Efficiency of Muscular Exercise in Health andDisease: Some Perspectives

Susan A. Goldstein through the production of exhaust steam. Automobile en-Columbia University gines typically operate at 20-25% efficiency, compared to

the 30-35% efficiency of diesel engines.

Elsworth R. Buskirk Any living creature can be considered as a mechanicalPennsylvania State University device (machine) fueled by chemical energy. Efficiency would

therefore be calculated as the ratio of work performance to

EXERCISE EFFICIENCY is defined as the ratio of work done energy expended. Clearly, the transformation of chemicalEat a steady-state to the energy expended to accomplish a energy into mechanical energy represents a conceptualgiven exercise. In animals (humans), the rate of oxygen thermodynamic system [11. Although no living organism is inconsumption (VO2 = cm3/min) is a measure of energy true equilibrium with its environment, steady-states duringexpended, while steady-state work output is measured in rest and exercise provide good approximations [2]. The termterms of power (force x distance/time). To obtain a meaning- "exercise" efficiency (or "work" efficiency) is believed to beful ratio, both quantities are converted to equivalent caloric the best description of human muscular efficiency because(heat energy) values. Standard methods of measuring V02 the original term, "mechanical" efficiency, was designatedand power output (work/time) are described below. for machines and does not necessarily refer to the efficiency

Efficiency was first described in machines during the period of muscular contraction and body movement [6].of the industrial revolution when it was termed "mechanical One of the original studies observing the efficiency ofefficiency." The mechanical efficiency of a machine is exercise was conducted in 1846 by the English engineerdetermined by measuring the ratio of the actual work done by James Prescott Joule who worked with horses [3]. Hea machine to its theoretical work potential. calculated equine efficiency at 25% by comparing the energy

Useful work output will always be less than work input, as value of a mixture of oats and corn consumed in one day withenergy is lost through various types of friction: i.e., the the work performed by the horse. This efficiency rating wassecond law of thermodynamics holds. According to the first considerably higher than the 10% efficiency he calculated forlaw of thermodynamics, energy cannot be created or de- the best Cornish steam engine of that time. Numerous studiesstroyed; it is possible, however, for energy to be converted of energy expenditure in humans using various crude meth-from one form to another. "Wasted" energy refers to the ods were performed thereafter; however, it was not untilenergy lost during conversion. The greater the amount of 1914 that experiments were conducted to calculate humanwasted energy, the lower the efficiency of the machine. A efficiency during exercise [4]. Benedict and Cathcart [51 werelow-efficiency machine will lose large amounts of energy the first to show a relationship between the mechanical workthrough heat storage and loss involving radiation, conduc- done by the human body in a steady-state and the equivalenttion, convection, and friction. Steam engines lose energy energy expended by both trained and untrained cyclists using

a stationary cycle ergometer with an electric brake. Morerecent data show that when the body is performing optimally,

Susan A. Goldstein is a Clinical Researcher in the efficiency of moderate-intensity exercise will be 20-25%.the Metabolism Unit at The Presbyterian Hos- During sedentary activities such as sitting or standing,pital in New York City and Adjunct Associate efficiency will be close to 0% because no external movementProfessor at Bridgeport University. She re- is performed, while energy is lost as heat.ceived the BS in biology from Syracuse Uni- There are different methods of calculating exercise effi-versity In 1980, the MS in nutriton from ciency, which vary according to their baseline correctionColumbia University in 1982, and Is presently factor, and must therefore be interpreted differently. Foura PhD candidate at the Institute of Human definitions of work efficiency have been described [7]: (1)Nutriton of Columbia University. (Department "gross" efficiency, which uses no baseline correction; (2)of Anesthesiology, Columbia University. 630 "net" efficiency, in which resting energy expenditure is usedWest 168 Street, New York, NY 10032). as base-line correction; (3) "work" or "cycling" efficiency,

which uses the energy cost of loadless pedaling as base-linecorrection ("work efficiency" can be used as a general term,

