Neuromuscular Control And Fatigue With Roger Enoka PhD

In this episode of Science of Ultra, Dr. Shawn Bearden interviews Dr. Roger Enoka, a leading expert in neuromuscular control from the University of Colorado. They delve into the mechanisms of neuromuscular control, focusing on motor units and their role in muscle activation and fatigue. Dr. Enoka explains the differences between eccentric and concentric contractions and their implications for ultramarathon runners, particularly regarding muscle damage and fatigue. They also discuss the concept of perceived fatigability versus performance fatigability, emphasizing the importance of feedback signals from the body during endurance events. Additionally, the conversation touches on training adaptations, the impact of aging and sex differences on muscle performance, and practical strategies for improving neuromuscular control, such as downhill running to enhance motor neuron activation and extracellular matrix resilience.

• Guest Introduction:
  • Roger Inoka, PhD, is the director of the Neurophysiology of Movement Laboratory at the University of Colorado.
  • Expert on neuromuscular determinants of movement in health and disease.
  • Authored numerous research articles and books on neural mechanics, human movement, and fatigue mechanisms.

"Essentially, I'm interested in how the nervous system controls human movement. And this arose from when I was a young man and interested in sports and various athletic activities."

• Dr. Inoka's interest in neuromuscular control stems from a personal background in sports and biomechanics.

• Motor Neurons and Motor Units:
  • Movements are controlled by activation signals from the spinal cord to muscles via motor neurons.
  • Each motor neuron activates a few hundred muscle fibers, forming a motor unit.

"The anatomical connection between one motor neuron that's one nerve cell and its muscle fibers is known as the motor unit."

• The motor unit is the final common pathway for activation signals to muscles.

• Rate Coding and Motor Unit Recruitment:
  • Force is controlled by varying the number of activated motor units (recruitment) and the rate of activation signals (rate coding).
  • Activation signals are electrical impulses known as action potentials.

"The rate at which these are sent from the spinal cord can vary from very low rates of about five of these electrical signals per second, up to 100 or so."

• The combined rate and number of action potentials determine muscle force production.

• Size Principle:
  • Motor units are activated in a stereotyped order from smallest, weakest, and slowest to largest, strongest, and fastest.
  • This principle is based on the intrinsic properties of motor neurons.

"When your brain decides that it wants to have one of your muscles produce a force, it sends down an activation signal to the spinal cord. And these motor units are activated in a relatively stereotyped order."

• The size principle ensures efficient and graded force production without conscious effort.

• Types of Contractions:
  • Concentric contractions involve muscle shortening when force produced is greater than opposing force.
  • Eccentric contractions involve muscle lengthening when force produced is less than opposing force.

"If the activation signal produces a muscle force that is greater than the opposing force, the muscle will shorten. And conversely, if the activation produces a force that's less than the opposing force, the muscle will lengthen."

• The type of contraction depends on the relative forces involved and the required movement.

• Gait Cycle and Muscle Contractions:
  • Running involves both concentric and eccentric contractions.
  • Uphill running emphasizes concentric contractions, while downhill running emphasizes eccentric contractions.

"If we're looking at some of the larger groups, say of the thigh muscle groups of the thigh, if we're going uphill, we're thinking about a lot of concentric contractions and we're going downhill, especially if we're running downhill fast, there's potentially a greater amount of eccentric load."

• Understanding these differences can help in training and managing fatigue during different running conditions.

• Force Generation and Fatigue:
  • Force generation capacity varies between eccentric and concentric contractions.
  • Eccentric contractions can produce more force but may lead to different fatigue mechanisms compared to concentric contractions.

"When it's shortening, the amount of force that it can produce is less than the amount of force it can produce when it's being lengthened."

• The different force capacities and fatigue mechanisms need to be considered in endurance training and performance.

These notes encapsulate the key ideas and discussions from the transcript, providing a comprehensive overview of neuromuscular control principles, force generation mechanisms, and their application to running.

• Structural components within muscle fibers experience greater stress during the lengthening phase.
• Muscles are more prone to damage during eccentric (lengthening) contractions compared to concentric (shortening) contractions.

"The structural components within the muscle fiber experience greater stress during the lengthening phase."

