Upper Limits of Performance

Two possibilities for such a "governor" or controller have been proposed. One school favors limitations imposed at the central nervous system level (a central governor) while the other feels the throttle on energy output is located peripherally, within the muscle cells or heart cells themselves. The debate between these two schools, ongoing for several decades, rages on. Why would evolution favor such a governor? The speculation is that excessive activity risks injury to the heart or the active muscles, presumably from low arterial oxygen levels or the byproducts of metabolism.

The suggestion of the existence of a "peripheral governor" was proposed in 1924 and is well articulated in Wikipedia, with references. I will quote directly below.

First is the cardiac protective rationale: "The 1922 Nobel Prize in Physiology or Medicine winner Archibald Hill proposed in 1924 that the heart was protected from anoxia in strenuous exercise by the existence of a governor...the heart is able to regulate its output, to some extent, in accordance with the degree of saturation of the arterial blood...we suggest that, in the body (either in the heart muscle itself or in the nervous system), there is some mechanism which causes a slowing of the circulation as soon as a serious degree of unsaturation occurs, and visa versa. This mechanism would tend, to some degree, to act as a 'governor', maintaining a reasonably high degree of saturation of the blood: the breathing of a gas mixture rich in oxygen would produce a greater degree of saturation of the blood and so allow the output to increase until the 'governor' stopped it again. We realize the danger of a hypothesis partly suggested by teleological reasoning: in this case, however, we can see no other explanation of our experimental results..."

Then an alternative speculation on a peripheral muscle protective function: "...further research upon exercise fatigue was modeled in terms of it being due to a mechanical failure of the exercising muscles ("peripheral muscle fatigue"). This failure was caused either by an inadequate oxygen supply to the exercising muscles, lactic acid buildup, or total energy depletion in the exhausted muscles..."

The more recent theory, suggesting a "central governor", was first articulated by TD Noaks, and is again well represented in Wikipedia which I will quote. "....the power output by muscles during exercise is continuously adjusted in regard to calculations made by the brain in regard to a safe level of exertion. These neural calculations factor in earlier experience with strenuous exercise, the planning duration of the exercise, and the present metabolic state of the body. These brain models ensure that body homeostasis is protected, and an emergency reserve margin is maintained. This neural control adjusts the number of activated skeletal muscle motor units, a control which is subjectively experienced as fatigue. This process, though occurring in the brain, is outside personal control....the rising perception of discomfort produced by exhausting exercise progressively reduces the conscious desire to over-ride this control mechanism, which, if it were to be reduced, would lead to the recruitment of more motor units. Thus the presence of conscious over-ride would be undesirable because it would increase or maintain the exercise intensity, thereby threatening homoeostasisÉ as exercise performance is centrally regulated by the CNS, then fatigue should no longer be considered a physical event but rather a sensation or emotion, separate from an overt physical manifestationÑfor example, the reduction in force output by the active muscles. Rather we now suggest that the physical manifestation of any increasing perception of fatigue may simply be an alteration in the subconsciously regulated pace at which the exercise is performed. Hence the novel suggestion is that the conventional understanding of fatigue is flawed because it makes no distinction between the sensation itself and the physical expression of that sensation which, we suggest, is the alteration in the subconsciously regulated pacing strategy consequent on changing motor unit recruitment/derecruitment by the CNS."

Thus he suggests that this central governor is a subconscious process based on prior experience (training) as well as ongoing inputs from various internal sensors, and can have the effect of decreasing the number of motor units being stimulated at any given time (via outputs of the motor neuron system). These inputs, the internal measures that are your body's monitors (your dash board) of how the "engine" is functioning might include such things as "..lactate levels, VO2max, heart rate, heart-rate variability, rapid morning heart rate, recovery heart rate, hormone levels (cortisol, testosterone, etc.), red cell counts (hemoglobin, hematocrit, red cell indices), immunity (white blood cells, interleukins, inflammation), muscle damage (creatine kinase, oxidative stress), blood pressure, and much more."

