Muscular force in running turkeys: the economy of minimizing work.

During running, muscles and tendons must absorb and release mechanical work to maintain the cyclic movements of the body and limbs, while also providing enough force to support the weight of the body. Direct measurements of force and fiber length in the lateral gastrocnemius muscle of running turkeys revealed that the stretch and recoil of tendon and muscle springs supply mechanical work while active muscle fibers produce high forces. During level running, the active muscle shortens little and performs little work but provides the force necessary to support body weight economically. Running economy is improved by muscles that act as active struts rather than working machines.

Faster top running speeds are achieved with greater ground forces not more rapid leg movements.

We twice tested the hypothesis that top running speeds are determined by the amount of force applied to the ground rather than how rapidly limbs are repositioned in the air. First, we compared the mechanics of 33 subjects of different sprinting abilities running at their top speeds on a level treadmill. Second, we compared the mechanics of declined (-6 degrees ) and inclined (+9 degrees ) top-speed treadmill running in five subjects. For both tests, we used a treadmill-mounted force plate to measure the time between stance periods of the same foot (swing time, t(sw)) and the force applied to the running surface at top speed. To obtain the force relevant for speed, the force applied normal to the ground was divided by the weight of the body (W(b)) and averaged over the period of foot-ground contact (F(avge)/W(b)). The top speeds of the 33 subjects who completed the level treadmill protocol spanned a 1.8-fold range from 6.2 to 11.1 m/s. Among these subjects, the regression of F(avge)/W(b) on top speed indicated that this force was 1.26 times greater for a runner with a top speed of 11.1 vs. 6.2 m/s. In contrast, the time taken to swing the limb into position for the next step (t(sw)) did not vary (P = 0.18). Declined and inclined top speeds differed by 1.4-fold (9.96+/-0.3 vs. 7.10+/-0.3 m/s, respectively), with the faster declined top speeds being achieved with mass-specific support forces that were 1.3 times greater (2.30+/- 0.06 vs. 1.76+/-0.04 F(avge)/ W(b)) and minimum t(sw) that were similar (+8%). We conclude that human runners reach faster top speeds not by repositioning their limbs more rapidly in the air, but by applying greater support forces to the ground.

Does the application of ground force set the energetic cost of cross-country skiing?

We tested whether the rate at which force is applied to the ground sets metabolic rates during classical-style roller skiing in four ways: 1) by increasing speed (from 2.5 to 4.5 m/s) during skiing with arms only, 2) by increasing speed (from 2.5 to 4.5 m/s) during skiing with legs only, 3) by changing stride rate (from 25 to 75 strides/min) at each of three speeds (3.0, 3.5, and 4.0 m/s) during skiing with legs only, and 4) by skiing with arms and legs together at three speeds (2.0-3.2 m/s, 1.5 degrees incline). We determined net metabolic rates from rates of O2 consumption (gross O2 consumption - standing O2 consumption) and rates of force application from the inverse period of pole-ground contact [1/tp(arms)] for the arms and the inverse period of propulsion [1/tp(legs)] for the legs. During arm-and-leg skiing at different speeds, metabolic rates changed in direct proportion to rates of force application, while the net ground force to counteract friction and gravity (F) was constant. Consequently, metabolic rates were described by a simple equation (metab = F . 1/tp . C, where metab is metabolic rates) with cost coefficients (C) of 8.2 and 0.16 J/N for arms and legs, respectively. Metabolic rates predicted from net ground forces and rates of force application during combined arm-and-leg skiing agreed with measured metabolic rates within +/-3. 5%. We conclude that rates of ground force application to support the weight of the body and overcome friction set the energetic cost of skiing and that the rate at which muscles expend metabolic energy during weight-bearing locomotion depends on the time course of their activation.

High-speed running performance is largely unaffected by hypoxic reductions in aerobic power.

We tested the importance of aerobic metabolism to human running speed directly by altering inspired oxygen concentrations and comparing the maximal speeds attained at different rates of oxygen uptake. Under both normoxic (20.93% O2) and hypoxic (13.00% O2) conditions, four fit adult men completed 15 all-out sprints lasting from 15 to 180 s as well as progressive, discontinuous treadmill tests to determine maximal oxygen uptake and the metabolic cost of steady-state running. Maximal aerobic power was lower by 30% (1.00 +/- 0.15 vs. 0.77 +/- 0.12 ml O2. kg-1. s-1) and sprinting rates of oxygen uptake by 12-25% under hypoxic vs. normoxic conditions while the metabolic cost of submaximal running was the same. Despite reductions in the aerobic energy available for sprinting under hypoxic conditions, our subjects were able to run just as fast for sprints of up to 60 s and nearly as fast for sprints of up to 120 s. This was possible because rates of anaerobic energy release, estimated from oxygen deficits, increased by as much as 18%, and thus compensated for the reductions in aerobic power. We conclude that maximal metabolic power outputs during sprinting are not limited by rates of anaerobic metabolism and that human speed is largely independent of aerobic power during all-out runs of 60 s or less.

