I have a wicked street fighting past. You know that under your clothes you can wear a good sized chain held together with a simple s hook. With practice you will certainly defend yourself against someone with a knife. Chains are excellent weapons used even by gladiators.
The good thing about a sap is that you can carry those parts into any building with a metal detector.
For an older person who may have weak hands, a roll of pennies is cheap and will give a punch the effect close to brass knuckles. ...
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No, it will give the effect of a broken hand and fewer options.
Broken hand likely a broken jaw too. Once the attacker is injured. It's not a time to stop. Once an attacker is down use feet. Stomp on their throat. That's helpful if someone is down.
What I have found is people using self defense stop. They let the attacker get a second chance and it is that second chance that dooms the defender.
The goal is to end the attack permanently. Which means keep going until the attacker is a wet spot. Actually that goes for any age. Even young people stop. They try to run away instead. Women especially try and run away screaming for help. They get an advantage and abandon it.