[体适能]Efficiency, Economy and Endurance Performance

jacky

Introduction
So far, if you read the two previous articles regarding "The System", you know that high level endurance performance depends on 1) a high maximal oxygen consumption, or VO2 max, and 2) a high lactate threshold, or point of OBLA. Your VO2 max sets the upper limit for your sustainable work potential. For the elite endurance athlete, a high VO2 max is like the invitation to the big dance. Having an invitation to the dance does not ensure you will dance with the prettiest girl. But, not having one ensures you won't! The lactate threshold tells us something about how much of the cardiovascular capacity you can take advantage of in a sustained effort. It is determined by skeletal muscle characteristics and training adaptations. Multiplying VO2 max x LT (Oxygen Consumption at Lactate Threshold) gives us a measure of the effective size of your endurance engine. Now we come to efficiency. What does efficiency have to do with endurance performance? Victory goes to the person with the biggest endurance engine right? Well, let's use a racecar analogy.

If I build a powerful, well-tuned engine that can run at redline RPMs for hours, and then drop it into a Ford truck chassis, the truck might go 120 mph. But if I drop it into a streamlined Ferrari chassis, I might hit 200 mph (in theory, personally I am afraid I would soil my pants and hit the brakes long before I reached 200 mph).  That is a big difference.  Engine performance didn't change, but performance velocity did. To some extent, the same efficiency effect is observed in every endurance sport. Efficiency is critical to maximizing performance velocity!

Physiological Efficiency Defined

In an exercise setting, efficiency is defined as the percentage of energy expended by the body that is converted to mechnical work (another form of energy).   

Work Efficiency = Mechanical work  / Chemical energy expended

We can measure the mechanical work performed using an ergometer, like a bicycle ergometer, or rowing machine.  We can measure the energy expended by the body indirectly via its oxygen consumption at sub maximal workloads.   With some basic biochemistry we can convert the oxygen consumption we measure during exercise to a standard measure of energy like kJoules, or Calories. And, we can do the same for the work we measure on the ergometer.  Work/time = power.  Power is measured in watts and is a measure of the intensity of work.  Intensity (watts) x exercise duration (minutes) gives us total work, again measured in kJoules or Calories.  

If we take a group of cyclists, or a group of rowers and perform sub maximal testing on them to determine how much energy they consume when performing a standard sub maximal workload, we find that overall work efficiency will range between about 17 and 26%, with an average somewhere in the middle of that range.  In other words for every 100 Calories of energy burned, we manage to convert 20 Calories of that energy to useful work on the pedals of the ergometer, or as pulling power on the rowing machine.   Now, if your goal is to lose body fat during exercise, then I suppose it pays to be inefficient, since it is Calories burned that matter.  However, if your goal is to move your body faster than the other guy, than being 25% efficient is way better than 18%!   So, what are the sources of inefficiency and what, if anything can we do about them?

Sources of Inefficiency in the Performance Machine

Let's use a 40 km time-trial in cycling as an example. The goal is to propel you body on a bicycle over land, and through air at the fastest sustained speed.   So where do the energy losses occur in the path from chemical energy stored in pasta to velocity sustained on the bicycle over a 40km distance?

1.      Chemical energy conversion losses -Your body must generate ATP for muscle contraction by chemically converting food energy, using a process that ultimately requires oxygen (hence the need for a big oxygen delivery capacity), while minimizing the production of lactic acid (high LT). All of the chemical energy in food is not transferred to ATP. About 60% is lost as heat energy. This is why you get hot during exercise. This source of inefficiency is the same in everyone.

2.      Fiber type differences in converting ATP energy to contraction force- The next source of energy loss is in the step in which the chemical energy trapped within the ATP molecule is converted to mechanical energy via muscle contraction. There is some recent data from one laboratory in the United States suggesting that fiber composition of the muscle influences efficiency of muscle contraction (Coyle et al., Medicine and Science in Sports and Exercise. 24:782-788, 1992). Cyclists with a high percentage of slow twitch fibers appear MORE efficient. This was observed as a smaller increase in oxygen consumption for a given increase in cycling power output, in a group of cyclists whose fiber composition varied between 35 and 76% slow twitch. The higher efficiency was also observed when performing repetitive leg extensions, suggesting the source of efficiency was in the muscle, not the riding technique.  The way they get at this is to measure the DELTA efficiency which is just the change in energy demand for a given change in power.  By measuring the change in oxygen consumption an athlete requires to increase his work rate from say 150 to 200 watts, while keeping his cadence and body position the same, this specific efficiency of the muscles can be determined.  The differences are small. Perhaps a cyclist with 80% slow twitch fibers would have a Delta efficiency of 25%. A person with only 50% slow twitch fibers might be 21% efficient. So within the endurance community, the numerical difference in muscular efficiency seems small, but the impact on power output in a 40km time trial can be 8-10% independent of other variables. For example, data from Horowitz et al. (Int. J. Sports Medicine, 15:152-157, 1993) compared two groups of seven cyclists. The average VO2 maintained during 1 hour of cycling (a function of VO2 max and Lactate threshold) was the same in the two groups (4.48 vs. 4.46 l/min). However, the group with the higher average % slow twitch fibers (73 vs 48) achieved higher power output during the hour of cycling at voluntary maximal power (342 watts vs 315). They achieved 8% higher power output for the same physiological cost. How does this translate to velocity on a bike that actually moves?

