Endurance training sessionsshould be performed a minimum of threedays per week for 30-60 minutes at mod-erate to high intensity to achieve this train-ing benefit. Some authors recommend lowintensity aerobic exercise for fat loss, be-cause at low intensity a greater percent-age of utilized energy is derived from fat.While this is true, low intensity aerobicexercise is not effective in eliciting themetabolic adaptations which bring abouta shift in energy substrate utilization pat-terns. Furthermore, low intensity aerobicexercise does relatively little to improvecardiovascular and respiratory fitness.While bodybuilders appropriately shouldfocus their training on resistance exercise,they will achieve a higher degree of mus-cularity and leanness if they also includea component of vigorous aerobic exer-cise.IntroductionOptimal endurance training is ofgreat interest not only to endurance ath-letes but to bodybuilders as well. Thisseries of articles will focus on how to useendurance training to help you achieveyour physique goals. Aerobic exercise isthe most effective way to lose body fat,and I’ll explain how to train optimally toburn fat without sacrificing muscle. We’llalso talk about ways to maximize your en-durance performance.General PrinciplesTwo general concepts underpinany successful exercise training program.
The Overload Principle describes the ideathat an exercise stimulus must be of somethreshold intensity to bring about a train-ing adaptation (1,2). Exercise representsa form of stress, and the body adapts tothat stress by getting stronger. To forcethe body to continue to adapt, the stimu-lus must continually become more intense.This is known as Progressive Overload.We can increase the training intensity byincreasing the load (the resistance), theworkout frequency, the workout dura-tion, or the power output (work performedper unit time). The most effective way toproduce increases in muscle size andstrength is to increase the load. The mosteffective way to improve endurance per-formance is to increase workout duration.The best way to improve speed is to in-crease power output during the workout.The Overload Principle (sometimes calledThe Intensity Principle) applies to endur-ance training as well as to resistance ex-ercise.The Specificity Principle statesthat the metabolic adaptations that occurin response to a training stimulus are spe-cific to the type of overload applied (1,2).Resistance training causes increases inmuscle size and strength (if it’s intenseenough) and aerobic exercise causes im-provements in cardiovascular endurance,with surprisingly little carry over betweenthe two (1).
Specific exercise elicits spe-cific adaptations creating specific train-ing effects (1).Metabolic AdaptationsAerobic conditioning results inmetabolic adaptations that improve energyproduction (1). Mitochondria from skel-etal muscle acquire a greatly increased ca-pacity to generate ATP by oxidative phos-phorylation. Mitochondria are the smallfurnaces inside cells where food is burned(oxidized) to produce energy. Oxidativephosphorylation is the biochemical path-way mitochondria use to combine fuelsubstrate molecules from food with oxy-gen, resulting in a release of energy whichis used to form ATP. Aerobic trainingmakes mitochondria more efficient at thisprocess, which means they can makemore ATP to power muscle fiber contrac-tions. This is a benefit of aerobic exercisethat you don’t get from weight lifting.Associated with the increased capacity formitochondrial oxygen uptake is an increasein the size and number of mitochondriaand a potential two-fold increase in thelevel of aerobic energy producing enzymesystems (1).
These adaptations are re-quired to sustain a high percentage of aero-bic capacity during prolonged exercisesessions. Animal studies have shown thatskeletal muscle myoglobin can increaseby as much as 80%. Myoglobin is a protein very similar to hemoglobin, exceptmyoglobin is found in muscle cells whilehemoglobin is found in red blood cells.Like hemoglobin, the function of myoglo-bin is to bind oxygen, and an increase inmyoglobin can facilitate oxygen deliveryto mitochondria.Aerobic training causes an in-crease in the muscle’s ability to mobilizeand oxidize fat. This occurs by an in-crease in blood flow within the muscleand in the activity of fat-mobilizing andfat-metabolizing enzymes (1). At anysubmaximal work rate, a trained indi-vidual uses more free fatty acids forenergy than an untrained person (1,2).This is a key point and deserves someemphasis. Aerobic exercise training en-hances the muscle’s ability to use fat asa fuel source and causes a shift in en-ergy substrate (fuel) selection such thatthe trained muscle learns to rely moreon fat as an energy source and less oncarbohydrate. This is important to en-durance athletes because increased useof fat as an exercise fuel has a carbohy-drate sparing effect – the more fat wecan burn the longer the carbs will last.Since carbohydrate (glycogen) depletionis a major factor limiting endurance, thismeans improved performance.
