Low cost of locomotion in lizards that are active at low temperatures

The nocturnality hypothesis of K. Autumn and coworkers states that nocturnal geckos have evolved a low energetic cost of locomotion (C(min)). A low C(min) increases maximum aerobic speed and partially offsets the decrease in maximum oxygen consumption caused by activity at low nocturnal temperatures. We tested whether a low C(min) is unique to nocturnal geckos or represents a more general pattern of convergent evolution among lizards that enables nocturnality and/or cold-temperature activity. We measured C(min) in four carefully selected lizard species from New Zealand (two nocturnal and two diurnal; n=5-9 individuals per species), including a nocturnal and diurnal gecko (a low C(min) is a gecko trait and is not related to nocturnality), a nocturnal skink (a low C(min) is related to being nocturnal), and a diurnal skink active at low temperatures (a low C(min) is related to being active at low body temperatures). The C(min) values of the four species measured in this study (range=0.21-2.00 mL O(2) g(-1) km(-1)) are lower than those of diurnal lizards from elsewhere, and the values are within or below the 95% confidence limits previously published for nocturnal geckos. A low C(min) increases the range of locomotor speeds possible at low temperatures and provides an advantage for lizards active at these temperatures. We accepted the hypothesis that nocturnal lizards in general have a low C(min) and provide evidence for a low C(min) in lizards from cool-temperate environments. The low C(min) in lizards living at high latitudes may enable extension of their latitudinal range into otherwise thermally suboptimal habitats.     

Secondarily diurnal geckos return to cost of locomotion typical of diurnal lizards

Previous studies showed that nocturnal geckos evolved a low energetic cost of locomotion (Cmin), which increases maximum aerobic speed and partially offsets the decrease in maximal oxygen consumption caused by activity at low nocturnal temperatures. Because the advantage of a low Cmin should apply at high diurnal temperatures as well as at low nocturnal temperatures, I hypothesized that Cmin remained low in geckos that have secondarily evolved diurnality. I measured Cmin in two secondarily diurnal gecko species, Rhoptropus bradfieldi (4.7 g+/-0.71 SE) and Phelsuma madagascariensis (23.9 g+/-3.7 SE), during steady exercise on a treadmill and rejected the hypothesis that secondarily diurnal geckos retain the low Cmin of their nocturnal ancestors. The Cmin in R. bradfieldi (2.468 mL O2 g-1 km-1+/-0.489 SE) and P. madagascariensis (1.389 mL O2 g-1 km-1+/-0.119 SE) returned to values typical of ancestrally diurnal lizards. This suggests that there is a trade-off that outweighs the performance advantage of low Cmin in a diurnal environment and that may cause an evolutionary association between Cmin and activity time (diurnality/nocturnality).     

The effect of body temperature on the locomotory energetics of lizards

Oxygen consumption (VO2), carbon dioxide production (VCO2), and stamina were measured in the lizard Tupinambis nigropunctatus running at sustainable and non-sustainable velocities (v) on a motor-driven treadmill. Three experimental groups were measured: field-fresh animals at body temperature (Tb) = 35 degrees C and laboratory-maintained animals at Tb = 35 and 25 degrees C. Mean preferred Tb was determined to be 35.2 degrees C. At 35 degrees C, field-fresh animals had a greater maximal oxygen consumption (VO2max corr) (4.22 vs 3.60 ml O2 g-0.76h-1) and a greater endurance. The net cost of transport (slope of VO2 on v) did not differ between the groups (= 2.60 ml O2 g-0.76)km-1). Velocity at which VO2max is attained (MAS) is 0.84 km h-1. The respiratory exchange ratio (R) exceeded 1.0 at v above MAS, indicating supplementary anaerobic metabolism. At 25 degrees C, VO2max corr was lower (2.34 ml O2 g-0.76h-1) as was endurance, MAS occurring at 0.5 km h-1. Net cost of transport was not significantly different than at 35 degrees C. The effect of Tb on locomotory costs was analyzed for this lizard and other species. It was concluded that the net cost of transport is temperature independent in all species examined and the total cost of locomotion (VO2 v-1) is temperature dependent in Tupinambis (Q10 = 1.4-2.0) and all other species examined except one. The energetic cost of locomotion [(VO2active-VO2rest)v-1], previously reported to be temperature independent in lizards, is temperature dependent in Tupinambis (Q10 = 1.3-1.6) and in two other species.2r     