3.       The energy cost of moving the limbs- If you sit on a bicycle ergometer with the load set at zero and pedal at 80 rpms, you will discover that even though you are not doing any measurable mechanical work, your are still WORKING.  It costs energy to just move your limbs, support your body, hold your balance, etc. The same is of course true for ANY movement, like running or skiing, or rowing.   When this “unloaded cost of movement” is included in our measure of the mechanical work to energy expenditure ratio, then we get the GROSS Efficiency.  Here, the word “gross” means “overall”, not “icky”.

One factor that impacts gross efficiency is movement frequency.  That can be cycling cadence, or rowing stroke rate, or stride frequency in XC skiing.  Higher cadences tend to cost more energy in general.  And heavier limbs have been shown to be less efficient to move. However, there is a balance such that trained athletes tend to zero in on an optimal cadence for their body type and anatomy.  When they are pushed away from that cadence, they use more energy to do the same work.   Therefore, it is important to realize that the ideal movement frequency is not a universal, but varies from individual to individual.  So, you should not try automatically to mimic your training partner’s cadence if they are much taller or shorter, or more or less muscular than you.  

Having said that, in sports like rowing and cross-country skiing, there is a general tendency that the best athletes with the big engines use it by pulling or pushing harder each stroke, not revving up their movement frequency.  This makes sense.  Pushing harder each stroke means that more of the total energy goes to propelling the body and less to moving the limbs back and forth.  And, you have no doubt seen the truth of this in watching how smoothly the great rowers, or skiers, or runners generate speed.  Their technique looks controlled and powerful, not frantic or hurried.  There is a fine balance though.  If you try to work with huge powers at too low frequencies, then the muscles become overloaded, blood flow gets compromised and fatigue results.  So, the endurance athlete seeks a balance between the efficiency of lower movement frequencies and the decreased muscle tension and blood vessel compression of higher frequencies for a given workload.  Exactly where this balance point lies varies from athlete to athlete.

In sports that are very technique intensive, like cycling, XC skiing, or rowing, there is much to be gained in perfecting the biomechanics of the movement.  However, in cycling, with its relatively basic movement pattern, there does not seem to be any difference in overall cycling efficiency between elite cyclists and cyclists that have not been training very long.   Sure, efficiency is an advantage, but the research indicates that cycling efficiency does not get better and better with years of training.  

Efficiency vs Economy

The difference between efficiency and economy in an exercise setting is that economy is measured as movement velocity for a given energy consumption, while efficiency is mechanical power output for a given energy consumption.  When we measure economy we connect the power produced to the movement intended, like cycling as fast as possible over 40km.

This brings us  to the Truck vs. Ferrari analogy. Having the biggest engine doesn't guarantee the fastest performance in car racing or bicycle racing (or rowing, running, and swimming). The Ferrari goes faster because it is lighter and slices very cleanly through the air, reducing aerodynamic drag. So does the cyclist who perfects an aerodynamic riding position. (Click here for a discussion of cycling aerodynamics from expert Jim Martin). The best distance runners display high running economy. This means that they can run at a given speed with less oxygen demand. A high economy can make up for a relatively lower VO2 max. For example, Derek Clayton ran an incredible 2:08 marathon in 1969. His VO2 max was "only" 69 ml/min/kg (well it was probably a bit higher than that, but this was data from one non-peak season test). Thanks to his high running economy, that time stood for 12 years and was not matched by talented runners such as Craig Virgin, Gary Tuttle, and Bill Rodgers, whose VO2 max values ranged from 78 to 82 ml/min/kg! In rowing, both the hydrodynamics of the racing shell and the technical mastery of the rower contribute to rowing economy. However, even on a stationary ergometer, elite rowers are more efficient than well-trained but non-elite oarsman. This is not due to a difference in fiber composition. So, it appears that subtle changes in rowing technique can continue to contribute to improve rowing efficiency and performance with additional years of training.

In no sport is efficiency more important than in swimming. The best swimmers in the world do not stand out in physiological tests of raw endurance capacity when compared to other endurance athletes. This suggests that high efficiency, achieved through a combination of ideal anatomical structure and technical perfection of the stroke is critical.

The Big Picture Going back to the performance model, I identified several anatomical and cellular characteristics that contribute to 1) maximal oxygen consumption, 2) relative work intensity at lactic acid threshold, and 3) efficiency of transfer of physiological work to movement velocity. The details differ with each sports discipline and the event duration. But these are the BIG THREE variables. Maximal oxygen consumption is limited by central cardiovascular function, but also dependent on the peripheral adaptations that occur in the trained muscles. A high lactate threshold is due to peripheral adaptations improving the muscle's ability to generate energy aerobically. And, a high efficiency/economy creates the link between the physiological engine and the actual performance goal, to maximize average velocity.   




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