At a yoga workshop recently the teacher was telling us to be aware of our stance and whether our feet were unbalanced on the ground, with more weight on the heels than on the front of the feet, or vice versa. She told us that the English language told us the effects of posture and the distribution of weight on the feet. If you 'dig your heels in', she said, you are stubborn and obstinate - not a good thing to be, whereas if your weight is on the balls of your feet then you are lively, alert and 'on the ball'.
Well, that might be a good image to evoke to help yoga students stand correctly but the etymology is wrong. The expression 'on the ball' comes from baseball and was first used in the early 20th century. There was an earlier expression 'to put something on the ball' which meant be slick and speedy. An advertisement then encouraged sportsmen to 'always keep your eye on the ball'. The phrase arrived in Britain, and the second part of the idiom took on a life of its own.