Eur J Appl Physiol (2005) DOI 10.1007/s00421-005-0077-5
O R I GI N A L A R T IC L E
Karim Zameziati Æ Guillaume Mornieux David Rouffet Æ Alain Belli
Relationship between the increase of effectiveness indexes and the increase of muscular efficiency with cycling power
Accepted: 25 September 2005 Springer-Verlag 2005
Abstract We determined the index of effectiveness (IE), as defined by the ratio of the tangential (effective force) to the total force applied on the pedals, using a new method proposed by Mornieux et al. (J Biomech, 2005), while simultaneously measuring the muscular efficiency during sub-maximal cycling tests of different intensities. This allowed us to verify whether part of the changes in muscular efficiency could be explained by a better orientation of the force applied on the pedals. Ten subjects were asked to perform an incremental test to exhaustion, starting at 100 W and with 30 W increments every 5 min, at 80 rpm. Gross (GE) and net (NE) efficiencies were calculated from the oxygen uptake and WExt measurements. From the three-dimensional force’s measurements, it was possible to measure the total force (FTot), including the effective (FTang) and ineffective force (FRad+Lat). IE has been determined as the ratio between FTang and FTot, applied on the pedals for three different time intervals, i.e., during the full revolution (IE360), the downstroke phase (IE180Desc) and the upstroke phase (IE180Asc). IE360 and IE180Asc were significantly correlated with GE (r=0.79 and 0.66, respectively) and NE (r=0.66 and 0.99, respectively). In contrast, IE180Desc was not correlated to GE or to NE. From a mechanical point of view, during the upstroke, K. Zameziati (&) Æ G. Mornieux Æ A. Belli Departement des Sciences et Technologies des Activite´s Physiques et Sportives-Unite´ PPEH, Universite´ de Saint Etienne, Saint Etienne, France E-mail:
[email protected] Tel.: +33-477-120733 Fax: +33-477-127229 K. Zameziati Dipartimento di Scienze e Tecnologie Biomediche, Sezione di Fisiologia, Universita` di Udine, Udine, Italy D. Rouffet UFR-STAPS, Universite´ Lyon I, Lyon, France K. Zameziati Me´decine du Sport et Myologie, Hoˆpital Bellevue, CHU Saint Etienne, 42055 Saint-Etienne Cedex 2, France
the subject was able to reduce the non-propulsive forces applied by an active muscle contraction, contrary to the downstroke phase. As a consequence, the term ‘passive phase’, which is currently used to characterize the upstroke phase, seems to be obsolete. The IE180Asc could also explain small variations of GE and NE for a recreational group. Keywords Efficiency Æ Pedalling effectiveness Æ Pedal forces
Introduction Muscular efficiency has been extensively studied in the literature (e.g., Whipp and Wasserman 1969; Di Prampero 2000) and is classically defined as the ratio of the mechanical work produced to the overall metabolic energy expenditure (gross efficiency, GE). To take into account the oxygen uptake at rest (about 0.3 l min1 for a subject sitting on the bicycle), Gaesser and Brooks (1975) introduced the concept of net efficiency (NE), i.e. the ratio of the mechanical power to the energy expenditure above that of rest. Both GE and NE, during cycling, increase significantly with increasing intensity below 70% of the maximal aerobic power (MAP) at constant pedalling rate. Indeed, GE increased from 10 to 19%; and NE from 15 to 22%, for an increase in external power output (WExt) from 33 to 133 W (Gaesser and Brooks 1975; Chavarren and Calbet 1999; Mourot et al. 2004). Cavanagh and Kram (1985) described two possible mechanisms whereby the relationship between WExt and energy expenditure could be modified: (i) by the link between muscles and the skeleton (i.e., muscle action may be mechanically influenced by joint geometry and corresponding lever arms and fibre length) and (ii) by the link between the human and the ergometer. The second link seems to be very relevant during cycloergometric exercise. Indeed, applying optimally oriented forces on the pedal is a major component of skilled performance, thus leading to the introduction of the term ‘force effectiveness’, in
cycling, to quantify the ratio between total force applied (FTot) and the only useful component, i.e., the propulsive force, which is tangential to the crank rotation (FTang). The effectiveness has been computed during the full revolution (IE360) (Davis and Hull 1981) or only during the propulsive phase (IE180Desc) (Coyle et al. 1991). Coyle et al. (1991) obtained a value of IE180Desc of 75% at 330 W, while Patterson and Moreno (1990) described a significant increase in IE360, with an increasing workload (e.g. 27 and 40% at 100 and 200 W, respectively). Therefore, an increase in exercise intensity seems to be associated with an optimization of the force applied on the pedals, and a decrease in effectiveness leads to an increased oxygen uptake (Patterson and Moreno 1990). However, to the best of our knowledge, the possible link between the orientation of forces on the pedals and the change in the muscular efficiency, during sub-maximal cycling tests, are quite rare in the literature. Only Lafortune