The force-posture interaction can be quite complex,

just like the other things we've been talking about.

But in this case, it's complex because of the way in which muscles interact with

each other, and with the skeletal lever system to produce the movement.

In this photo, here, you see the change in the length of the lever arm,

with the baseball picture here.

However, many other things also affect the force the athlete can produce.

And we're only going to be able to address a few of these, so

I'll pick the most important ones.

This is an example of the strength curve for

a multi-joint movement that is shown under the curve.

The athlete is lifting a barbell from below the knees to the overhead position.

And notice how muscular force varies over the full range of the joint-motion.

Maximal force is exerted on the barbell depends on the height of the bar.

It is at its maximum when the bar is just slightly above the knee height.

It is lowest when the bar is above shoulder height.

Now, three strategies are used to train muscles when the amount of force they

can produce changes over the range of motion.

And you're trying to train all aspects of that range of motion.

And they include the peak contraction principal.

I'm going to come back and talk about these.

We can use the accommodating resistance strategy or you can focus on accentuation.

So let's see how you implement these approaches.

Start with peak contraction principle.

This is the traditional approach.

And the focus of the strength development is on the weakest point of

the movement strength curve.

And the weakest points are shown here with the blue arrows,

beginning with the movement on one end, when the bar is below the knee and

the movement on the other end when it's above the head.

And if you use the peak contraction principle,

you would design exercises to improve the muscle's ability in

both these positions, to generate the forces that they need,

and to improve the ability to produce force in those two positions.

Now, the accommodating resistance refers to a focus

on maximizing tension throughout an entire range of motion.

And there are two ways of doing this.

Both require specially designed machines.

You've got isokinetic training.

It's popular with physical therapy.

It's really used by lead athletes.

This equipment is expensive.

And the other disadvantages is that it can only control angular velocities

below 3,000 degrees per second.

Athletic movements can be about 5,000 degrees per second.

And as well,

most isokinetic training devices only permit a single joint movement.

Isokinetic training has a very low specificity, therefore,

although it can be of some value when you're rehabbing an athlete

from an injury and you want to target a specific muscle.

There are lever and cam machines that are designed to maximize

muscle activity throughout the range of motion.

However, once again, these exercises typically involve single joint movements.

And this means that they have a low transfer effect.

When you're going to the free weights,

chains are a more recent strategy for varying the training

stimulus throughout the range of motion of the free weight itself.

However, I want to point out the research about the effectiveness of chains on

improving the athletes sport specific strength remains scant.

The idea is that as she's lifting here, she's lifting more and

more of the chain links off the floor.

And the third method is the accentuation.

Now, the definition of accentuate means to focus on something or

make something stand out.

And accentuation in strength training Involves an effort to design

the resistance exercise so it matches the range of motion of the force production.

Here, you see a device designed to accentuate the delivery phase of a discus

throw.

And if you look at the guy who's out there throwing,

you can see that the guy who's on the resistance machine,

their positions look quite similar.

Now, drawing a bow is another excellent example of a need to accentuate

force application of muscles over a range of motion.

In this case, of archery, the goal is to minimize the use of the arm and

shoulder muscles as much as possible while producing a high force to draw the bow and

stabilize the position prior to arrow release.

The muscles used to draw the bow back, stabilize the wrist,

the elbow and the shoulder joints are not very big.

And this means their ability to produce force is limited, and they easily fatigue.

And an archer has to do this hundreds of times during a competition.

Now, biomechanically, the best of line of force is through the bone and joint lines.

However, this is not possible to accomplish because the body and

the head are in the way.

The goal, therefore, is to bring the bone and

joint line to as close as possible as the force line of the arrow.

So you're trying to move everything as close as possible there.

The bow shoulder is a particularly important joint.

It is stabilized by very small muscles.

The lower the torque around the shoulder, the wrist in the other joints,

the lower the required muscle activity needs to be.

The archer attempts to maximize the use of the bones by allowing

the adjacent bones to compress into the sockets, so

less muscle activity is needed to stabilize joint.

And this saves the fatigue on those small muscles.

Now, when a force does not go through a joint, there's no choice but

to use the muscles to stabilized it.

And the correct technique that I've got illustrated here is

drawing the bow you see the shoulder is close to the arrow.

The arrow is the red line.

And in the incorrect technique, the shoulder joint is further away.

Now, according to top archery coaches,

beginners have a tremendously difficult time positioning their

bow shoulder close to the arrow, without bending their elbow.

When they're told to keep their arms straight,

they move their shoulder away from the arrow.

They just can't coordinate those two actions.

Keeping the arms straight and the shoulder close to the arrow

is apparently very, very difficult to accomplish.

Now, elite archers also use another trick.

They recruit their abdominal muscles to pull the rib cage down towards the hips.

And apparently this helps bring the bow shoulder joint closer to the arrow,

while reducing the stress on the arm and the shoulder muscles.

Now, imagine all the intramuscular coordination needed just to draw a bow.

The archer must consciously control three sets of muscles.

They've got to control the abdominal muscles, the wrist and elbow muscles or

the arm muscles and they also have to control the shoulder muscles.

So the shoulder stabilizes the actual shoulder joints.

Now, apparently, it's difficult to design a resistance exercises, or any exercise

that transfers to the type of muscle activity needed while drawing a bow.

And a specially designed piece of equipment similar to the notion of the one

designed for training muscle strength over the final delivery phase for the discus

throw might be useful in not only teaching the correct posture of the archer.

But also for strengthening the small muscles of the arm and

shoulder by allowing the coach to design progressive overload into the equipment.

The bent elbow is a particular problem for young archers to overcome.

They simply don't have the feel,

now the shoulder muscle development, to align the shoulder blades correctly.

Now, this is a really nice example of a problem a coach has when training

an athlete's strength to match the force production needs of the sport movement.

Force-Posture Interaction

Science of Training Young Athletes Part 2

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