Relative to body weight, humans have the heftiest brains in the animal kingdom.

This explains everything.

Scientists asked 72 men, ranging in age from 23 to 80, to tap their index fingers as fast as they could for 10 seconds. The researchers also did brain scans to measure in each subject the amount of myelin — a fatty sheath of insulation that coats nerve axons and allows for signaling bursts in our brains.

Both the tapping speed and the amount of myelin was found to decline "with an accelerating trajectory" after age 39.

Study leader George Bartzokis, professor of psychiatry at UCLA, called the results "pretty striking" and said: "That may well be why, besides achy joints and arthritis, even the fittest athletes retire and all older people move slower than they did when they were younger."

The myelination of brain circuits was known to peak in middle age. Bartzokis and others have long argued that brain aging might be primarily related to the myelin breakdown.

Learning more about this decline in fine-motor-skills speed offers some hope for treating the aging brain.

"Since in healthy individuals brain myelin breakdown begins to occur in middle age, there is a decades-long period during which therapeutic interventions could alter the course of brain aging and possibly delay age-driven degenerative brain disorders such as Alzheimer's," Bartzokis said.

The findings are detailed in the online version of the journal Neurobiology of Aging.