Unit 2 – What Is Fatigue?

There are two nervous systems that work together to help create muscle contraction.

The two nervous systems are…

CNS (Central Nervous System) and the PNS (Peripheral Nervous System).

The following steps will walk you through an overview of the process that creates a muscle contraction by working from the CNS to the PNS…



The CNS includes the brain and the start of the nerves that innervate (connect) the muscles. The CNS affects the following in order as a communication system.

  • First Step- The fatigue that is associated with CNS is the excitatory input to high motor centers (high motor centers = the brain).- This means the brain is sending a signal to the required muscles to contract.
  • Second Step- The excitatory drives to the lower motor neurons (pathway to the muscle). - This is simply connecting the electrical signal sent from the brain to the muscle.
  • Third Step- Motor neuron excitability (the amount of stimulation a muscle requires to contract). - This simply represents the amount of needed electrical signal required to allow step 4 to occur.
  • Fourth Step- Neuromuscular transmission (transmission of the electrical signal to the muscle). - This is where the actual connection between the CNS and PNS occurs. This means that the signal goes from being electrical (action potential) to chemical (the release of acetylcholine which is needed for a muscular contraction to occur.)



PNS is all the nerves outside of the spinal cord. The PNS affects the following in a specific order providing the actual muscle contraction.

  • Fifth Step- The Sarcolemma excitability (transmits action potential to the t-tubule) - This is the pathway from neuron to t-tubule’s and sarcoplasmic reticulum. T-tubule’s (a pathway) connect the signal from sarcolemma to the sarcoplasmic reticulum. The sarcoplasmic reticulum is where calcium is stored.
  • Sixth Step- Excitation contraction coupling (when the electrical signal converts to a mechanical response) - This is simply a length of time needed to convert the electrical signal (action potential) to mechanical response. Fatigue would increase the needed time to convert from electrical to mechanical causing a delay in contraction.
  • Seventh Step- Contractile mechanisms (shortening of the muscle) - This is simply the shortening (contracting) of the muscle.
  • Eighth Step- Metabolic energy supply (energy required to perform the task). - This is the amount of usable energy stored in the muscle. This energy plays a part in allowing the muscle to continually contract at a high rate.
  • Ninth- Metabolite accumulation (the waste of muscle contraction) - This means waste products (hydrogen ions and inorganic phosphate) effect future muscle contractions. If there are an increase in these waste products the muscle will not continue to contact as effectively.

Now that we have a background on the depth of the process that is muscle contraction let’s look at an applicable example surrounding how UCL injury can occur due to fatigue.

First we need to note the importance of the muscles surrounding the elbow joint.

Through research Buffi Et Al. states the importance of how musculature surrounding the elbow play a role in stabilizing the elbow…

"Therefore, during this time period, the flexor-pronator muscles were primarily contracting eccentrically while the triceps compartments were contracting concentrically… FCR, FCU, and FDS are likely to be important elbow stabilizers during dynamic tasks."

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What this study and others have shown is that during the throwing motion the flexor muscles located in the forearm work to stabilize the elbow during the throwing motion. By contracting these muscles disperse torque and reduce stress from being applied more directly to the UCL.



When we consider these flexor muscles fatigued, their ability to protect the UCL through contraction is reduced.

When fatigued the process of muscle contraction through the CNS and PNS does not operate as efficiently thus leaving the forearm flexors contraction rate decreased and the UCL more exposed to more direct torque.

To draw the connection more directly we can see that when these flexor muscles are fatigued the UCL absorbs greater force thus leading it to higher risk of injury. UCL damage depending on grade of tearing will typically will result in Tommy John Surgery.

Our goal through workload is to mitigate injury risk. To do so it is obvious the importance of mitigating fatigue. If we effectively manage workload we effectively mitigate fatigue enabling us to keep athletes healthy long term.

In our next units we will cover the calculation and application of workload management.

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