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Obesity in the US: 2011
According to the CDC, in 2011, Mississippi had the highest adult obesity rate: 34.9%. The state with the lowest rate was Colorado: 20.7%. (source: JAMA, October 17, 2012)


Exercise Myths

Author: Stan Reents, PharmD
Original Posting: 06/06/2009 02:06 PM
Last Revision: 12/10/2018 05:44 AM

Several exercise myths never seem to go away. In this review, I'll explain why they don't make sense.

1)  "Crunches and/or sit-ups will help me lose weight around my waist."

FALSE.  Believing that you can stimulate weight loss from the waist by doing crunches and sit-ups is about as illogical as thinking that you can keep your car running strong by simply washing it.

The reason why crunches and/or sit-ups don't help you lose weight around your waist is because "spot reducing" just isn't possible.  If you do enough crunches or sit-ups to promote fat loss around your waist, it's because fat loss is occurring throughout your body (eg., buns, thighs, etc.).  For some people, it may appear that fat loss is predominantly occurring around the waist because that's where they have the largest fat stores.  But they could also shrink their waistline if they rode a bicycle 90 minutes per day, 5 days per week, and didn't do any sit-ups or crunches.

So, don't be misled by those infomercials selling abdominal exercise machines.  Also, keep in mind that exercise doesn't have to be excruciating to achieve fat loss. Pretty much any activity (even non-sports activities such as mowing the lawn or moving furniture) will help you burn calories.  Any time you burn more calories than you take in, you will lose weight. This is why avoiding sitting all day long is probably a better way to prevent weight gain for many people than exercising hard for 30 minutes.

2)  "Aerobic and strength exercise should not be done on the same day."

FALSE.  Try telling this to a decathlete. In Olympic competition, 5 decathlon events are completed on day #1, and the remaining 5 events are completed on the very next day.  Day #1 combines the shot put with the 100-meter dash and the 400-meter run, a brutal test of muscle performance. Sure, these athletes are spent at the end of the competition, but it doesn't mean that you can't mix different types of training exercises on the same day.

However, having stated that, there is a right and wrong way to go about it....

• Aerobic exercise before strength exercise: Muscle cells need carbohydrates during strength exercise. Prolonged aerobic exercise, such as running, cycling, exercising on an elliptical trainer, swimming, etc. depletes glycogen (the stored form of carbohydrates). The human body only has about 2000 kcal of glycogen stored up. This can be substantially depleted after about 20-30 minutes of aerobic exercise. If you do your aerobic exercise first, followed by weight-lifting, your muscles may not have enough energy. In this case, your body may attempt to break down protein to meet energy needs. (NOTE: Fat stores are certainly your largest energy reservoir, but it takes a lot longer to break down fats for energy.)

• Strength exercise before aerobic exercise: Here, I can share a personal experience:

When I was at the University of Florida, I participated in a strength-training research study.  Periodically during the study, isometric assessments of leg strength were made.  This was done on a knee extension machine by exerting maximum effort at 6 different knee angles.  After one of those assessment sessions (the testing only took about 10-15 minutes), I played in a tennis tournament several hours later.

Or, I should say, I attempted to play.  My thighs felt like wood.  No, actually, they felt like lead.  I couldn't run or jump or change direction.  I'll never make that mistake again. (Yes, I lost that match...)

Joseph Warpeha, a PhD candidate in exercise physiology at U. Minnesota, has stated that it all depends on how intense the first activity is, regardless of the order.  Either activity, if done at a high intensity level, will deplete energy systems and cause fatigue for the one that follows (Warpeha JM.)

So, for the average person, it's OK to do both types of exercise on the same day.  However, if the first session is intense, try waiting several hours before doing the 2nd session. At least in theory, a lengthy bout of aerobic exercise, followed by weight-lifting might be counter-productive. If only for safety reasons, if you're going to do both, it makes sense to perform heavy weight-lifting prior to any prolonged aerobic exercise session.

However, never do either form of exercise just prior to a tennis match, basketball game, or swim meet!

3)  "Exercise turns fat into muscle."

