Understanding the muscular system at a cellular level Part 1 - The Basics

We never really stop to think just how amazing the muscular system is in the body. Not only do simple daily activities and physical movement depend on the muscular system, but on a broader scale, how amazing is it that the human body can do so much. The girls in the following video are around 52kg and are lifting near 100kg OVER THEIR HEAD.

Watching this, it is obvious that a rigorous training regimen, a strong motivation, and an amazing neuro-muscular system is to thank here. This type of sport performance involves huge forces acting through lever systems of the skeleton, to move and coordinate all body parts into one synchronized lift. These skeletal muscles are in control by the cerebral cortex, which communicates through motor neurons, and involves energy systems involving quick energy, as well as constant supply of oxygen for these lifts to be possible.

At the most basic level, a strength and conditioning professional is concerned with maximizing physical performance, and is attempting to improve the function and control of the motor unit (functional unit of a muscle), while also considering the cardiovascular, respiratory, neurological, and endocrine systems.

In order for strength and conditioning specialists to maximize the development of their athletes, they must have a basic understanding of not only skeletal muscle function, but the functional units and processes that occur with each movement. This is where the nerd alert hits. If you are into learning, growing, and re-teaching yourself concepts that you may have forgotten. Dive into the following videos and explanations which are foundations for simple take-aways at the end of this article, and in articles to come!

This explanation of the muscular systems is simple and sums up the basic structure of a skeletal muscle well:

In summary of the video, we have the epimysium (outer layer surrounding muscle fascicles), the perimysium (surrounding each fasiculus or group of fibers), and the endomysium (surrounding individual muscle fibers).

Next, a motor unit. This video explains the concept visually and quickly:

As mentioned in the first video above, the two proteins contained at the microscopic level of a muscle are called actin and myosin. Myosin is the thick filament, and actin is the thin filament, which together give muscle a striated appearance. For a muscle contraction to occur, an action potential, released due to a cue from the brain, causes the release of calcium at the muscle level from a structure called the sarcoplasmic reticulum, and causes binding and tension development between myosin and actin. This is perhaps a strange visual to imagine two microscopic fibers sliding past eachother to develop muscle tension to lift your mug of coffee, but some refer to this process as the Sliding Filament Theory of muscle contraction. Below, the video describes it in a step-process nicely:

Put into words, the Sliding Filament Theory states that sarcomeres (see below) slide inward on myosin filaments (thick filament), pulling the Z-lines/discs towards the center of the sarcomere, and thus shortening the muscle fiber.

In the Resting phase of this contraction, there is little calcium present in the myofibril, as it is stored in the sarcoplasmic reticulum (SR). As well, very few of the myosin cross-bridges are bound to actin and no tension is in the muscle.

In the Excitation-contraction coupling phase, the SR is stimulated, and releases calcium, which further binds to troponin (situated on the actin - thin - filament). This causes tropomyosin (which is also located on actin), to shift so that the myosin head can now attach to the actin filament. This is important, because the amount of force a muscle can generate is directly related to the number of myosin cross-bridge heads bound to actin.

The Contraction phase is described in the video above quite nicely, which involved the breakdown (hydrolysis) of ATP to ADP and Pi, catalyzed by ATPase. In order for the myosin head to detach from actin, another ATP must replace the ADP on the cross-bridge site (for the head of the mysosin to detach and re-cock into place). If calcium is available, contractions continue. If not, muscle relaxes.

The Recharge phase describes the process of the contraction phase being able to repeat itself and produce measurable muscle shortening - but only occurs if there is calcium available, as well as ATP to assist in uncoupling.

Lastly, the Relaxation phase occurs when the motor nerve ceases to be stimulated (no brain input). Calcium is then pumped back into the SR, preventing actin and myosin linkage.

Now, this might be super dry to you but if you've stuck with me through all that science, the next part is quite applied and transferable to every day training, and understanding of the muscular system. There happens to be an OPTIMAL length, at which the actin and myosin overlap is enough, but not too much, so as to produce the most force development possible. This is interesting, because when stretched muscle (elongated muscle) tries to contract, there is a lower force potential produced.

Imagine you go for a long stretch before you go sprint. In theory, you would be at the right hand side of the above Force-Length relationship, where very few of your myosin and actin filaments would be overlapping at the microscopic level. When you go to sprint, your brain will send a signal to your muscle to contract, but even with the release of calcium and the availability of ATP, some muscle fibers will not have the optimal overlap, and will fail to contract. Practically, this means that more of a dynamic movement and less static stretching will help you perform your best, especially when the exercise involves a maximal contraction such as a heavy lift, a sprint, or a box jump!

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Essentials of Strength Training and Conditioning. (2008). National Strength and Conditioning Association. Published by Human Kinetics.



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