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Basic Sciences: Original Investigations

Maximal lactate steady state during the second decade of age

BENEKE, RALPH; HECK, HERMANN; SCHWARZ, VOLKER; LEITHÄUSER, RENATE

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Medicine & Science in Sports & Exercise: December 1996 - Volume 28 - Issue 12 - p 1474-1478
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Abstract

Maximal lactate steady state (MLSS) identifies the time course of blood lactate concentration (BLC) during the constant workload representing the highest level that can be performed by oxidative energy metabolism(19,26). As previously reported, the determination of MLSS and various concepts of “anaerobic threshold” do not necessarily replicate identical levels of BLC and/or workload(5,19). MLSS represents the highest attainable equilibrium between lactate appearance and disappearance during prolonged constant workload and can be determined during subsequent constant load tests performed on separate days(5,6,18,19,28,29).

Various authors(1,2,9,23,25,32,35,40) have described changes in aerobic and anaerobic metabolism during growth. The second decade of life has appeared as a key period in the change of energy metabolism(2,3,13-17,20-22,30,31,34). During this time males usually go through puberty (38). Muscle lactate production was suggested to be related to the sexual maturity level of pubescent boys and to the testosterone level in the blood of rats(13,14,24). Controversial results concerning BLC at submaximal and maximal workload in relationship to age have been published(10,15,16,18,23,28,29,40). However, previous studies addressing MLSS in children and adults cannot be compared interchangeably (18,28,29,40). Differences in results may reflect methodological effects(6). Therefore, the purpose of the present study was to investigate the MLSS related to age in males during the second decade of age.

METHODS

Subjects. Thirty-four males (mean ± SD age: 15.4 ± 2.8 yr, range: 11-20 yr; height: 171.8 ± 14.9 cm, range: 134-191 cm; body mass: 59.6 ± 15.5 kg, range: 27-90 kg) volunteered to be subjects. They represented a randomized sample of healthy pre- to post-pubescent subjects normal in physical characteristics related to age(38), who underwent a routine fitness check in a sports medical unit. Parental permission and informed consent were obtained prior to all testing.

Procedure. The subjects were submitted to an incremental load test and to several constant load tests on an electronically braked cycle ergometer (Zimmermann, Leipzig, Germany; Elema Schönander 380, Siemens, Berlin, Germany). Pedaling frequency was fixed at 60 rev·min-1. The subjects were instructed not to engage in strenuous activity for one day before an exercise test. Tests were conducted at the same time of the day with at least 48 h between each test.

Incremental load test. Two comparable (19) incremental load tests were adopted. Boys aged 14 yr or less were subjected to initial workload of 25 W or 50 W, and the workload was increased by 25 W every 2.0 min until exhaustion. Subjects older than 14 yr underwent the initial workload of 50 W or 100 W, which was increased by 50 W every 3.0 min until exhaustion. Based on sports history at both test protocols, the initial workload depended on the expected final workload. The incremental load tests lasted 8-16 min in boys aged 14 yr or less, and 12-18 min in subjects older than 14 yr, respectively.

Constant load test. Each constant load test lasted 30 min. The workload intensity of the first constant workload was set corresponding to a BLC of approximately 3.0 mmol·l-1 measured during the incremental load test. If a steady state of BLC was achieved or BLC decreased during constant workload, subsequent constant load tests at 5% to 10% higher workloads were performed on separate days until a continuous increase in BLC was observed.

Blood lactate concentration. Capillary blood samples (20 μl) were taken from the hyperaemic (Finalgon forte, Thomae, Biberach, Germany) earlobe while the subjects continued to exercise. During incremental workload blood samples were drawn before the test and at the end of each stage. During the constant load tests the BLC was measured before and at the end of every 5th min of the 30 min constant workload. The BLC was analyzed by the enzymatic method (Boehringer, Mannheim, Germany).

Heart rate. Heart rates were monitored continuously throughout all testing (Sport tester PE 3000, Polar, Finland). Data corresponding to the time of blood sampling times were analyzed.

Maximal lactate steady state. Maximal lactate steady state (MLSS) was defined as the highest BLC that increased by no more than 1.0 mmol·l-1 during the final 20 min of a 30-min constant workload test. The MLSS was calculated as average value of the BLC measured at minutes 15, 20, 25, and 30 of the MLSS workload(5,18,19).

Statistics. Data are reported as mean values and standard deviations (SD). The relationships between dependent variables and age were examined by analysis of linear regression. For all statistics, the significance level was set P ≤ 0.05.

RESULTS

MLSS, MLSS workload, MLSS heart rate measured during the constant load tests, the maximal workload determined at the end of the incremental load test, and MLSS intensity related to maximal workload are depicted inTable 1.

MLSS and MLSS intensity were independent of age(Figs. 1 and 2). MLSS heart rate showed an inverse relationship with age (P < 0.01) (Fig. 3), whereas MLSS workload and maximal workload expressed in absolute values(Figs. 4 and 5) and in relationship to body mass(Figs. 6 and 7), increased (P < 0.001) during the second decade of life.

DISCUSSION

The present results suggest that during the second decade of age MLSS is independent of age. In contrast, numerous studies reported that BLC and/or muscle lactate were lower in prepubescent and pubescent children than in adults at the same relative workload related to ˙VO2max or at the same heart rate both during incremental and constant workloads(13-16,23).