and Cavanagh (1983) have obtained a correlation, albeit not significant (r=0.43), between changes in NE and the index of effectiveness (IE). This lack of relationship could be partly explained by the methods utilized to measure the total force applied to the pedals. In addition, changing the dimensions of the pedals (e.g., by fixing force transducers) can also mechanically modify the parameters, thus inducing slight changes in the oxygen consumption (Too 1990). However, a recent method allowed us to improve the accuracy of effectiveness measurement, by assessing the forces in three dimensions without modifying the mechanical properties of the pedals (Mornieux et al. 2005). The aim of this study was then: (i) to measure different IEs using a new method, while simultaneously determining the muscular efficiency at different workloads in sub-maximal cycling tests and, (ii) to verify whether a part of the changes in muscular efficiency could be explained by a better optimization of the force applied on the pedals.
Methods Subjects and training A group of ten healthy male subjects volunteered to participate in this study. Mean (SD) age, body mass, : body height, maximal oxygen uptake ðVO2 maxÞ; maximal external power output (WExt.max) and [La] were 26 (1) years, 69 (12) kg, 1.76 (0.04) m, 58 (6) ml min1 kg1, 273 (32) W and 14 (2) mmol l1, respectively. All subjects were highly trained in different physical activities (e.g., soccer, running, race and sprinting). However, highly trained cyclists were not selected to avoid adaptations due to their specific training (Coyle et al. 1991; Takaishi et al. 1998). All subjects were informed of the experimental protocol and of its potential risks and gave written consent prior to their participation.
General laboratory testing sequence Before the test, a 10-min warm up was performed on a cycle ergometer at 80 rpm, with a friction force not exceeding 15 N. This was followed by 5 min of rest, with the subjects remaining seated on the ergometer to : determine their resting oxygen consumption ðVO2 RestÞ: Each subject was asked to perform an incremental test to exhaustion, starting at 100 W, with 30 W increments every 5 min, at a constant pedalling rate of 80 rpm, imposed by an auditory metronome. Oxygen uptake : ðVO2 Þ and the pedal forces were measured at 4 min 30 s during 30 and 16 s, respectively. Blood samples were taken from the fingertip for lactate determination at the end of each step. Instrumentation :
Oxygen consumption (: VO2 in ml min1 kg1), carbon dioxide production ( VCO2 in ml min:1 kg1: ) and the respiratory exchange ratio (R), i.e., VCO2 =VO2 , were monitored during the entire test by means of a gas analyzer system (CPX/D, Medical Graphics, St. Paul, MN, USA) and stored on a personal computer. The : : VO2 ; VCO2 and (R) were averaged during the last 60 s of each power stage at a steady state. The gas analyzer was calibrated using gases of known concentrations, both before and after each test. Blood lactates concentration [La] (in mmol l1) was analyzed by an electro-enzymatic technique (YSI 2300, Yellow Spring Instrument, Yellow Spring, USA). A typical friction-loaded cycle ergometer (Monark type 818E, Stockholm, Sweden) was equipped with specific transducers, according to Arsac et al. (1996). The friction force, applied by the tension of the belt that surrounds the flywheel, was measured by means of a strain gauge (FGP Instrumentation type FN3030 0-20daN, les Clayes sous Bois, France). The flywheel displacement was calculated with an accuracy of 11,815 points per pedal revolution (gear ratio of 52:14), using an incremental encoder (Hengsler type RI32-0100 AR11, 2 channels, 100 points per turn, Aldingen, Germany) fixed on a castor (Ø65 mm) linked to the flywheel. The distance traveled by one point on the rim of the flywheel was 6 m for one pedal revolution, allowing a simple conversion from angular to linear velocity. The rear horizontal position of the crank of the right pedal could be detected by a magnetic transducer (Omron type E2EG-X5MB1, Fontenay sous Bois, France). The pedals were equipped with toe clips. The crank length was 0.17 m. The mass and the radius of the flywheel were 22.5 kg and 0.26 m, respectively. To obtain an accurate measurement of the flywheel inertia, the method proposed by Lakomy (1986) was used. In order to calculate the three-dimensional forces exerted on the pedals, the chainring, gear mechanism, the cranks and the pedal system were separated from the bicycle frame and tightly fixed on a force platform
(Kistler type 9281B, Wintertur, Switzerland), by means of a specially designed steel part providing high stiffness and rigid connection. The bicycle frame, including the flywheel and friction system, was fixed aside from the force platform, in order to avoid any mechanical contact with either the gear mechanism or the steel part, and in such a way that the original dimensions of the whole bicycle were maintained. The force platform used was equipped with four piezo-electric transducers, each measuring the three-dimensional forces applied (for details see Mornieux et al. 2005).