Elsworth R. Buskirk is Professor of Applied but indicates a more specific type of efficiency in the presentPhysiology at Pennsylvania State University. paper); and (4) "delta" efficiency, which uses two exerciseHe received the BS in biology from the Univer- intensities and measures the ratio of the change in intensityity of Wisconsin at St. Olaf in 1950, and from to the change in expended energy. The energy required tothe University of Minnesota the MS in 1951 move the legs and to overcome the resistance of the wheeland the PhD in 1954 in physiological hygiene. can be measured during loadless pedaling (zero workrate), as

in "work" efficiency. Using this method, one can avoid anunderestimation of efficiency. Certain investigators [11, how-ever, believe that loadless pedaling does not always representthe energy cost of moving the legs. "Delta" efficiency is,therefore, considered the most accurate estimation of effi-

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ciency, because it compares the change, in energy expendi- TABLE Iture from one exercise intensity to another, thus eliminating Gross exercise efficiency for men and women performing differentthe energy cost of moving the legs. activities

This paper will discuss exercise efficiency as it applies to ________activities of normal healthy adults and children and also to Activtysome conditions in the clinical setting. A few basic points willbe posited here to provide a framework for this discussion: Walking 20.6-43.0% [361

1. The efficiency of muscular exercise= power (or work) output Running 40.0-50.0% [14]power(orwork)input Swimming 2.9-7.4% [18] 2.7-9.4% [1812. A higher exercise efficiency for a given task means a Cycling

lower energy expenditure. (75-300 kgm/min) 7.4-12.2% [22] 11.0-22.3% [20]3. To be less efficient means that the caloric cost of an Cycling

activity is greater: i.e., in an endurance event such as (400-600 kgm/min) 14.9-18.8% [22]marathon running, a reduced efficiency is a considerabledrawback.

4. An ergometer is an apparatus designed to measure thework performed by a group of muscles; the most common of efficiency was greatest when the individual was running atthese devices is the cycle ergometer, which measures the his natural stride length. To maintain running speed, it iswork performed by the leg muscles. generally more efficient to shorten rather than lengthenHuman exercise efficiency is dependent on a number of running stride, and then increase the rate of steps.

factors. These include type of activity, such as walking, Swimming can be intensive, since almost all muscles of therunning, and cycling, and individual variation, which is body are used and energy must be expended to maintain bodyinfluenced by body size, fitness level, and skill in performing a position and overcome water resistance. It is therefore notgiven task. Training will usually increase exercise efficiency in surprising that V02 per unit distance is higher in swimmingperforming most activities, but not always. For example, than in other activities [18]. In fact, the energy cost ofOlympic medal bicyclists and untrained persons had the same swimming a given distance is about four times greater thanexercise efficiency [81 on a cycle ergometer. The effects of that of running that same distance [19]. Nevertheless,training will depend on the age, health, and coordination of swimming can be an easy task for obese or older, fatterthe subject. In general, the variability in efficiency between individuals, who float easily due to a high proportion of bodyindividuals is greater than the overall increase in efficiency fat. For the same reason, women, who normally havethat results from certain types of training, considerably more body fat than men, can swim a givenMost studies of human exercise efficiency have been distance with about 70% of the energy that men would use

performed on adults, with only a few reported on children. [191. Small increases in efficiency will occur with trainingEstimated energy expenditures for children have commonly because swimming style and technique is quite important.been extrapolated from adult values. The studies carried out The most accurate studies of exercise efficiency have beenon children show that they have higher values of V02/kg body carried out using steady-state exercise on the cycle ergome-weight as compared to adults when running at the same ter. Workrate on a cycle ergometer is relatively independentspeed [9, 10, 111. Children are reportedly less efficient than of the subject's weight, and can thus be more accuratelyadults. This may be due to inferior technique, smaller size, determined than on a treadmill or steps. Efficiency of cyclingshorter legs, and/or changing body size. Girls are slightly but is estimated at 25%, but varies with speed of pedaling,consistently more efficient than boys. external workrate, and training. Women are reported to beWalking or running on level ground is not considered true more efficient than men when cycling at a given pedaling

