“Inertia is the resistance of any physical object to any change in its state of motion (this includes changes to its speed, direction or state of rest). It is the tendency of objects to keep moving in a straight line at constant velocity.”
This applies to athletes 100%. The human body will keep moving in a certain direction until a force acts upon it to slow it down or change the direction. Typically, this is the person's muscles, ligaments, tendons, and bones (and occasionally the opposing team...). Guess what? Weak muscles and ligaments do a poor job of changing a body's inertia; strong ones, however, are pretty darn good at it.
In addition to making people stronger and more awesome, we work on speed** and change of direction with our athletes. Below is a drill that teaches acceleration, deceleration, and changing direction quickly.
Most parents want their kids to get faster, frankly, most kids want to run faster which is why they come to us. However, the other side of the commonly thought of "speed work" is being able to decelerate safely and then redirect force in a different direction (forward, backward, or sideways relative to the original direction). Most non-contact injuries happen because the brain has decided to change directions but the body is not prepared to do so. For example, ACL tears frequently happen when an athlete tries to change directions but they are unable to decelerate properly before trying to do so. It was really hard to find a video of an ACL tear (non-contact), the best I could find was this compilation. See the athletes at markers :11 (basketball), :29 (baseball), 1:05 (tennis), and 1:20 (football).
So, since decelerating is just as important as accelerating we do drill such as this one:
And from the side:
This is a simpler drill since the only movement is linear (forward/backward), no lateral/sideways motion, since I want him to learn how to slow his body down safely and then change direction.
The progression to this drill would be to adding some sort of lateral movement from which the athlete either has to slow down from or change direction into.
Again, speed training should encompass both acceleration and deceleration; failing to train and practice the deceleration component sets up athletes for injuries during actual sport practices and games.
This is a really important point, don't skip it:
*"Speed" often improves, almost exclusively at first, by getting an athlete stronger (with nary a "speed" drill in sight) because stronger athletes can a) apply more force to the ground and thus propelling themselves further with each step and b) can slow themselves down more quickly in order to change directions. Strength training also eliminates "power leaks" i.e. a weak and noodle-y core is just going to flop around with lots of wasted motion instead of translating force. Honestly, 99% of "slow" kids are actually just weak. Once an athlete has an actual strength base, then we can start working on speed and change of direction drills.