Apparatus for measuring cycling technique Signals from the strain gauge, displacement encoder and position transducer were sampled at 200 Hz on a PC computer, via a specially designed interface card, which included a strain gauge signal conditioner (Analog Device 1B31AN, Norwood, MA, USA), a 12-bit A/D converter (Analog Device AD574AJD, Norwood, MA, USA) for force measurement and a 12-bit counter (Hewlett Packard type HCTL-2000, Palo Alto, CA, USA) for displacement and position measurements. At the same time, the 12 force signals (four transducers · three dimensions) from the force platform were connected to an amplifier (Kistler type 9861A, Wintertur, Switzerland) and simultaneously sampled at 200 Hz on the same PC, via a 12-bit A/D converter (Keithley Metrabyte type DAS-8, Taunton, MA, USA). The instantaneous force and displacement data were low pass filtered (Butterworth fourth order with no phase lag) with a cut-off frequency of 25 Hz.
the range of WExt to estimate GE and NE was lower than 70% of MAP and (iii) the level of [La] did not exceed 4.5 mmol l1. Pedal forces computation From three-dimensional pedal forces and torques, computed at the force platform level and at the chain level, and knowing all dimensions of the apparatus and the instantaneous crank angle (a) in relation to the rear horizontal position, the three-dimensional pedal forces could be determined by applying the six fundamental ~ ¼ 0) with an mechanical equations (i.e. R~ F ¼ 0; RM accuracy better than 4%. The details of the pedal forces computation have been described elsewhere (Mornieux et al. 2005). From the three-dimensional pedal forces (i.e. the vertical force (FV), the anterior–posterior force (FH) and the medio-lateral force (FLat)), it was possible to measure the total force (FTot) applied on each pedal. From the vertical and horizontal pedal forces, it was possible to measure a resultant force (FR) applied on each pedal in the plane of the crank as well as the angle (b) between FR and the horizontal line. The effective force (FTang), tangential to the crank displacement, was measured according to trigonometric laws, using FR and the angle combination from a and b (Fig. 1). The radial force (FRad) along the crank was defined from FR and FTang, using basic geometry. FLat obtained directly from the three-dimensional pedal forces measurement was added to FRad to measure the ineffective force (F(Rad+Lat)).