Ambulatory estimates of maximal aerobic power from foot -ground contact times and heart rates in running humans.

Seeking to develop a simple ambulatory test of maximal aerobic power (VO(2 max)), we hypothesized that the ratio of inverse foot-ground contact time (1/t(c)) to heart rate (HR) during steady-speed running would accurately predict VO(2 max). Given the direct relationship between 1/t(c) and mass-specific O(2) uptake during running, the ratio 1/t(c). HR should reflect mass-specific O(2) pulse and, in turn, aerobic power. We divided 36 volunteers into matched experimental and validation groups. VO(2 max) was determined by a treadmill test to volitional fatigue. Ambulatory monitors on the shoe and chest recorded foot-ground contact time (t(c)) and steady-state HR, respectively, at a series of submaximal running speeds. In the experimental group, aerobic fitness index (1/t(c). HR) was nearly constant across running speed and correlated with VO(2 max) (r = 0.90). The regression equation derived from data from the experimental group predicted VO(2 max) from the 1/t(c). HR values in the validation group within 8.3% and 4.7 ml O(2) x kg(-1) x min(-1) (r = 0.84) of measured values. We conclude that simultaneous measurements of foot-ground constant times and heart rates during level running at a freely chosen constant speed can provide accurate estimates of maximal aerobic power.

Peak oxygen deficit during one- and two-legged cycling in men and women.

The objectives of this study were to determine the relationships of estimated active muscle mass and gender to anaerobic capacity, as measured by the peak oxygen deficit, and to compare these relationships with those for peak oxygen uptake (VO2peak). Fat-free leg volumes (FFLV), and one- and two-legged cycling peak oxygen deficit and VO2peak were determined in young, physically active men (N = 11) and women (N = 9). For men and women, mean (+/- SD) peak oxygen deficit for one-legged cycling (2.27 +/- 0.30 and 1.18 +/- 0.18 l) was 52% of that for two-legged cycling (4.40 +/- 0.62 and 2.25 +/- 0.28 l). For all subjects and both modes of exercise, there was a strong linear relation between peak oxygen deficit (1) and estimated active muscle mass (FFLV) (r = 0.94). This relation was the same in one- and two-legged cycling, but was different for men and women. For a given FFLV, the peak oxygen deficit was significantly higher (P < 0.05) in men than women by an average of 0.44 l. The relation of peak oxygen deficit to FFLV was significantly stronger than the relation of VO2peak to FFLV (r = 0.80). We conclude: (a) that the peak oxygen deficit is strongly related to the estimated active muscle mass during cycling; (b) that for a given estimated active muscle mass (FFLV), the peak oxygen deficit is higher in men than women; and (c) that the peak oxygen deficit is more strongly related than VO2peak to the estimated quantity of active muscle.

Peak oxygen deficit predicts sprint and middle-distance track performance.

The purpose of this study was to determine the value of the peak oxygen deficit (POD) as a predictor of sprint and middle-distance track performance. POD, peak blood lactate, VO2peak, lactate threshold, and running economy at 3.6 m.s-1 were measured during horizontal treadmill running in 22 male and 19 female competitive runners of different event specialties. Subjects also completed running performance trials at 100, 200, 400, 800, 1500, and 5000 m. Correlations of track performances with POD (ml.kg-1) (-0.66, -0.71, -0.71, -0.62, -0.52, and -0.40) were moderately strong at the sprint and middle distances, accounting for 44-50% of the performance variance at the three shortest distances. Correlations of track performances with peak blood lactate concentration were lower than with POD and accounted for approximately one-half as much of the performance variance (21-26%) at the three shortest distances. Multiple regression analyses indicated that the POD was the strongest metabolic predictor of 100-, 200- and 400-m performance, and that VO2peak was the strongest metabolic predictor of 800-, 1500-, and 5000-m performance. We conclude that the POD is a moderately strong predictor of sprint and middle-distance track performance.

Validation of the 12-min swim as a field test of peak aerobic power in young men.