work because there is no impeding force or slope of incline frequency and resistance [20, 21]. This advantage has been(Work = force x distance). Body weight, speed, gradient, attributed both to women's generally smaller size and toand surface type [121 will alter the efficiency of either of gender-based differences in substrate and/or hormonal re-these activities. Wind resistance will also influence energy sponses to exercise.expenditure, especially in running, and can increase the total Up until now, we have used exercise efficiency in a generalenergy cost at sprint speed by as much as 13% [131. Walking sense to mean "gross" efficiency. However, because cycleis more efficient than running as long as the speed is kept ergometry has been studied in more detail than the otherbelow about 4 km/hr. Running, however, is more efficient activities, it will be discussed in terms of "gross", "net",than walking at higher speeds (Table I). This higher efficiency "work", and "delta" efficiency calculations. Although re-is attributed to the elastic properties of skeletal muscle,which stores energy when stretched and recovers it whenshortened [14]. As compared to walking, the forward mo- 4.60mentum of the body during running is consequently enhancedwithout a proportional increase in energy expenditure. The E 4.40combination of a relatively lower energy expenditure and _higher work output results in a relatively high running xefficiency. 4 20Enhanced efficiency is also associated with a greater ,L

percent of slow-twitch as opposed to fast-twitch muscle ' 4.00- Xfibers [1 51. Marathon runners, who have a greater quantity of ° -_Islow-twitch fibers and more highly developed slow muscles, C,I I I I I I, Iare 5-10% more efficient than middle-distance runners [161. 138 142 146 150 154 158 162 166 170Furthermore, a naturally long running stride is associated with Stride length. cmgreater speed and efficiency [81. Increasing stride to an Figure 1. Oxygen uptake during running at a speed of 16 km/hr withunnatural length, however, can decrease speed and effi- different stride lengths. The encircled cross represents the freelyciency. Hogberg [ 1 71 measured "02 while a subject ran at 16 chosen length of stride. Reproduced with permission from Hogberg,km/hr with different stride lengths (Fig. 1), and found that 1952 1161.

8 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1986

ported absolute values for efficiency are inconsistent, acertain trend for each definition can be described. Investiga- Ergometry-tors have consistently found that "gross" efficiency in- -______creases with increasing work rates at all pedaling speeds,while increasing pedaling frequency has been reported to Armdecrease "gross" efficiency (Table II) [7, 211. "Net" effi- Armciency increases less dramatically with increasing workrateexcept at the lowest pedaling speeds of 30-50 rpm, where Legefficiency decreases [21]. When pedaling rate is increased,the "net" efficiency decreases at all work intensities. (upright)"Work" efficiency increases as the workrate is increasedfrom light to moderate intensity, but decreases with higherworkrates [5, 7, 22]. As pedaling speed increases, "work" Leg (supine)efficiency also increases [7]. Although the data on "delta"efficiency are more limited and slightly less consistent, mostinvestigators would agree that the mean "delta" efficiency isapproximately 28%, and that it decreases with increasingworkrate [22, 23] and increasing pedaling speed [7] (Table 11).

Different muscle groups and posture also appear to influ-ence the efficiency of muscular exercise (Fig. 2). Powers etal. [24] demonstrated that arm ergometry results in trends of"gross", "work", and "delta" efficiencies (as a function ofincreasing power output) similar to those of leg exercise, butis less efficient [22]. The relatively greater energy expendi-ture during arm exercise has been attributed to the somewhatlarger fraction of energy required to stabilize posture. Supineas compared to upright exercise results in an increased Figure 2. Arm ergometry in upright position; leg ergometry in supineefficiency, a larger oxygen deficit [23, 241 and a less marked and upright positions.change in efficiency as the severity of exercise is increased[221. The differences observed during supine exercise havebeen associated with gravitational factors limiting muscle "useful work" performed per unit of time during steady-stateblood flow. exercise, and can be measured in terms of power (force x

MEASUREMENTS distance/time).Power or Work Output In cycle ergometry, a friction-type cycle ergometer isPoweror Work Outpu commonly used, which is able to increase or decreaseMeasuring exercise efficiency involves the measurement of commonly ue whichi ablet increase tode a

powr otpu anenrgyinpt. owe ouputis qua totheresistance, or workload. The cycle is calibrated to give apower output and energy input. Power output is equal to thedirect read-out in power units, i.e., watts or kilogram-meters

TABLE 11 per unit time. The power output must then be converted toTABLEwi the same unit of measure as the energy input. The factor toExercise efficiency (gross, net, work, and delta) (%) with pedaling convert kcal/minute into kilogram-meters/minute is 426.8

frequency and power intensity in men kgm/kcal.

