Would you have guessed...

…that missing a free throw or slicing a golf ball might be due to activity happening in your brain before you even move a muscle?

Scientists reported in the journal Neuron this week that the reason we can’t exactly replicate a motion, no matter how hard we try, has more to do with our brain than our muscles.

Krishna Shenoy, a professor of electrical engineering and neurosciences at Stanford University, and co-authors Dr. Mark Churchland and Afsheen Afshar also at Stanford, cite basketball legend Larry Bird, who shot 71 consecutive free throws over two months. Why did he miss the 72nd shot? The scientists think it has something to do with “motor preparation”—the activity in the brain that occurs just as you get ready to move.an muscles.

Motor preparation happens when the brain knows that the body must do something and can plan ahead for it. For example, when a basketball player makes a free throw he or she takes a moment at the foul line preparing. During this time, the brain is making a plan for motion. “Areas of your brain are getting you ready to get that arm to move,” says Shenoy. With motor preparation, Shenoy says, “you will be able to make that movement more accurately, and more quickly.”

Shenoy and co-authors trained two rhesus monkeys to reach for targets on command. They wanted to know if the speed of the monkey’s reach could be predicted by the activity of brain cells in the preparatory phase. The scientists measured the activity of neuron cells in the monkeys’ brains before the command was given.

They found that neuronal activity correlated with the motion that followed: on average, the more electrical signals released per second by a neuron before the command, the faster the monkey’s arm moved; fewer electrical signals, called action potentials, correlated on average with a slower movement.

The finding recasts the common understanding of movement variability, which is that muscles are the culprits of inconsistent motion. “This is the first evidence to show that at least half of your movement variability is due to the fact that your brain simply can’t plan your movement in the same way,” says Shenoy.

So why does practice make us better, if not perfect? The scientists say they’re not quite sure. One possibility is that practice allows you to become better at executing the mental planning process, says Afshar.

Our inability to execute motions perfectly may seem frustrating, but it could be advantageous to humans: flexibility in the brain allows us to respond better to new experiences. Afshar explains, “Only in a highly contrived situation, do you really try to do the same thing each and every time.”

This is what differentiates us from computers. “Computers are designed to do the same computation exactly the same way exactly correctly every single time,” says Shenoy. “While a computer can be programmed to do a variety of things, it’s a piece of junk compared to an ant. An ant can contend with hugely unexpected things in its environment.”