Estimates of muscular efficiency can be derived from the slope of lines that describe the relationship between energy input and power output as well as the ratio of energy input to power output. The curvilinear association between energy input and power output suggests that muscular efficiency decreases as functions of cycling frequency and power output(5,10,24). Poole et al.(20) showed parallel increments in leg and whole body˙VO2 in response to increments in power output. Thus, they established that estimates of muscular efficiency calculated from whole body˙VO2 do, in fact, approximate efficiency of working limb muscles in humans during leg cycling.
In the laboratory setting, men are reported to be most efficient at pedaling cadences between 50 and 60 rpm(5,10,22). The literature also contains reports that competitive cyclists often pedal at cadences of 90-100 r·m-1 to sustain very high power outputs during road training and competition (15,16). At high power outputs, many competitive cyclists often “spin” (i.e., pedal at high frequencies) in an effort to minimize leg fatigue as pedaling becomes labored. Inasmuch as competitive cyclists are reported to “adapt” to higher frequencies of leg movement following training(7,15), we suspected that differences may exist in both the metabolic response and cycling efficiency of competitive cyclists when compared with recreational cyclists at similar power outputs and cycling frequencies.
Only three reports in the literature have addressed the relationship between training and muscular efficiency. Bönig et al.(4) found no differences in Δ efficiency between competitive and recreational cyclists during leg ergometry exercise at similar power outputs and cycling frequencies. In contrast to others(10,24), Böning et al. (4) reported that Δ efficiency increased as a function of power output. Stuart et al. (24) also observed no differences inΔ efficiency between trained distance runners and sprinters during leg cycling. Unlike Bönig et al., they reported that Δ efficiency decreased with increments in power output. Although these two studies reached similar conclusions concerning the relationship between training and muscular efficiency, the association between power output and Δ efficiency drew divergent results. Recently, Marsh and Martin (18) found that prior cycling experience had no effect on either the preferred or most economical pedaling cadences in trained cyclists and noncyclists (trained distance runners). Because no differences were found between the most economical pedaling cadences, Marsh and Martin (18) inferred that whole body muscular efficiencies must also be similar. Interestingly, there was no efficiency data provided to support this assertion.
In this study, we were interested in determining whether competitive cyclists were more efficient (i.e., had higher whole body muscular efficiencies) than recreational cyclists during graded and submaximal endurance exercise. We hypothesized that local muscle adaptations or more efficient recruitment patterns of leg musculature engendered from previous cycling experience would allow for improved whole body muscular efficiencies in our competitive group. However, our data did not support the hypothesis. Instead, we observed that competitive cyclists had greater submaximal power outputs and endurance at each pedaling frequency despite similar whole body muscular efficiencies when compared with recreational cyclists. Furthermore, whole body muscular efficiency and endurance were inversely related in both groups.
Subject selection. All test procedures were performed in accordance with University of California Human Subjects Committee Guidelines(Protocol # 91-836). Twelve healthy male nonsmokers between 18 and 30 yr of age volunteered for this study. Prior to participation, each subject provided informed written consent. Classification of subjects was based upon previous cycling experience. Competitive (23.7 ± 1.3 yr; 77 ± 4.4 kg;N = 6) cyclists (CC) included individuals with a minimum of 2 yr of competitive cycling experience. Recreational cyclists (RC) (23.2 ± 1.6 yr; 81 ± 6.5 kg; N = 6) included individuals who cycled for recreational purposes or as part of a regular fitness program. Competitive cyclists averaged approximately 4.3 ± 1.2 yr of competitive cycling experience.
Test protocols: ˙VO2peak. Separate tests of peak oxygen consumption (˙VO2peak) at cadences of 50 and 80 rpm were conducted initially, followed by two submaximal endurance exercise tests at 75% of ˙VO2peak. One endurance test was conducted at 50 rpm and the other at 80 rpm. During exercise, pedaling frequency was kept synchronized with a metronome (Franz LM-4, New Haven, CT). All exercise tests were performed on a Corvail 400 electronically braked, manufacturer calibrated cycle ergometer (Lode, B.V. Groningen, The Netherlands) equipped with foot straps and adjustable handlebars. Each subject was allowed 6 wk to complete all four exercise protocols.
