Fatigue Factors

We’ve all heard the quote “Fatigue makes cowards of us all.” 

Nowhere is this more true than in sport. Preventing or resisting fatigue is the whole reason sports training even became a discipline. 

• But what is fatigue? 

• What are its causes? 

• Are there different forms of fatigue? 

• If so, what are they?

Steve Magness wrote a great post on the origins of fatigue and what’s called “central governor theory.” Summing it up, fatigue is a perception, rather than a physiological process and is dictated by our brain trying to protect us.

Physiological Causes

That being said, let’s dive into the different physiological mechanisms that cause fatigue.

During training or competition, our bodies can experience two different forms of fatigue - central and peripheral.

Peripheral fatigue occurs at the muscle fiber and cellular level - i.e. past the neuromuscular junction where synapses become actions. 

Central fatigue occurs in the spinal cord and central nervous system - i.e. before the neuromuscular junction where synapses are still just action potentials.

The third major factor in the development and regulation of fatigue is temperature - or thermoregulation - essentially how your body reacts to stress by changing its core temperature to meet the demands of the activity. The following excerpts are pulled directly from my Coaches' Conditioning Course.

Peripheral Fatigue

Peripheral fatigue is defined as the loss of contraction force or power caused by processes distal to the neuromuscular junction.

This is primarily due to biochemical changes.

As cellular homeostasis is disrupted by training, especially once intensity increases above Lactate Threshold 2, it results in a cascade of biochemical affects:

1. Blood lactate accumulations > 4 mmol/L

2. Accumulation of hydrogen ions

3. Accumulation of ammonia in the blood

4. Accumulation of heat, leading to increased sweat secretion

These collective changes lead to increases in heart rate, respiratory rate, and eventually dehydration.

Muscle fiber changes

Similar to cells, muscle fiber function is impacted significantly by intensities above LT2 as follows: 

1. Accumulation of inorganic phosphate causes a decrease in contractile force

2. Accumulation of hydrogen ions can also cause a decrease in contractile force

3. Decline of glycogen stores (in extreme cases) and blood glucose

Environmental changes to muscle fibers all generally affect fibers' ability to contract quickly and with appropriate force.

Central Fatigue

Central fatigue is defined as the loss of contraction force or power caused by processes proximal to the neuromuscular junction

CNS effects

Fatigue can also present with central nervous system function (hence the term "Central Fatigue"). The following are some causes of central fatigue and their effects.

1. Neural drive can be affected by mechanoreceptor feedback in Type IA and Type II muscle spindles or Type IB Golgi tendon organs

2. Stimulation of different types of nerves can cause a decrease in motor neuron firing rate and motor output inhibition

3.  Accumulation of serotonin, GABA, glutamate and dopamine

As far as the downstream effects of central fatigue, these include: loss of motor control, coordination, and deficits in cognition.

Thermoregulation

In an effort to maintain homeostasis during exercise, the body works hard to maintain a steady core temperature.

As core temperature increases up to 40º C (104ºF), central motor drive decreases.

This is potentially due to two factors:

1. cardiovascular system stress limiting blood flow to the brain (lower blood flow leading to a decreased ability of the brain to dissipate heat)

2. increased temperature causing sensations of fatigue and increased perceived exertion

One important thing to note: in studies involving thermoregulation and exercise, an enzyme called NO-synthase (nitric oxide synthase) which increases oxygen delivery through stimulating vasodilation was artificially decreased to simulate exercise intensity.

Essentially this demonstrates that athletes with more developed aerobic systems are generally less susceptible to the effects of heat stress during training and competition than athletes with less developed aerobic systems.

Takeaways

The more I learn about physiology and biochemistry, the more important the aerobic system becomes. This is more than confirmation bias - this is concrete evidence that the direction our field needs to move is toward more volume, better low - moderate intensity conditioning methods, and a disregard for the minimalist trends that are trying to take over.

Come back next week and we’ll talk more about how our bodies perceive fatigue and steps we can take to improve this perception.

Thanks for reading,

Tim

PS my good buddy Glenn Revell started a podcast! It’s called The Gray Matters and we think you guys will dig it. Give us a listen on Spotify!