Europ. J. appl. Physiol. 34, t91--197 (1975) 9 by Springer-Verlag 1975

Blood Flow in Thigh Muscle during Bicycling Exercise at Varying Work Rates* Flemming Bonde-Petersen, Jan Henriksson and Bj6rn Lundin Department of Physiology, Gymnastik-och IdrottshSgskolan, Stockholm, and Laboratory for the Theory of Gymnastics, August Krogh Institute, University of Copenhagen, Copenhagen Received January 28, 1975

Abstract. t6 male subjects exercised at 25, 50, 75, 90, 100 and 120% of l?oam~xon a yon D5beln bicycle ergometer. The muscle mass was measured in a whole body counter. Muscle blood flow (MBF) estimated from the rate of 13aXe clearance from m. rectus femoris showed a levelling-off at about 0.5 1 of blood per rain and liter of muscle tissue (equal to an irrigation coefficient of 0.5 rain-1) at work rates above 50 to 60 % of ~ max. This concurs with clearance data from the literature. However, when MBF is calculated from 17o2, muscle mass, and reliable values for a- voz differences, MBF in the present subjects would: 1. Not level off before 90 to 100% ~ 2 ~ , 2. reach a value of t.0 min-L The underestimation of MBF calculated from 13~Xe clearance and the levelling-off shown by this method may be due to a systematic error inherent in the method, the 13aXe clearance being diffusion limited at high flow rates. Key words: laaXe Muscle Clearance -- Muscle Mass -- ~~ Power -- Limiting Factors to Local Muscle Blood Flow.

-- Maximal Aerobic

A s m u s s e n et al. (i939) c a l c u l a t e d from v a r i a t i o n s in l?o~ d u r i n g isehaemic exercise t h a t m u s c l e blood flow (MBF) p e r k g of exercising muscle w o u l d r e a c h a

maximum of i.0 1 • kg -I • rain -I ( = an irrigation coefficient, k, of i.0 rain-l). A similar value could be calculated on the basis of data from Jorfeldt and VvTahren (1971), who measured local limb blood flow with a dye dilution technique. The d a t a also i n d i c a t e a r e c t i l i n e a r r e l a t i o n b e t w e e n M B F a n d exercise p o w e r u p t o l)o2max. W i t h t h e s a m e m e t h o d W a h r e n et al. (1974) found, in e l d e r l y men, t h a t b l o o d flow to t h e leg rose in l i n e a r p r o p o r t i o n to e x t e r n a l exercise power, e x c e p t a t t h e h e a v i e s t w o r k loads where it t e n d e d to level off. This r e c t i l i n e a r r e l a t i o n is s u p p o r t e d b y m e a s u r e m e n t s using t h e clearance r a t e o f l~aXe from exercising quach'iceps m u s c l e ( G r i m b y e~ al., 1967; P i r n a y et al., i972). H o w e v e r , m a x i m u m k-values m e a s u r e d were o n l y a b o u t 0.5 rain -1. Clausen a n d L a s s e n (1971) also u s e d laaXe clearance a n d o b t a i n e d a m a x i m u m k of 0.5 rain -~, b u t t h e y i n d i c a t e a levelling-off of M B F a t s u b m a x i m a l values as also f o u n d b y B o n d e - P e t e r s e n et al. (i970). W e d e s i g n e d t h e s e e x p e r i m e n t s for t w o reasons. F i r s t we w a n t e d to o b t a i n m o r e i n f o r m a t i o n a b o u t t h e o r d e r o f m a g n i t u d e o f b l o o d flow p e r v o l u m e of m u s c l e tissue, s e c o n d it was t h e o b j e c t to s t u d y w h e t h e r or n o t M B F is levelling o f a t s u b m a x i m a l flow rates. * This study was supported by a grant from the Swedish Medical Council and from Statens l~egevidenskabelige forskningsr&d, Denmark, project no 512-667 and 512-1156.

192

Flemming Bonde-Petersen et al.

Table I. ]~ean values (~), standard deviation (SD), and range of 16 subjects for maximal oxygen uptake rate (l?o2 ~ ) and muscle mass 2 Age, years Height, cm Body weight, kg 1 • mir~-1 ~ max ml x rain-1 x kg body weight-1 }Iuscle { kg mass in per cent of body weight l?o~. . . . ml • rain-1 • kg muscle mass-1

8D

25 6.8 '182 4.9 71.7 6.6 3.67 0.46 51.2 5.0i 27.4 3.6 38.4 5.3 ~33.9 15.4

Range 19--40 17'i --i90 59.4--82.4 3.09 --4.4 42--58 2~ A --35.8 27.7 --47.9 106--164