To perform useful work on a treadmill, a subject walks orPower (kgm/min) runs up a grade greater than zero degrees, i.e., he lifts his

weight to a given height. The calculations of "gross", "net",Frequency (rpm) 75-150 200-400 600-880 and "delta" efficiency on the treadmill are similar to thosedescribed for ergometry. "Work" efficiency for treadmill

Gross exercise, however, is calculated using the energy cost of30-50 1 1.9 16.4 19.8 horizontal walking or running instead of that of loadless60-80 - 14.0 19.6 pedaling as the baseline correction. Percent grade is defined60-80 -

14.0 159. as units of climb per 100 horizontal units. Power output is

equal to the weight of the subject multiplied by the verticalNet distance walked up the treadmill incline over a period of time.

30-50 24.8 25.0 23.6 Energy or Work Input60-80 15.9 20.3 23.2 The measurement of energy expenditure usually involves100 - 12.4 17.2 some form of calorimetry. The heat lost by the human body

100 - 12.417.2 can be measured by direct calorimetry or estimated byWork indirect methods.

40 - 25.9 26.3 One type of direct calorimeter consists of a thermally60-80 35.2 29.3 26.9 insulated chamber containing a multi-layered wall. Heat100 - 28.0 2867 leaving the body by radiation, convection, and conduction, is

100 2.08. absorbed and carried away by a stream of sub-skin-tempera-Delta ture water flowing through pipes attached to the inner walls.

40-60 33.8 27.3 26.2 The increase in the water temperature is then measured. The9O _ 15.6 15 4 ~~~~~~~~productof the increase in water temperature, the specific90 15.6 15.4 ~~~~~~~~~heatof the water, and the water flow rate gives the amount

of heat carried away. To this is added the heat content ofrpm = revolutions per minute water vaporized from the subject's body, giving the total heatBased on information from Gaesser and Brooks [71, Hesser et al. energy generated. Thermocouples mounted in the walls

[22], Goldstein et al. (201 and Weissman et al. [231. monitor chamber temperature continuously, and the parallel

JUNE 1986 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE 9

walls are maintained at nearly constant temperature either by leg mass, 20-30 percent to a lack of physical training, and 5-water circulating through coils or by an electrical heating 1 5 percent to the increased work of breathing.system. These devices minimize any flow of heat between It appears that not only altered body weight, but nutritionalthe walls, making the chamber an adiabatic system. Direct intake as well can affect muscular efficiency. An increase incalorimeters are expensive to construct and difficult to energy expenditure (dietary induced thermogenesis), whichoperate, and as a consequence this technique is seldom used occurs to varying degrees after a meal (dietary inducedin studies of exercise efficiency. thermogenesis), maintains its magnitude during exercise,With indirect calorimetry, energy expenditure is calculated thereby reducing gross efficiency [30, 31].

from measurements of 02 consumption and CO2 production Hospitalization often includes periods of semi-starvation,by the subject. There are two methods: closed circuit and with patients receiving only a 5 percent dextrose solution onopen circuit. In the closed circuit system, the subject preoperative, post-operative, and special test days. Evenbreathes and rebreathes from a prefilled container (or spiro- short periods of semi-starvation will result in an adaptation tometer) of 02. The exhaled CO2 is absorbed by a canister of starvation, i.e., reduced metabolic rate with eventual weightpotassium hydroxide in the breathing circuit. A kymograph loss. A mere four days of semistarvation with 5 percentattached to the spirometer records changes in the volume of dextrose cause a significant increase in gross, net, and deltathe system as 02 iS consumed. Since the subject is re- efficiencies during submaximal cycle exercise [23, 301. Bybreathing only the gas in the spirometer, this is a closed contrast, semistarvation with an isocaloric amino acid infu-system. Closed circuit spirometry is not an optimal method sion results in an increase in metabolic rate and a small butfor measuring V02 during exercise because the machine is significant decrease in efficiency, as compared to a controlcumbersome, may impede breathing, and requires the subject diet [231. Since the body does not appear to adapt to semi-to remain close to the equipment. The system is generally starvation with amino acids, weight loss would occur at ainadequate for measurements during moderate and severe more rapid pace than it would with 5 percent dextrose. Thus,exercise. Consequently, the open circuit method has become amino acid infusions as the sole nutrient could be contraindi-the preferred method of gas analysis during exercise. cated in some hospitalized patients. On a short-term basis,With the open circuit system, the subject inhales room air however, this means of nutritional support might prove