FALSE. As someone improves from sedentary to "fit," typically there is a simultaneous increase in muscle and a decrease in visible (subcutaneous) fat. Fat is a major energy reservoir, particularly when the activity persists for longer than 30-40 minutes. This causes fat cells to shrink. While it's understandable that some people believe that fat is turned into muscle, a direct conversion isn't occurring.

Further, research shows that these fat cells don't simply disappear:

• Subcutaneous fat was examined in distance runners. Runners who were never fat were compared to runners who had been fat in the past. At the time of the study, both groups had relatively similar body fat percentages. However, when their sub-q fat samples were examined under a microscope, it was found that the "formerly fat" runners had more fat cells per gram of fat. In other words, their fat cells were smaller (Tremblay A, et al. 1984). Or, to reword it yet another way, exercise does not cause fat cells to melt away like ice cubes on a hot sidewalk. A better analogy would be like grapes shrinking into raisins.

4)  "Lactic acid is bad."

NOT EXACTLY. The burning sensation that you experience in your muscles when you exercise hard is thought to be due to the build-up of lactic acid.

Lactic acid forms in muscles when exercise intensity is fairly high. When exercise is intense, skeletal muscles need a lot of energy and they need it fast. Three different energy systems supply energy to exercising muscles. One of them -- the one that produces lactic acid -- is called "the glycolytic system" because glycogen is broken down as the energy source.  If enough oxygen is present, then pyruvic acid is generated. However, if exercise intensity requires more oxygen than can be supplied (here, think of the 100-meter dash...), then, lactic acid is produced. The build-up of lactic acid is thought to contribute to muscular fatigue.

The build-up of lactic acid is one of the factors that limits exercise performance.  The exercise intensity at which lactic acid starts to build up rapidly is called the "lactate threshold." Marathon runners know that, if they can keep their pace just below their lactate threshold, they'll avoid lactic acid-induced muscle fatigue. It is believed that, if 2 athletes have the same VO2max, the one who has the higher lactate threshold (ie., the one who hits his/her lactate threshold at a higher level of exercise intensity) has the advantage.

This is the classical view on lactic acid (ie., that lactic acid is bad because it limits performance). However, it's not really correct to think of lactic acid as "bad." In fact, there is a new school of thought regarding lactic acid:

George Brooks, a PhD in exercise physiology at UC Berkeley, came up with a different perspective on lactic acid beginning in the early 1970's (Brooks GA, et al. 1973).  He found that muscle cells actually use lactic acid as an energy source.  For many years, his new theory on lactic acid was met with resistance by the scientific community.  Now, it is known that, as fitness improves, muscle cells -- specifically, the mitochondria inside muscle cells -- adapt in order to utilize more lactic acid for fuel.  (For those who want to read a good scientific paper on this, see the report by Hashimoto and Brooks in the March 2008 issue of Medicine & Science in Sports & Exercise.)

So, while it's understandable why athletes and trainers think of lactic acid as "bad", it does, nonetheless, have some beneficial effects.

5) "Huge muscles represent the epitome of fitness."

FALSE. This one is definitely false and it's important to clarify why.  Although body composition is a component of "fitness" (as fitness and health are inseparable), it is incorrect to view a heavily-muscled body as the epitome of fitness.  An extremely low body fat percentage can lead to serious health problems just as readily as when body fat percentage is too high.  Second, many heavily-muscled people focus only on weight-training. During resistance exercise, blood pressures can hit astronomical levels. In one report, BP in a male athlete hit 480/350 during leg press! (MacDougall JD, et al. 1985). When resistance exercise is the only type of exercise a person performs, the walls of the arteries adapt to these extremely high blood pressures by becoming stiffer.  This is known as "decreased compliance" and it can occur in as little as 4 months (Miyachi M, et al. 2004).  Decreased compliance of the circulatory system is generally thought to be an undesirable health finding.

6)  "Runners have more knee problems in old age than non-runners."

FALSE. This one never seems to go away despite research refuting it over a span of more than 30 years:

According to the November 2005 issue of Runner's World magazine (p. 27): "There isn't any data to show a connection between running and osteoarthritis." In fact, the evidence shows that runners actually have fewer knee problems in old age compared to non-runners:  A 14-yr study by Stanford researchers showed that elderly runners had less pain and less arthritis than non-runners (Bruce B, et al. 2005).  When the bones and joints of older runners were examined, it was found that runners had better bone density (Lane NE, et al. 1986) and better functioning knee cartilage (Roos EM, et al. 2005) than non-runners.