Calculations Calculation of muscular efficiencies
GE ð%Þ ¼ 100
:
WExt
VO2 Eq 0
! ð1Þ
WExt
1
A NE ð%Þ ¼ 100 @ : : VO2 VO2 Rest Eq
ð2Þ
:
where V:O2 is the oxygen consumption at steady state (l s1), VO2 Rest was the oxygen consumption at rest (l s1) and Eq=20.9 kJ l1 is the energetic equivalent for O2 (which is strictly true only for a respiratory quotient of 0.96). To obtain a valid determination of muscular efficiency, the oxygen consumption was measured at steady state and under strict aerobic conditions, i.e., at an intensity lower than the anaerobic threshold (restriction of lactate accumulation). To this aim: (i) steady state time was set at 4 min for each step until exhaustion, (ii)
Fig. 1 Resultant force (FR) applied to the pedal in the plane of the crank; effective force (FTang) tangential to the crank displacement and radial force (FRad) along the crank. In the figure, a is the instantaneous crank angle in relation to the horizontal pedal position and b is the angle between FR and the horizontal
External power output External power output at the flywheel level (WExt in Watt) was computed as the product of total force and flywheel velocity: WExt ¼ ðFI þ FF Þ V
ð3Þ
where FF (N) is the friction force, FI (N) the force to overcome flywheel inertia and V (m s1) the flywheel linear velocity, determined using a first order derivation of the flywheel displacement. Calculation of the pedalling effectiveness The following equations were used to calculate the pedalling effectiveness as the percentage of the applied linear impulse, which was used to generate angular impulses in three different intervals, i.e., during the full revolution (IE360), the downstroke phase (IE180Desc) and the upstroke phase (IE180Asc), according to Coyle et al. (1991): R 2p FTang ðhÞ dh IE360 ð%Þ ¼ R02p 100 ð4Þ 0 FTot ðhÞ dh Rp FTang ðhÞ dh 100 ð5Þ IE180 Desc ð%Þ ¼ R0p 0 FTot ðhÞ dh R 2p FTang ðhÞ dh 100 IE180 Asc ð%Þ ¼ Rp2p F ðhÞ dh Tot p
non negligible involvement of anaerobic metabolism (Sjo¨din and Jacobs 1981). Therefore, they were excluded from further analysis. The mean value of [La] for all remaining subjects, in the sub-maximal condition was [mean (SD)] 2.7 (1) mmol l1.
ð6Þ
where both FTang and FTot were measured as the sum of both pedals for IE360 and as the components of the downstroke pedal, for IE180Desc, and the upstroke pedal, for IE180Asc. The crank angle was h.
Below 60% of maximal oxygen uptake Below 60% of maximal oxygen uptake (corresponding to W: Ext of [mean (SD)] 164 (29) W), the average values of VO2 ; GE and NE [mean (SD)] were 34 (7) ml min1 kg1, 19 (2) and 22 (4) %, respectively. The mean values of IE360, IE180Desc and IE180Asc were [mean (SD)] 43 (7), 70 (2) and 34 (14) %, respectively. GE and NE were significantly and positively correlated with WExt (r=0.79, P £ 0.01; r=0.64, P £ 0.01, respectively; Fig. 2). Gross efficiency was significantly and positively correlated with IE360 and IE180Asc (r=0.79, P £ 0.01; r=0.66, P £ 0.01, respectively; Fig. 3a–c). NE was significantly and positively correlated with IE360 and IE180Asc (r=0.66, P £ 0.01; r=0.54, P £ 0.01, respectively; Fig. 3a–c). However, GE and NE were not correlated with IE180Desc (r=0.23 and 0.28, respectively; Fig. 3b). Beyond 60% of maximal oxygen uptake Beyond 60% of maximal oxygen uptake (corresponding to WExt of [mean (SD)] 268 (35) W), the average values of IE360, IE180Desc, and IE180Asc were [mean (SD)] 59 (7), 73 (4) and 4 (23) %, respectively. The FTang360, FTang180Desc and FTang180Asc values were [mean (SD)] 191 (32), 197 (20) and 4 (15) N, respectively. The F(Rad+Lat)360, F(Rad+Lat)180Desc and F(Rad+Lat1)80Asc values were [mean (SD)] 131 (21), 181 (31) and 47 (8) N, respectively.
Statistics Pearson’s correlation test was then used to check eventual changes in GE, NE, IE360, IE180Desc, IE180Asc, FTang360, FTang180Desc, FTang180Asc, F(Rad+Lat)360, F(Rad+Lat)180Desc and F(Rad+Lat)180Asc, with increasing WExt. Pearson’s correlation test was then used to find out whether the variation in GE and NE could be linked to effectiveness changes (i.e. IE360, IE180Desc and IE180Asc). Statistical significant level was accepted for P £ 0.05.