This isalso very important to bodybuilders be-cause it offers a way to shift your me-tabolism into a fat-burning mode. Aero-bic training teaches your muscles to burnmore fat and less carbs. This happensat rest as well as during submaximal ex-ercise. (During maximal exercise, carbsare still the main fuel.) Notice what hap-pens if you combine this approach witha very low fat diet. The aerobic trainingshifts your muscle’s fuel selection intofat-burning mode, and your body be-comes a fat burning machine. But there’sno fat in your diet. So where does thefat come from to fuel your muscles?From stored body fat. By combiningproper training and nutrition techniquesyou can teach your body to draw on itsown stored fat as a primary energysource.Cardiovascular andRespiratory AdaptationsThe weight and volume of theheart increase with long-term aerobictraining (1). This is characterized by anincrease in the size of the left ventricularchamber and by a thickening of its walls.The left ventricle is the chamber of theheart which pumps blood out to the body,and intense exercise makes it get biggerand stronger, just like any other muscle.This means it can pump harder and de-liver a larger volume of blood per minuteto working muscles.
This in turn meansmore oxygen delivery, more energy pro-duction, and more muscular power out-put. The heart’s stroke volume increasessignificantly at rest and during exercise.Stroke volume is the volume of blood theleft ventricle can eject in one beat. Sincethe left ventricle is larger and stronger, itcan pump out more blood in a single beatthan before training. Resting andsubmaximal heart rate are decreased dur-ing aerobic training. Since the heart canpump more blood with each beat, itdoesn’t need to beat as often and heartrate is decreased compared to beforetraining. Plasma volume and total hemo-globin content of the blood increase withendurance training. This also improvesoxygen delivery.One of the most significantchanges in cardiovascular function is anincrease in maximal cardiac output (1,2).Cardiac output is the volume of blood theheart can pump in one minute. The in-creased cardiac output is mediated largelyby the increase in stroke volume. Train-ing also produces a significant increase inthe amount of oxygen extracted from cir-culating blood (1,2). This is determinedby measuring the oxygen concentrationin arterial blood supplying a muscle andin venous blood leaving the muscle. Thedifference is referred to as the arterio-venous oxygen gradient, and it is increasedby endurance training because themuscles become more efficient at extract-ing oxygen from the blood.
This is prob-ably due to the increased capillary supplyof muscle fibers, as well as their increasedmyoglobin and mitochondrial content.Regular aerobic training reduces bloodpressure. Endurance exercise increases theventilatory capacity of the lungs by in-creasing both breathing frequency and tidalvolume (the volume of air per breath). Insubmaximal exercise the trained athleteventilates less than before training (mara-thon runners don’t get out of breath fromclimbing a flight of stairs).One of the most important adaptations toendurance exercise is an increase in thenumber of capillaries surrounding eachmuscle fiber (2). Endurance training canincrease capillary density of muscles by15% (and probably more, I suspect). Thisallows greater exchange of gases, heat,wastes, and nutrients between the bloodand working muscle fibers (2). This fa-cilitates not only energy production, butalso fat metabolism and muscular growth.These increases occur within the first fewweeks to months of aerobic training. Ifyou want to grow big muscles, you needto deliver nutrients to them. The nutrientsare delivered by capillaries. Do your aero-bics. Endurance is a term that actuallydescribes two separate components:muscular endurance and cardiorespiratoryendurance. Muscular endurance is theability of a muscle or muscle group tosustain high intensity repetitive exercise(2). Muscular endurance is highly relatedto muscular strength and anaerobic con-ditioning. An example is how many rep-etitions you can do with agiven weight on the benchpress.
Technically speaking,strength is defined as yourone rep maximum (1RM).Let’s say your one rep maxat bench is 225 pounds.That means you can prob-ably do 185 pounds for 8reps or so. If you trainbench for several weeks at185 pounds, pretty soonyou’ll be able to do 9 repsat 185. This is an increasein muscular endurance at185 pounds. From a strictlytechnical point of view, thisis not an increase instrength. To demonstratean increase in strength, youneed to increase your 1RM.Going from 8 reps at 185pounds to 9 reps at 185pounds probably won’t in-crease your 1RM by much, if any. How-ever, if you keep training soon you’ll beable to do 12 reps at 185, then the nexttime you test your 1RM you’ll find youcan push up 230 with no problem. Sowhile muscular strength and enduranceare separate concepts, they are closelyrelated. Another example of muscular en-durance is a static muscular contraction,such as a wrestler trying to pin his oppo-nent to the mat (2). Another examplewould be holding a leg extension in thefully extended position. Let’s say you canhold a leg extension at 150 pounds fullyextended for 10 seconds before you startto fail and lower the weight. After severalweeks of training you may be able to holdit for 15 seconds. This is an increase inmuscular endurance. (This technique,along with forced negatives, is in my bagof tricks for breaking through plateaus.)Whereas muscular endurancerefers to individual muscles, cardiorespi-ratory endurance refers to the body as awhole (2). It describes your body’s over-all ability to sustain prolonged rhythmicexercise.