Energetics of locomotion in African pygmies

The energy cost of walking (Cw) and running (Cr), and the maximal O2 consumption (VO2max) were determined in a field study on 17 Pygmies (age 24 years, SD 6; height 160 cm, SD 5; body mass 57.2 kg, SD 4.8) living in the region of Bipindi, Cameroon. The Cw varied from 112 ml.kg-1.km-1, SD 25 [velocity (v), 4 km.h-1] to 143 ml.kg-1.km-1, SD 16 (v, 7 km.h-1). Optimal walking v was 5 km.h-1. The Cr was 156 ml.kg-1.km-1, SD 14 (v, 10 km.h-1) and was constant in the 8-11 km.h-1 speed range. The VO2max was 33.7 ml.kg-1.min-1, i.e. lower than in other African populations of the same age. The Cr and Cw were lower than in taller Caucasian endurance runners. These findings, which challenge the theory of physical similarity as applied to animal locomotion, may depend either on the mechanics of locomotion which in Pygmies may be different from that observed in Caucasians, or on a greater mechanical efficiency in Pygmies than in Caucasians. The low Cr values observed enable Pygmies to reach higher running speeds than would be expected on the basis of their VO2max.     

Exercise training enhances aerobic capacity in juvenile estuarine crocodiles (Crocodylus porosus)

Aerobic capacity (VO2max) of endothermic vertebrates is known to increase with exercise training, but this effect has not been found to-date in non-avian reptiles. We exercised juvenile estuarine crocodiles (Crocodylus porosus) to walk at 0.75-0.88 km/h on a treadmill for up to 20 min a day over 16 weeks, and compared their aerobic performance with that of unexercised crocodiles. In the exercised group, VO2max increased from 6.9 to 8.5 mLO2/kg/min (+28%), and locomotor endurance increased from 3.8 to 6.9 min (+82%). Neither VO2max nor endurance changed significantly in the sedentary group. This finding extends the exercise training effect onto another vertebrate clade, and demonstrates that ectothermic amniotes are capable of elevating their aerobic capacity in response to exercise training. We propose that differences in cardiopulmonary structure and function in non-avian reptiles may be responsible for the absence (in squamates) or presence (in crocodilians) of a strong training effect on aerobic capacity.     

LOCOMOTOR PERFORMANCE AT LOW TEMPERATURE AND THE EVOLUTION OF NOCTURNALITY IN GECKOS

Nocturnal geckos are active at body temperatures 10–35°C below the thermal optima for maximum rate of aerobic metabolism of diurnal lizards. Therefore, given ancestral (diurnal) lizard physiology, nocturnality causes a substantial thermal handicap in locomotor performance. In prior studies, we hypothesized that a low minimum cost of locomotion (C_min) in geckos was an adaptation that increased locomotor endurance capacity at low, nocturnal temperatures. However, C_min is only part of an integrated system that, in conjunction with the maximum rate of oxygen consumption, sets the maximum speed that can be sustained aerobically (termed the maximum aerobic speed or MAS). We conducted the first phylogenetic analysis of MAS and lizards and found that the greatest changes in MAS, C_min and (at activity temperatures) in the evolutionary history of lizards all coincided with the evolution of nocturnality in geckos. Geckos active at 15–25°C did not become optimized for nocturnal temperatures, or fully offset the thermal effects of nocturnality by evolving maximal rates of oxygen consumption comparable to diurnal lizards active at 35°C. Geckos did evolve MAS twice that of diurnal lizards running at low temperatures by evolving a remarkably low C_min. Allometric analysis and phylogenetically independent contrasts of , C_min, and MAS indicate a 72% evolutionary decrease in , (at activity temperatures) and a 50% evolutionary decrease in C_min concordant with the evolution of nocturnality in geckos. Experimental measurements show that decreased C_min in six species of gecko increased MAS by 50–120% compared to diurnal lizards at low temperatures. Thus, geckos sufficiently overcame the near paralyzing effects of nocturnal temperatures, but only offset about 50% of the decrease in MAS resulting from the low maximum rate of oxygen consumption. Although the nocturnal environment remains severely suboptimal, the evolution of a low cost of locomotion in the ancestor of geckos was highly adaptive for nocturnality. We also present a generalized approach to ecophysiological evolution that integrates phylogeny with the causal relationships among environment, physiology, and performance capacity. With respect to a clade, two hypotheses are central to our integrative approach: (1) a change of an environmental variable (e.g., temperature) causes a performance handicap; and (2) evolution of a physiological variable (e.g., minimum cost of locomotion [C_min]) increases performance in the derived environment. To test the hypothesis that evolution of a physiological variable is adaptive in nature, we suggest determining if individuals in nature perform at levels exceeding the performance capacity of their hypothetical ancestors and if this additional performance capacity is due to the evolution of the physiological variable in question.     

