The question of the superiority of low intensity/extended duration over high intensity/short duration exercise has been Vigorously debated. Current exercise literature uses the questionable term "intensity." "High density/low density" or high/low work rate would be more suitable terms. To keep things simple, I'll be using the term "high/low density" throughout this article.
On one side of the debate you have aerobic instructors and the so-called exercise 'gurus' of our times spewing forth propaganda which claims that jumping up-and-down for 30-60 minutes to the sound of Richard Simmons is more efficient than leaving that small reservoir of perspiration on the floor and equipment following a high density work-out.
What Is The Difference?
To understand the difference between the two methods we must first distinguish between the way they work. Low Density/Long Duration (LD/LD) workouts are typically performed in the range of 50-65% of one's max heart rate ( 220-age = Max Heart Rate).
There are two different ways to calculate your maximum heart rate and your target heart rates. The method I just explained is the simple method. Read the full article here.
Target Heart Rate Calculator
Using the 220 - Age formula.
The LD/LD method usually derives its source of energy from the oxidative, otherwise known as the Aerobic, system. The Aerobic (i.e., in the presence of oxygen) system uses fat (tryglycerides found in adiposities) as its primary source of fuel during exercise via the Krebs Cycle.
Part of the reason fat is used as a primary source of fuel during LD/LD is because fat molecules contain large quantities of energy per unit of weight. Slow twitch fibers, commonly known as Type I fibers are stressed during this type of activity.
These contain a large amount of oxidative enzymes, and are also surrounded by many capillaries which supply the muscle with myoglobin (oxygen-binding protein). During LD/LDs both carbohydrates and fats are being used; hence, the saying, "fats burn in the flames of carbohydrates." Low muscle and liver stores of glycogen will limit the effect LD/LD exercise has on the "fuel mix" of fat/carbohydrates.
[Note: the body rarely uses blood glucose directly as a means of fuel as its purpose is to maintain the glucose level of the blood for other body tissues, especially the brain].
High Density/Short Duration (HD/SD) workouts are usually performed in the range of 70-90% of one's max heart rate, so they can also be labeled as interval and anaerobic training methods. The HD/SD method derives its source of energy primarily from the phosphagen and glycolitic systems.
[Note: all systems are used simultaneously, and at no time during exercise or rest does a single energy system provide the complete supply of energy].
There appears to be a shift from fat metabolism to carbohydrate metabolism as the density of the exercise increases. The movement is towards muscle glycogen being used as the primary source of fuel while muscle tryglicerides, Plasma FFA (free fatty acids) and blood glucose take a back seat. The sudden shift, as the density of exercise increases, is due, in part, to a increase in fast twitch fibers as well as a release into the bloodstream of certain hormones like "epinephrine".
As the density of exercise rises there is a progressive rise in this hormone. "Epinephrine" increases muscle glycogen breakdown as well as an increase in lactate levels (lactic acid). Lactate increase will inhibit fat metabolism as a substrate.
Another factor is that fast twitch fibres have an abundance of glycolitic enzymes but few mitochondria and lypolitic enzymes (enzymes responsible for fat breakdown). In other words, fast twitch fibers are more efficient at metabolizing carbohydrates than fats. So an increase in fast twitch recruitment will bring about a greater metabolism of carbohydrates and less fat metabolism.
It has been recommended that LD/LD training sessions be the optimal method for reducing one's body fat stores. Though studies have shown that LD/LD may be the choice of many, the body is very adaptive and will become more efficient in conserving energy in relation to energy expenditure.
Have you ever wondered why aerobic instructors, in general, don't sport a lean physique? It's a bit of an exercise mystery since they usually teach several, often many, classes a week which often last for upwards of an hour. Charles Poliquin, a Canadian strength coach, identifies this as the "Chunky Aerobics Instructor Syndrome."
Despite the frequency of aerobic exercise most aerobic instructors carry a high percentage of body fat. The fitness magazine, IDEA, released a study several years ago indicating that aerobic instructors had, on an average, a body-fat percentage of 20%... yes, that's right... 20%! This is surprisingly high for people who train for extended periods of time at a low density level. An individual who trains using primarily LD/LD is like a Honda Civic which can stretch its fuel consumption for extended periods [making fuel consumption sufficient.]
Even though there is a large emphasis placed upon carbohydrate metabolism and low levels of fat metabolism in HD/SD exercise there are studies showing that it is the density of exercise which should be taken into consideration:
"Impact of high-intensity exercise on energy expenditure, lipid oxidation and body fatness".
