By Arnie Baker, MD
Intensity is the load or speed of work performed. It is the “how hard” of a cycling workout.
Perceived exertion, speed, cadence, heart-rate, power, and torque monitoring all have important roles to play in assessing work intensity. Other measures of intensity, including blood lactate and oxygen consumption, have their place in physiology labs, sport science, and experimentation, but are generally not used in training.
Measures of intensity during bicycling can be used to:
- Design training and racing programs
- Analyze perceptions of training and racing.
- Help keep efforts easy and hard enough.
- Help motivation.
Measures of intensity for cycling are outlined below. More details about heart-rate, power, and torque monitoring follow the general outline.
Athletes have always used perceived exertion to measure workout intensity.
Although objective measures of workout intensity are the most common measures used by “scientific” athletes and coaches, psychological intensity does not always correlate directly with these measures. Perceived exertion is related to many factors including breathing rate and depth, and muscle tension, burning, and heaviness.
There are a number of methods of rating perceived exertion.
The basic method, used by generations of athletes, is simply that work is easy, moderate, or hard.
Easy work is below the aerobic threshold, generally less than 65% of maximum heart rate. Moderate work is aerobic work, generally 66% to 85% of maximum heart rate. Hard work is sustained high-level aerobic work (above 80% of maximum heart rate), or anaerobic work.
The most common “scientific” rating system is the Borg scale, from 6 to 20. For individuals with heart rate maximums of 200 beats per minute, the scale roughly corresponds to 1/10 of heart rate. The scale assigns descriptors, such as “somewhat hard” for value number 13.
Perceived Exertion Summary
Some advantages of intensity measurement by perceived exertion are that the measure is descriptive, intuitive, and free.
The disadvantages are that the measure is subjective, and that its reliability and validity can be poor, especially in beginners. Using perceived exertion to regulate workload, it is easy to work too hard or too easy. Most athletes underestimate the intensity of their efforts.
Fatigue and/or glycogen depletion increases perceived effort relative to objective measures of intensity. Warm-up, group, and environmental changes alter the perception of perceived as a Borg 13.
Speed in miles (or kilometers) per hour as a role in objectively measuring intensity. Speed is unreliable when environmental factors such as wind or elevation change cannot be controlled or accounted for.
Speed is useful in designing programs for time trialists when intensity of effort is prescribed by pace- whether on level ground or climbs, whether 40K (25 mile) riders or 4K track racers.
Cadence, or pedal revolutions per minute, may be a reflection of muscular tension of neuromuscular work. For these kinds of efforts, cadence can be an important measure of work intensity.
Heart rates are easily monitored with heart-rate monitors.Advantages of heart rate monitors include their relative low cost and that they provide a quantifiable measure of intensity. They are best used in measuring aerobic intensity.
Heart rate monitoring is relatively poor in quantifying the intensity of strength and leg speed work. If you work on leg speed, for example, and spin flat-out as fast as you can in an easy gear for 5 minutes, your heart rate may be very high, but your power output may be only moderate,
On the other hand, if you sprint in a moderately hard gear for 20 seconds flat-out as hard as you can, your power output may be maximum, but your heart rate may not have time to “catch up” to a maximum effort.
Monitors lag in the measurement of anaerobic work. Heart rates of more than 90% of maximum are sufficient, but not necessary, to document anaerobic work.
Power is the rate of work. Power measurement—traditionally available on laboratory ergometers—has been available on new- generation portable “consumer” devices at least a decade.
Some have argued that fitness, a response to training, responds more closely to power training than any other method of intensity training.
Most of the newer devices are quite valid and accurate. Some can measure changes in small fractions of a pedal stroke. Reliable units tend to be expensive ($500 to $2,000+).
Power is arguably the best measure of muscular work. Power measurement provides immediate, effort sensitive feedback. Unlike speed, it is unaffected by environmental conditions such as wind or elevation change. It is objective and accurate.
Torque is rotational force. Power measuring devices measure crank or wheel torque and multiply by cadence or speed to determine power. Said differently, torque is power per pedal stroke.
Although closely related, they are not the same. Sometimes torque training is different from power training.
Blood lactate measurements, traditionally performed only in physiology labs, can be measured in the field. Portable consumer units are still generally unreliable and not valid measurements of effort.
Blood lactate testing has other disadvantages. It is invasive (requires blood samples). There is a delay in the body’s processing of muscle lactic acid to blood lactate, and the technology requires time to process a blood sample.
Devices are expensive ($300 to $2,000+). Each test also requires testing strips or chemical reagents that cost several dollars.
There are also more fundamental and theoretical problems with this measure of intensity. The measurement of lactate in the blood is less important, and removed, from what is going on with lactic acid production in working muscles. When and to what degree lactic acid or blood lactate limits performance is also subject to debate.
Oxygen uptake can be used as a measure of intensity, but generally only in physiology labs under testing conditions. These devices are cumbersome. Oxygen update lags effort. They are poor at reflecting anaerobic work.
Oxygen uptake can be estimated from power output. (Arnie’s formula: VO2 = 12 x watts / kg + 3.3. Oxygen uptake in milliliters per kilogram body mass equals power output per kilogram times twelve, plus 3.3.)
Common methods of measuring cycling intensity are summarized in Table 3.
There are many methods of subjectively or objectively measuring workout intensity. No single measure of intensity gives the full story. Using several measures gives a clear picture of effort.
Perceived effort, using informal or formal scales, is the method most commonly used. For most cyclists, heart rate monitoring is a good, objective measure of aerobic intensity. Power measurement is a relatively expensive and underused method of measuring muscular work intensity. Power measurement will probably become more popular when the price of these devices falls.
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