of a known composition, i.e., 21% 02, 0.03% CO2 and 79% beneficial to the obese patient with life threatening complica-nitroo,c;. Analysis of the difference in composition between tions who must lose weight quickly.exhaled air and the ambient air reflects the body's gasexchange. Three techniques of open circuit calorimetry are CLINICAL DISORDERScommonly used: (1) A rigid transparent head chamber or Almost all patients with chronic disease have some workcanopy connected to the spirometer, which is worn over the intolerance (Table ll), perhaps due to one or more impair-head throughout the activity. (2) A valve system and light- ments in the 02 transport and utilization chain [32]. Interfer-weight, portable spirometer also worn during the activity. (3) ence with the normal coordination of cardiovascular, respira-A valve system and Douglas bag for intermittent collections tory, and/or metabolic mechanisms may result in inefficientof expired air. gas transport and reduced exercise efficiency.Comparative studies using both men and animals confirm Cardiac Disorders

that direct and indirect calorimetry methods provide equally Many cardiac conditions have little or no effect on workaccurate results and will often agree to within less than 1 % efficiency at submaximal exercise intensities. The cardiac[251. patient's primary problem during exercise is the deficit of 02

delivered to the body tissues. Even at a relatively low exerciseNUTRITION intensity, these patients show a reduced amount of 02The effects of nutritional deprivation on work efficiency extracted from the blood delivered to the tissues by each

have been studied in undernourished adults and children [23, heart beat (02 pulse).26, 271. An elaborate study by Spurr et al. [25] looked at the Respiratory Disorderseffects of marginal malnutrition (early onset) on efficiency in Respiratory patients with obstructive lung disease, depend-658 boys. Measurements of V02 were carried out during ing on the severity, can have a greatly increased V02 duringsubmaximal treadmill walking (15% grade) in boys 6 to 16 exercise and reduced exercise efficiency. The ratio of V02years of age. Gross efficiency increased with age and was predicted to V02 measured in patients with airflow obstruc-significantly lower in low-weight-for-age-and-height as com- tion during steady-state ergometry is given in Table IV. Thesepared to normal boys. However, "delta" efficiency showed elevated values are due to both the increased work tono significant differences between nutritional groups. It was ventilate the lungs (as a result of increased airway resist-concluded that marginal malnutrition had no effect on the ance), and inefficient respiratory muscle activity.efficiency of submaximal treadmill walking. It has been calculated [33] that the 02 cost of breathing atKeys' classic study [281 showed that following six months rest in normal individuals and those with chronic obstructive

of restricted food intake, with a resultant 25% body weightdecrease in men, V02 decreased not only at rest, but also TABLE IIIduring treadmill exercise. With refeeding, however, gross Gross exercise efficiency (GEE) in relation to chronic diseaseefficiency returned to normal. Chronically malnourishedwomen with anorexia nervosa also demonstrate a reducedresting V02 and increased gross efficiency during muscular DISEASE GEE DISEASE GEE DISEASE GEEexercise. It is evident that the metabolic response to starva- Cardiac Pulmonarytion includes not only adaptations during rest, but alsoincreased efficiency during muscular work. Periph. vascular - Circulation - Anorexia 1The effects of obesity on energy expenditure during rest Cardiac muscle - Airflow obstr. 4 Obesity 4

and exercise are well established. Obese persons, as com- Valvular - or 4 Fibrosis - or 4pared to normal weight subjects, expend more energy during Coronary artery - Chest wall - or 4rest as well as during treadmill and bicycling exercises [29].It is believed that 60-70 percent of this energy over- Revised from Wasserman and Whipp 132]expenditure in obese subjects is due to movement of a larger - = normal 4 = Decrease I = Increase

10 IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE JUNE 1986

TABLE IV 1978.Steady-state exercise in respiratory patients 3. Scoresby W and Joule JP: Mechanical powers of electro-magnetism:steam and horses. Philos Mag Vet, 28:448, 1846.