• Explanation: This highlights why eccentric contractions can cause more muscle damage due to increased stress on muscle fibers.

• Recent research indicates a predominance of central fatigue over peripheral fatigue after ultramarathons.
• Central fatigue refers to fatigue originating in the central nervous system, while peripheral fatigue originates in the muscles.

"There seems to be, after an ultramarathon, a predominance of central fatigue over peripheral fatigue."

• Explanation: Central fatigue is more significant than peripheral fatigue in ultramarathon runners, suggesting the brain and nervous system play a larger role in prolonged endurance events.

• Fatigue should be considered as a symptom, similar to pain, and measured through subjective reports.
• The traditional division of fatigue into central and peripheral categories is being reconsidered.

"The most reasonable way to think about fatigue is to think of it as a symptom. And it's parallel to pain."

• Explanation: Fatigue, like pain, is subjective and should be measured by asking individuals about their experience.

• Performance fatigability involves physical declines in muscle activation or force capacity.
• Perceived fatigability involves the subjective experience of fatigue influenced by homeostatic and psychological factors.

"Performance fatigability... could be due to changes within the muscle related to the contractile properties or due to the activation signal that the muscle receives from the nervous system."

• Explanation: Performance fatigability is measurable through physical tests and is influenced by muscle and nervous system factors.

"Perceived fatigability... has to do with mechanisms that challenge the integrity of the performer, such as homeostasis or psychological factors."

• Explanation: Perceived fatigability is the subjective experience of fatigue and is influenced by the body's internal state and mental factors.

• Feedback signals from the body, such as substrate depletion and dehydration, influence perceived fatigability.
• These signals help the runner make decisions about pacing and energy expenditure.

"It's these feedback signals that allow the performer, the runner in this case, to derive these perceptions."

• Explanation: Feedback from the body informs the runner's perception of fatigue and influences their pacing strategy.

• Ultramarathons involve significant mental challenges in addition to physical ones.
• Runners experience cycles of energy and fatigue, influenced by central perception mechanisms.

"Ultra marathons are 50% physical and 90% mental."

• Explanation: Mental endurance plays a crucial role in ultramarathons, often more so than physical endurance.

• Effective pacing strategies can help runners avoid extreme lows in energy and maintain performance.
• Understanding and managing perceived fatigability is key to optimizing pacing.

"How can you figure out the right pacing strategy or effort strategy to try to not hit those lows, so low that you have to back so far off to make a recovery?"

• Explanation: Proper pacing strategies are essential to avoid significant drops in performance and maintain a steady effort throughout the race.

• Early improvements in running form are largely due to neuromuscular adaptations.
• These adaptations include learning and optimizing muscle recruitment and order of activation.

"The initial gains in strength were largely due to adaptations in the activation signal sent from the nervous system."

• Explanation: Early improvements in running efficiency are primarily due to the nervous system learning to better control muscle activation.

• Continued neuromuscular adaptations can potentially sustain performance in long-duration events.
• Research is needed to explore the extent of these adaptations in endurance sports.

"I don't think anybody has really looked at this in long duration events, about the relative potential of adjustments in neural activity to sustain performance."

• Explanation: There is a gap in research regarding long-term neuromuscular adaptations in endurance events, indicating a potential area for further study.

• Strength training results in increased muscle size or reduced intramuscular fat.
• Running economy is not significantly influenced by changes in neural control.

"Most of the subsequent increases in strength are largely due to increases in muscle size or reductions in intramuscular fat and so on."

• Muscle strength improvements are more related to physical changes than neural adaptations.

• Downhill running involves eccentric contractions with different neural activation signals compared to controlled weight lowering.
• Fast downhill running can improve motor neuron activation rates.

"When I'm running downhill and I'm doing an eccentric contraction to accommodate foot strike, there's no control there. It's just an activation signal, a big burst of activity that turns the muscle on."

• Downhill running does not require precise motor unit control but a burst of neural activity.

"Training with fast contractions or fast running downhill can indeed improve the ability of motor neurons to generate these activation signals very quickly."

• Downhill running can enhance the nervous system's ability to rapidly generate activation signals.

