During the execution of Damiens, guilty of having given a blow to King Louis XV with a penknife (quartering for a blow with a penknife...!), it was reported by witnesses that the executioner Sanson had to cut the tendons of his shoulders and hips because they did not want to give way! Damiens was very muscular, but this shows that shoulders and hips are very strong.
There are always exceptions to the rule. In exactly how many 'draw and quarters' do you see that notation as compared to how many there were?
To take it to the NFL (National Football League for you non-American) Frank Gore had a career 16 year career at running back. The average career at that position is 3. You can't cite Frank as an example for your average or median running back to make your argument.
I am not disagreeing with you that shoulders and hips are strong. But your assertion that the ligaments there are somehow stronger than say a a knee or elbow is not really born out by injury statistics. Shoulders are easy to dislocate because of the wide range of motions they are required to produce. Hips and knees are a bit harder because their range of motion is much more limited. When you are taught to attack a knee you are taught to come at from directly in front or from the side. generally while you may knock a person down (using the same amount of force) from behind you are likely to not inflict any damage because the knee is designed to bend that way where it isn't in the first two instances.
When you go to push a person down if they are coming at you properly balanced you are not likely to do so. Hit that same person with the same amount of force as they are perpendicular to you you will probably succeed,
It is never merely an application of force against strength it is always HOW do you apply force against a weakness.
And then of course it is also a factor of muscle tissue (as you pointed out), prior damage to the area in question and the generally physical health of the victim.
Of course the whole conversation I theoretical since I doubt we'll get someone we could try it on to see what happens.
And your would be assassin was an IDIOT as you have to generally get to a depth of 4 inches (10-11 cm) before you hit anything vital. Very few pen knives have that blade length.
kisses
willowfall