And whether it's relevant or not, this is worth mentioning: Dean Karnazes began his ridiculously-extreme ultra-distance running at age 30. One accomplishment he is famous for in 2006 (at age 44) is running 50 marathons in 50 50 different states!  Yep, that's right:  every day for 50 consecutive days, he ran a marathon in a new state. His final marathon was the NYC Marathon. After that, he ran back to St. Louis where he started: a final run of 1300 miles! In 2009, Karnazes stated that he has had his knees X-ray'd and they were found to be normal (source: Competitor magazine, June 2009).

Well, at least his knees are.....

7)  "Stretching prevents injuries."

IT DEPENDS.  For this statement, it's more a matter of clarification than a myth.  In my opinion, this issue was best explained by Erik Witvrouw, PhD, and colleagues at Ghent University in Belgium.  They believe that whether or not stretching prevents injuries depends on the sport or activity.  For sports that provide limited "shock" to the muscle-tendon unit (eg., cycling, rollerblading, swimming, etc.), stretching prior to these activities probably does little to prevent injuries.  However, for sports that stress the muscle-tendon unit a lot more (eg., basketball, football, rugby, soccer, tennis, volleyball, or any activity that involves lots of jumping and/or change-in-direction running), then, stretching does seem to help prevent injuries (Witvrouw E, et al. 2004).


Readers may also be interested in:


Brooks GA, Bruaner KE, Cassens RG. Glycogen synthesis and metabolism of lactic acid after exercise. Am J Physiol 1973;224:1162-1166.  Abstract

Bruce B, Fries JF, Lubeck DP. Aerobic exercise and its impact on musculoskeletal pain in older adults: a 14 year prospective, longitudinal study. Arthritis Res Ther 2005;7:R1263-R1270.  Abstract

Hashimoto T, Brooks GA. Mitochondrial lactate oxidation complex and an adaptive role for lactate production. Med Sci Sports Exerc 2008;40:486-494.  Abstract

Lane NE, Bloch DA, Jones HH, et al. Long-distance running, bone density, and osteoarthritis. JAMA 1986;255:1147-1151.  Abstract

MacDougall JD, Tuxen D, Sale DG, et al. Arterial blood pressure response to heavy resistance exercise. J Appl Physiol 1985;58:785-790. Abstract

Miyachi M, Kawano H, Sugawara J, et al. Unfavorable effects of resistance training on central arterial compliance. Circulation 2004;110:2858-2863.  Abstract

Roos EM, Dahlberg L. Positive effects of moderate exercise on glycosaminoglycan content in knee cartilage. Arth Rheum 2005;52:3507-3514.  Abstract

Tremblay A, Despres JP, Bouchard C. Adipose tissue characteristics of ex-obese long-distance runners. Int J Obes 1984;8:641-648. Abstract

Warpeha JM. Aerobic and resistance training in the same day. NSCA's Performance Training Journal. vol. 6, no. 2, pp. 6-7.  (no abstract)

Witvrouw E, Mahieu N, Danneels L, et al. Stretching and injury prevention: an obscure relationship. Sports Med 2004;34:443-449.  Abstract


Stan Reents, PharmD, is available to speak on a variety of exercise-related topics. (Here is a downloadable recording of one of his Health Talks.) He also provides a one-on-one Health Coaching Service. Contact him through the Contact Us page.


Stan Reents, PharmD, is a former healthcare professional. He is a member of the American College of Lifestyle Medicine (ACLM) and a member of the American College of Sports Medicine (ACSM). In the past, he has been certified as a Health Fitness Specialist by ACSM, as a Certified Health Coach by ACE, as a Personal Trainer by ACE, and as a tennis coach by USTA. He is the author of Sport and Exercise Pharmacology (published by Human Kinetics) and has written for Runner's World magazine, Senior Softball USA, Training and Conditioning and other fitness publications.

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