Material and Methods 16 healthy male subjects were employed in the study. Personal data including ~ : max and muscle mass are given in Table 1. Muscle blood flow (MBF) was calculated from the clearance rate of laaXe dissolved in isotonic saline (Kety, t949; Holzman et al., 1964; Lassen et al., t964) and injected into the belly of the m. rectus ]emoris about 15 cm proximal to basis patellae. A light-weight scintillation crystal detector (Meditronic) was strapped by adhesive tape to the thigh. The counts per l0 sec (C) were printed out and later plotted against time on semilogarithmic paper. The irrigation coefficient (k) was calculated from the steepest slope of the curve according to the equation: k = -- d log C/dt • 2.30 • rain-~, where ~ = 0.7 (Conn, 1961) is the tissue/blood partition coefficient. We rejected any curve indicating a rectilinear slope for less than 75 % of the depot. Muscle mass was determined from measurements of total body potassium in a whole body counter, where the 4~ content of the body was measured. The muscle mass was calculated as = 0.4 K--0.28 W--0.9 S where K is potassium in grams, W and S body weight and skeletal weight in kg, respectively (yon DSbeln, 1964). Skeletal weight was calculated from body height, radio-ulnar and femoral condylar breadths. Maximal oxygen uptake rate (Fo2~,~) was determined according to the levelling-off criterion during exercise on a friction ergometer at 60 rpm (Astrand and Saltin, 1961 ; Binkhorst and van Leeuwen, 1963). Expired air was collected in Douglas bags and analyzed with a modified Haldane gas analyzer for CO2 and 02 content. The expired volume was measured in a balanced Tissot spirometer.

Experimental Procedure T h e y o n D 5 b e l n ergometer was also used w h e n the 18aXe cIearance was measured. T h e subjects mair~tained a pedal f r e q u e n c y of 60 r p m with the aid of a m e t r o n o m e . T h e loads applied corresponded to 25, 50, 75, 90, ~00 a n d i 2 0 % of t h e load sufficient to elicit the I?o2 max. One or two work loads were performed on a g i v e n day. W h e n two works loads were performed on the same d a y a iest period of I to 2 hrs separated t h e experimeuts. Six of the subjects h a d only one leg studied, while t h e r e m a i n i n g l 0 subjects h a d b o t h legs s t u d i e d s i m u l t a n e o u s l y on each load with a scintillation detector on each thigh. On those l 0 subjects only the higher flow v a l u e of t h e two registered at each load was used i n the data, so t h a t the d a t a comprised only one leg from each subject. E a c h e x p e r i m e n t s t a r t e d with the subject sitting o n t h e bicycle with the detectors a t t a c h e d to one or b o t h thighs. The b a c k g r o u n d a c t i v i t y was first

Muscle Blood Flow and Aerobic Power

193

counted for 5 min. The b a c k g r o u n d values for t0 see counting periods were so low, however, in every case in comparison with the counts registered during the experiment t h a t t h e y were disregarded in the calculation of MBF. After the isotope had been injected into one or both legs, the subjects started to exercise immediately. I n this w a y the injection artifact was disregarded, as preliminary investigations had shown t h a t it was of no importance during exercise hyperaemia in muscles (Bonde-Petersen, unpublished results). The work time was 10 rain, except in case of h e a v y loads in which the exercise terminated at the point of fatigue. The wash-out of the isotope was followed over the entire exercise period.

Results The mean laSXe clearance increased with increasing l?o2 from 25% to 50% of ~ m a x (Table 2). Above this level only a small arLd insignificant increase was seen, from 0.402 rain -1 at 50% to 0.454 rain -1 at t 2 0 % l?o:max. I n the same interval between 50% and t20~ ~)~ max the 17o2h a d increased from 1.84 to 3.67 1 • m i n - L F r o m the present results on 1?o2max and muscle mass it was also possible to estimate the M B F during exercise. I f data for a- vo~ differences during exercise from the literature are used (Astrand et al., i964; P e m o w et al., 1965) it is possible to calculate a mean M B F dm~ing exercise from 17o2. The bulk of active muscle substantially contributing to the increase in 1?o2 during bicycling can be p u t at maximally 70% of the total muscle mass. This means t h a t 27.4 • 0.7 = 19.2 kg muscle (Table t) is involved in the exercise. The a-v differences over working muscle a~ 50, 75 and t 0 0 % [?o2max can be p u t at 0A50, 0.t70, and 0.190 10~ per liter of blood respectively. The corresponding 17o~(Table 2) is 1.8, 2.8, and 3.7 1 • m i n - L I f a resting 17o2of 0.3 1 • rain -1 is subtracted, M B F can be calculated as follows: At 50% 12o~.max, M B F = 1.4 • 0.15 -1 • t9.2 -~ ~ 0.5 rain-l; at~ 75% l?o2rnax, 3 ' [ B F = 2 . 4 • 0 . t 7 - 1 • i 9 . 2 - 1 = 0 . 7 m i n - 1 ; and at t 0 0 % l?o~max, M B F = 3 . 3 • 0.t9-* • t9.2 -1 = 0.9 rain -1. I f less t h a n 70% of the total muscle mass is involved in the exercise, the calculated values for M B F on the different relative loads will increase correspondingly. The fat content of h u m a n muscles will influence the clearance rate of lsaXe in a nega~ice direction, as t h e tissue blood partition coefficient for f a t t y tissue is about t0 (Lindbjerg et al., t966). The levelling-off of 13aXe would, therefore, be