Rather than being limited by theendurance of a particular muscle, your car-diorespiratory endurance is limited by yourbody’s energy producing ability, which isin turn limited by your ability to deliveroxygen to working muscle tissue, whichis in turn limited by your cardiovascularand respiratory systems. Most exercisephysiologists regard VO2 Max as the bestindicator of cardiorespiratory endurancecapacity (2). While strength, defined asthe one rep maximum, is the best way tomeasure performance improvements inresistance training, VO2 Max is the bestway to measure aerobic power (2). VO2Max is defined as the highest rate of oxy-gen consumption attainable during maxi-mal exhaustive exercise (2). You certainlycan exercise at intensities higher than yourVO2 Max, but this recruits the anaerobicenergy producing pathways. After aminute or two at this intensity fatigue willset in and muscular failure will occur. YourVO2 Max represents the highest level ofexercise intensity that you can sustain fora prolonged period of time. The VO2 Maxdictates the rate of work or the pace youcan sustain (2). Aerobic conditioning re-sults in an average increase of 20% VO2Max following six months of condition-ing. This is brought about bya combination of two factors.An increase in cardiac outputresults in more blood, andthus more oxygen, being de-livered to tissues. Second, anincrease in the arteriovenousoxygen gradient means thatmore of this oxygen is beingextracted from the blood bythe muscle.
This means thatmore oxygen is being used bythe muscle to produce energy,and more energy productionmeans more muscle powerand endurance.Lactate ThresholdWhen glucose is me-tabolized anaerobically (with-out oxygen) it is converted topyruvate and subsequentlyinto lactate (lactic acid). Lac-tic acid buildup inside muscle cells is oneof the factors that makes your musclesburn when you train a set of biceps curlsto failure. At lower intensity exercise, youreally don’t recruit the anaerobic energysystem because you don’t need it. (Referback to our series on cellular energy pro-duction.) During endurance exercise, yourbody can supply oxygen fast enough tothe muscles so that you can produce allthe energy you need from the oxidationof glucose and fat, without producing lac-tic acid. As exercise intensity increases,you eventually reach a level where theaerobic energy producing pathway ismaxed out, and anaerobic energy produc-tion begins. At that point, lactate is produced inside muscle tissue and begins toappear in the blood as a waste product.The lactate threshold is the point whereblood lactate begins to appear. Like VO2Max, this is a measure of cardiorespira-tory fitness. Endurance training increasesthe lactate threshold, which means ahigher level of energy production can oc-cur by the aerobic pathway before theanaerobic pathway is called into play.Trained endurance athletes can performexercise at a higher VO2 Max before bloodlactate appears.
This means that they canexercise at a higher intensity (they canproduce more power aerobically) beforeanaerobic metabolism begins.At first it might sound like VO2Max and lactate threshold are really twoways of measuring the same thing, butthey’re not. While they both reflect en-durance performance, they are looking atdifferent aspects. VO2 Max is a descrip-tion of the maximal aerobic energy pro-ducing ability of an athlete. Lactate thresh-old describes the percentage of VO2 Maxat which the athlete can train beforeanaerobic metabolism begins. The increasein lactate threshold, at a given percentageof VO2 Max, is probably due to a greaterability to clear lactate produced by themuscle (due to increased capillary den-sity of the muscle tissue bed), an increasein skeletal muscle enzymes involved inaerobic energy production, and a shift inmetabolic substrate to a fuel mix involv-ing a higher proportion of energy derivedfrom fat.These concepts lay the basicground work you need for a thoroughunderstanding of endurance exercisephysiology. Next month we’ll talk abouttraining intensity, respiratory quotient, fatmetabolism, and specific strategies onhow to incorporate endurance training intoyour program to maximize fat loss with-out losing muscle.
References1. McArdle WD, Katch FI, and Katch VL.Exercise Physiology: Energy, Nutrition,and Human Performance. Lea & Febiger,Philadelphia, 1991.
2. Wilmore JH and Costill DL. Physiol-ogy of Exercise and Sport. Human Kinet-ics, Champaign, IL, 1994.