Physics + kinesiology = biomechanics

I've been trying to figure out for awhile how much impact there is on leg bones manipulating two variables: pace and surface. Turns out gait style has a role as well. Biomechanics book has some clues ---

Newton's third law states that for every action, there's an equal and opposite reaction. As feet hit the ground, the ground responds with an upward reaction force. The harder you land, the harder that Ground Reaction Force pushing back is.

By age 2 or 3, kids develop an adult-like gait pattern (legs may "eggbeater" a bit still) with heel striking. In kids age 5-12, the magnitude of the GRF is related to speed. (Does this hold true for adults?)

Basic physics: any force is split into horizontal and vertical components. The resultant force (total force) you find with trigonometry.

The magnitude of the vertical component of the GRF for running FLAT is roughly 2-3 x body weight. The range is because it depends on gait (midfoot, heelstrike, rearfoot). "Toeing out" increases ground reaction force mediolaterally (I am pretty sure this means if you toe out a bit running, you get more force on the middle/outer edge of the foot. Stability shoes are supposed to correct things like toeing out, pronation, supination).

A graph of force for rearfoot vs midfoot strikers shows a spike in force at the start of the footstrike for a rearfoot striker, which then settles and roughly joints the same curve as a midfoot striker. On the whole, midfoot striking is slightly less impact.

Other factors that influence GRF patterns:
pace
duration of run
knee flexion angle at contact
stride length (more on that later)
fatigue
what shoe you're wearing
surface stiffness
surface smoothness
light intensity (I have no idea what this means, will have to ask in class)
grade (incline / hill)

Of note: if you run downhill at a 9% grade, you have 54% MORE impact than running flat. (Also you get the eccentric contractions that cause muscle soreness!)

Another of note: FATIGUE slightly reduces impact! Know why? It's cool. As you get tired, you start to "shuffle" a bit more - think that last 10k of a marathon, it's actually a relatively efficient gait. Not kicking your heels up behind you too much means it's more efficient, and less impact. So, fatigue -- "shuffling" (use the term loosely), less impact. Another reason it really is good to be tired.

That, however, is secondary in the ways-to-reduce-impact. The top two: shorter strides, and increased knee flexion at contact.

HERE'S SOMETHING REALLY INTERESTING ABOUT SURFACES THAT I DON'T QUITE BELIEVE:
"Although it seems logical that running harder surfaces would generate larger amounts of ground reaction forces, this has not been documented. When encountering surfaces of different stiffness, runners typically make individual adjustments in running kinematics that tend to maintain GRFs at a constant level. This may be explained to some extent by the runner's sensitivity to the shock waves resulting from every heel strike that propograte upward, dynamically loading the musculoskeletal system."

As the magnitude of the GRF increases, dynamic loading of the musculoskeletal system increases at a rate of five times the rate of the GRF increase. (oh lookit we get calculus in there! related rates and derivatives methinks)

Supposedly, what happens is the muscles increase their activity to "minimize soft-tissue vibrations"

STRIDE LENGTH
As you run faster, most people increase stride length. THIS IS COUNTERPRODUCTIVE.
Longer strides have a higher GRF and a larger horizontal GRF component, which slows you down --- it's just plain inefficient; "a retarding horizontal force of 6% body weight increase the metabolic cost of running by 30%"

Longer strides also place more impact on the knees, as there is a more extended knee angle at contact.

Anyone else have thoughts or knowledge on this? It's interesting.