Table 2. Mean value (k), standard deviation (SD), and range of 16 subjects for laaXe clearance from m. rectus femoris at different exercise powers on a bicycle ergometer expressed relative

tO go2max ]~, rain-1 SD taaXe clearance (rain -1) at 25% of I~o2 m~x( = 0.92 1. rain-1) 50 % of ?o, m~x( = 1.84 1. rain-1) 75 % of l?o~m~x( = 2.76 I. rain-1) 90 % of ~ m~x( = 3.30 1. rain-~) 100 % of IYo2m~x( = 3.67 1. mia-1) 120% of 17o2m~

0.2t9 0.402 0.404 0.409 0.427 0.454

0A~10 0A74 0.150 0.154 0.186 0.121

l~ange 0.023--0.449 0.079--0.630 0A~18--0.693 0.226--0.729 0.268--0.811 0.267--0.693

194

Flemming Bonde-Petersen et al.

assumed to be influenced by this effect. We postulated that the mean values of :3aXe clearance at levelling-off would be negatively correlated to the individual fat content of the body expressed as the reciprocal of the relative muscle mass in per cent of body weight. No such correlation was found.

Discussion The muscle blood flow (MBF) in the present investigation was examined in two ways. MBF estimated on the basis of k values from :3aXe clearance indicated at 50% of I?o~max a k equal to 0.40 to 0.45 rain -1. From calculations based on measurements of the subjects' muscle mass and Vozmax, MBF is not levelling-off until ~2 max is reached at a flow rate of 0.9 rain-:, or two times the k value just mentioned. The maximum k values obtained for 13aXe clearance (Fig, l) concur with those reported in the literature (Grimby et al., 1967; Bonde-Petersen et al., 1970; Clausen and Lassen, 197i ; P i m a y et al., i972). Clausen and Lassen (197i) found a levelling-off at 70%, but their results are only comparable with the present ones to a certain extent because they did not measure the ~2 max, but determined the load which would fatigue the subject within 6 rain (W6'). Also, a curve drawn through their data. suggests a levelling-off at 60~/o and not at 70% W6' as they state. Grimby et al. (1967) and Pirnay et al. (1972) found increasing k values up to i00% l~o~max. However, pooling of :~aXe clearance data from the literature, together with the present data, support the impression that :aaXe clearance rate does level off at submaximal values exceeding 60 % I?o~max. I f MBF is calculated on the basis of 17o2 and muscle mass engaged in the bicycle exercise, a maximum value of at least 0.9 min -1 is expected.. Behind this calculation is the assumption that 70 % of the total muscle mass is involved in bicycle exercise. However, it is unlikely that all of the active muscles are involved to the same extent. Therefore, differences in muscle perfusion would be expected. Together with the hip extensors, m. quadriceps femoris is one of the muscles most directly involved in this kind of exercise. The calculated value for MBF of 0.9 rain -1 for exercising muscle at I?o2max, therefore, rather represents a minimum value for these muscles. Further: on the basis of these calculations no levelling-off of MBF should occur before ~'o2max is reached. Also data from blood flow measured over the single legs during bicycle exercise (Jorfeldt and Wahren, 197i) reveal a rectilinear relation between blood flow to the legs and the exercise power. The muscle mass working during bicycle exercise distal to the sample site in the experiments of Jorfeldt and Wahren (1971) can be estimated to about 6 kg in each leg. I f this is so, ?~BF will attain considerably higher levels than indicated by the :8~Xe clearar~ce rate (Fig. i). From published values of a-vo~ differences over the legs (Pemov/et eL, i965), it is possible to complete the data of Jorfeldt and W~,hren (i97t) even at l?o~max and above (Fig. l). Also data on cardiac output (Q) from Astrand et al. indicate a rectilinear relation between 0 and exercise power. It is obvious that these increases in ~ are diverted to muscle, as skin and splanchnic vascular beds are constricted, at least during exercise of short duration (c/. Rowell, 1974).