The purposes of this study were to validate the 12-min swim as a field test of VO2max and to compare its validity with that of the 12-min run. Thirty-six young men completed 12-min swim, 12-min run, tethered swimming (TS) VO2peak, and treadmill running (TR) VO2peak tests within 3 wk. Mean (+/- SD) 12-min swim and run distances were 581 +/- 88 and 2797 +/- 290 m, and mean TS and TR VO2peak values were 50.3 +/- 6.2 and 57.2 +/- 5.5 ml.kg BW-1.min-1, respectively. Correlation coefficients and standard errors of estimate for predictions of TS VO2peak from the 12-min swim (0.40 and 5.7 ml.kg BW-1.min-1) and run (0.74 and 4.2 ml.kg BW-1.min-1) and for predictions of TR VO2peak from the 12-min swim (0.38 and 5.1 ml.kg BW-1.min-1) and run (0.88 and 2.6 ml.kg BW-1.min-1) indicated that the 12-min run was a more accurate predictor of TS or TR VO2peak than the 12-min swim. We conclude that the 12-min swim has relatively low validity as a field test of peak aerobic power and that it should not be considered an equally valid alternative to the 12-min run in young male recreational swimmers. However, the accuracy of predicting VO2peak from the 12-min swim is as good as some other commonly used methods, and, therefore, it may be adequate for fitness classification in situations in which a high level of accuracy is not needed.

Metabolic determinants of 1-mile run/walk performance in children.

The 1-mile run/walk test is the field test of choice for evaluating maximal aerobic power (VO2max) in school-aged children. The objective of this study was to determine the relative importance of selected metabolic determinants of mile run/walk performance in children 6-14 yr of age. Mile run/walk time (MRWT), VO2max, running economy (VO2 in ml.kg-1.min-1 at 8.05 km.h-1; VO2econ), and the percentage of VO2max utilized at the average mile run/walk speed (%VO2max) were measured in 59 children (33 boys and 26 girls); 27 6-8 yr olds (group 1), 17 9-11 yr olds (group 2), and 15 12-14 yr olds (group 3). Partial correlations between MRWT and VO2max, VO2econ, and %VO2max, holding constant the effects of age and sex, were as follows: group 1: -0.26, 0.03, and -0.82; group 2; -0.43, 0.09, and -0.88; and group 3, -0.60, 0.45, and -0.80. Multiple regression analysis indicated that the combination of the three metabolic measures accounted for 90%, 97%, and 90% of the variance in MRWT in the three age groups, respectively. Standardized regression coefficients for VO2max, VO2econ, and %VO2max in group 1 (-0.66, 0.19, and -0.83), group 2 (-0.45, 0.33, and -0.92), and group 3 (-0.76, 0.27, and -0.50) indicated that the %VO2max utilized at the average mile run/walk speed was the most important determinant of MRWT variance in children 6-11 yr old, whereas VO2max was the most important determinant for children 12-14 yr old. We conclude that the relative importance of the metabolic determinants of the 1-mile run/walk test, as typically administered in the schools, changes with age.

Effect of varying levels of hypohydration on responses during submaximal cycling.

The effect of varying levels of hypohydration on hemodynamic, cardiorespiratory, and metabolic responses to progressive incremental submaximal cycling was examined in nine male subjects. Subjects cycled in a neutral (22 degrees C) environment under euhydration (EU), moderate hypohydration (MH), and severe hypohydration (SH). To achieve the desired level of hypohydration, subjects cycled at 50% VO2max for 1.5 h in a 38 degrees C environment on two separate occasions, 36 h prior to testing. Mean (+/- SE) percent losses in body weight from baseline during EU, MH, and SH were 0.6 +/- 0.3%, 3.3 +/- 0.1%, and 5.6 +/- 0.4%, respectively. Ventilation, O2 uptake, respiratory exchange ratio, heart rate, plasma free fatty acids, plasma glycerol, blood lactate, and hematocrit were not significantly altered by hypohydration. During EU, hemoglobin concentration was significantly lower than during both MH and SH, but no significant difference was observed for plasma volume loss. Plasma glucose was significantly higher during SH compared with EU and MH. These results suggest that hypohydration of up to 5.6% caused by exercise and fluid manipulation over 36 h does not alter cardiorespiratory or blood lactate responses during progressive incremental submaximal cycling in a neutral environment. However, hepatic metabolism may be altered during hypohydration as indicated by higher plasma glucose levels.

The application of ground force explains the energetic cost of running backward and forward.