4. Atwater WO and Rosa EB: Description of a new respiration calorimeterSubject Age Diagnosis % Measured/PredictedV02 and experiments on the conservation of energy in the human body. U.S.

Department of Agriculture, Office Exp Sta Bull, No. 63, 1899.1. 53 Emphysema 191 % 5. Benedict FG and Cathcart EP: A metabolic study with special reference to

2. 35 Asthma 11 0% the efficiency of the human body as a machine. Carnegie Institute of2. 35 Asthma 110% Washington, Publication No 187, 1913.3. 52 Emphysem8 129% 6. Kuttinger HG: Force, work, power, and exercise. MedSci Sport, 10:227-4. 35 Asthma 126% 228, 1978.5. 33 Chronic Bronchitis 125% 7. Gaesser GA and Brooks GA: Muscular efficiency during steady-stateexercise: effects of speed and work rate. J Appi Physiol 38(6):1132-1139,

1975.Revised table from Wasserman and Whipp [321 8. Astrand PO and Rodahl K: Textbook of Work Physiology, 2nd Edition,McGraw-Hill, New York, p. 436, 1977.

9. Astrand PO: Human physical fitness with special reference to sex andage. Physiol Rev, 36:307, 1956.

pulmonary disease (COPD) is approximately 4% and 15%, 10. Bar-OrO, Shephard RJ and Allen CL: Cardiac output of 10-13 year-oldresp of t l 02 rboys and girls during submaximal exercise. J Appl Physiol, 30(2):219-223,respectively, of total 02 requirement. Levison and Cherniak 1971.

[331 found that, during exercise, normal subjects used 10- 11. Malina RM and Roche AF: Manual of Physical Status and Performance,15% total V02 for the work of the respiratory muscles. In Vol. 2, Plenum Press, New York, p. 151, 1983.contrast, the respiratory muscles of patients with COPD use 12. Givoni B and Goldman RF: Predicting metabolic energy cost. J Applcontrast, theareVr2. The extent of increased work can vary Physiol, 30(3):429, 1971.35-40% of total V02. The extent of increased work can vary 13. Pugh LGCE: Wind resistance in running and walking and the mechanicalconsiderably. On the average, however, V02 at any given efficiency of work against horizontal or vertical forces. J. Physiol, 213:255-exercise level was increased by about 250 cm3/min in 276, 1971.patients with severe airway obstruction [34]. 14. Cavagna GA, Saibene FP, and Margaria R: Mechanical work in running.Respiratory disease becomes progressively worse with JAppIPhysiol, 19121:249-256, 1964.15. Davies RE: Bioenergetics of muscular contraction. In Control of Energyincreasing age, and increasing age is associated with a Metabolism, B Chance, RW Estabrook and JR Williamson (eds.), Academicdecline in exercise efficiency [351. Aging thus compounds the Press, New York, pp. 388-392, 1965.effect of COPD on exercise efficiency, resulting in an 16. Fox EL and Costill DL: Estimated cardiorespiratory responses duringexaggerated reduction in efficiency which may contribute to marathon running. Arch Environ Health, 24:315-324, 1972.

1 7. Hogberg P: How do stride length and stride frequency influence energythe unexplained weight loss observed in older COPD patients. output during running? Arbeitsphysiol, 14:437, 1952.Exercise efficiency is usually not affected by other clinical 18. Pendergast DR, diPrampero PE, Craig AB et al.: Quantitative analysis ofdisorders, with the aforementioned exceptions of obesity and the front crawl in men and women. J Appl Physiol, 43(3):475-479, 1977.19. Holmer 1: Physiology of swimming in man. In Exercise and Sportanorexia, where it is decreased and increased, respectively. Science Reviews, HS Hutton and Dl Miller (eds.), vol. 7, Franklin Institute Press

