Beginners' Library ( Part II )

Beginners' Library ( Part II )


Take a look at this first;

http://www.youtube.com/watch?v=gJ309LfHQ3M&feature=related


Remember the chemicals used.



How do muscles produce force?



Motor Unit Recruitment aka Motor Unit Summation


As the name implies, this is about the force obtained by recruiting more and more motor units.

Some like to call it Fiber Summation. But, you gotta remember that you can't use a fiber without activating the neuron that controls the fiber, and you can't recruit just certain number of the fibers. Once this neuron is activated, all fibers under control will come into play, which is something I have stated earlier.

When we perform an exercise, say, calf raises, the MUs in our calves are not recruited in a random fashion. They are called upon according to what is known as Size Principle.

Calves have Large MUs which contain larger amount of fibers and small MUs which are of small neurons and few muscle fibers.

How many large MUs and Small MUs there are is genetically determined.

For fine, delicate movements low threshold MUs will be used. The control small numbers of slow twitch fibers.

So, when we calf raise light weights, these small MUs which have low threshold for activation will start working first. After a few warm up sets with light loads, we slam on more plates. As there is a demand for more force, necessary amount of larger MUs are recruited.

Over time, some of the MUs will start to drop out as they become fatigued, hence, more and more MUs will be called upon to keep the weights going.



Rate Coding

The second Mechanism is Rate Coding * Rate Coding refers to how many times motor neurons fire in a certain period of time, the faster they fire, the faster the fibers they control will contract, resulting in greater tension in each fiber, hence, more force output *.

In a given motor unit there is a range of firing frequencies. Remember ? Small MUs operate at a lower frequency range than Larger MUs *.

Within that range, the force generated by a motor unit increases with increasing firing frequency.


MUs you have recruited can handle the loads you are lifting, they won't fire at maximum frequency. But, when you need more force at the same time, you can't recruit anymore MUs, then, the guys working will start to increase their firing frequency to the max.



Tetanus


When fibers are forced to contract before it relaxes completely from previous contraction, they will twitch faster than normal, hence, there will be increased tension/force. In addition, there will also be motor unit summation, that is, if there are MUs left unused, they will be called upon as necessary.

This is known as Tetanus.