In modern fiction there is an erroneous notion of what a strong female character is: that a strong female character implies that the character be strong and female, to be aggressive or otherwise capable at enduring and inflicting violence. Strength is not the same thing as force: and a strong female character is like a strong male character: they have realistic motivations, an inner life, hopes, fears, all of the characteristics of an actual person. A female character like Katniss from Suzanne Collins YA series The Hunger Games, for example, is strength depicted as the capability to inflict violence. This is a misunderstanding of what makes characters strong: it is not their physical strength, though physical strength is an attribute of strong female characters, however it is not their strength that makes them strong, it is their humanity and realism.
A poor female characterization could be a caricature of the masculine notion of strength as the capability to inflict violence and outperform others in feats of physical strength. Strong in this definition refers to level of character, rather than relative measures of physical strength. Again, this has been used to great effect with strong female characters in many works of literature, but a modern misinterpretation of this is the portrayal of women as simply violent, aggressive, as a poor means of conveying strength of characterization. To use physical strength as a character trait must work within the overall character arc, male or female, and it would be foolish to conclude that without being traditionally strong or physically able, a male character would be weak because of this inability to project physical force. If we limited strong characters to those most capable of successfully beating up their foes, it would be a poor definition of strength.
There are different types of strength, moral, emotional, and psychological. The criticism for the lack of strong female characters is not that there are not female characters who can kick ass, though traditionally casting women as helpless and without agency has been a problem, but that there are poorly written characters whose motivations, fears and hopes don’t strike the audience as genuine or engaging. Character strength based poorly on violence is a weak character, whether they’re Hercules or Athena. This is not a criticism of depicting women as strong fighters, but of making the point that it is poor character writing to rely on physical strength as the hinge of a strong character.