1980.20. SA Goldstein, C Weissman, J Askanazi, DH Elwyn and JM Kinney:

Ventilatory patterns during low-level supine exercise in males and females. FedCONCLUSIONS Proc, 43:320, 1984.Exercise efficiency is seldom measured directly, being more 21. Thompson EM: A study of energy expenditure and mechanical effi-often extrapolated from 02 uptake during steady-state exer- ciency in young girls and women. Dissertation for PhD, Columbia University,New York, 1940.cise. Consequently, power output should be measured so 22. Hesser CM, Linnarsson D and Bjurstedt: Cardiorespiratory and meta-that the influences on exercise of disease, training, sex, age, bolic responses to positive, negative and minimum-load leg exercise. Resp

etc., can be compared. In activities such as walking, running, Physiol, 30:51-67, 1977.cycling and swimming, efficiency is greatest during running 23. Weissman C, Goldstein SA, Askanazi J et al.: Semistarvation andcyclin andsimmin, effiiency s gretest dring rnning exercise. J Appl Physiol, (Submitted).[131 and lowest during swimming [1 7]. Cycle ergometry has 24. Powers SK, Beadle RE and Mangum M: Exercise efficiency during armbeen studied most extensively, primarily because this exer- ergometry: Effects of speed and work rate. J Appl Physiol, 56(21:495-499,cise can be performed in a relatively stationary position, and 1984.yields accurate measurements of power output and input. 25. McArdle WD, Katch Fl and Katch VI: Exercise, Physiology, Energy,Nutrition, and Human Performance, Lea and Febiger, Philadelphia, 1981.Children are less efficient than adults, and women more 26. Spurr GB, Barac-Nieto M, Reina JC and Ramirez R: Marginal malnutri-efficient than men during exercise. Training generally en- tion in school-aged Columbian boys: Efficiency of treadmill walking in submaxi-hances efficiency for many types of activities and in most mal exercise. Am J Clin Nutr, 39:452-459, 1984.individuals, but the increases with training are small com- 27. Barac-Nieto M, Spurr GV, Dahner HW et al.: Aerobic work capacity and

individuals,but the increases with training are small com endurance during nutritional repletion of undernourished men. Am J Clin Nutr,pared to variations of efficiency among individuals. Gross, 33:2268-2275, 1980.net, and delta efficiencies increase in response to semi- 28. Keys A, Brozek J, Henschel H et al.: The Biology ofHuman Starvation.starvation with a 5% dextrose solution or mixed diet, but University of Minnesota Press, Minneapolis, 1950.

29. Anton-Kuchly B, Pogor P and Varene P: Determinants of increaseddecrease during semi-starvation with amino acid infusions,energy cost of submaximal exercise in obese subjects. J Appl Physiol: RespThe clinical impact of these findings is important for both the Environ Exercise Physiol, 56(1):18-23, 1984.

malnourished and the obese hospitalized patient. Disease 30. Bray GA, Whipp BJ and Koyal SN: The acute effects of food intake onstates will influence exercise efficiency; a reduction is energy expenditure during cycle ergometry. Am J Clin Nutr, 27:254-259,commnly oun in atiets ith hronc ostrutiveand 1974.commonly found in patients with chronic obstructive and 31. Dagenais GR, Oriol A and McGregor M: Hemodynamic effects ofrestrictive lung diseases. This reduced efficiency is attributed carbohydrate and protein meals in man: Rest and exercise. J Appl Physiol,to the increased work of breathing, which is in turn a result of 21:1157-1162, 1966.

increased airway resistance and inefficient, uncoordinated 32. Wasserman K and Whipp BJ: Exercise physiology in health and disease.respiatorymusceactvity The educion i execise ffi- 33. Levison H and Cherniak RM: Ventilatory cost of exercise in chronicciency Is less prevalent in cardiac and other disease states. obstructive pulmonary disease. JAppiPhysiol, 25:21-27, 1968.

34. Jones NI, Jones G and Edwards RHT: Exercise tolerance in chronicairway disease. Am RBev Resp Dis, 102:477-491, 1971.~~~~~~~~~~~~~~~~~35.Patrick JM, Bassey EK and Fentem PH: The rising ventilatory cost ofRFRENCES bicycle exercise in the seventh decade. Clin Sci, 65:521-526, 1983.1. Whipp BJ and Wasserman K: Efficiency of muscular work. J Appl 36. Donovan CM and Brooks GA: Muscular efficiency during steady statePhysiol, 26151:644-648, 1969. exercise. 2. Effects of walking speed and work rate. JAppIPhysiol, 43(3):43 1-2. Barham JN: Mechanical Kinesiology. C. V. Mosby, St. Louis, p. 271, 439, 1977.

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