An Educational Experience
Lesson 3: Ball Kicking/Kneeing
This is very similar to ball punching except that the feet and knees are more powerful than the hands. It does not matter whether or not you wear shoes or not. Shoes make the blow wider so that the testicles have a harder time getting away, which is why using shoes makes the kick more painful but it is no less safe than kicking barefoot. Although it is possible to rupture a testicle with a kick it is practically unheard of. We do not possess the coordination or skill to make such as kick. It is like being able to balance a clothes basket on one's head while walking place to place. It is possible because it has happened but it would requires years of training. Unless you are a good kick boxer your kicks are not going to be anywhere near that 60psi limit. Most kicks will be around 0-30psi. With practice the kicks might even get up to 50psi but unless you are real training and practice there is no risk of rupturing a testicle.
The only exception is with a full force kick. A full force kick takes practice and training. It is where you put all of your force into the kick. Most people cannot even do a full force kick if they wanted to.
It does not matter what part of your foot connects because it can be the toes, heel, ankles, top of the foot, or leg. Kneeing the testicles generates a great deal less force but than again it requires a great deal less coordination. Unless you have very strong legs and you are an expert at kneeing people so much so that you could use it as a winning move in a fight, you do not need to worry about hurting the testicles with knees. Few people can do a knee with 60 psi.
To summarize, as long as you do not hit the testicles into the body it does not matter how hard you hit. The testicles will swing and redirect the force of the blow. If you are very skilled at kicks or knees you should not kick or knee as hard as you can. Otherwise, there is little chance that you could cause an injury.