In addition, MLSS heart rate decreased with increasing age. However, the age dependent decrease of submaximal and maximal heart rates demonstrates that lower levels of BLC at given heart rates do not say anything about changes in the relationship between aerobic and anaerobic energy metabolism.

The second decade of life appeared to be a key period associated with changes in muscle metabolism. During puberty, changes in testosterone levels were expected to increase anaerobic capacity(2,3,13,14). Lower activities of phosphofructokinase and lactate dehydrogenase in children combined with a reduced ability to reach very low pH values or to lower the base excess to the extent found in adults were published(13-15,21,37). However, some of the latter results were determined in deconditioned subjects(13-15). Other studies investigating healthy subjects did not show a relation between age and glycolytic enzymes or BLC at maximum workload and age (7,10,30).

Small muscle mass, decreased cardiac output and circulation time, and higher levels of arteriovenous O2 difference, rate of fatty acid utilization, liver and muscle perfusion, and relative mitochondrium volume have been discussed as factors that may cause differences in energy metabolism between children and adults (4,33). At MLSS intensity a RER of approximately 1.0 can be assumed (36). This indicates that the rate of fatty acid utilization and gluconeogenesis by the liver can be expected to be of minor importance to MLSS intensity(26,27,36,39). In spite of changes in muscle mass, cardiac output, circulation time, arteriovenous O2 difference, and mitochondrium volume, age dependent changes in the˙VO2max related to body mass(ml·kg-1·min-1] did not occur in trained subjects during growth (1,2,30). In untrained subjects and females a decrease in ˙VO2max related to body mass has been described during growth (2).

Compared to adults, the kinetics of ˙VO2 were rarely found to be accelerated in children (25,32,41). Changes in ˙VO2 kinetics may affect the O2 deficit and the time constant (τ) of lactate elimination (41). However, whether there is an age dependent effect on ˙VO2 kinetics or not, changes of τ modify the time that passes until MLSS is reached but not the level of MLSS (12,26,27).

In agreement with previous reports, the absolute maximal workload increased with age(2,3,13-16,23,35). The latter seems to confirm an age related increase in MLSS workload. However, there is little support for the observation that maximal workload and MLSS workload related to body mass increase with age(2,3,13-16,34). Previous data (41) reported a significant difference in maximal workload related to body mass between children aged 7 to 9 yr and adults aged 26 to 42 yr. This difference in relative maximal workload was combined with a significant increase in working efficiency(41). A lower working efficiency in children may explain the increase in relative workload combined with a presumably unchanged level of relative ˙VO2(16,34,41). A second reason for the increase in workload capacity related to body mass may be reflected in the specific sample of subjects. In spite of random selection, the younger subjects represented a large spectrum of school children, whereas the older subjects were active recreational sportsmen and some were national class athletes.

MLSS represents a level of workload intensity that, after an initial period with a minor percentage of anaerobic metabolism, is exclusively performed by oxidative energy production. However, the level of MLSS represents the highest individual rate of anaerobic glycolysis at which the amount of pyruvate produced can be used by aerobic oxidation(8,19,26,27).

An incremental load test as performed in the present study gives limited information concerning anaerobic metabolism. According to di Prampero(11) at a representative incremental load test (workload: 50 W to 250 W, maximal BLC: 12 mmol·l-1), approximately 12% of the power output is produced by anaerobic net lactate production during the final stage.

The Wingate Anaerobic Test, not conducted in the present study, presumably is a more valid test of anaerobic metabolism(2,3,16). Previous investigators pointed out that peak power related to body mass increases with age(2,3,16) and that the late pubertal period may accentuate anaerobic capacity (2,3). Relative peak power was found to increase by approximately 74% in children (age 6-8 yr) to adolescents (age 14-15 yr), whereas the corresponding increase in peak BLC was approximately 26% (16). The latter data support the hypothesis that the difference in anaerobic exercise testing between children and adolescents may be an effect of neuromuscular factors and mechanical working efficiency rather than an indicator of a reduced anaerobic capacity in children. This supports the theory that neuromuscular factors may contribute to increases in anaerobic power with physical maturity(3). An improved efficiency of motor patterns and a higher percentage of muscle fiber activation, resulting in more effective inter- and intra-muscular coordination, have been postulated as an effect of growth(34). However, data to support this theory are missing.

The present study demonstrated that during the second decade of life MLSS and MLSS workload intensity are age independent. Both MLSS workload and maximal workload increased with age. The age independence of MLSS supports the theory that with physical maturity neuromuscular factors may contribute to the changes in response to selected exercises more than changes in oxidative metabolism and/or glycolysis.

T1-6
F1-6
Figure 1-Maximal lactate steady state (MLSS) related to age (•::
N = 1, ○: N = 2).
F2-6
Figure 2-MLSS heart rate related to age (•::
N = 1, ○: N = 2).
F3-6
Figure 3-MLSS intensity related to age (•::
N = 1, ○: N = 2).
F4-6
Figure 4-MLSS workload related to age (•::
N = 1, ○: N = 2).
F5-6
Figure 5-Maximal workload related to age (•::
N = 1, ○: N = 2, ♦: N = 3, ▪: N = 4, □: N = 5).
F6-6
Figure 6-Relative MLSS workload related to age (•::
N = 1, ○: N = 2).
F7-6
Figure 7-Relative maximal workload related to age (•::
N = 1, ○: N = 2).

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Keywords:

BLOOD LACTATE; CONSTANT WORKLOAD; AEROBIC; ANAEROBIC; CHILDREN

©1996The American College of Sports Medicine