physiology of systems for muscle-powered propulsion -- Roberts

Ken Roberts

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This page is about oxygen and fuel and the non-muscular support systems in the body. For forces and geometry and propulsive power, see most of the rest of this website.

components + capacities

peripheral / muscle-specific

aerobic combustion: things to utilize carbohydrate fuel with oxygen to make the muscle cell contract and do work.
anaerobic combustion: things to utilize carbohydrate fuel without oxygen to make the muscle cell contract and do work.

This process is produces less work from the fuel, and produces lactic acid.

A little lactic acid is OK, but too high a concentration of reduces the effectiveness of the muscle to do further work -- so it has to be dealt with somehow.

recycling lactic acid

A little lactic acid near the muscles is OK (maybe even good?). But too high a concentration of reduces the effectiveness of the muscle to do further work -- so it has to be dealt with somehow.

The main way to do this is to use oxygen, so the accumulation of lactic acid from anaerobic combustion is sometimes called "oxygen debt". If you didn't have enough oxygen around at the moment you were burning the fuel, you still have to get and use the missing oxygen later, to deal with the results.

Also it seems that sustained high concentrations of lactic acid are associated with longer-lasting reductions in the effectiveness of the muscle -- that persist even after the lactic acid concentration has been reduced. Not clear to me if these longer-lasting effects are the result of exposure to high lactic acid concentration or are an independent consequence of sustained anaerobic work.

Anyway it likely helps to ship some of the lactic acid out to some other place in the body, before it causes more persistent negative effects locally. It would be interesting to know what the rough proportion is between recycling lactic acid locally versus non-locally in various performance situations.

Not clear that lactic acid is inherently bad for the muscles, and seems likely that other chemicals are involved than just lactic acid, but lactic acid is the one that's easy to measure, so that's what the popular exercise books always talk about. So I will often talk that way too, even though I'm suspicious of it.

transport of blood carrying oxygen and fuel into the muscle.

a critical bottleneck of performance is getting oxygen from the visible blood vessels into the muscle cells.

the truly "peripheral" muscle-specific aspects of this peripheral transport network are:

(a) the density and number and width of the capillaries. This can be increased greatly in response to training stress.

Also membranes between capillaries and muscle cells -- but I haven't heard that there's any way to improve the performance of these membranes, so I won't mention them in further discussion.

There are two other drivers of the effectiveness of peripheral oxygen transport which are not muscle-specific:

(b) hematocrit concentration in the blood -- The more oxygen that can be carried by the same volume of blood, the more oxygen can be transported into the muscle through the same size network of capillaries. But only up to a point, since there are side-effects to increasing hematocrit concentration.

(c) blood pressure -- the higher the pressure on the blood, the faster it will flow through the same network of capillaries, and so the more oxygen will arrive per second to the muscle.

transport of blood carrying lactic acid and other waste products out of the muscle.

capacity considerations are mostly the same as for transport of oxygen, except that hematocrit concentration is not relevant.

nearby storage of carbohydrate fuel

nearby storage of non-carbohydrate fuel

conversion of non-carbohydrate into carbohydrate fuel

central / shared

hematocrit density in the blood

This is well-known and highly-targeted in modern training programs. What I think needs to be pointed out is that it's critical impact for endurance performance is on all the muscle-specific peripheral transport systems.

pressure of the blood

Some aspects of higher blood pressure during athletic performance can help peripheral transport, but I doubt it's a good idea to try to target training of healthy athletes for any aspects of blood pressure. Because (a) too much blood pressure could be harmful; (b) I suspect the body of a healthy athlete will naturally develop sufficient capability to produce healthy aspects of performance blood pressure, in response to training stresses targeted for peripheral adaptations.

maximum blood volume flow rate

This maximum for this is related to the current size of the chambers of the heart, and the maximum frequency of heart contractions per minute.

It also depends on how much force the heart muscles can generate, the viscosity of blood, and how many and how wide the "pipes" are in the circulatory system -- and even partly on the capacity of the peripheral transport network of capillaries.

recycling lactic acid

A little lactic acid near the muscles is OK (maybe even good?). But too high a concentration of reduces the effectiveness of the muscle to do further work -- so it has to be dealt with somehow.

Seems to me that just because the lactic acid was generated in a specific muscle does not mean that it has to be recycled in the same place. It should be straightforward for other locations in the body to recycle the lactic acid using their own supply of oxygen -- just have to use the peripheral and central transport systems to get it to some other processing location. Advantages of "shipping out" the lactic acid to another location are: (a) quickly reduces concentration in the muscle before it results in more persistenct negative impacts, (b) leaves more of the muscles own local oxygen free for other purposes.

