Energy cost and efficiency of riding aerodynamic bicycles

Summary Traction resistance (R _t) was determined by towing two cyclists in fully dropped posture on bicycles with an aerodynamic frame with lenticular wheels (AL), an aerodynamic frame with traditional wheels (AT), or a traditional frame with lenticular wheels (TL) in calm air on a flat wooden track at constant speed (8.6–14.6 m·s^–1). Under all experimental conditions, R _t increased linearly with the square of air velocity (_a ²); r ² equal to greater than 0.89. The constant k = R _t/_a ² was about 15% lower for AL and AT (0.157 and 0.155 N·²·m^–2) than for TL bicycles (0.184 N·²·^–2). These data show firstly, that in terms of mechanical energy savings, the role of lenticular wheels is negligible and, secondly, that for TL bicycles, the value of k was essentially equal to that found by others for bicycles with a traditional frame and traditional wheels (TT). The energy cost of cycling per unit distance (C _c, J·m^–1) was also measured for AT and TT bicycles from the ratio of the O₂ consumption above resting to speed, in the speed range from 4.7 to 11.1 m·s^–1. The C _c also increased linearly with _a ², as described by: C _c = 30.8 + 0.558 _a ² and C _c = 29.6 + 0.606 _a ² for AT and TT bicycles. Thus from our study it would seem that AT bicycles are only about 5% more economical than TT at 12.5 m·s^– the economy tending to increase slightly with the speed. Assuming a rolling coefficient equal to that observed by others in similar conditions, the mechanical efficiency was about 10% lower for aerodynamic than for conventional bicycles, amounting to about 22% and 25% at a speed of 12.5 m·s^–1. From these data it was possible to calculate that the performance improvement when riding aerodynamic bicycles, all other things being equal, ought to be about 3%. This compares favourably with the increase of about 4% observed in world record speeds (over distances from 1 to 20 km) after the adoption of the new bicycles.     

Energy cost and efficiency of sculling a Venetian gondola

Summary Oxygen uptake was measured on four male subjects during sculling gondolas at constant speeds from 1 to 3 m · s^–1. The number of scullers on board in the different trials was one, two or four. Tractional water resistance (drag,D, N) was also measured in the same range of speeds. Energy cost of locomotion per unit of distance (C, J·m^–1), as calculated from the ratio of O₂ uptake above resting to, increased with v according to a power function (C=155.2· ^1.67;r=0.88). AlsoD could be described as a power function of the speed:D=12.3· ^2.21;r= 0.94). The overall efficiency of motion, as obtained from the ratio ofD toC increased with speed from 9.2% at 1.41 m· s^–1 to 14.5% at 3.08 m·s^–1. It is concluded that, in spite of this relatively low efficiency of motion, the gondola is a very economic means. Indeed, at low speeds ( 1 m·s^–1), the absolute amount of energy for propelling a gondola is the same as that for waking on the level at the same speed for a subject of 70 kg body mass.     

Effects of altitude on top speeds during 1 h unaccompanied cycling

The present world record for 1 h unaccompanied cycling (55.291 km) was set by T. Rominger in November 1994 at sea level (Bordeaux, France). However, maximal aerobic cycling performances can be expected to increase at altitude because, for a given air temperature, air density decreases more than VO_2max. The combined effect of these opposite trends results in an improvement of performances. In this study, based on the aerodynamics of track cycling, and assuming an average decrease of VO_2max with altitude as from the literature, we show that the ideal altitude for Rominger is 4000 m where he could cover 60.1 km in 1 h. To our knowledge, only two cyclists attempted at close time intervals to set the 1 h record at sea level and at altitude (Mexico, 2230 m above sea level): F. Moser and J. Longo. Their increase of performance with altitude was only about 50% of that predicted on the basis of similar calculations as performed on Rominger. This suggests that the decrease of VO_2max resulting from altitude is greater for athletes than for average trained subjects and/or that the fraction of VO_2max that can be maintained throughout 1 h decreases with altitude.     