Following a 4-min rest period, each subject pedaled at the predetermined cadence in the absence of an external workload (i.e., power output = 0 W). Immediately following unloaded cycling, the power output was increased 50 W. Subsequent power outputs were increased by 50-W increments every 4 min until volitional fatigue or ˙VO2peak was reached. The ˙VO2 was measured during the last 30 s of each 4-min interval.
Submaximal endurance exercise. Two submaximal endurance exercise bouts were performed at power outputs which elicited 75% of the˙VO2peak measured initially at 50 rpm. During each submaximal exercise bout, subjects cycled for 4 min each from 50 W to 200 W following rest. Subjects then cycled at 75% ˙VO2peak until exhaustion. Following each submaximal exercise bout, times to exhaustion were recorded for each subject.
Measurements. Ventilation, ˙VO2, ˙VCO2, and respiratory exchange ratio (RER) were displayed and monitored on line as described previously (5,10,13,22). Values obtained during the last 30 s of each interval were taken as representative of the power output and pedaling cadence maintained over that interval.
Calculations of energy input, peak power output, and total work. Peak power output was recorded as the highest power output attained during each graded exercise test. Estimates of energy input at each power output and cycling frequency were obtained from ˙VO2 and RER indirectly, using the tables of Zuntz and Schumburg as modified by Lusk (9). Total work, defined as the product of submaximal energy input and endurance, was expressed in kilowatt hours (kw·h-1).
Computation of muscular efficiency. The term“muscular” does not describe the efficiency of the musclesper se. Rather, muscular refers to the efficiency of whole body locomotion with inference to efficiency of muscle contraction(23). Estimates of gross efficiency were computed during each continuous, graded, and submaximal exercise bout using the ratio of power output and energy input. Delta (Δ) efficiency estimates were computed using the slopes of lines that described the change in energy input (ΔE) relative to the change in power output (ΔW) between successive workloads(10). Effects of cycling frequency on Δ efficiency were determined using the ratio of successive changes in both energy input and power output between 50 and 80 rpm.
Statistical analyses. An analysis of covariance (ANCOVA)(Superanova, Berkeley, CA) was performed to assess whether differences in˙VO2, energy input, or RER were present between recreational and competitive cyclists during each graded exercise test. Because competitive and recreational cyclists have responded differently to increases in power output(6,26), the effect of power output, the covariate, on both ˙VO2 and energy input was removed during the analysis. For competitive and recreational cyclists, analysis of covariance allowed comparisons of the slopes of lines describing the relationship between energy input and power output and of ˙VO2 and power output, respectively. Comparisons were made at each cycling frequency and power output with a significance level of P < 0.05. Differences in the effects of power output on computed estimates of gross and Δ efficiency were compared at each cycling frequency using an unpaired t-test(Statview II; Berkeley, CA). An unpaired t-test was also used to compare differences in ˙VO2, energy input, and gross efficiency between competitive and recreational cyclists at each time point during constant-load submaximal exercise. Effects of pedaling frequency on Δ efficiency were made by one-way analysis of variance (1-way ANOVA; Statview II). Differences in the mean ˙VO2, power output, energy input, RER, and gross efficiency produced by competitive and recreational cyclists, as well as endurance times and total work at each cycling frequency, were also compared using one-way ANOVA. All differences were determined to be significant at P < 0.05. The data presented in tables are reported as the mean ± SEM; data in figures are given as means only.
In each group and at each pedaling frequency, the relationship between˙VO2 and power output was essentially linear (Fig. 1). Peak power outputs were significantly (P < 0.01) greater in competitive cyclists, equaling 333 ± 10.5 W versus 250 ± 2 W. Moreover, ˙VO2peak was greater (P < 0.05) in competitive cyclists at 50 rpm only (3.74 ± 0.08 vs 3.22 ± 0.15 l·min-1). During each graded exercise bout, submaximal˙VO2 was not different between groups at rest or power outputs between 50 and 250 W at either cycling frequency.
During each submaximal endurance exercise, power outputs were greater(P < 0.01) in competitive cyclists (258 ± 10W vs 210± 4W;). At 50 rpm, the mean ˙VO2 was greater (P< 0.05) in competitive cyclists (3.08 ± 0.02 l·min-1 vs 2.78 ± 0.05 l·min-1) (Fig. 2). The mean ˙VO2 in competitive cyclists remained higher (P < 0.01) at 80 rpm, measuring 3.14 ± 0.01 l·min-1 compared with 2.70 ± 0.04 l·min-1.
