Biomechanical analysis is divided into two components. First, there is the analysis of movement through description. This is called kinematics. Next, there is the study of the forces that causes motion. This is known as kinetics.

Kinematics = Describing motion

Kinetics = What is causing the motion (forces)

Before I get into further discussion on these two concepts, let's talk briefly about scalar v. vector quantities. This is important to understand as I discuss the terms associated with kinematics and kinetics.

A scalar quantity is a quantity that is just a magnitude (how big? how far?). A vector quantity is a quantity that has a magnitude and a direction associated with it.

For example, the definition of speed is just distance/time. There is no direction associated with it. Speed is an example of a scalar quantity.

In contrast, velocity is a vector quantity. Velocity has a direction associated with it. For example, when I was discussing the velocity after contacting the table, I discussed the "vertical velocity." If I had just said "velocity" and did not indicate a direction (vertical or horizontal), my terminology would have been improper. To go even further, I should have said "positive" vertical velocity so that you knew that the gymnast was moving in the positive (upwards) vertical direction. By just stating "vertical velocity," it could very well have been negative (in which the gymnast would be going downwards). Of course, as coaches, I knew that you understood what I was talking about. But, just stating vertical velocity with no "positive" or "negative" associated with it would not "fly" with the science community, especially if they were not knowledgable of gymnastics.

So, let's talk briefly about some key kinematics terms.

Position - Position is just where you're at in space. In biomechanics, we break

everything into (x,y) coordinates for 2-dimensional analysis and

(x,y,z) coordinates if we're analyzing in 3-dimensions. This is how

we track movement. Basically, the fancy software we use

references everything into a grid system and as the body moves,

those coordinates obviously change. That's the basics.

For 2-dimensions, the math is actually pretty easy, while it gets

nastier for 3-dimensions. Fortunately, the software does all of that

these days.

Displacement - This refers to a change in position in a particular DIRECTION.

So, displacement is a vector. It has a direction associated with

it. (i.e. positive horizontal displacement) The equation is:

final position - initial position (change in position)

Distance is a scalar quantity. It's just a magnitude (how far?)

with no regard for direction.

Here's an example. If I run from one end of the football field to

another, my displacement would be 100 yards. A straight line

from start to finish is always 100 yards. However, my distance

could be well beyond that if I ran a zig-zag the entire length of

the field and not a straight line. So, maybe now I've run 200

yards. Make sense?

Displacement is always measured in meters by the way.

Velocity - This refers to how long it took you to change position. So, there is

a time component. The equation for velocity is the following:

displacement / change in time

So, if I ran in a straight line for 60m and it took me 10s, my

velocity would be 6 m/s. So, in a single second, I was able

to cover 6 meters. (My initial position was 0 and my initial

time was 0)

Acceleration - This refers to how long it took to change in velocity. Or, in other

words, it is the rate of change of velocity. So, did I speed up or

slow down?

change in velocity / change in time

So, if I were moving at a constant velocity of 6 m/s at the start

and increased to 8 m/s after 10s, then my acceleration would

be 0.2 m/s/s.

Velocity at finish = 8 m/s

Velocity at start = 6 m/s

Change = 2 m/s

Time at finish = 10/s

Time at start = 0

Change in time = 10s

So, 2 m/s / 10s = 0.2 m/s/s

This would mean that during each second, I increased my velocity

by 0.2 m/s. So, this is the acceleration or the rate of change in

velocity. And, again, acceleration can be positive or negative and is direction dependent. So, if I throw a ball up into the air, it may be going upwards, but it will slow down as it approaches the top before it comes back down. So, the ball is actually experiencing a negative acceleration. It is slowing down even though it is moving in a positive direction. On the flip side, it will actually be positively accelerating on the way down even though it is moving in a negative direction.

Now, you don't have to be able to calculate this to apply it to gymnastics coaching. But, understanding the concepts is important when you happen to come upon a research article that is actually quantifying these things. You will have the tools to understand the concepts being talked about and you'll be able to put them into coaching concepts and a practical sense.

So, these terms all describe what the body is doing. They are kinematic variables. In my next post, I'll talk more about kinetics. Then, we'll get into angular kinetics and angular kinematics, projectile motion, and eventually move into more gymnastics-specific content.

Let me say, however, that most gymnastics skills are extremely complex and extremely hard to analyze. Few skills have been analyzed because of their complexity. So, if you ask me to analyze a skill, I cannot fully guarantee that I can completely explain why a particular skill is/has to be performed a particular way, but I will do my best to provide some insight and a mechanical rationale.