Habit

The primary cause of unnecessary tension is habit.
You have spent a lifetime overexerting and now it feels perfectly natural and normal.

Being relaxed yet strong (without trying) seems counter-intuitive; it simply does not make sense to you.
It goes against everything you think concerning strength.
This psychological habit is what causes the physical problem to occur.

All change must begin in the mind - that is why tao/zen reading is absolutely vital.