In both groups, the gas exchange R increased as a function of power output, but decreased with increments in cycling frequency. Competitive cyclists maintained a lower RER (P < 0.01) at 50 rpm (0.93 ± 0.02 vs 1.02 ± 0.01) and 80 rpm (0.96 ± 0.01 vs 1.09 ± 0.01).
Both competitive and recreational cyclists showed improvements (P< 0.05) in times to exhaustion with increased cycling frequency(Table 1). At 50 (27 ± 5 min vs 14 ± 2 min;P < 0.05) and 80 rpm (35 ± 4 min vs 20 ± 4 min;P < 0.05), competitive cyclists cycled for longer periods of time. The endurance times of competitive cyclists were 48% longer at 50 rpm and 43% longer at 80 rpm than those observed for recreational cyclists.
Total work was approximately double (P < 0.01) in competitive cyclists at each cycling frequency (Table 1). Recreational cyclists showed a 30% (within group) increase in endurance compared with competitive cyclists' 23% increase as cycling cadence was increased from 50 to 80 rpm.
Effects of power output and pedaling frequency on estimates of muscular efficiency. Because no significant differences in energy input were observed between 50 and 250 W, the slopes of lines that described the relationship between energy input and power output were similar. Analysis of covariance with effects of power output removed confirmed that the associations (slopes) of ˙VO2 and energy input on power output were not significantly different. Moreover, no significant differences in estimates of gross efficiency were observed between groups at any power output or cycling frequency during graded exercise tests (Fig. 3). When subjects rode at constant power outputs, ˙VO2 remained constant. Therefore, no significant changes in submaximal gross efficiency were detected (Table 2). Mean gross efficiencies during submaximal exercise in competitive cyclists equaled 23.4 ± 0.3% at 50 rpm and 22.6 ± 0.2% at 80 rpm. In recreational cyclists, mean gross efficiencies during submaximal exercise equaled 23.6 ± 0.5% at 50 rpm and 21.9 ± 0.2% at 80 rpm, respectively.
Consistent with previous results, we detected no significant differences inΔ efficiency between competitive and recreational cyclists at any power output or cycling frequency (Table 3;Fig. 4).
In both competitive and recreational cyclists, the relationship between˙VO2 and power output was not different during graded or submaximal(≈75%˙VO2peak) endurance exercise at either 50 or 80 rpm. Moreover, the slopes of lines that described the relationship between energy input and power output were similar in competitive and recreational cyclists at each power output and pedaling frequency. At 50 and 80 rpm, we observed differences in ˙VO2, endurance time, and submaximal power output at 75% ˙VO2peak between groups. However, estimates of muscular efficiency were not different owing to cycling experience. As such, we interpreted our results to mean that whole body muscular efficiencies were not different. Therefore, our findings did not support the hypothesis that previous cycling experience improves whole body muscular efficiency.
In both groups endurance times were increased at 80 rather than 50 rpm. The increased endurance times in competitive cyclists are consistent with reports of increased endurance resulting from previous cycling experience(1,3,6,8,15). However, we found that endurance and muscular efficiency do not respond similarly to increased pedaling frequencies. As such, factors that affect endurance times may not be directly related to whole body muscular efficiency.
Increases in ˙VO2 as a function of pedaling frequency have been well-documented in competitive and recreational cyclists(1). We found that ˙VO2 was increased at 80 rpm compared with 50 rpm in both groups. Thus, in male cyclists an increased˙VO2 at higher cycling frequencies was highly reproducible(3,5,14,22).
The present results are consistent with earlier results of Stuart et al.(24) and Böning et al. (4) who showed no effect of training experience on muscular efficiency in runners. Our findings were also consistent with conclusions of Marsh and Martin(18) who studied the effects of cycling experience on cycling economy. We also observed that competitive cyclists had greater submaximal power outputs and endurance at each pedaling frequency despite no differences in whole body muscular efficiencies when compared with recreational cyclists. Consistent with Horowitz et al.(15), we also observed that submaximal power output and endurance capacity were increased owing to training. Such changes were found by Coyle et al. (6,8) to result from greater recruitment of more efficient Type I muscle fibers during exercise. Previously, Δ and gross efficiency have been reported to be well correlated with% Type I fiber number and proportion(8,15,16).