Energy cost and efficiency of Venetian rowing on a traditional,flat hull boat (<Emphasis Type="Italic">Bissa</Emphasis>)

The total net metabolic power output (, kW) required to scull a traditional, flat hull boat—the “Bissa”, 9.02 m long and weighting about 500 kg including the crew—was assessed at different constant speeds (ν) ranging from 2.44 to 3.75 m s⁻¹. increased with the speed:  = 0.417 × e ^0.664v ; r ² = 0.931. The amount of metabolic energy spent per unit distance (C, J m⁻¹) to move the “Bissa”, calculated by dividing by the corresponding ν, was a linear function of ν: C = 0.369 ν –0.063; r ² = 0.821. The hydrodynamic resistance met by the boat in the water—drag (D, N)—was estimated by analysing the decay of the reciprocal of ν as a function of time measured during several spontaneous deceleration tests carried out in still water and by knowing the total mass of the watercraft plus crew. D increased as a square function of speed: D = 12.76 v ². This allowed us to calculate the drag efficiency (η_d), as the ratio of D to C: η_d increased from 8.9 to 13.7% in the range of the speeds tested. The “Bissa” turned out to be as economical as other flat hull, traditional watercrafts, such as the bigger Venetian gondola, and her η_d was similar to that of other modern and traditional watercrafts.     

Energy cost of running in similarly trained men and women

Summary The energy demand of running on a treadmill was studied in different groups of trained athletes of both sexes. We have not found any significant differences in the net energy cost (C) during running (expressed in J·kg⁻¹·m⁻¹) between similarly trained groups of men and women. For men and women respectively in adult middle distance runnersC=3.57±0.15 and 3.65±0.20, in adult long-distance runnersC=3.63±0.18 and 3.70±0.21, in adult canoeistsC=3.82±0.34 and 3.80±0.24, in young middle-distance runnersC=3.84±0.18 and 3.78±0.26 and in young long-distance runnersC=3.85±0.12 and 3.80±0.24. This similarity may be explained by the similar training states of both sexes, resulting from the intense training which did not differ in its relative intensity and frequency between the groups of men and women. A negative relationship was found between the energy cost of running and maximal oxygen uptake expressed relative to body weight (for menr=−0.471,p<0.001; for womenr=−0.589,p<0.001). In contrast, no significant relationship was found in either sex between the energy cost of running and . We conclude therefore that differences in sports performance between similarly trained men and women are related to differences in . The evaluation ofC as an additional characteristic during laboratory tests may help us to ascertain, along with other parameters, not only the effectiveness of the training procedure, but also to evaluate the technique performed.     

Effects on efficiency in repetitive lifting of load and frequency combinations at a constant total power output

Sumary Boxes were lifted and lowered repetitively at three different combinations of load and frequency. These combinations were chosen such that the total mechanical power generated was constant. Effects of the varying load or frequency conditions (but constant total mechanical power) on the rate of energy expenditure (M) and on the mechanical efficiency (ME) were measured. Mechanical power was determined from film analysis and separated into external power (generated to lift the load) and internal power (to raise the lifter's body mass). The M was determined from oxygen consumption measurements. The ME was calculated in two ways, depending on the definition of mechanical power, including either the external power only (ME_ext) or the total power output (ME_tot). Despite a constant total mechanical power, M increased at higher loads and lower frequencies. This might be explained by the increasing isometric force required in postural and load control. The M increase resulted in a decrease of ME_tot. However, at higher loads and lower frequencies ME_ext increased, indicating that more external work can be done at the same energy costs at higher loads or lower frequencies, which could be of interest from the point of view of occupational physiology. It would seem that at higher loads or lower frequencies the increased costs for isometric muscle action do not outweigh the benefit of raising the body less frequently. Furthermore, it was found that the ME,, in lifting was much lower than the values reported for other kinds of activity. This was due to the large proportion of total power output that was internal power in repetitive lifting [e.g. 83.1% (at a load of 6 kg) in the present study].     

