Part 4 - Stretch-shortening cycle

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From the course by University of Florida

The Science of Training Young Athletes

188 ratings

University of Florida

The Science of Training Young Athletes

188 ratings

Seventy percent of kids drop out of sports before their high school graduation. Only 15% leave because they feel they are not good enough. Almost 70% leave because they were not having fun, or due to problems with the coach. Injuries cause 30% to give up sports. This course is packed full of practical sports science information that provide youth coaches and parents with the practical pediatric sports science insights to successfully retain young athletes and develop their sport potential while avoiding injury and overtraining. We begin by examining the multidimensional nature of coaching, the relevant sport motor performance abilities, the impact of growth and development on motor skills, the gene versus practice controversy, and briefly overview the body structures strengthened through training. Then we explore the athlete's energy supply, where this energy comes from, and how it matures along with the athlete. Finally, we examine the development of strength, power, anaerobic capacity, coordination and flexibility through the life span. The optional text manual for this course is available at: http://www.learnitez.com/HighPerformanceScience/manuals/

From the lesson

Week 4: Enhancing the athlete’s physical work capacity

The level of expertise with which an athlete is able to perform sports skill depends on how well the coach molds the correct ratio of endurance, strength, power and speed to meet the demands of the sport. In this section you will learn the science behind developing these important motor performance abilities as the young athlete moves through puberty.

Part 1 - Introduction to the physiology of strength and power6:10

Part 2 - Muscle components affecting strength and power4:40

Part 3 - Intrinsic sensors8:11

Part 4 - Stretch-shortening cycle6:49

Part 5 - Strength and muscle fiber type4:42

Part 6 - Key points of the physiology of strength2:23

Meet the Instructors

Dr. Chris Brooks
Instructor
Coaching Science Coordinator for USA Track and Field

Over 100 years ago it was noticed that a countermovement

produced a higher jump than when a countermovement was not used.

A counter movement is a movement in the opposite direction to the goal direction.

This was initially called the wind up movement, and

in 1979 it was renamed the stretch-shortening cycle.

The stretch-shortening cycle occurs when the active muscle is first lengthened,

which is called an eccentric contraction, immediately before shortening.

And this shortening is called the concentric contraction.

Several possible explanations are given for

this stretch shortening cycle effect on force production.

And the first is motor units and muscles work more effectively with

the pre-stretch, and therefore more power is produced.

The second reason is that a countermovement permits time for

the central nervous system to fully activate the sarcomere contractile

machinery.

The third reason, the elastic protein woven throughout the muscle fiber and

the tendon stores energy during the ecentric contraction phase,

and then reuses that energy during the concentric contraction phase.

And the muscle spindle stretch occurring during the countermovement

triggers spinal reflexes that increase the muscle contraction to

a level above what is possible when no pre-stretch occurs.

And finally, the pre-stretch or the active muscle potentially alters the properties

of the sarcomere's contractile machinery, and this will enhance the force produced.

This enhancement is called potentiation and

increases with the speed of pre-stretch.

The word potentiation describes the enhancement of one agent by another so

that the combined effect is greater

than the sum of the effect of each agent by itself.

It's a word that's commonly used to describe the interaction between two or

more drugs.

In our case, the word has been stolen from the drug industry, so

to speak, and used in reference to the interaction between

the stretch of the tendon and the enhanced force outcome.

The stretch-shortening cycle is particularly useful for

storing energy in a tendon.

That in turn enhances the maximal force production, and

we talked a little bit about this already.

Energy stored in the stretched achilles tendon, for

example, permits jumpers to produce a higher force at take off.

Throwers use the elastic tissue woven into their chest and shoulder muscles

by pre-stretching the relevant muscles just before final implement delivery.

And a highly developed and effective stretch shortly cycle in the quadriceps

muscles improves the athletes reactive leg strength by stiffening the touchdown leg.

Athletes with a stiffer leg can run faster, jump higher,

or longer, due to the shorter ground reaction time.

One theory about the value of plyometric training is that it

teaches the brain how to quickly recruit the fast type II fibers.

Another theory is that plyometric training teaches the brain how to better predict

the force of landing, enabling it to set up the relevant muscles into

a higher level of pre-activity before the foot touches the ground.

If you ever encountered an unexpected step down and your knee gave way,

it is because the brain was not prepared to stimulate to required muscular

force to stop the knee from collapsing due to the force of gravity.

And the idea that the brain learns to pretense

relevant muscles before the foot strikes the ground comes from drop jump studies.

Athletes who have the highest jump after they drop off a bench also have

a very active vastus lateralis muscle before ground contact.

Now here's a graph that is based on the data from this particular study.

The green line is the activity of the vastus lateralis muscle

during the highest jumps.

And the black line is the activity of

vastus lateralis muscle during the poorest jumps.

And here is the point of touchdown.

And this is the takeoff point.

And as you can see here, with this little arrow,

that the vastus lateralis muscle was quite active well before touch down when

the jumps with very high and had a lower activity when the jumps were quite low.

Vastus lateralis muscle is also much higher in its activity throughout

the ground contact phase for those subjects who had the highest jumps.

The active vastus lateralis muscle has the effect of stiffening the leg and

this results in a far stretch reflex that in turn results in a higher jump.

The stretch-shortening cycle is also important for

helping accelerate legs when there is no contact on the ground.

When clearing a hurdle, for example, the hip flexor stretch reflex

is thought to help accelerate the leg through for the next stride.

Another example is a long jumper who also uses the hip flexor stretch reflex for

a powerful swing leg at takeoff.

The shot and discus and javelin thrower use the stretch reflex

in the chest muscles to accelerate the arm during the final delivery phase.

In all these cases shown here the limbs are not physically connected to the ground

and the stretch reflex serves to magnify the natural force of the muscle.

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