Physiology Query

[jcharry]

I've noticed that there seem to be a number of people posting who have extensive knowledge of exercise physiology. I have a question that's been bugging me for a while as I've seen conflicting information. I've read that lower intensity, aerobic exercise causes the body to develop a better distribution system (building capillaries and new blood vessels). So if people who have a poor aerobic base begin training at higher intensities, do they compromise the development of a better distribution system? Do they "teach" their bodies to prefer glycogen over fat as a fuel source?

~Judi

[Legspeed]

The idea that low intensity aerobic exercise promotes capillarization is based on science thats about 25 years old. While it may be true that low intensity aerobics promotes more capillarization adaption than sitting on the couch, recent studies performed, say, in the last 10 years clearly show that high intensity aerobic exercise (defined as intensities just below threshold) provide a significantly greater adaptation effect.

Keep in mind that de-trained individuals have a low threshold relative to their maximum heart rate. So when they're working at 75-80% of estimated MHR, they are usually fairly close to threshold. That's why it's common to see rapid improvement in a short time. As they become more fit, their thresholds increase. Assuming they persist at working at the 75-80% level, it's not unreasonable to observe performance improvements decelerate.

Oh, and I'm pretty sure it's impossible to "teach" yourself to use a specific metabolic pathway.

[Todd S]

Just a few comments before I run off...

Last I checked, the only part of your body capable of 'learning' anything is your brain. The energy system your muscles utilize at any given point in time is strictly a matter of chemistry and thermodynamics. However, your body does make long term adaptations to imposed training stresses that will influence metabolic processes.

What do you want the answer to be? If I give this a little thought I can probably make a strong argument for a couple of alternative answers to your question(s). Probably no 'right' answer to what you're asking... Simply arguments to either support or disprove.

Capillary density and mitochondrial density are proportional to VO2max. Once a moderate level of fitness is achieved, most exercise protocols intended to increase VO2max involve relatively high levels of training intensity.

If you want to be a glycogen 'sparer' and a fat 'burner', either be born with and/or focus your training on maximizing your power (cycling) or pace (running) at VO2max. The lower your chosen power output or pace as a percentage of your power or pace at VO2max, the less glycogen you'll burn.

The value of 'base building' is the adaptations it causes within your working skeletal muscles. The only thing 'magical' about low base building exercise intensities is that these lower relative intensities allow you to build training volume. Simply training at low, non stressful intensities will do little to improve fitness for folks with even low to moderate fitness levels unless they use that low intensity training to build training frequency and/or duration. You'll find many proponents of 'long, slow distance' training, but you won't find many who push 'short, slow distance' training which is what a steady diet of 40 minute Spinning classes at less than 75% MHR soon becomes.

Hope that helps.

[Todd S]

And if you want to be totally confused I dug this up... Like Fox News, I want to be fair and balanced. This kind of shows how if you don't think a little bit some of the science can lead someone to an incorrect conclusion.


Simoes, H. G., Campbell, C. S., & Kokubun, E. (1998). High and low lactic acidosis training: Effects upon aerobic and anaerobic performance. Medicine and Science in Sports and Exercise, 30(5), Supplement abstract 932.

Summary:
Two groups of four volunteer runners trained similarly for four weeks. During that period, one group trained three times per week eliciting blood lactate concentrations of at least 10 mM and the other maintained lactate levels below 6 mM for a similar number of training sessions. Before and after the training period, Ss were tested for an all-out 3,000 m run, 4 mmol/l threshold, steady state heart rate while running at 200 m/min velocity, mean velocity in 5 x 30-m sprints, mean velocity in a 60-m sprint, and mean velocity in a 300-m run.

The low-acidosis training group showed anaerobic gains, an increase in 300-m velocity, and aerobic improvement with a lower heart rate during the 200m/min run and non-significant faster 3,000-m time. The high-acidosis training group also showed anaerobic gains with higher velocity and lactate levels in the 300-m run. However, aerobic performance was compromised. Time for 3000-m run was slower and a higher heart rate was evidenced in the 200 m/min run.

It was concluded that high acidosis training compromises aerobic fitness.

Implication:
Too much lactate tolerance (high acidosis) training can cause aerobic performance to decline. Anaerobic training that only stimulates moderate accumulated lactate levels enhances anaerobic performance and maintains or improves aerobic fitness.

Deeper implication:
The training intensity of the 'high-acidosis training group' of at least 10 mmol/l could probably be more commonly referred to as sprint training. Probably more intense and for shorter intervals with much more recovery required between intervals than the training done in even the most intense Spinning class.
The 6 mmol/l upper limit (slightly higher than what would commonly be referred to as lactate or anaerobic threshold) of the 'low-acidosis training group' would probably translate in HR terms to a max of somewhere in the upper 80's to maybe 90 percent of max HR for a relatively seasoned Spinning class participant. Sound familiar? Close to the upper HR limit of a good, hard interval class.