Determinants of endurance in well-trained cyclists EDWARD F. COYLE, ANDREW R. COGGAN, MAR1 K. HOPPER, AND THOMAS J. WALTERS Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas, Austin, Texas 78712 COYLE, EDWARD F., ANDREW R. COGGAN, MARI K. HOP- PER, AND THOMAS J. WALTERS. Determinants of endurance in well-trained cyclists. J. Appl. Physiol. 64(6): 2622-2630,1988.- Fourteen competitive cyclists who possessed a similar maxi- mum 02 consumption (VO 2 max range, 4.6-5.0 l/min) were com- pared regarding blood lactate responses, glycogen usage, and endurance during submaximal exercise. Seven subjects reached their blood lactate threshold (LT) during exercise of a relatively low intensity (group L) (i.e., 65.8 t 1.7% 7jozrnax), whereas exercise of a relatively high intensity was required to elicit LT in the other seven men (group H) (i.e., 81.5 k 1.8% TOM,, P 0.001). Time to fatigue during exercise at 88% of VOLT, was more than twofold longer in group H compared with group L (60.8 k 3.1 vs. 29.1 k 5.0 min P 0.001). Over 92% of the variance in performance was related to the %VO~ mm at LT and muscle capillary density. The vastus lateralis muscle of group L was stressed more than that of group H during submaximal cycling (i.e., 79% V0 2 mm), as reflected by more than a twofold greater (P c 0.001) rate of glycogen utilization and blood lactate concentration. The quality of the vastus lateralis in groups H and L was similar regarding mitochondrial enyzme activity, whereas group H possessed a greater percentage of type I muscle fibers (66.7 & 5.2 vs. 46.9 t 3.8 P 0.01). The differing metabolic responses to submaximal exercise observed between the two groups appeared to be specific to the leg extension phase of cycling, since the blood lactate responses of the two groups were comparable during uphill running. These data indicate that endurance can vary greatly among individuals with an equal VOW,,, The factors associated with a high %VO~ mm at LT were years of cycling experience (r = 0.75 P c 0.01) and percent type I muscle fibers (r = 0.55 P c 0.05). It appears that intense cycle training performed for ~5 yr com- pared with 2-3 yr promotes continued adaptations that reduce muscle glycogenolysis specifically when cycling. lactate threshold capillary density human performance mi- tochondria cycling neuromuscular recruitment training spec- ificity IT IS OUR CONCEPT (13,16, 19) as well as others (25-27, 34) that athletic performance velocity during endurance exercise (i.e., lasting from -10 min to several hours) is determined by the highest steady-state rate of 02 con- sumption (VO,) that can be tolerated and the biome- chanical economy of movement, defined as the velocity achieved for a given rate of I-702. The To2 maintained during competition is related to the VOW at which lactate begins to accumulate in blood [i.e., lactate threshold (LT- VOg)] (13, 16, 19), which we interpret to be reflective of muscle glycogenolysis and lactate production (21, 23). Within this framework, differences in endurance per- formance observed between individuals and improve- ments with training can largely be explained in terms of LT-VOW and economy. Maximal VO~ (Vozmax) has long been recognized as an important determinant of endur- ance performance (2), generally because it sets the upper limit for steady-state VOW and LT-VOW (13). Although VOW,, is a good predictor of performance in heteroge- neous groups of athletes (11, 16), it has frequently been recognized that individuals with a similar VOzrnax can differ in performance velocity (27). In many cases, these performance differences can largely be explained by dif- ferences in biomechanical economy rather than by LT- Tjo, or the percent of VOW,,, that can be maintained during competition (8, 19). Along these same lines, successful endurance athletes have been observed to achieve a high Vozrnru after their initial few years of intense training (29,3O), and therefore other adaptations are likely to contribute to their contin- ued performance improvements. Surprisingly little direct data are available, however, regarding the factors respon- sible for continued performance improvements in already well-trained endurance athletes. Daniels et al. (14) found that running economy and VOW mar did not change during a period of increased training volume that improved 2- mile run time. This indicates improvements in the % VOW mm maintained in competition and possibly the LT- . vo i��� popular theory is that differences in performance velocity and LT-VOW, which cannot be explained by Vozrnax or economy, may be related to differences in the quality of the trained skeletal musculature. There is much evidence to indicate that training-induced in- creases in respiratory capacity of the muscle fibers result in slower utilization of muscle glycogen and less lactate production and that these adaptations play an important role in improving performance (21). Indeed, muscle res- piratory capacity is highly related to LT-VOW when cross- sectional observations are made of men in varied states of physical training (24) as well as when endurance athletes stop training and thus display large reductions in mitochondrial enzyme activity (12). However, it is not clear if increased mitochondrial enzyme activity is re- sponsible for increases in LT-VOW and performance in already well-trained individuals (6, 17, 32, 33). In the present study we examined fourteen cyclists, all of whom had been training intensely for 3-12 yr and who possessed similarly high values for I702 mm (i.e., -4.8 l/ min). They differed considerably, however, in the dura- tion they could cycle at 88% Vozrnmm We sought to determine 1) whether performance is related to the blood 2622 0161-7567/88 $1.50 Copyright 0 1988 the American Physiological Society

