If there is anything else to use besides bones, connective tissue and muscle, what is it?
Ted Williams once said something to the effect that hitting a baseball was the hardest thing to do in sports. When the ball leaves the pitchers hand it has to travel roughly 55 feet to reach the plate. The batter has less than 1/2 second to decide whether or not to swing at a 100 mph fastball. Add to that the fact that the pitcher has a variety of pitches at his disposal and there's an awful lot of information for the batter to process in that less than 1/2 second.
So what differentiates an exceptional batter from an average batter? Assuming they're roughly comparable in physical development, mind. The exceptional batter will have a higher degree of mind/body coordination than the average batter. Integration of mind and body is an important factor effecting performance in all human activities. And Aikido is no exception.
When practicing push tests we see a noticeable performance difference when the student consciously lets the mind waver, loosening coordination of mind and body, as opposed to when the student "centers" the mind and closely coordinates mind and body.