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Author(s): David R. Caprette, PhD
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Motor Units and Musculoskeletal System

This slide shows three individuals, each performing a different task. The individual on the left is balancing on one leg in a static state; the individual in the center is riding a bicycle at a leisurely pace; the person on the right is in the process of “exploding” to run a sprint. This variety of performance is made possible by the diversity of specific groups of muscle fiber that comprise our muscle groups. These different groups of muscle fibers are connected to the higher centers of the brain that control all of our bodily movements by motor neurons.

A cluster of muscle fibers that is innervated by a single motor neuron is referred to as a motor unit. Three types of motor units (slow-oxidative, fast oxidative, and fast-glycolytic) have been identified in most animal species, including humans.
A slow oxidative motor unit possesses unique properties (details will follow later) that enable individuals to balance with stability and oppose gravity for relatively long durations without fatiguing. See how long you can stand and balance yourself on one leg.

Fibers of a fast oxidative motor unit contract somewhat faster than those comprising the slow unit and are able to sustain contractions for long durations. Fast oxidative motor units contain high levels of energy generating systems (called mitochondria), which use substrates, such as fats and carbohydrates, in the presence of oxygen to make energy available for muscle contraction. Think of the fats and carbohydrates that you eat as fuels to drive contraction (i.e., something similar to the gasoline that you put into your automobile to drive the pistons). When these units are recruited during activities of relatively moderate intensity (e.g., cycling, distance running, swimming, playing soccer etc.), such activities can be sustained for relatively long duration.

The final motor unit to consider is the fast-glycolytic motor unit. The fibers comprising this unit contract very quickly and a large number of them are linked to each neuron. Thus, these units are called upon to perform intense movements of high power output, as in sprinting or trying to “slam dunk” a basketball. The shortcoming to these units is that the fibers do not possess a high number of mitochondria. Thus, one cannot sustain the activity for long duration. Test yourself to see how long you can sprint when you run all out at full speed.

One final thought. Through evolution and via our recent ancestry, we all possess different levels of expression of these motor units. Most humans inherently express a mixture of the fast and slow units in nearly equal proportion. However, there are individuals, such as those who may excel in a marathon, who express as much as 80% slow fibers (probably due to both genetics and training); whereas, individuals who may excel in sprinting and power events express mainly the fast units.

Question: Can you predict the fiber-typing for either a lion or a cheetah? How about a sloth?

Suggested Reading:
Caiozzo, V. J. ,Haddad, F., Baker, M. J., Herrick, R. E., Prietto, N., & Baldwin K. M. (1996). Microgravity induced transformations of myosin isoforms and contractile properties of skeletal muscle. J. Appl. Physiol.  81: 123-132.

What you see in this slide are three individuals. The one on the left is basically assuming an anti-gravity posture. That individual, while assuming that posture is using (through the nervous system) a series of fibers that we refer to as slow fibers. The slow fibers are innervated by common neurons that innervate each of these fibers. While the individual is standing in the balanced position, the motor units are being activated to enable this individual to assume that posture. If you think this is easy (and it looks like something easy to do), why don’t you try that posture while looking at this slide presentation? You will see that this is no easy task. The individual in the middle is riding a bicycle. I am sure at some stage we have all ridden a bicycle, and it seems pretty easy to do, but if you are going to sustain that bicycle movement over time, it is generally more intense than the posture that the individual is assuming on the left part of the slide. The individual (on the bicycle) recruits fibers that are a little bit different than those slow fibers. These fibers are essential for sustaining our locomotion, whether it is running or, in this case, cycling, for fairly long durations, up to an hour or more. The person on the right is getting ready to perform bursts or power activities, such as sprinting or “exploding” away from the ground, as in jumping and so forth. In those types of movements, a different set of muscle fibers, activated again by motor neurons, are called into play. All the things that are being done here, are being carried out under the overall umbrella of gravity. Gravity is the constant force that is being imposed on the body, acting as a force to pull the legs and the body and more or less anchor the body to the ground.

I want you to think about what gravity means to you. Do you experience gravity in your daily activities? Are certain activities more strenuous than others?