Ventilatory threshold and mechanical efficiency in endurance runners

Summary In order to evaluate changes in parameters at the ventilatory threshold (VT) and in mechanical efficiency (ME) during training in the years 1982 and 1983 we tested seven top-class endurance runners on a treadmill. The VT and ME were assessed during their training period (January 1982 and 1983) and during their competitive period (March and July 1982). The maximal functional variables were almost constant during the training year, the maximal change in being about 5%. Similarly, at the VT was almost constant; the maximal change in at VT was also about 5%. Substantially greater changes, about 10%, were recorded in the velocity of running at the VT, at which the maximum was attained in July (18.9±0.8 km·h^–1 or 5.25±0.22 m·s^–1); this value was significantly higher than values assessed during the remaining tests. The greatest change, about 23%, during the training year was found in ME, for which the maximum was attained in July (35.7%±2.1%). This was not significantly different from the value recorded in March (34.5%±3.3%), but both values were significantly higher than those recorded during the training period. We can therefore conclude that in highly trained endurance runners the times needed to attain the optimal conditions for sports performance differ from the point of view of special speed training and from the point of view of mechanical-metabolic readiness.     

Effect of the underwater torque on the energy cost,drag and efficiency of front crawl swimming

Underwater torque (T′) is defined as the product of the force with which the swimmer's feet tend to sink times the distance between the feet and the centre of volume of the lungs. It has previously been shown that experimental changes ofT′, obtained by securing around the swimmer's waist a plastic tube filled, on different occasions, with air, water or 2-kg lead, were accompanied by changes in the energy cost of swimming per unit of distance (C_S) at any given speed. The aim of this study was to investigate whether the observed increases of C_S withT′ during front crawl swimming were due to an increase of active body drag (D_b), a decrease of drag efficiency (η_d) or both. The effect of experimental changes ofT′ on C_S, D_b and η_d were therefore studied on a group of eight male elite swimmers at two submaximal speeds (1.00 and 1.23 m · s⁻¹). To compare different subjects and different speeds, the individual data for C_S, D_b,η_d andT′ were normalized dividing them by the corresponding individual averages. These were calculated from all individual data (of C_S, D_b, η_d andT′) obtained from that subject at that speed. It was found that, between the two extremes of this study (tube filled with air and with 2-kg lead),T′ increased by 73% and that C_S, D_b and η_d increased linearly withT′. The increase of C_S between the two extremes was intermediate ( ≈ 20%) between that of D_b (≈ 35%) and of η_d ( ≈ 16%). Thus, the actual strategy implemented by the swimmers to counteractT′, was to tolerate a large increase of D_b. This led also to a substantial (albeit smaller) increase of did, the effect of which was to reduce the increase of C_S that would otherwise have occurred.     

Energy cost of swimming of elite long-distance swimmers

The aim of this study was: (1) to assess the energy cost of swimming (C_s, kJ km^–1) in a group of male (n=5) and female (n=5) elite swimmers specialised in long-distance competitions; (2) to evaluate the possible effect of a 2-km trial on the absolute value of C_s. C_s was assessed during three consecutive 400-m trials covered in a 50-m pool at increasing speeds (v₁, v₂, v₃). After these experiments the subjects swam a 2-km trial at the 10-km race speed (v_2km) after which the three 400-m trials were repeated at the same speed as before (v₅=v₁, v₆=v₂, v₇=v₃). C_s was calculated by dividing the net oxygen uptake at steady state by the corresponding average speed (v, m s^–1). was estimated by using back extrapolation technique from breath-to-breath recorded during the first 30 s of recovery after each test. C_s increased (from 0.69 kJ m^–1 to 1.27 kJ m^–1) as a function of v (from 1.29 m s^–1 to 1.50 m s^–1), its values being comparable to those measured in elite short distance swimmers at similar speeds. In both groups of subjects the speed maintained during the 2-km trial (v_2km) was on the average only 1.2% faster than of v₂ and v₆ (P>0.05), whereas C_s assessed at the end of the 2-km trial (v_2km) turned out to be 21±26% larger than that assessed at v₂ and v₆ (P<0.05); the average stroke frequency (SF, cycles min^–1) during the 2-km trial turned to be about 6% (P<0.05) faster than that assessed at v₂ and v₆. At v₅, C_s turned out to be 19±9% (P<0.05) and 22±27% (0.1<P=0.05) larger than at v₁ in male and female subjects (respectively). SF was significantly faster (P<0.05, in male subjects) and the distance per stroke (Ds=v/SF) significantly shorter (P<0.05) in female subjects at v₅ and v₆ than at v₁ and v₂. These data suggest that the increase of C_s found after the 2-km trial was likely related to a decrease in propelling efficiency, since the latter is related to the distance per stroke.     

