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Is there a conflict between minimizing effort and energy expenditure with increasing velocities of muscle contraction in humans?

Baoyuan Chen, N. L. Jones and K. J. Killian

MS 8687 Received 3 September 1998; accepted after revision 6 May 1999.

	ABSTRACT

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1. The present study examined the possibility that minimizing effort conflicts with minimizing energy expenditure at different velocities of muscle contraction during cycling.

2. Six normal subjects underwent incremental exercise on an electrically stabilized cycle ergometer. Power output increased by 45 W every 3 min to exhaustion at pedalling frequencies of 40, 60, 80 and 100 r.p.m. on separate days. Energy expenditure (oxygen uptake), leg effort and dyspnoea (Borg 0-10 scale) were measured in parallel at the end of each minute.

3. All six subjects completed 10 min of exercise achieving 180 W for all four pedalling frequencies. Two-way analysis of variance indicated that oxygen uptake (P < 0·0001), leg effort (P < 0·0001) and dyspnoea (P < 0·0001) increased with duration of exercise and power output; oxygen uptake (P < 0·0001) and leg effort (P < 0·05) were significantly different between pedalling frequencies; the interactions were not significant. Oxygen uptake was minimal at 60 r.p.m., and increased at both higher and lower pedalling frequencies. Both leg effort and dyspnoea were minimal at 80 r.p.m.; leg effort intensified at higher and lower pedalling frequencies; and dyspnoea was most intense at 100 r.p.m.

4. There was a conflict between minimization of energy expenditure and leg effort at power outputs less than 180 W. Minimizing effort occurred at the expense of an increase in energy expenditure.

	INTRODUCTION

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The capacity to generate power, which is the product of force and velocity, decreases above and below an optimal velocity. The relationships between force, velocity and the energy consumed by muscle were first elucidated by the classical studies of Hill (1951). Wilkie (1950) extended these studies to human muscle. In animals, muscle groups have different force-velocity relationships associated with differences in muscle morphology (Bodine et al. 1982), fibre type (Edgerton et al. 1986) and biochemical characteristics (Barany, 1967). In humans, these findings are essentially similar (Wickiewicz et al. 1984).

Cyclists select the force relative to velocity by choosing specific gear ratios. Slow twitch fibres are inefficient at higher velocities of contraction and influence gear selection (Christie, 1934). As power output increases, cyclists generally increase the pedalling frequency (velocity) (Seabury et al. 1977; Moffatt & Stamford, 1978; Coast & Welch, 1985). Untrained cyclists select lower pedalling frequencies than trained subjects (Hagberg et al. 1981; Pivarnik et al. 1988). Elite cyclists pedal at frequencies greater than 90 r.p.m. All cyclists increase the pedalling frequency and decrease the tension developed during each pedal stroke when climbing a hill. This reduces the leg effort for the same total power output. However, there is an added energy cost to lifting the legs more frequently (Hagberg et al. 1981). The present study examined the possibility that minimizing effort may conflict with minimizing energy expenditure at different velocities of muscle contraction. A psychophysical method (Borg, 1982) was used that rates the magnitude of effort required to pedal and to breathe and energy expenditure was measured by oxygen uptake during incremental exercise to exhaustion at four different pedalling frequencies.

	METHODS

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Six normal male volunteers (means ± S.D.: age, 35·8 ± 7·27 years; weight, 70·2 ± 8·33 kg; height, 176·5 ± 7·15 cm; forced expired volume in 1 s (FEV₁), 4·7 ± 0·59 l s^-1) were studied; five took part in regular exercise and were fitter and more familiar with the use of the Borg scale than the remaining subject; none were competitive cyclists. The nature and possible risks of the experiment were explained and informed consent was obtained; the study was approved by the Ethics Committee of McMaster University.

Exercise was performed on an electrically stabilized cycle ergometer (Siemens Ergo Med 740, Siemens Electric Ltd), calibrated by torque balance to establish that power was constant at all power settings and constant between the pedalling frequencies used in this study. All subjects completed four progressive maximal tests on the cycle ergometer on four visits separated by at least a 1 day interval. Each test was performed at one of four pedalling frequencies (40, 60, 80 and 100 r.p.m.) in random order. The first workload was set at 45 W and was incremented by 45 W every 3 min until exhaustion. Heart rate was recorded with a standard 3-lead system cardiac monitor. Ventilation, breathing frequency, tidal volume, oxygen uptake and carbon dioxide output were measured using a calibrated universal exercise system (SensorMedics MMC Horizon, Anaheim, CA, USA). At the end of each minute the subjects were asked to rate the magnitude of leg effort and dyspnoea (discomfort associated with the act of breathing) by pointing to a Borg scale (0-10 scale) (Borg, 1982), displayed in view of the subject and observers. The rating statement chosen by the subject was repeated by the observer and confirmed by the subject.