DETERMINANTS OF CYCLING ENDURANCE 2623 lactate responses and glycogen utilization during sub- maximal exercise, 2) the extent to which glycogen utili- zation and time to fatigue during exercise *at a given %VO, max can vary in athletes with the same Vo2 max, and 3) whether or not muscle mitochondrial enzyme activity, capillary density, fiber type, or cycling skill are associated with differences in glycogen utilization, 1 .actate produc- tion, and endurance performance among cyclists with a similar V02 m8X. METHODS Subjects and training. Fourteen male endurance ath- letes were studied after giving their written informed consent. This study was approved by the Human Studies Committee of the University of Texas. The physical characteristics of these men are presented in Table 1. These men were selected cyclists who could reach because they were competitive . vo 2max during cycling and be- - cause they were homogeneous regarding VO, maX (l/min). The total number of years that these athletes had been performing endurance training, which included cycling, -running and swimming, ranged from 3 to 12 yr. To familiarize these subjects with high-intensity sta- tionary cycle ergometry, they all performed the following interval training program 3 days/wk during the 2-mo period before study. This consisted of six 5min exercise . bouts of 90-100% of Vo 2max with 3 min of rest between bouts. Measurement of VO 2mQx and blood lactate threshold. w vo 2 max was measured during continuous exercise tests lasting between 8 and 10 min. Testing was performed during both treadmill running (at constant speed with TABLE 1. Characteristics of subjects who reached lactate threshold at a high (group H) and a low (group L) percent of VOZ,, Subj No. Training Time Age, Wt, VO VO Plnax, 9-2, Yr kg Cycling, Total, 2y��� ml.kg-l. l/mm at LT, Yr v min��� % Group H 1 2 3 4 5 6 7 Means 24.7 71.1 5.1 7.6 4.87 68.6 81.5 &SE k1.4 t1.4 *0.9* k1.0 t0.04" t1.2 k1.87 Group L 8 9 10 11 12 13 14 Means 25.1 72.1 2.7 7.0 4.75 66.0 65.8 *SE kO.8 k1.5 20.7 kO.8 to.03 AA.2 t1.7 25 68.0 8 8 4.90 72.1 85.5 20 75.7 3 5 5.02 66.3 81.0 25 73.0 7 7 4.72 64.7 86.0 32 75.1 7 12 4.98 66.3 84.7 24 65.3 3 5 4.75 72.7 81.0 22 72.0 6 6 4.85 67.4 80.4 25 72.0 2 10 4.87 70.8 72.0 28 65.3 3 8 4.81 73.8 68.0 26 72.0 2 8 4.81 66.8 63.0 22 74.2 2 8 4.81 64.8 70.0 24 73.9 7 7 4.60 62.2 71.2 24 71.0 2 3 4.79 67.5 68.0 25 78.4 1 9 4.73 60.3 61.5 27 70.0 2 6 4.67 66.7 59.1 iTo2max, maximum O2 consumption LT, lactate threshold. Group H is significantly greater than group L using Student���s t test: * P 0.05 fP 0.001. the grade increasing 2% every 2 min) and during cycle ergometry (York rate increasing every 2 min). Running and cycling VO grnax were within 0.1 l/min in all 14 sub- jects. This indicates that incremental cycling elicited true . vo 2 max9 as opposed to peak VO,, in these subjects who were tested on several occasions. A clear leveling off of VO, occurred in all subjects. The subjects breathed through a Daniels valve expired gases were continuously sampled from a mixing chamber and analyzed for O2 (Applied Electrochemistry S3A) and CO2 (Beckman LB- 2). Inspired air volumes were measured using a dry gas meter (Parkinson-Cowan CD4). These instruments were interfaced with an Apple II-plus computer, which calcu- lated \jo2 every 30 s. Blood lactate threshold (LT) was determined as pre- viously described (Ref. 13 i.e., 1-mM increase in blood lactate concentration above base line) by graphing an individuals��� venous blood lactate concentration measured after 10 min of cycling at five intensities ranging between 50 and 90% of iT02 max. The bouts were performed during two testing sessions during the week before the perform- ance evaluation. During cycle testing, a Quinton cycle ergometer (model 845) equipped with toe clips was used. This ergometer provides a constant power output inde- pendent of pedal frequency. However, the subjects were required to maintain a pedal frequency of 75-85 rpm during all the testing. Venous blood samples were ob- tained from a catheter in an antecubital vein before and immediately after exercise and assayed for lactate con- centration (18). 30, was averaged during the last 4 min of this steady-state testing. The subjects were fan cooled during all exercise tests. The blood lactate thresholds of 11 subjects were also determined while running at various speeds on a tread- mill set at a 10% grade, which elicited 60-90% of VO, maxa This test was incorporated to provide an additional com- parison of the subjects during an activity other than cycling, which stresses the quadricep muscles (10). The influence of cycling with and without toe clips and shoe cleats was also determined. During the week of the performance evaluation the subjects��� training and diet were standardized. On Mon- day the subjects exercised for 20 min at intensities slightly above LT after which Vo2,,, during cycling was verified. On Wednesday endurance performance was evaluated and on Friday glycogen utilization was deter- mined as described below. All subjects consumed a high- carbohydrate diet (500 g/day) and exercised moderately for 30 min on Tuesday and Thursday. All testing was done in the morning after an overnight fast. Evaluations during treadmill running were generally performed dur- ing the next week. Endurance performance. Endurance performance was defined as the length of time that cycling could be main- tained at an intensity requiring -88% of iT02,,* Tjo2 was monitored periodically to ensure that the work rate remained between 87 and 91% of VOW mm. The subjects were not aware of elapsed time or of the other subjects��� performances. The test was terminated when the subject was unable to maintain the required work rate. At ex- haustion they all complained of local fatigue in their leg musculature. A venous blood sample was drawn imme-