If you want to be a professional football player, you'd better start practicing your 40-yard dash. It's the gold standard for assessing a player's speed and ability to accelerate, as NBC Learn's segment on kinematics, motion, speed and acceleration shows.

Human beings need about 10 yards to reach maximum velocity, so the 40 is really a test of both acceleration and speed—unlike a longer sprint, such as the 100, which is more about a runner's ability to maintain maximum speed. Acceleration depends on how much force runners can put into the ground (and thus receive back) relative to their mass. For this reason, the smaller you are the easier it is to accelerate rapidly. That's why gymnasts, for example, are generally small—they must be able to generate a large amount of force relative to mass to accelerate enough to run and perform multiple flips in a row. Imagine an offensive lineman trying to do that! Wide receivers, running backs and defensive backs are not as massive as linemen, and therefore are very good accelerators, which is one reason they can handily outrun the latter in a 40-yard dash.

At present, no standard method or variable exists to quantify a human or animal's top acceleration. One reason: the variable changes with every step until top speed is reached, making a tangible value a moving target. As a result, comparing the top accelerations of humans and other animals is difficult. Nevertheless, it's true that smaller animals are better at accelerating—think of how quickly a squirrel can dart up a tree trunk, for example.

Speaking of squirrels, imagine a 40-yard dash that races a wide receiver, a safety, an ostrich, an elephant and a pig—who would win? "The ostrich wins pretty easily," says Peter Weyand, a professor of applied physiology and biomechanics at Southern Methodist University. "And then would probably come the wide receiver, the safety, squirrel, the pig and, finally, the elephant."

The ostrich, although bigger than a human, is built for speed. "The easiest way to explain why the ostrich is fast is that it has long legs," Weyand says. It also runs on its toes, and what looks like a backward knee is actually its ankle. Most of the bird's leg muscles reside on short thighbones, so the task of accelerating and maintaining speed is left to long, light limbs.

Having longer legs, particularly in relation to mass, also allows one more time to put force on the ground. "The only time you can change your velocity is when your foot is on the ground. You push backwards on the ground, and the reaction force causes you to go forward," Weyand explains. Fast people hit the ground harder in relation to how much they weigh, and for a shorter period of time.

Handicapping the 40-Yard Dash What are the top speeds that our contenders hit? Weyand offers his assessment:

Ostrich: The big birds peak at around 35 miles per hour.

Wide receiver: The most fleet-footed receivers top out around 25 mph.

Safety: Safeties are usually not as quick, because they tend to be bigger and wear more pads, so they probably max out at around 22 mph.

Pig: Varies—an undomesticated pig could probably hit about 16 mph, give or take three or four mph.

Squirrel: The nutty rodents can hightail it at up to 14 mph—on a good day

Elephant: Although much more massive, they can sprint at roughly the same speed as a squirrel