• Studies show sex differences in fatiguing contractions, but these are based on artificial tasks.
• Real-world activities like running have not been extensively studied for sex differences.

"There are statistically significant differences between men and women in the kinds of adjustments."

• Laboratory studies show sex differences, but their relevance to real-world activities is unclear.

"Given that the body types tend to be more similar for ultra marathon people between men and women... there's less likely to be a big influence of sex differences."

• Ultra marathon runners' similar body types may reduce the impact of sex differences.

• Active older adults maintain muscle mass equivalent to young adults.
• Continuous physical activity is a countermeasure to age-related muscle loss.

"Master athletes who have been active their entire lives... their muscle mass is equivalent to young adults."

• Lifelong activity helps preserve muscle mass and strength in older adults.

"If people remain active for their entire life, this is a great countermeasure to this age-related loss of muscle mass and decrease in strength."

• Regular activity is essential to combat age-related declines in muscle mass and strength.

• Older individuals can gain muscle mass but require higher relative effort in workouts.

"Older individuals are capable and able to gain muscle mass. One of the main issues is that the relative effort in workouts to gain that equivalence is much higher."

• Older adults must exercise at higher intensities to achieve similar muscle gains as younger individuals.

"When we measure physical performance in old adults, we're also measuring the influence of reduced daily levels of activity."

• Reduced activity levels in older adults affect their physical performance measurements.

• Muscle cramps originate in the nervous system, not the muscle.
• Various antidotes for cramps work by stimulating the nervous system.

"Muscle cramps begin in the nervous system. They do not begin in muscle."

• Cramps are a result of increased neural activity sent from the spinal cord to the muscle.

"When the player gets cramped in the middle of the game, they give them a glass of vinegar to drink, and the cramps go away."

• Substances like vinegar can alleviate cramps by affecting the nervous system.

"A muscle cramp represents a transient increase in the amount of activity sent from the spinal cord to the muscle."

• Cramping is due to a temporary surge in neural signals to the muscle.

• Muscle cramps involve sensory receptors that send inhibitory signals to spinal motor neurons, reducing their activation.
• The exact evolutionary reason for this wiring is not clearly understood.

"It's like when a person has a disease, like multiple sclerosis, for example, why does one muscle group get affected more so than other muscle groups? It's the same kind of a question."

• The quote highlights the complexity and specificity of muscle cramps, comparing it to neurological diseases.

• Caffeine is used to influence perceptions of fatigue, especially in endurance events.
• Studies on caffeine's effect on long-duration events are limited.
• Caffeine's impact may be related to circadian rhythms and variations in wakefulness.

"Variation in wakefulness is one of the factors that can influence perceived fatigability."

• This quote explains how fluctuations in wakefulness can affect perceived fatigue and how caffeine might modulate this.

• Different muscle groups may have varying feedback mechanisms to the brain.
• Theoretical constructs in physiology are poorly defined, limiting understanding.

"If I'm running an ultramarathon, and I'm getting fatigued... is that mainly due to, for example, an increase in core temperature, or is it mainly due to dehydration?"

• This quote underscores the complexity of determining the causes of fatigue and the need for better-defined physiological constructs.

• During isometric contractions, motor units may stop being active, and new ones are recruited.
• The nervous system compensates for fatigue by activating additional motor units.

"To keep the same force, the nervous system has to activate additional motor units."

• This quote explains the mechanism by which the nervous system maintains force during fatiguing contractions.

• Downhill running can provide a good training stimulus for both motor neurons and the extracellular matrix.
• Strengthening the extracellular matrix may reduce the likelihood of damage during long-duration runs.

"When the runners train by going downhill quickly, that not only does this provide a good stimulus for the motor neurons, it also provides a good training stimulus for the extracellular matrix."

• This quote emphasizes the dual benefits of downhill running on neuromuscular control and connective tissue strength.

• The discussion ties together various aspects of neuromuscular control and training.
• The importance of understanding and incorporating different training stimuli is highlighted.

"It's wonderful that you touched on that to sort of start tying some of these threads together and weave a better picture for people and how it all comes together."

• This quote reflects the aim to provide a holistic understanding of how different training methods can benefit runners.