Muscle Blood Flow and Aerobic Power

t95

min-I

1.0 F,.

Z

v

0.8

/

1,1,1

5 If. u_ W

0.6

/

o lJ

B

o

< 0.4 re w

~

o/4

/

/

/

/

/

O//zx /

,-

Ov

0.2

O ~

25

5~0

;5 i

o

,io

9EO 100 r

'120 ~

,:o

max

m,.-,

Fig. I. Muscle blood flow in rain-1 against exercise power expressed both in absolute values and relative to l?o2 .... Filled symbols indicate 13aXeclearance measurements, unfilled symbols arc data based on direct or indirect measurements of muscle blood flow. Data from Grimby et at. (1967) = A, Clausen and Lassen (t97t) = I , Pirnay et al. (1972) = v, present investigation =@. Data calculated from dye dilution method by Jorfeldt and Wahren (1971) = iX. Data calculated from 17o2and muscle mass in present investigation and a- vo2 differences from the Iiterature by Astrand et al. (1964) and Pernow et al. (t965)= 9 data calculated by Asmussen et al. (t939) = v

I t has been claimed by Clausen and Lassen (t97f) that levelling-off of MBF can be explained (i) by: recruitment of additional muscle groups, and (2) b y the increasing intramuscular pressure caused by the contraction. However, regarding (t), EMG studies (Henriksson and Bonde-Petersen, t974) have shown t h a t both m. rectus and m. vastus lateralis ]emoris function in a manner reetilinearly proportional to the exercise power up to 120% of I?o~max. I t is, therefore, not likely t h a t an important recruitment of additional muscle groups at heavier exercise power takes place during bicycle exercise. Concerning (2), it has been demonstrated b y Hermiston and Bonde-Petersen (1975) t h a t dynamic exercise does not occlude MBF, until a force of 60 % or more of m a x i m u m isometric strength is exerted. During bicycle exercise even at t20 % of ~2 max, the pressure on the pedal will not exceed 60 kg (Hoes et al., t968; Gollnick et al., 1974) which is about 45 to 50% of isometric m a x i m u m strength. I n s u m m a r y there is no reason to believe t h a t MBF in exercising man should level off at 60 % of 17o2max. Second it is assumed t h a t the MBF estimated from the k value calculated from the laaXe clearance method at high flow rates, are far too low. The discrepancy between the clearance values and the flow values obtained with other methods is likely due to systematic errors in the clearance method. The

196

Flemming Bonde-Petersen et al.

reliability of the 13SXenon m e t h o d has been questioned b y different authors (Kjellmer et al., t967; Bonde-Petersen and Siggaard-Andersen, 1969; Bolme and Edvall, i97i). The p a t t e r n of levelling-off of the k values ir~ spite of increasing work rates, where M B F still increases, m u s t be caused b y a diffusion restriction for the X e n o n molecule at high flow rates. F r o m the present data, therefore, it must be concluded t h a t lasXe is n o t reliable as a flow tracer in intact h u m a n s during h e a v y exercise. However, if the m e t h o d of xssXe clearance is regarded as semiquantitative flow tracer method, it can provide useful information within the field of exercise physiology.

