Understanding Muscle Force Production

Although mentioned in a few articles in passing, I have not covered the subject of force production in the muscles to create power against varying levels of resistance. Although the general anatomy of the muscle and the exact physiological workings of the muscle contraction are explained well in my Muscle Contraction article, what is not explained is through what criteria muscle force production occurs. In this article we will explore force production, the phases of single twitch tension production, and more importantly, how our muscles gradate force depending on context/resistance.

What is force?

The term “force” is often used interchangeably with “power”. However, in terms of an applicable definition, we can simply say it is the energy to enact change, or, in this case, movement [1].

How do muscles produce force?

Muscles produce force by contracting muscle fibers and creating tension; now, while this has been explained mechanistically in the Muscle Contraction article, we will now go into a bit more of understanding the criterion method of the total muscle belly as opposed to focusing on a single muscle fiber. However, to understand this in its entirety, it would be advantageous to also educate oneself on the anatomy and mechanistic function of the muscle fiber described in the Muscle Contraction article.

That said, muscles produce force (aka, enough energy to create movement) by creating tension in the muscle fiber. It does this by moving the myosin chain toward the Z-disks (ends) of the sarcomere. Once the myosin chain has reached the Z-disks, the sarcomere, and by association, the muscle fiber is fully contracted. Now, there is no “in-between” as in a partial contraction within a single muscle fiber; muscle fibers either contract or do not contract [2]. Why is this the case?

This is the case, because once the muscle fiber is stimulated, it releases and reabsorbs maximal amounts of all the ingredients necessary for contraction to take place. As such, the movement of the actin-myosin chain is considered an “all of none” principle. In summary, the nerve that connects to the muscle fiber must reach a certain level of activation, and once reached (called “reaching threshold”), the nerve activates the muscle fiber fully, and once activated, the muscle fiber releases a series of chemicals fully to force the movement of the myosin-actin chains which, in turn, move fully across the sarcomere to each end [3].

 

This is further explained by looking into the three phases each muscle fiber goes through during a “twitch”.

The top diagram indicates the threshold met (+30mV) by the neural impulse to move signaling forward to the muscle fiber.

The blue section in the lower diagram shows the latent period with no tension as the fiber prepares to contract. 

The green sections of the lower diagram indicate contraction of the fiber by moving the myosin chain toward the ends of the sarcomere, the Z-disks, as seen in the image above.

Latent Phase

This is the period of time in which the muscle fiber is receiving the message from the neural impulse. The period in which the impulse travels across the terminal cisternae, releasing calcium, etc – basically the “preparatory” step before contraction of the fiber [5].

Contraction Phase

This is the physical act of the fibers contracting via the movement of the myosin-actin cross bridge moving myosin closer to the Z-disks of the sarcomere [3][4]. This step takes anywhere between 10-100 ms, depending on fiber type [5][6].

Relaxation Phase

The act of moving the myosin chain back to its original position, away from the Z-disks, back to the original tension [4][5]. Time of occurrence is highly dependent on circumstances as we will soon understand.

Now we should be clear on the facts that muscle fibers contract to produce force via “twitching”, they can only contract maximally or not at all, and the different phases for contraction, as well as the estimated time for each phase to occur.


How do muscles produce maximal and submaximal force?

So, we should now fully understand how a single muscle fiber contracts to produce tension and in turn produces force. However, this still leaves a large question unanswered: if the muscle fibers can only produce force maximally, then how does the body gradate power output to attenuate varying degrees of need? For example, how is the body able to decrease force production to pick up a pencil and increase it to lift a heavy box off of the ground?

There are two answers to this question: recruitment and summation [5].

Recruitment

While it still holds true that each muscle fiber, when activated, is fully contracting, the body is able to still create a type of gradation in the amount of total force produced by simply recruiting more or less fibers [2][5].

For example, the body would, hypothetically, recruit 10 muscle fibers contracting at 100% to lift a pencil, and use 1000 muscle fibers contracting at 100% to lift the aforementioned box.


Summation

On the other hand, it does not simply boil down to the amount of recruitment, but also on the activation. Again, it is true that activation is maximal, but the frequency of activation can be gradated. So, fibers can be twitched causing a contraction, but instead of allowing the fiber to relax, it is possible to “twitch”, activate the fiber again to maintain contraction and not allow it to relax [5]. This means that there is a sustained contraction until fatigue sets in which is called tetanus [5].

Increased stimulus leads to more frequent tension/contraction and less frequent relaxation until maximal tension is achieved (tetanus) and the fiber tires.

So, in review, varying degrees of force are established by the increase or decrease of muscle fibers, in a muscle, being stimulated and by the frequency of stimulation up to maximal frequency called tetanus.

SUMMARY

Overall, we spoke about how force was just another way of describing power production leading to movement (change), and the body being able to do that by contracting the muscle fibers maximally, but in cases of needing less or more force production by the muscle belly, the body can increase or decrease the number of muscle fibers stimulated, as well as the frequency of stimulation to create maximal or submaximal force production.

Writer: Nicolas Verhoeven

                                                                                                Citations

[1] Force | Definition of Force by Merriam-Webster. (n.d.). Retrieved from http://www.merriam-webster.com/dictionary/force

[2] Lab #9: Muscle Physiology [PDF]. (n.d.). Retrieved from http://www.indiana.edu/~nimsmsf/P215/p215notes/LabManual/Lab9.pdf

[3] Verhoeven, N. A. (2015). Omni Focus Fitness. Retrieved from http://www.omnifocusfitness.com/#!muscle-contraction/c1sb1

[4] Human Physiology - Muscle. (n.d.). Retrieved from http://people.eku.edu/ritchisong/301notes3.htm

[5] Exercise 6: Muscle Physiology II – Twitch & Summation [PDF]. (n.d.). Retrieved from http://www.cabrillo.edu/~mhalter/Biology5/6_FingerTwitch.pdf

[6] Verhoeven, N. A. (n.d.). Omni Focus Fitness. Retrieved from http://www.omnifocusfitness.com/#!muscle-fibers/z2paj

 

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