Author: Yoshioka, M., Doucet, E., St-Pierre, S., Almeras, N., Richard, D., Labrie, A., Despres, J.P., Bouchard, C., Tremblay, A.,
Volume: 25 Issue:3, Page: 332-9 Y ear: 2001 Mar.
"Impact of exercise intensity on body fatness and skeletal muscle metabolism".
Author: Tremblay, A., Simoneau, J.A., Bouchard, C.,
Volume: 43 Issue:7, Page: 814-8 Year:1994 Jul.
It has also been found that, possibly, HD/SD workouts do a better job of promoting the release of human growth hormone (HGH) from the pituitary gland. HGH is a potent stimulator of fat metabolism.
|"A number of studies indicate that LD/LD exercise increases energy expenditure in the period of time immediately after activity."|
Research has not supplied firm proof for this, so there may yet be a threshold which one may have to work over before benefiting from this effect. LD/LD exercise (around 50% VO2 max) seems less likely to make a difference, while moderate or intense are more likely to.
For example, one review concluded that light exercise could be expected to lead to burning an extra 5-10 calories afterwards; moderate exercise to an extra 12-35 calories. In contrast, strenuous exercise was shown to increase post-exercise energy burning by a huge 180 calories ("Excess post-exercise oxygen consumption - magnitude, mechanisms and practical implications", Bahr, et al, Acta Physiol. Scand., suppl. 605, pp 1-70).
The reduction in body fat, observed in results from high intensity exercise could be explained, perhaps, by an increase in the post-exercise Resting Metabolic Rate. Most studies that have found an elevated resting metabolic rate have performed their measurements within 24-39 hours of the exercise session.
[Note: studies were preformed on men and woman and it was found that women did not benefit from a post-exercise rise in their metabolic rate. This indicates that women have to work twice as hard as men when it comes to fat-loss.It appears that women conserve energy more efficiently (ie, they burn fewer calories) at rest and in response to exercise.]
So, the verdict is in. It has been proven that LD/LD exercise metabolizes fat more efficiently than HD/SD exercise. On the other hand, HD/SD exercise has, over an extended period of time, shown a greater decreases in fat stores. Each method is useful depending upon individual circumstances, and factors such as time, current fitness level and the presence of potential cardiovascular problems.
Individuals who want to lose weight (who are fit incardiovascular terms) should profit from a step-up in their workout intensities. ("lmpact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism", "Metabolism", vol. 43(7), pp 814-818, 1994).
The time at which to perform each method is another topic. Some studies show that higher density exercise will elicit a better effect if performed in the afternoon (Bernard et al. 1998, "European Journal of Applied Physiology", 77, pp. 133-138).
These findings suggest that the best time of day for high density training is in the afternoon or early evening. This is consistent with other research into circadian rhythms showing that heart rate, body temperature and muscular strength are all higher in the afternoon than in the morning.
How Does Exercise Work Its Effects?
At the website of Peak Performance Online (http://www.pponline.co.uk), Janet Stansfeld explains how the mechanisms of the body relate to energy expenditure.
- Increased hormonal activity Some investigations have found that increased resting metabolic rate following endurance exercise is associated with higher blood levels of adrenaline and noradrenaline. These hormones are controlled by the nerves embedded within muscles, known as sympathetic nerves.
Some scientists have looked directly at sympathetic nerves in muscles and found that exercise stimulates the nerve activity. These hormones tend in turn to stimulate various metabolic processes which have the net effect of raising RMR. Other hormones may also play a role. There's some evidence that exercise leads to an increased production of thyroid hormone, which in turn steps up general metabolic activity.
- Protein resynthesis several lines of evidence suggest that increased metabolic rate following exercise is associated with altered protein metabolism. Some data show that exercise increases protein breakdown. To keep protein status constant, protein synthesis would need to be stepped up following exercise.
There's some indication that exercise causes an increase in the levels of enzymes involved with making proteins. However, so far there's no direct experimental evidence confirming a direct link between increased protein turnover and metabolic rate.
So now you can see how a combined effort using either HD/SD (high density /short duration), or LD/LD (low density long duration) exercises combined with resistance training and a proper eating regimen can impact a individual and help him/her towards a leaner, healthier body.
- Westcott, W. Be Strong, 1993.
- King, J. Brad, Fat Wars, 2000.