking at ways to handle them without use of excessive force. Often this takes a great deal of patience, talking slowly and in an unthreatening manner until the person calms down.