At high power outputs and cycling frequencies, sources of decreased muscular efficiencies include both unmeasured and increased internal work(10,22,24,25). Neither process is directly related to external work performance. Increases in internal energy production indirectly affect the slow, rather than fast, component of˙VO2 kinetics over time (26). Factors that affect internal work include increased ventilation, cardiac function, body temperature, catecholamine secretion, and substrate (futile) cycling(13,14,25,27).
Changes in unmeasured work consider both the energy input and crank force necessary to overcome frictional resistance of the flywheel and crankset on an electronically braked cycle ergometer at lower frequencies of leg movement(i.e., 50-60 rpm). Patterson and Moreno (19) and others(17,21) report a lessening of leg force per pedal stroke at higher pedaling cadences (80-100 rpm). In competitive and recreational cyclists, Sanderson (21) observed a decline in the ratio of total applied force to force applied perpendicular to the pedal crank (index of effectiveness) with increasing pedaling frequency. Because optimal pedaling frequency increases as a function of power output(5), one might expect whole body muscular efficiency to respond similarly. In this study, we found that whole body muscular efficiencies increased as a function of power output but declined as a function of pedaling frequency.
In contrast, both groups cycled longer at 80 rpm than at 50 rpm. Although the total energy input is higher when cycling at 80 rpm, presumably there is a lower energy requirement to move the legs at higher cycling frequencies. High resistance/low cycling frequency exercise is associated with increased recruitment and glycogen depletion in Type II muscle fibers(2,11,12). Furthermore, increased recruitment of Type II fibers as a function of power output and cycling frequency has produced a net decrease in whole body muscular efficiency(10,24). In the present study, it may be that the inverse relationship between endurance and whole body efficiency is caused by fiber type differences or local factors associated with the exercising musculature. However, we cannot draw such conclusions from our data. Therefore, it remains undetermined whether there are factors that link endurance and whole body efficiency.
Both competitive and recreational cyclists cycled longer at 80 rpm than at 50 rpm. We interpreted these findings to suggest that there are pedaling cadences that optimize biomechanical efficiency (i.e., reduce peripheral fatigue of leg musculature) in lieu of whole body metabolic efficiency. Marsh and Martin (18) have reported preferred cadences (88.4± 9.9 rpm) differ markedly from those traditionally viewed as most economical (59.5 ± 5.8 rpm). Hence, our observation that whole body muscular efficiencies did not improve with cycling experience extends those findings of Marsh and Martin pertaining to cycling experience and selection of optimal pedaling cadences.
Based on our results and the work of others(4,10,18,24), one would conclude that training per se had no effect on muscular efficiency. We note, however, that our competitive cyclists were not subjected to a controlled training protocol. Böning et al. (4) and Stuart et al. (24) also failed to incorporate controlled training regimens. Moreover, Böning et al. (4), Marsh and Martin (18), and Stuart et al. (24) did not examine whole body muscular efficiencies during submaximal exercise. Inasmuch as our present estimates of submaximal whole body muscular efficiencies were not different, we believe that our findings more appropriately suggest that leg cycling efficiency is not improved by previous training experience.
To our knowledge, this study is the first to demonstrate that competitive cyclists do not become more efficient at higher pedaling frequencies and power outputs as a function of prior cycling experience despite increases in submaximal power outputs and endurance. We have shown that whole body muscular efficiency is not different between competitive and recreational cyclists during graded and submaximal leg ergometry exercise despite the presence of greater endurance at higher pedaling frequencies. In the competitive group, prior cycling experience resulted in increased submaximal power outputs and endurance but did not improve whole body muscular efficiency. In summary, we conclude that prior cycling experience was of minor importance when comparing muscular efficiency of competitive and recreational cyclists. Furthermore, factors contributing to the inverse relationship between whole body muscular efficiency and endurance have yet to be determined.
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Keywords:©1996The American College of Sports Medicine
EXERCISE EFFICIENCY; ECONOMY; TRAINING; EXERTION; CYCLING; TRAINING; MUSCLE