We compared backward with forward running to test the idea that the application of ground force to support the weight of the body determines the energetic cost of running. We hypothesized that higher metabolic rates during backward versus forward running would be directly related to greater rates of ground force application and the volume of muscle activated to apply support forces to the ground. Four trained males ran backward and forward under steady-state conditions at eight treadmill speeds from 1.75 to 3.50 m x s(-1). Rates of oxygen uptake were measured to determine metabolic rates, and inverse periods of foot-ground contact (1/tc) were measured to estimate rates of ground force application. As expected, at all eight speeds, both metabolic rates and estimated rates of ground force application were greater for backward than for forward running. At the five slowest speeds, the differences in rates of ground force application were directly proportional to the differences in metabolic rates between modes (paired t-test, P<0.05), but at the three highest speeds, small but significant differences in proportionality were present in this relationship. At one of these three higher speeds (3.0 m x s(-1)), additional measurements to estimate muscle volumes were made using a non-invasive force plate/video technique. These measurements indicated that the volume of muscle active per unit of force applied to the ground was 10+/-3% greater when running backward than forward at this speed. The product of rates of ground force application and estimated muscle volumes predicted a difference in metabolic rate that was indistinguishable from the difference we measured (34+/-6% versus 35+/-6%; means +/- s.e.m., N=4). We conclude that metabolic rates during running are determined by rates of ground force application and the volume of muscle activated to apply support forces to the ground.

Effects of varying levels of hypohydration on ratings of perceived exertion.

To investigate the effects of varying levels of hypohydration on ratings of perceived exertion (RPE) during moderate and heavy submaximal exercise, and at the lactate threshold (LT) and ventilatory threshold (VT), 9 male subjects cycled under states of euhydration (EU), moderate hypohydration (MH), and severe hypohydration (SH). The desired level of hypohydration was achieved over a 36-hr period by having subjects cycle at 50% VO2max in a 38 degrees C environment on two occasions while controlling fluid intake and diet. During submaximal exercise, oxygen uptake, ventilation, heart rate, blood lactate, and RPE were not significantly different among treatments. Hypohydration did not significantly alter LT or VT, or perceptual responses at LT or VT. It is concluded that hypohydration of up to 5.6% caused by fluid manipulation and exercise in the heat over a 36-hr period does not alter RPE or the lactate or ventilatory threshold, nor RPE at the lactate and ventilatory thresholds measured during moderate and heavy submaximal cycling in a neutral (22 degrees C) environment.

Energetics of bipedal running. I. Metabolic cost of generating force.

Similarly sized bipeds and quadrupeds use nearly the same amount of metabolic energy to run, despite dramatic differences in morphology and running mechanics. It has been shown that the rate of metabolic energy use in quadrupedal runners and bipedal hoppers can be predicted from just body weight and the time available to generate force as indicated by the duration of foot-ground contact. We tested whether this link between running mechanics and energetics also applies to running bipeds. We measured rates of energy consumption and times of foot contact for humans (mean body mass 78.88 kg) and five species of birds (mean body mass range 0.13-40.1 kg). We find that most (70-90%) of the increase in metabolic rate with speed in running bipeds can be explained by changes in the time available to generate force. The rate of force generation also explains differences in metabolic rate over the size range of birds measured. However, for a given rate of force generation, birds use on average 1.7 times more metabolic energy than quadrupeds. The rate of energy consumption for a given rate of force generation for humans is intermediate between that of birds and quadrupeds. These results support the idea that the cost of muscular force production determines the energy cost of running and suggest that bipedal runners use more energy for a given rate of force production because they require a greater volume of muscle to support their body weight.

Validation of the 12-minute swim as a field test of peak aerobic power in young women.

The purposes of this study were to validate the 12-min swim as a field test of VO2 peak in female recreational swimmers and to compare its validity with that of the 12-min run. The results are contrasted with those previously reported on a comparable group of male recreational swimmers. Thirty-four young women completed 12-min swim, 12-min run, tethered swimming VO2 peak, and treadmill running VO2 peak tests within 3 weeks. Mean (+/- SD) 12-min swim and run distances were 597 +/- 82 and 2,313 +/- 317 m, and mean tethered swim and treadmill run VO2 peak values were 39.2 +/- 4.9 and 45.4 +/- 6.3 ml.kg BW-1.min-1, respectively. Correlation coefficients and standard errors of estimate for predictions of swimming VO2 peak from the 12-min swim (.42 and 4.5 ml.kg BW-1.min-1) and run (.58 and 4.1 ml.kg BW-1.min-1) and for predictions of treadmill run VO2 peak from the 12-min swim (.34 and 6.0 ml.kg BW-1.min-1) and run (.87 and 3.2 ml.kg BW-1.min-1) indicated that the 12-min run was a more accurate predictor of tethered swim or treadmill run VO2 peak than the 12-min swim. These data are in close agreement with our previous study on young male recreational swimmers. We conclude that the 12-min swim has relatively low validity as a field test of peak aerobic power and that it is not an equally valid alternative to the 12-min run in young adult female recreational swimmers.(ABSTRACT TRUNCATED AT 250 WORDS)