Energy cost and intracyclic variation of the velocity of the centre of mass in butterfly stroke

The purpose of this study was to examine the relationship between the intra-cycle variation of the horizontal velocity of displacement (dV) and the energy cost (EC) in butterfly stroke. Five Portuguese national level swimmers performed one maximal and two sub-maximal 200-m butterfly swims. The oxygen consumption was measured breath-by-breath by portable metabolic cart. A respiratory snorkel and valve system with low hydrodynamic resistance was used to measure pulmonary ventilation and to collect breathing air samples. Blood samples from the ear lobe were collected before and after each swim to analyse blood lactate concentration. Total energy expenditure ( E _tot) and EC were calculated for each swim. The swims were videotaped in the sagittal plane with a set of two cameras providing dual projection from both underwater and above the water surface. The APAS system was used to analyse dV for the centre of mass. The E _tot increased linearly with the increasing V, presenting a significant correlation coefficient between these parameters ( r =0.827, P <0.001). The increase in EC was significantly associated with the increase in the dV ( r =0.807, P <0.001). All data were presented as the mean value and the standard deviation. It is concluded that high intra-cycle variation of the velocity of the centre of mass was related to less efficient swimming and vice versa for the butterfly stroke.     

Metabolic cost, mechanical work, and efficiency during walking in young and older men

Aim: To investigate mechanical work, efficiency, and antagonist muscle co‐activation with a view to better understand the cause of the elevated metabolic cost of walking (C_W) in older adults. Methods: Metabolic, mechanical and electromyographic measurements were made as healthy young (YOU; n = 12, age = 27 ± 3 years) and older (OLD; n = 20, age = 74 ± 3 years) men of equivalent body mass and leg length walked on a treadmill at four speeds (ranging from 0.83 to 1.67 m s^?1). Results: Net (above resting) C_W, determined by indirect calorimetry was 31% higher (average across speeds) in OLD (P < 0.05). The integrity of the passive pendulum like interchange of mechanical energies of the centre of mass (COM_B), an energy‐saving mechanism, was maintained in OLD. Furthermore, total mechanical work, determined from fluctuations in mechanical energy of COM_B and of body segments relative to COM_B, was not significantly elevated in OLD. This resulted in a lower efficiency in OLD (?17%, P < 0.05). Co‐activation, temporally quantified from electromyography recordings, was 31% higher in OLD for antagonist muscles of the thigh (P < 0.05). Thigh co‐activation was moderately correlated with C_W at three speeds (r = 0.38–0.52, P < 0.05). Conclusion: Healthy septuagenarians with no gait impairment have an elevated C_W which is not explained by an elevation in whole body mechanical work. Increased antagonist muscle co‐activation (possibly an adaptation to ensure adequate joint stability) may offer partial explanation of the elevated C_W.     