Analysis of results

Regression analysis (single and multiple) and analysis of variance (two way and repeated measures) were used. Power output and duration of exercise were systematically related. Because oxygen uptake, leg effort and dyspnoea increased with duration at any given power output, the effect of pedalling frequency was analysed relative to the duration of exercise. Leg effort, dyspnoea (Borg scale) and oxygen uptake (ml min^-1 STPD (standard temperature and pressure dry)) were dependent variables, duration of exercise (and thus power output) and pedalling frequency were independent contributors. The number of time points was initially confined to the 10 min of exercise completed by all six subjects at all four pedalling frequencies for the two-way analysis of variance. This was required because of the variation in exercise capacity between subjects. Repeated measures analysis of variance was also performed across the four pedalling frequencies achieved by each individual subject. The number of time points increased to 15, 16, 15, 12 and 13 for the individual subjects because of the differences in their exercise capacity. Post hoc analysis was performed using Newman-Keuls test.

	RESULTS

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Duration of exercise and maximum power output achieved (MPO) were, respectively, 14·2 ± 2·32 min and 217 ± 33·9 W at 40 r.p.m., 15·5 ± 3·02 min and 255 ± 46·5 W at 60 r.p.m., 16·0 ± 2·83 min and 255 ± 46·5 W at 80 r.p.m. and 15·2 ± 3·37 min and 240 ± 46·5 W at 100 r.p.m. (P = 0·10 and P < 0·01). The MPO at 40 r.p.m. was significantly lower than the MPO at 60, 80 and 100 r.p.m.

Oxygen uptake

The mean oxygen uptake per minute is plotted against the duration of exercise and power output for each of the four pedalling frequencies in Fig. 1. All subjects contributed to the first 10 min of exercise. Over this period, the mean oxygen uptake per minute increased significantly with duration (F = 323; P < 0·0001; Table 1); the mean oxygen uptake per minute changed from 1439 ml min^-1 at 40 r.p.m. to 1391 ml min^-1 at 60 r.p.m. to 1544 ml min^-1 at 80 r.p.m. and to 1684 ml min^-1 at 100 r.p.m. (F = 62; P < 0·0001; all values were significantly different from each other; P < 0·05); the interaction was not significant.

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Figure 1 Oxygen uptake (VO2) increased with duration and power output (P < 0·0001) and as pedalling frequency increased (P < 0·001). The inset represents the mean oxygen uptake for each pedalling frequency.

Table 1. Mean oxygen uptake per minute at four different pedalling frequencies

Repeated measures analysis of variance with pedalling frequency was performed following the further inclusion of oxygen uptake values at the higher power outputs achieved by each individual subject at all four pedalling frequencies. The mean oxygen uptake per minute achieved was 1780 ml min^-1 at 40 r.p.m., 1720 ml min^-1 at 60 r.p.m., 1840 ml min^-1 at 80 r.p.m. and 1960 ml min^-1 at 100 r.p.m. (F = 63; P < 0·0001) (inset of Fig. 1). The oxygen uptake decreased from 40 to 60 r.p.m. (P < 0·001), increased from 60 to 80 r.p.m. (P < 0·0001) and increased from 80 to 100 r.p.m. (P < 0·0001). The changes in oxygen uptake with pedalling frequency were significant up to 10 min of exercise (180 W) (P < 0·05); at the higher power outputs they were only significant at minutes 13 and 14 (Table 1).

Sense of effort

Leg effort intensified with duration of exercise for each of the four pedalling frequencies (Fig. 2). All subjects contributed to the first 10 min of exercise. Over this period, the mean leg effort increased significantly with duration (F = 65; P < 0·0001; Table 2); the mean leg effort changed from 2·44 at 40 r.p.m. to 2·05 at 60 r.p.m., to 1·78 at 80 r.p.m. and to 1·95 at 100 r.p.m. (F = 2·8; P < 0·05); leg effort at 40 r.p.m. was significantly different from leg effort at 80 r.p.m. (P < 0·05); the interaction was not significant.

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Figure 2 There was a significant increase in leg effort with duration (P < 0·0001) and power output (P < 0·0001) and a significant decrease as pedalling frequency increased (P < 0·0001). The inset represents the mean leg effort for each pedalling frequency.