References Asmussen, E., Christensen, E. H., Nielsen, M.: Die 02-Aufnahme der ruhenden und der arbeitenden Skelettmuskeln. Skand. Arch. Physiol. 82, 2t2--220 (1939) Astrand, P. 0., Cuddy, T. E., Saltin, B., Stenberg, J. : Cardiac output during submaximal and maximal work. g. appl. Physiol. 19, 268--274 (1964) Astrand, P. O., Saltin, B. : Maximal oxygen uptake and heart rate in various types of muscular activity, g. appl. Physiol. 16, 977--981 (1961) Binkhorst, R. A., Leeuwen, P. Van: A rapid method for the determination of aerobic capacity. Int. Z. angew. Physiol. 19,459--467 (t963) :Bolme, P., Edwall, L.: Dissociation of tracer disappearance rate and blood flow in isolated skeletal muscle during various vascular reactions. Aeta physiol, scand. 82, 17--27 (197t) Bonde-Petersen, F., Nielsen, ]3., Nielsen, S. L., Vanggaard, L. : 1ssXe clearance from musculus quadriceps femoris during concentric and eccentric bicycle exercise at different temperatures and loads. Aeta physiol, seand. 80, 16--iTA (1970) Bonde-Petersen, F., Siggaard-Andersen, J.: Simultaneous venous occlusion plethysmography and Xe 1aS clearance in patients with arteriosclerosis of the lower extremities. An attempt to evaluate the blood flow in skin and muscle. Scand. J. thorae, cardiovasc. Surg. 3, 20--25 (1969) Clausen, g. P., Lassen, N. A.: Muscle blood flow during exercise in normal man studied by the 13aXenon clearance method. Cardiovasc. Res. 5, 245--254 (1971) Corm, H. L. : Equilibrium distribution of radioxenon in tissue: Xenon-hemoglobin association curve, g. appl. Physiol. 16, 1065--t070 (1961) yon D6beln, W. : A simple bicycle ergometer. J. appl. Physiol. 7, 222--224 (1954) yon D6beln, W.: Determination of body constitutions. In: Symposia of the swedish nutrition foundation. II. Occurence, causes and prevention of overnutrition (G. Blix, Ed.). Uppsala: Almquist & Wiksell t964 Gollniek, P. D., Piehl, K., Saltin, B. : Selective glycogen depletion pattern in human muscle fibres after exercise of varying intensity and at varying pedal rates. J. Physiol. (Lond.) 241, 45--57 (1974) Grimby, G., tt~ggendal, E., Saltin, B.: Local Xenon13a clearance from the quadriceps muscle during exercise in man. J. appl. Physiol. 22, 305--310 (1967) Henriksson, J., Bonde-Petersen, F.: Integrated electromyography of quadriceps femoris muscle at different exercise intensities. J. appl. Physiol. 86, 218--220 (1974) Iiermiston, 1~. T., Bonde-Petersen, F. : The influence of varying oxygen tensions in inspired air on 1saXemuscle clearance gnd fatigue levels during sustained and dynamic contractions. Europ. J. appl. :Physiol. (1975, to be submitted) Hirzel, I-L G., Krayenbuehl, It. P. : Validity of the 1saXenon method for measuring coronary blood flow. Comparison with coronary sinus outflow determined by an electromagnetic flowprobe. Pfliigers Arch. 849, 159--t69 (1974) Hoes, M. J. A. g. M., Binkhorst, 1~. A., Smeekes-Kuyl, A. E. M. C., Wissers, A. (3. A. : Measurements of forces exerted on pedal and crank during work on a bicycle ergometer at different loads. Int. Z. angew. Physiol. 26, 33--42 (1968) Itolzman, G. B., Wagner, H. N., Iio, M., l~abinowitz, D., Zierler, K. L.: Measurements of muscle blood flow in the human forearm with radioactive Krypton and Xenon. Circulation 80, 27--34 (t964)

Muscle Blood Flow and Aerobic Power

197

Jorfeldt, L., Wahren, J. : Leg blood flow during exercise in man. Clin. Sci. 41, 459--473 (197l) Kety, S. S. : )/Ieasurement of regional circulation by the local clearance of radioactive sodium. Amer. Heart J. 38, 322--328 (1949) Kjellmer, I., Lindbjerg, I., Pr~rovsk:~, I., Tonnesen, K. I-I. : The relation between blood flow in an isolated muscle measured with the Xe la~ clearance and a direct recording technique. Acta physiol, scand. 69, 69--78 (t967) Lassen, 1~. A., Lindbjerg, I. F., Munck, O.: Measurement of blood flow through skeletal muscIe by intramuscular in~ection of 133Xenon. Lancet 1964 I, 686--689 Lindbjerg, I. F., Andersen, A. M., Munck, 0., Jorgenscn, M. : The fat content of leg muscles and its influence on the ~a3Xenon clearance method of blood-flow measurements. Scand. J. clin. Lab. Invest. 18, 525--534 (1966) Pernow, B., Wahren, J., Zetterquist, S. : Studies on the peripheral circulation and metabolism in man. IV. Oxygen utilization and lactate formation in the legs of healthy young men during strenuous exercise. Acta physiol, scand. 64, 289--298 (1965) Pirnay, F., Marechal, R., Radermecker, R., Petit, J. M.: Muscle blood flow during submaximum and maximum exercise on a bicycle ergometer. J. appl. Physiol. 82, 2t0--2t2 (1972) Rowell, L. B.: Human cardiovascular ~djustments to exercise and thermal stress. Physiol. Rev. ~4, 75--159 (1974) Wahren, J., Saltin, B., Jorfeldt, L., Pernow, B.: Influence of age on the local circulatory adaptation to leg exercise. Scand. J. clin. Lab. Invest. 83, 79--86 (1974) Dr. Flemming Bonde-Petersen Gymnastikteoretisk laboratorium August Krogh Instituter Universitetsparken :13 DK-2100 Copenhagen, Denmark