Energy cost of front-crawl swimming at supra-maximal speeds and underwater torque in young swimmers

The energy cost of front-crawl swimming (C_s, kJ · m⁻¹) at maximal voluntary speeds over distances of 50, 100, 200 and 400 m, and the underwater torque (T′) were assessed in nine young swimmers (three males and six females; 12–17 years old). C_s was calculated from the ratio of the total metabolic energy (E _s, kJ) spent to the distance covered. E _s was estimated as the sum of the energy derived from alactic (AnAl), lactic (AnL) and aerobic (Aer) processes. In turn, AnL was obtained from the net increase of lactate concentration after exercise, AnAl was assumed to amount to 0.393 kJ · kg⁻¹ of body mass, and Aer was estimated from the maximal aerobic power of the subject. Maximal oxygen consumption was calculated by means of the back-extrapolation technique from the oxygen consumption kinetics recorded during recovery after a 400-m maximal trial. Underwater torque (T′, N · m), defined as the product of the force with which the feet of a subject lying horizontally in water tends to sink times the distance from the feet to the center of volume of the lungs, was determined by means of an underwater balance. C_s (kJ · m⁻¹) turned out to be a continuous function of the speed (v, m · s⁻¹) in both males (C_s=0.603 · 10^{0.228 v} , r ²=0.991; n=12) and females (C_s=0.360 · 10^{0.339 v} , r ²=0.919; n=24). A significant relationship was found between T′ and C_s at 1.2 m · s⁻¹; C_s=0.042T′ + 0.594, r=0.839, n=10, P < 0.05. On the contrary, no significant relationships were found between C_s and T′ at faster speeds (1.4 and 1.6 m · s⁻¹). This suggests that T′ is a determinant of C_s only at speeds comparable to that maintained by the subjects over the longest, 400-m distance [mean (SD) 1.20 (0.07) m · s⁻¹]. Accepted: 25 August 2000     

Train driving efficiency and safety: examining the cost of fatigue

This study investigated the effects of fatigue on train driving using data loggers on 50 locomotives operated by pairs of male train drivers (24-56 years) on an Adelaide-Melbourne corridor. Drivers' work history was used to calculate a fatigue score using Fatigue Audit Interdyne Software. Trains were assigned to one of three groups, based on drivers' maximum fatigue score: low (n = 15), moderate (n = 22) or high (n = 13) fatigue. Changes in driving parameters at different fatigue levels were investigated. A significant (P < 0.05) increase in fuel use was observed. Drivers in the moderate fatigue group used 4% more, and drivers in the high group used 9% more fuel than drivers in the low group. As these trains run daily, taking horsepower into account, this represents an approximate extra weekly cost of AUD$3512 using high compared with low fatigue drivers. High fatigue-group drivers used less throttle and dynamic brake and engaged in more heavy brake and maximum speed violations. Comparison of three, 100 km track sub-sections with undulating, flat, and hilly grade indicated that fuel use increases occurred primarily during the undulating sub-section, and heavy brake and maximum speed violations occurred primarily in the flat sub-section. Fatigued driving becomes less well-planned, resulting in reduced efficiency (e.g. increased fuel consumption) and safety (e.g. braking and speeding violations). Fatigue may manifest differentially depending on track grade. In certain areas, fatigue will cause increased fuel use and economic cost, and in others, reduced safety through driving violations. These factors should be carefully examined in future railway operator research.     

Energy sour es and mechanical efficiency of anaerobic work in dog gastrocnemius

Thermally isolated dog gastrocnemii were stimulated to exhaustion by rhythmic isotonic tetani of 0.2 s duration (30 min^–1) in complete occlusion of blood flow. Total enthalpy change,H=heat+work, work output,w, (kJ·kg^–1) and average mechanical efficiency over the working period, =w/H, were determined from deep muscle temperature increase and amount of shortening, and lactate produced, La, (mol·kg^–1) from washout curves, respectively. Under these conditionsH is the sum of the enthalpies due to a) alactic sources (H _a1), i.e., net P depletion and muscle O₂ stores utilization, and b) La formation: . Thus, as , the molar enthalpy of La formation, amounts to 76 kJ·mol^–1,H _a1 could be calculated. It was observed that: i) ranged from 0.2–0.5 and was higher the lowerH, this inverse relationship being essentially due to the dependence of onH _a1 as described by:H _a1=H _o·10^–k+a (whereH _o=9.3,k=3.7 anda=0.15), and ii) the shortening speed of the first few tetani (v _i, mm·s^–1), was directly related toH _a1 as described by:v _i=63.3+30.7H _a1.H _a1 is essentially proportional to theP content of resting muscle. These findings suggest therefore that the efficiency of contraction and the shortening speed are both affected by theP concentration, the efficiency increasing, and the speed decreasing with decreasingP.     

