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Neural Fatigue and Swimming

Several months ago, Dr. Mullen touched on a rarely discussed, but very critical topic: neural fatigue.

“The devil, or forgotten Little Nicky, for optimal race results is neural fatigue. This forgotten and unknown variable rises higher in events with high force production. Make sure the nervous system has achieved proper time to recover. If the nervous system does not recover it will not have adequate time to react and will fail...no good! It is estimated the neural system takes seven to ten times the length of the muscular system to recover. (See Perfect Swimming Warm Down)

What is neural fatigue? Quite simply neurons get tired. It’s still not entirely clear how or why this happens, but we do know that fatigue is more than the muscular and cardiovascular systems. Some theorize neural fatigue is an evolved biological mechanism to prevent us from causing serious damage to our bodies, which is one
application of Dr. Noakes' Central Governor Hypothesis.  One way to address neural fatigue is via proper warm down, as described in the earlier post. Indeed some theorize that easy, over distance training is that much like a slow warm down, as easy swims can balance the nervous system and provide ongoing neural recovery.

Many swimmers train their easy days too hard and thus detract from optimal velocity on their harder days. Trying to squeeze a few seconds faster per 100 on an easy day adds only a negligible benefit for conditioning, but may chip away at the body’s neural readiness for the next hard workout. Alternatively, even if someone can habitually “dig deep” and attain optimal velocities on harder days, short term performance comes at the price of increased neural fatigue. As Gandevia (1996) writes,

“[D]during sustained maximal voluntary contractions, voluntary activation becomes less than optimal so that force can be increased by stimulation of the motor cortex or the motor nerve. Complex changes in excitability of the motor cortex also occur with fatigue, but can be dissociated from the impairment of voluntary activation. We argue that inadequate neural drive effectively 'upstream' of the motor cortex must be one site involved in the genesis of central fatigue.”


Likewise, greater fitness may actually increase one’s susceptibility to neural fatigue, which may partially explain why many athletes suffer setbacks during critical times in the season. When you’re not in peak form, the body is limited by both coordination and fitness. Coordination limits performance when the brain forgets how to optimally use muscles. Think of returning to pool after a layoff…it’s hard to make your muscles sore when your hand repeatedly slides through the water like pure air. Likewise, an undeveloped cardiovascular system limits your ability to work at a high percentage of maximum effort for an extended period. In contrast, peak fitness brings a perfect storm for neural fatigue. Finely tuned coordination and a strong catch and pull mean high recruitment of motor units; cardiovascular fitness means you can operate for an extended duration.

Consider a 2009 study by Ahtiainen comparing strength athletes with non-athletes in a knee extensor test to fatigue. Perhaps counter-intuitively, only the strength athletes demonstrated reduced muscle activation at the end of the experiment. Authors reasoned that “experienced strength athletes were capable to activate their muscles to a greater extent than their non-strength-trained counterparts indicated by neural fatigue during the exercise. Greater motor unit activation in strength athletes than in nonathletes may be due to training-induced neural adaptation, which manifested during fatiguing exercise.”

Additionally, males may also have more fatigue susceptibility than females (Hakkinen 1996). Perhaps due to greater percentage of muscle mass than females, males have greater propensity for neural fatigue and may require recovery (Should Female Swimmers Train Differently than Males).

Summary
The challenge with neural fatigue is how to measure it directly.  Subjective measures like mood, muscle soreness, appetite, and range of motion can sometimes correlate with neural fatigue but once you notice changes you’re probably too late!  Autonomic nervous system readiness can also be measured, but it simply measures the resting state. It doesn’t tell us HOW to address neural fatigue. Here are a few simple ways to address neural fatigue:     

  1. Warm down: (I know, sometimes hard to do with school/work shortly after practice, along with getting thrown out of your lane by the next group beginning practice).
  2. Recovery days easier: Remember, the nervous system requires longer recovery than the musculoskeletal and cardiovascular systems (to the extent we can isolate these systems as independent).
  3. Management of work to rest ratios: Some such as Dr. Rushall advocate ultra-short training, with short sprints and short rests. The total workload may be similar in 30 x 25 at goal pace set with 15 seconds rest compared to 7 x 100, but shorter workbouts are theorized as less impactful on neural fatigue, which is ongoing and not isolated to one workout. Work and rest lengths are especially important on dry-land where “metabolic conditioning circuits” and high rep/high intensity lifts add little to someone already swimming 10, 15, or 20+ hours per week already.
Without advanced measuring tools, neural fatigue is highly conceptual for coaches and athletes. Nonetheless, know that the nervous system plays a role in fatigue and affects the body in ways not obvious on the surface.

References
  1. Gandevia SC, Allen GM, Butler JE, Taylor JL.  Supraspinal factors in human muscle fatigue: evidence for suboptimal output from the motor cortex.  J Physiol. 1996 Jan 15;490 ( Pt 2):529-36.
  2. Ahtiainen JP, Häkkinen K.  Strength athletes are capable to produce greater muscle activation and neural fatigue during high-intensity resistance exercise than nonathletes.  J Strength Cond Res. 2009 Jul;23(4):1129-34. doi: 10.1519/JSC.0b013e3181aa1b72.
  3. Häkkinen K.  Neuromuscular fatigue in males and females during strenuous heavy resistance loading.  Electromyogr Clin Neurophysiol. 1994 Jun;34(4):205-14.
By Allan Phillips. Allan and his wife Katherine are heavily involved in the strength and conditioning community, for more information refer to Pike Athletics.
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