Seems clear to me the lower the lactic acid concentration in the blood going into the muscle (coming from the shared central supply), the more excess lactic acid it can pick up from the muscle, and remove it more quickly. So if the lactic acid is not only removed from the muscle, but also recycled somewhere else before the blood comes back, that helps too.

It would be interesting to know what the rough proportion is between recycling lactic acid locally versus non-locally in various performance situations.

my theory guess

My best guess is that:

the critical physiological bottleneck in most athletic endurance performance situations of duration 30 minutes to 3 hours is peripheral transport capacity.

The most critical resources which are shared across muscle groups are:

hematocrit density in the blood

pressure of the blood

? central transport flow volume?

In healthy progressively-trained endurance athletes, central transport volume capacity is not a bottleneck -- although in untrained humans, it might a bottleneck on a particular day, before the development process has time to respond to new demands, and in some unhealthy humans, central transport volume capacity could be an endurance performance bottleneck.

Central transport volume capacity

central transport volume capacity is not a significant limiter in most athletic endurance performance situations of duration 30 minutes to 3 hours.

The critical central parameters which are shared across muscle groups are:

hematocrit density in the blood
pressure of the blood

because these two are the ones that directly impact the critical bottleneck: peripheral transport of oxygen in the "last step" through the capillaries.

I suspect exercise writers get focused on flow volume because it's simple to understand, and it fits with our concepts of how automobile engines work. But automobile engines are old-fashioned highly-inefficient macro technology. Muscular propulsion is sophisticated efficient nano-technology. Someday human-designed nano-technology propulsion will surpass the natural kind (if the human species and civilization survive that long), but for now muscles are the leading edge. Anyway . . .

The real action in human nano-technology is in the muscle cells and tiny capillaries where it's difficult to know what's really going on -- not the big pipes where it's easy. So what do the exercise books talk about: the unimportant stuff that's easy.

increased central transport flow volume capacity is a result of other gains in peripheral systems, not their cause.

I strongly suspect the healthy body is programmed to develop a substantial over-capacity of central CV flow volume capacity in response to training stresses targeted for peripheral adaptations.

My guess is that the suggestion of using surgery to expand central transport volume capacity (e.g. by surgically enlarging chambers of the heart) is a major misunderstanding of the role of central CV system.

evidence against the "max central CV" theory

(a) the power output level at which substantial lactic acid is produced and observed is significantly lower than the V02max power output level.

If the critical performance bottleneck were the maximum rate of total oxygen transport through the central system, then the sum of the aerobic combustion capacities in all the peripheral muscles being used would have to be greater than the central maximum volume rate -- so the muscles would not use anerobic combustion until the demanded power rate was slightly over 100% of the V02max power rate.

At a power output 90% of V02max, the muscles would have more than sufficient aerobic combustion capacity to deliver the required power without using any anaerobic combustion -- so no lactic acid build-up would be observed at 90% of V02max.

In fact most athletes do produce significant lactic acid at 90% of V02max, which shows that the real problem for endurance performance in progressively-trained healthy athletes is not lack of central oxygen flow capacity.

(b) well-trained triathletes have different V02max volume rates for each of their different propulsion modes: running, bicycling, swimming.

questions

Q: If Central CV is not critical, then how come altitude training and EPO work?

Because altitude training and EPO effect hematocrit concentration in the blood, and hematocrit is a key driver of the effectiveness of all the peripheral transport systems.

Q: If Central CV is not critical, then how come well-trained endurance athletes have larger heights and bigger blood vessels?

Central CV volume and flow rate are not critical, because the normal physiological development and training-stress response processes of a healthy athlete's body do not allow it to become critical, when using a progressive training program.

My theory is that that a key (unconscious) strategy develpment process of a healthy body is to try to maintain a substantial over-capacity of Central CV volume and flow rate. One possible explanation is that this strategy is because getting "caught short" on Central CV capacity could have such serious consequences that it must not be allowed to happen. Another might be that the body is structured so that the biological cost of expanding Central CV volume is low compared with peripheral transport and other propulsive structures.

Central CV is very necessary and important -- so important that in healthy athletes we can trust that it will be taken care of unconsciously. So Yes, Central CV volume and flow should be appropriately monitored, to discover if the athlete is no longer healthy.