Results Subject characteristics and responses Two subjects reached [La] values higher than 4.5 mmol l1 in sub-maximal conditions (at 60 (10) % of maximal oxygen uptake), which was considered evidence for the
Fig. 2 Relationship between averaged WExt maintained during sub-maximal condition for all subjects and gross (open dots) and net (grey dots) efficiency
Entire range of work rates tested IE360 and IE180Asc were significantly and positively correlated with WExt (r=0.8, P £ 0.01; r=0.81, P £ 0.01, respectively; Fig. 4a–c). IE180Desc was not correlated with increasing WExt (r=0.02; Fig. 4b). Moreover, FTang360, FTang180Desc, FTang180Asc, F(Rad+Lat)360 and F(Rad+lat)180Desc were significantly and positively correlated with WExt (r=0.99, P £ 0.01; r=0.75, P £ 0.01; r=0.72, P £ 0.01; r=0.69, P £ 0.01; r=0.86, P £ 0.01, respectively; Fig. 5a–c). F(Rad+Lat)180Asc was not correlated with increasing WExt (r=0.2; Fig. 5c).
Discussion GE and NE mean values (19 and 22% at 164 W, respectively) increased significantly with WExt (Fig. 2), as was also reported by others at the same cadence of pedalling (Gaesser and Brooks 1975; Boning et al. 1984; Sidossis et al. 1992; Nickleberry and Brooks 1996; Chavarren and Calbet 1999). In addition, IE360 mean values (43 and 59% at 164 and 268 W, respectively) were similar to those reported by others at comparable WExt and pedalling frequencies (48% at 155 W and 60 rpm (Lafortune and Cavanagh 1983); 40% at 200 W and 80 rpm (Patterson and Moreno 1990). Figure 4a shows a significant linear increase in the overall effectiveness with WExt. Since the pedal forces applied to the pedals are due to muscular action, inefficiently applied pedal forces are likely to cause inefficiency in muscular work. However, Lafortune and Cavanagh (1983) found only a non-significant correlation (r=0.43) between the net physiological efficiency and IE. They concluded that even though some subjects were applying less force to work at an equivalent WExt, they were not necessarily more efficient. In this study, we proposed a new method to explore a possible link between muscular efficiency changes and effectiveness. Figure 3a shows that GE and NE increased significantly with IE360 (r=0.79, P £ 0.01; r=0.66, P £ 0.01, respectively). This suggests that IE, computed during the full revolution, could play a determinant role in modulating muscular efficiency changes at a constant pedalling rate. Force data also indicate that, with increasing WExt, the subjects tend to adopt a more effective pedalling strategy: i.e., effective propulsive force (FTang360) increased to a larger extent than the ineffective one (F(Rad+Lat)360) (Figs. 5a, 6a, b). As described by Coyle et al. (1991), high performances were mainly assumed to depend on the capacity to generate high ‘downstroke power’ between 0 and 180 crank angle. Between 0 and 180 crank angle, the present results of IE180Desc mean values (70 and 73% at 164– 268 W, respectively) are comparable to that reported by Coyle et al. (1991) (75% at 330 W). This is consistent
Fig. 3 Relationship between index of effectiveness (IE) and gross (open dots) or net (grey dots) efficiency. a Overall IE during a full revolution (IE360); b IE during the downstroke (IE180Desc); c IE during the upstroke (IE180Asc)
with the absence of any significant relationship between WExt and IE180Desc, as observed in this study (r=0.02; Fig. 4b). Figure 5b indicates that all subjects used a
Fig. 4 Relationship between index of effectiveness (IE) and WExt. a Overall IE during a full revolution (IE360); b IE during the downstroke (IE180Desc); c IE during the upstroke (IE180Asc)
Fig. 5 Relationship between WExt and effective (tangential) forces (grey lozenges) or ineffective (radial + lateral) forces (open lozenges) during a full revolution (IE360) (a); during the downstroke (IE180Desc) (b); during the upstroke (IE180Asc) (c)
similar strategy of effective force (FTang180Desc) and ineffective force (F(Rad+Lat)180Desc), during the downstroke. It can be concluded from the present results, as well as from those reported by Coyle et al. (1991), that,
during the downstroke, all subjects were able to develop a value of about 70% of maximal effectiveness and maintain it at a cadence of 80 rpm independent of WExt (e.g. 70 and 73% of IE at 164–268 W, respectively). This
Fig. 6 Average total FTot and tangential FTang forces of pedal loading recorded during cycling at a 160 W; b 260 W. The grey area situated between the two curves corresponds to the ineffective force F(Rad+Lat)