Table 2. Mean leg effort per minute at four different pedalling frequencies

Repeated measures analysis of variance with pedalling frequency was performed following the further inclusion of the estimates of leg effort at the higher power outputs achieved by each individual subject at all four pedalling frequencies. The mean leg effort was 3·86 at 40 r.p.m., 3·08 at 60 r.p.m., 2·81 at 80 r.p.m. and 3·03 at 100 r.p.m. (F = 17; P < 0·0001) (inset of Fig. 2). Leg effort at 40 r.p.m. was significantly higher than at all other pedalling frequencies (P < 0·0001); the differences between 60, 80 and 100 r.p.m. did not reach statistical significance. The changes in leg effort at individual time points during exercise did not reach statistical significance (Table 2).

Ventilation and pattern of breathing

All subjects contributed to the first 10 min of exercise. Over this period, the mean ventilation per minute increased significantly with duration (F = 61; P < 0·0001); the mean ventilation per minute increased with pedalling frequency: 36·2 l min^-1 at 40 r.p.m., 38·4 l min^-1 at 60 r.p.m., 40·1 l min^-1 at 80 r.p.m., and 46·8 l min^-1 at 100 r.p.m. (F = 22; P < 0·0001). Ventilation at 100 r.p.m. was significantly higher than at all other pedalling frequencies (P < 0·0001). The interaction was not significant.

Repeated measures analysis of variance with pedalling frequency was performed following the further inclusion of values at the higher power outputs achieved by each individual subject at all four pedalling frequencies. The mean ventilation per minute achieved was 45·4 l min^-1 at 40 r.p.m., 47·6 l min^-1 at 60 r.p.m., 48·5 l min^-1 at 80 r.p.m. and 55·5 l min^-1 at 100 r.p.m. (F = 99; P < 0·0001). Ventilation increased significantly for each increment in pedalling frequency (P < 0·0001) with the exception of 60 to 80 r.p.m. (n.s.). The increase in ventilation with pedalling frequency was significant up to 15 min of exercise (270 W) (P < 0·05). Ventilation (VE, l min^-1) increased with the duration of exercise (min) (P < 0·0001) and with pedalling frequency (r.p.m.; P < 0·0001):

(r = 0·91).

Ventilation (l min^-1) increased in a close relationship to carbon dioxide output (V_CO2, l min^-1) and the additional contribution of pedalling frequency did not reach statistical significance:

(r = 0·97).

The mean breathing frequency achieved was 23·1 breaths min^-1 at 40 r.p.m., 25·4 breaths min^-1 at 60 r.p.m., 24·1 breaths min^-1 at 80 r.p.m. and 26·7 breaths min^-1 at 100 r.p.m. (F = 25; P < 0·0001). All values were significantly different from each other. The mean tidal volume achieved was 1·97 l at 40 r.p.m., 1·90 l at 60 r.p.m., 2·04 l at 80 r.p.m. and 2·13 l at 100 r.p.m. (F = 25; P < 0·0001). All values were significantly different from each other. However, the increases in breathing frequency (breaths min^-1) and tidal volume (l) were significantly related to V_CO2 (l min^-1) but not to pedalling frequency.

(r = 0·56).

(r = 0·83).

Sense of dyspnoea

Dyspnoea intensified with duration of exercise for each of the four pedalling frequencies (Fig. 3). All subjects contributed to the first 10 min of exercise. Over this period, the mean dyspnoea increased significantly with duration (F = 13; P < 0·0001; Table 3). The mean dyspnoea was 1·02 at 40 r.p.m., 1·45 at 60 r.p.m., 1·30 at 80 r.p.m. and 1·66 at 100 r.p.m. (F = 1·9; n.s.); the interaction was not significant.

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Figure 3 There was a significant increase in dyspnoea with duration (P < 0·0001) and power output (P < 0·0001), but it did not achieve statistical significance (P = 0·06) as pedalling frequency increased. The inset represents the mean dyspnoea for each pedalling frequency.

Table 3. Mean dyspnoea per minute at four different pedalling frequencies

Repeated measures analysis of variance with pedalling frequency was performed following the further inclusion of the estimates of dyspnoea at the higher power outputs achieved by each individual subject at all four pedalling frequencies. The mean dyspnoea was 2·10 at 40 r.p.m., 2·26 at 60 r.p.m., 2·06 at 80 r.p.m. and 2·52 at 100 r.p.m. (F = 5·8; P < 0·001) (inset of Fig. 3). Dyspnoea at 100 r.p.m. was significantly higher than at all other pedalling frequencies (P < 0·0001); the differences between 40, 60 and 80 r.p.m. did not reach statistical significance. The changes in dyspnoea at individual time points during exercise did not reach statistical significance (Table 3).