But there's no target it specifically for training stress, because it will normally develop more than adequately in response to training stresses targeted for peripheral adaptation.

Q: If Central CV is not critical, then how come very short work-recovery intervals are recommended and effective?

I've seen special short interval sequences to target Central CV capacity, like

10x { work :40 / rest :20 seconds }
5x { work :60 / rest :60 seconds }
5x { work :80 / rest :40 seconds }

During the work segments, the power output goes significantly anaerobic, perhaps even above V02max power output level. I remember hearing reports that these short-interval workouts can result in developing substantial increase in V02max measurements very quickly.

my responses:

(a) Yes these workouts do stimulate development of Central CV capacity -- perhaps more and quicker than other workouts.

(b) Yes these workouts often result in higher measured V02max oxygen flow and power output (if the athlete did not already have a rather high V02max).

(c) Yes these workouts often result in faster speed in an endurance performance of around 30 minutes (if the athlete did not already have a rather high speed in endurance performance)

my interpretations:

(a) But is this max Central CV volume flow capacity then beyond what was needed to support the speed achievable by the athlete in the endurance performance?

Perhaps the max Central CV volume flow capacity was already beyond what was needed even before the special workouts.

What is the appropriate measure of Central CV volume + flow capacity? I don't think the usual VO2max measure effectively measures this. (see below)

Before we could say whether this kind of workout protocol works better for developing maximum Central CV flow volume capacity, we first should define a good measure of maximum Central CV flow volume capacity.

(b) Higher V02max measurements are the result of increases in the capacity of one or more peripheral systems.

The higher V02max number after development time in response to the workout, was observed because the workout was successful in improving one or more peripheral systems.

For most propulsive motion techniques for most athletes in most situations, the V02max protocol mainly measures the sum of the oxygen demands of the peripheral transport and muscle systems engaged in this motion technique. That's why the V02max numbers are different for running versus swimming versus bicycling by the same athlete -- because each motion technique engages a different ensemble of peripheral systems.

So the measurement puts a lower limit on Central CV oxygen volume flow capacity, but it says nothing about the maximum.

The key questions to ask are:

Why is this kind of workout successful in developing some kinds of performance capacity in peripheral systems?
Are those the kinds of capacity that fit the athlete's goals for endurance performance?

(c) Endurance performance: It seems clear that these short intervals can develop anaerobic and/or sprinting capabilities (whether they're better than all other possible workout sequences, I don't know)

Workouts that train Anaerobic capability often also train Lactate Threshold. What's best to develop some capacity can be different in different periods in the athletes year-long (or multi-year) training program. Possibly these short-work-rest-interval workouts somehow are the best training stress to stimulate development of Lactate Threshold during "final-weeks" or "sharpening" period of the program. So it could be that these workouts are effective for increasing speed in endurance performance because of Lactate Threshold, not Central CV.

Many endurance performance require a substantial anaerobic capability for competitive reasons -- especially mass-start races where air resistance is a significant factor: notably bicycle road-racing and inline-skate marathons. Because "pack tactics" requires that the leaders accelerate to try to "drop" the others drafting them -- or at least require them to work harder to keep up. And followers must accelerate rapidly to keep the draft and avoid being dropped.

In non-mass-start time-trails on terrain with rolling hills, it is advantageous to go (slightly?) anaerobic when getting near the top of climbing a hill, then use the following downhill to recover the "oxygen debt".

So it could be that these workouts are effective for increasing speed in endurance performance because of peripheral Anaerobic capacity, not Central CV.

Q: How come so many recognized experts say Central CV capacity is the bottleneck?

Because during the time they were becoming "recognized", they thought and said lots of interesting things about technique and equipment that made sense at the time.

Because really many of these motion techniques for propulsion are really really complicated -- and for a long time nobody could grasp all the complexity -- it took decades. In the meantime these experts needed to make some simplifying assumptions in order to feel like they had some control over it, and so they'd feel halfway confident about giving some helpful answers to lots of obvious-sounding questions about equipment and technique.

Then new people come along who are not "recognized" and they say new things about technique and equipment -- which call into question some of the old things they said. Which then leads to digging out and peeling away some of those old simplifying assumptions. Which is scary for a "recognized" expert, because it might call into question lots of those other helpful answers which they gave.

The Central CV capacity argument gives them a quick response to justify their status quo against lots of these new ideas. 90% of the new ideas are wrong anyway, so most of the time this works.