Effects of the transition time between muscle-tendon stretch and shortening on mechanical efficiency

The net mechanical efficiency of positive work (η_pos) has been shown to increase if it is immediately preceded by negative work. This phenomenon is explained by the storage of elastic energy during the negative phase and its release during the subsequent positive phase. If a transition time (T) takes place, the elastic energy is dissipated into heat. The aim of the present study was to investigate the relationship between η_pos and T, and to determine the minimal T required so that η_pos reached its minimal value. Seven healthy male subjects were tested during four series of lowering–raising of the body mass. In the first series (S ₀), the negative and positive phases were executed without any transition time. In the three other series, T was varied by a timer (0.12, 0.24 and 0.56 s for series S ₁, S ₂ and S ₃, respectively). These exercises were performed on a force platform sensitive to vertical forces to measure the mechanical work and a gas analyser was used to determine the energy expenditure. The results indicated that η_pos was the highest (31.1%) for the series without any transition time (S ₀). The efficiencies observed with transition times (S ₁, S ₂ and S ₃) were 27.7, 26.0 and 23.8%, respectively, demonstrating that T plays an important role for mechanical efficiency. The investigation of the relationship between η_pos and T revealed that the minimal T required so that η_pos reached its minimal value is 0.59 s.     

Submaximal arm crank ergometry: Effects of crank axis positioning on mechanical efficiency,physiological strain and perceived discomfort

Purpose: To evaluate the effect of the spatial orientation of the crank axis on mechanical efficiency, physiological strain and perceived discomfort in submaximal synchronous arm crank ergometry.

Methods: Twelve able-bodied individuals performed 12 submaximal exercise bouts of 3 minutes (women: 20 W/25 W; men: 25 W/35 W). The crank axis position was defined by elbow and shoulder angle.

Results: The results showed that a crank set-up with an elbow angle of 30° was more efficient than 15°; oxygen consumption and minute ventilation were significantly lower. No significant effects were seen for shoulder angle. Power output and gender showed obvious effects.

Discussion and conclusion: The magnitude of this effect and the absence of any significant shoulder angle effects may be due to the relative low exertion levels that were evaluated. An elbow angle of 30° flexion in arm crank exercise is favourable compared to an elbow angle of 15° in able-bodied untrained subjects.     

Influence of training,sex, age and body mass on the energy cost of running

Summary To highlight the influences of age, sex, body mass (m _b) and running training on the energy cost of running (C _r) young basketball players [38 boys (BB) and 14 girls (BG), aged 14.2 (SD 0.3) and 12.2 (SD 1.9) years, respectively] were selected to be compared to middle-distance runners [27 men (MR) and 14 women (FR) aged 23.7 (SD 3.4) and 23.9 (SD 4.1) years, respectively]. TheC _r was measured during a maximal treadmill test. In each groupC _r and body mass (m _b) and body height were negatively and significantly correlated. A stepwise regression showed that among both the body dimensions measured,m _b was the most important factor in determining the variations ofC _r For the whole group (n=93) the correlation coefficient was 0.72 (P<0.0001). For a givenm _b, there was no significant difference between theC _r of BG, BB and MR: this result would support the hypothesis that the differences inC _r currently attributed to age, running training or sex differences are mainly related tom _b. On the other hand, for a givenm _b, FR showed a significantly lower Cr than the basketball players (P<0.01 for BG and BB) and than MR (P<0.05), thus suggesting that women decrease theirC _r as a response to running training more efficiently than do men.     