And there is good support for this idea in several scientific studies. This study nicely demonstrates the anticipatory aspect of a decrease in power output. To quote: "Thus, reduced power output... during self-paced exercise in the heat occurs before there is any abnormal increase in rectal temperature, heart rate or perception of effort. This adaptation appears to form part of an anticipatory response which adjusts muscle recruitment and power output to reduce heat production, thereby ensuring that thermal homeostasis is maintained during exercise in the heat." This was confirmed more recently: "Thus, a hyperthermic-induced anticipatory reduction of muscle activation may have occurred during the hot exercise trials only."Of even more interest is the observation that we do have some conscious control over this central inhibition of muscular activity. In this study, which demonstrated a decrease in power output over a series of sprints in warm, humid conditions, revealed "...the ability of subjects to return power output to near initial values during the final of six maximal effort sprints." This is an excellent example of perceived exertion as a conscious awareness of the central governor - and our ability to override it for short periods. And it is all tied together with pacing as a strategy to balance the needs of the central governor to protect our CV and muscular system while leaving some reserve for that extra effort.

The most intriguing aspect of a central mechanism is that it provides a rational explanation for (and thus some ideas to treat and prevent) a diffuse set of overexercise and fatigue syndromes: "this new interpretation is the first to allow a more reasonable description of a number of phenomena that defy rational explanation according to the traditional "limitations" models of fatigue. These include, among many others, the chronic fatigue syndrome, in which affected individuals experience evident fatigue at rest, and the role of psychological and motivational factors, centrally (brain) acting pharmaceutical agents, hypnosis, shouting or sudden unexpected gunshots, or other forms of distraction including music or premeditated deception on human exercise performance."

After reading multiple papers and arguments both pro and con, I suspect that the reason neither camp has proven their point is that we dealing with a mixed system.

1. First, there is a centrally moderated, conscious governor that limits muscle over activity. As an example, when muscle pain occurs, we limit our activity. Likewise if we perceive an oxygen debt with its attendant generalized dysphoria (discomfort) we slow down involuntarily, even if just for a second.
2. Then there is a second, subconscious, central governor. When there is input (from internal sensors) of a need for more cardiac output for example, the heart rate involuntarily increases. Likewise with anxiety or stress our heart rate increases without our control. And we have no (or at least very little) control over these autonomic and subliminal changes.
3. And finally, changes at the peripheral level limit performance. Put a muscle in a more acidic medium, measure its contractility, and you will see changes. So local changes, for example areas of poorer arterial perfusion of actively contracting muscles that then become anoxic (and acidotic) earlier , lead to a deterioration in maximal performance.

Thus rather than a single unifying director or governor, it is the interplay of several central and peripheral processes that define our limits - a "virtual" governor if you will.

The peripheral barriers to improved performance can be modified with training (more vessels to the muscles which then lead to less anoxia and acidosis for any level of exertion and more efficient use of glycogen).

The conscious central ones can also be improved by training - you experience the discomfort of the exertion associated with interval training and the next time you find you can push on a bit further into the discomfort. You know someone else has ridden at that pace so you can suppress your body's negative "reports" i.e. dysphoria or the sensation that things are not right, and put just a little extra into the effort. Basically you are learning to ride through discomfort that you have experienced before.

The central unconscious modifiers are, as I said, generally outside our control. However, they cannot be forgotten. Even though we cannot train to improve them in a positive way as we can the two above, we ignore them at the peril of a decrement in performance as we experience those frustrating overtraining and chronic fatigue syndromes.

For those of you interested, this multifactorial process is nicely summarized by Abbiss and Laursen in their paper Models to explain fatigue during prolonged endurance cycling.

A "Central Governor" explains the Placebo Effect.

Their speculation that "...the brain is constantly taking note of physical changes using sensors throughout the body... it will then combine that information with memories of previous experiences, current mood, social feedback - and its estimates of the challenges to come.... (and) based on these calculations, it will determine how many muscle fibres to recruit and the intensity of the movements that the body can sustain. If the brain senses that we risk overexerting ourselves, it will create a subjective feeling of fatigue that deters us from further exercise..." sounds very much like the central governor concept discussed above.