optimal value of effectiveness seems to be constant (70%), because the subjects are not able to increase the propulsive force without increasing the non-propulsive force by an equivalent amount. Thus, the force effectiveness index during the downstroke could not be linked to changes in muscular efficiency (r=0.08; Fig. 3b). Between 180 and 360 crank angle, this stroke was commonly defined as a ‘passive phase’ (Davis and Hull 1981; Lafortune and Cavanagh 1983; Ericson 1988). A consistent conclusion of the previous studies was that cyclists do not pull on the pedal during the upstroke. The pedal force recorded was mainly caused by the weight of the limb, which is moved upwards, partly, by forces applied to the contralateral pedal (Ericson 1988). However, Coyle et al. (1991) found a reduced negative torque during the upstroke, than observed previously, and concluded that this reduction in negative torque was produced by pulling up on the pedal. The mean values of IE180Asc recorded in this study (35 and 4% at 164– 268 W, respectively) were similar to those reported in
the literature (Coyle et al. 1991; Sanderson et al. 2000; Sanderson and Black 2003). Figure 4c shows that IE180Asc increases significantly with WExt (r=0.81, P £ 0.01). In addition, the force data also indicate that, with an increasing WExt, all subjects adopt a more effective ‘recovery’ pedalling strategy during the upstroke; i.e., the effective force (FTang180Asc) decreased continuously whereas the ineffective force (F(Rad+Lat)180Asc) was small and relatively constant ([mean (SD)] 50 (10) N, Fig. 5c). Figure 5c shows that, during the upstroke, a few subjects were able to develop a positive value of effective force (FTang180Asc) at a high level of WExt, as reported by Coyle et al. (1991). Figure 4c shows that GE and NE increase significantly with the IE180Asc (r=0.66, P £ 0.01; r=0.54, P £ 0.01, respectively). It can be suggested from these results, as well as from those reported by other authors, that, during the upstroke, increased endurance may be achieved if training focuses on maintaining an effective ‘recovery’ force profile (Coyle et al. 1991; Sanderson and Black 2003).
However, as described by Sanderson and Cavanagh (1990), the IE computed during the full revolution was not sensitive to variations within the cycle. This is consistent with the IE and the pattern of forces applied (effective and ineffective), studied separately during the downstroke and upstroke phases, as observed in this study (Figs. 3b, c, 5b, c). Even if our results describe the upstroke as a key mechanical factor, its physiological impact could be discussed. Force data indicate that, with increasing WExt, pedal forces were mainly created by isometric, and weakly by concentric and eccentric, muscular contraction (i.e., inefficient forces (F(Rad+Lat)180Asc) were constant and effective propulsive forces (FTang180Asc) were small (Fig. 5c). Assuming a direct relationship between oxygen uptake and WExt, Patterson and Moreno (1990) reported that isometric or non-external work producing forces result in relatively small increases in oxygen uptake, as compared to concentric muscular contraction involved during the downstroke phase. Therefore, it can be concluded from the present results, as well as from those reported by Patterson et al. (1983), that it would be awkward to interpret the significant relationship between muscular efficiency and optimization of pedalling, without taking into account the relative importance of mechanical and physiological impact of the upstroke phase. Nevertheless, this study has demonstrated that IE180Asc is an important factor in determining variations in muscular efficiency, at least in recreational cyclists.
Conclusion In light of the previous findings, it may appear that the pattern of IE depends directly on the period of computation. From a mechanical point of view, during the upstroke, recreational subjects were able to reduce the non-propulsive forces applied by an active muscle contraction. This suggests for future experiments to redefine this period as being not only the passive phase but also the active phase. The term of passive phase currently used to describe the upstroke phase seems obsolete. Also, the IE180Asc could explain small variations of muscular efficiency. As a final point, the data presented here offer further insights into the comprehension of the optimal pattern of pedalling and improving effectiveness during the upstroke phase may be a key factor to make sure that subjects can achieve their maximum.
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