The mechanical efficiency of locomotion in men and women with special emphasis on stretch-shortening cycle exercises

Summary The mechanical efficiency of the leg extensor musculature of men and women was examined with a special sledge ergometer. The subjects (ten males and ten females) performed (a) pure positive work, (b) pure negative work and (c) a combination of negative and positive work (strech-shortening cycle). The mechanical efficiency of pure positive work was on average 19.8±1.2% for female subjects and 17.4±1.2% for male subjects (t=4.12, P<0.001), although the work intensity was equal in both groups. The mechanical efficiency of pure negative work was slightly lower in women than in men (59.3±14.4% vs 75.6±29.3%). The mechanical efficiency of positive work (₊) in a stretch-shortening cycle exercise was 38.1±6.8% in men and 35.5±6.9% in women. The utilization of prestretch was better for female subjects at low prestretch levels, whereas males showed greater potentiation of elastic energy at higher prestretch levels. Regarding absolute W _el (work due to elasticity) values, male subjects showed greater (P<0.001) values than females (189±44 J vs 115±36 J, respectively). Fundamental differences in neuromuscular functions in men and women might cause the differences in the results obtained.     

Differentiated perceptions of exertion and energy cost of young women while carrying loads

Summary Differentiated local ratings of perceived exertion from the legs and central ratings from the chest, and oxygen consumption, were determined during load carriage in seven young women. Subjects walked for 6 min at 3.22, 4.83, 6.44, or 8.05 km·h^–1 carrying (1) no load, (2) a load equal to 7.5% of body weight (mean: 4.66 kg) or (3) a load equal to 15% of body weight (mean: 9.32 kg). Thus, each subject underwent 12 separate tests. The external loads were in the form of lead pellets carried in a plastic scuba belt worn around the waist. A differentiation threshold was found at 6.44 km·h^–1 for the 0% and 7.5% loads and at 4.83 km·h^–1 for the 15% load. At speeds below the threshold, the perception of exertion was similar in the legs, chest and overall. At higher speeds, exertion was perceived to be more intense in the legs than overall and less intense in the chest than overall, suggesting that the local legs signal was the dominant factor in shaping the overall sensation of exertion. The oxygen uptake was greater for the 15% load than for either the 0% or 7.5% loads, but was similar for the 0% and 7.5% loads. Findings suggested a critical weight limit for external loads that could be transported without increasing the metabolic cost beyond that required to move the body weight alone. This limit fell between 7.5% and 15% of the body weight. When oxygen uptake was expressed per kg of total weight transported, there was no loss of metabolic efficiency while carrying loads up to 15% of the body weight.     

The interplay between propelling efficiency,hydrodynamic position and energy cost of front crawl in 8 to 19-year-old swimmers

The aim of this study was to investigate the interplay between the arm stroke efficiency (an index of propelling efficiency, η _P) and the static and dynamic position in water (indexes of hydrodynamic resistance, W _d) in determining the energy cost of front crawl (C) during a swimmer’s growth. These three parameters are indeed related by the following equation: where η _o is the overall efficiency of swimming. The experiments were carried out on 72 swimmers (38 M and 34 F; 8–19 years) who were asked to swim at 1 m s⁻¹. The static position in water was assessed by measuring the underwater torque (T′); the dynamic position in water by measuring the projected frontal area (A _eff). The ratio between the average values of the eldest to youngest class of age was 3.84 and 2.27 for T′, 2.13 and 1.68 for A _eff, and 1.13 and 1.24 for η _P (in M and F, respectively). The increase in T′ and in A _eff was larger than the increase in efficiency suggesting that, in this age range, C should increase, the more so in M than F. Indeed, C increased by 1.58 in male and 1.17 in female swimmers. Based on the values of C and η _P (and assuming a constant value of η _o) it is possible to estimate that, in this age range, W _d increases by about 1.97 in male and 1.32 in female swimmers, an increase which is proportional to the observed increase in A _eff.