	DISCUSSION

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Oxygen uptake was lowest at 60 r.p.m. There was an increase of 3·7 % at 40 r.p.m., 7·4 % at 80 r.p.m. and 14·0 % at 100 r.p.m. Leg effort and dyspnoea were both minimal at 80 r.p.m. Leg effort increased 30 % at 40 r.p.m., 9·5 % at 60 r.p.m. and 7·6 % at 100 r.p.m. Dyspnoea increased 1·8 % at 40 r.p.m., 9·4 % at 60 r.p.m. and 21·8 % at 100 r.p.m. (the differences between 40, 60 and 80 r.p.m. were not significant). Leg effort and oxygen uptake had a parabolic relationship with pedalling frequency but the optimum frequencies were not the same. At 80 r.p.m., leg effort and dyspnoea were both minimal while there was a 7·4 % increase in oxygen uptake indicating a conflict between the simultaneous minimization of effort and energy expenditure.

Energy expenditure

The energy cost of generating power has been studied repeatedly over the past 60 years with a particular interest in the optimal pattern of muscle contraction (Dickinson, 1928; Garry & Wishart, 1931). The present study is in general agreement with previous studies (Banister & Jackson, 1967; Seabury et al. 1977; Lollgen et al. 1978; Hagberg et al. 1981; Powers et al. 1984; Coast & Welch, 1985; Coast et al. 1986; Pivarnik et al. 1988). The following is a summary of their results. Oxygen uptake increases linearly with power output and in a parabolic relationship with pedalling frequency. Oxygen uptake is minimal at pedalling frequencies of 50-80 r.p.m., and increases below 40 r.p.m. and above 100 r.p.m. The pedalling frequency at which energy expenditure is minimized increases from 50 to 80 r.p.m. as power output increases (Coast & Welch, 1985). In elite cyclists the changes in oxygen uptake due to pedalling frequency are seen at both low and high power outputs (Seabury et al. 1977; Coast & Welch, 1985). In non-elite cyclists the effect of pedalling frequency on oxygen uptake is greater at lower power outputs and decreases as power output approaches the maximal capacity of the individual to exercise (Banister & Jackson, 1967; Pivarnik et al. 1988). In essence, there is no conflict between the present study and these previous studies.

Actin and myosin cross-bridging, calcium uptake and release, and active ion transport or other non-muscular demands throughout the body contribute to energy expenditure. The changes in oxygen costs between pedalling frequencies may be attributed solely to the added energy required to lift the legs more frequently. The oxygen costs of loadless pedalling increase from 7·5 ml kg^-1 at 60 r.p.m. to 19·3 ml kg^-1 at 120 r.p.m. amounting to 13·8 ml r.p.m.^-1 in a 70 kg subject (Hagberg et al. 1981). The increases in oxygen costs reported in the present study were less because the weight of the legs actually contributes to the work performed. The increase in oxygen uptake from 60 to 40 r.p.m. was attributed to the poor co-ordination of cycling at low rates. The changes in oxygen uptake between pedalling frequencies were only seen up to 180 W but there were progressively fewer subjects due to variation in exercise capacity between subjects. Furthermore, it is increasingly more difficult to discriminate a modest added oxygen uptake due to the added pedalling frequency from values of less than 400 ml to values of greater than 3000 ml. The lack of significance for the changes in oxygen consumption at higher power outputs is commonly attributed to limitation in oxygen extraction when the muscle tension is highest and, in this study, to fewer subjects at the highest power outputs.

Effort

The perceived exertion associated with generating power has been studied with a particular interest in the pattern of muscle contraction associated with minimal leg effort. The present study is in agreement with these previous studies (Pandolf & Noble, 1973; Cafarelli, 1978; Lollgen et al. 1980). The following is a summary of the results. Leg effort increases in a positively accelerating relationship with power output and in a parabolic relationship with pedalling frequency. Perceived exertion is minimal at pedalling frequencies of 50-80 r.p.m., and increases below 40 r.p.m. and above 100 r.p.m. The pedalling frequency at which perceived exertion is minimized increases from 40-50 r.p.m. at low power outputs to 80-90 r.p.m. at very high power outputs. Individual subjects may experience minimal leg effort at a slightly lower or higher pedalling rate.

Borg (1982) developed a rating scale using quantitative semantics which he linked to numbers that allowed a measurement of absolute sensory magnitude, while maintaining the ratio characteristics indicated by Stevens's power law (Stevens, 1957). During cycling, leg effort increases as a power function of the percentage of the maximal power output (MPO) (Killian et al. 1992), and with the duration of a given level of muscular activity (min) (Kearon et al. 1991):

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Corresponding author

K. J. Killian: Ambrose Cardiorespiratory Unit, McMaster University Medical Centre, 1200 Main Street West, Hamilton, Ontario, Canada L8N 3Z5.

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