Increased capacity for sustained locomotion at low temperature in parthenogenetic geckos of hybrid origin

The evolution of parthenogenesis is typically associated with hybridization and polyploidy. These correlates of parthenogenesis may have important physiological consequences that need be taken into account in understanding the relative merits of sexual and parthenogenetic reproduction. We compared the thermal sensitivity of aerobically sustained locomotion in hybrid/triploid parthenogenetic races of the gecko Heteronotia binoei and their diploid sexual progenitors. Endurance times at low temperature (10 degrees , 12.5 degrees , and 15 degrees C, 0.05 km h(-1)) were significantly greater in parthenogenetic females than in sexual females. Comparison of oxygen consumption rates during sustained locomotion at increasing speeds (0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 km h(-1), 25 degrees C) indicated that parthenogenetic lizards have higher maximum oxygen consumption rates and maximum aerobic speeds than do female sexual geckos. In addition, parthenogenetic geckos showed greater levels of voluntary activity at 15 degrees C than did sexual geckos, although this pattern appears strongest in comparison to male sexual forms. Parthenogenetic lineages of Heteronotia thus have an advantage over sexual lineages in being capable of greater aerobic activity. This result is opposite of that found in prior studies of parthenogenetic teiid lizards (genus Cnemidophorus) and highlights the idiosyncratic nature of phenotypic evolution in parthenogens of hybrid origin.     

Additive effects of training and high-fat diet on energy metabolism during exercise

This study was conducted to obtain additional information about the adaptations after 12 wk of high-fat diet (HFD) per se or HFD combined with endurance training in the rat using a two [diet: carbohydrate (CHO) or HFD] by two (training: sedentary or trained) by two (condition at death: rested or exercised) factorial design. Adaptation to prolonged HFD increases maximal O2 uptake (VO2max; 13%, P less than 0.05) and submaximal running endurance (+64%, P less than 0.05). This enhancement in exercise capacity could be attributed to 1) an increase in skeletal muscle aerobic enzyme activities (3-hydroxyacyl-CoA dehydrogenase and citrate synthase in soleus and red quadriceps) or 2) a decrease in liver glycogen breakdown in response to 1 h exercise at 80% VO2max. When training is superimposed to HFD, the most prominent finding provided by this study is that the diet-induced effects are cumulative with the well-known training effect on VO2max, exercise endurance, oxidative capacity of red muscle, and metabolic responses to exercise, with a further reduction in liver glycogen breakdown.     

The energy cost of walking or running on sand

Oxygen uptake (VO2) at steady state, heart rate and perceived exertion were determined on nine subjects (six men and three women) while walking (3-7 km.h-1) or running (7-14 km.h-1) on sand or on a firm surface. The women performed the walking tests only. The energy cost of locomotion per unit of distance (C) was then calculated from the ratio of VO2 to speed and expressed in J.kg-1.m-1 assuming an energy equivalent of 20.9 J.ml O2-1. At the highest speeds C was adjusted for the measured lactate contribution (which ranged from approximately 2% to approximately 11% of the total). It was found that, when walking on sand, C increased linearly with speed from 3.1 J.kg-1.m-1 at 3 km.h-1 to 5.5 J.kg-1.m-1 at 7 km.h-1, whereas on a firm surface C attained a minimum of 2.3 J.kg-1.m-1 at 4.5 km.h-1 being greater at lower or higher speeds. On average, when walking at speeds greater than 3 km.h-1, C was about 1.8 times greater on sand than on compact terrain. When running on sand C was approximately independent of the speed, amounting to 5.3 J.kg-1.m-1, i.e. about 1.2 times greater than on compact terrain. These findings could be attributed to a reduced recovery of potential and kinetic energy at each stride when walking on sand (approximately 45% to be compared to approximately 65% on a firm surface) and to a reduced recovery of elastic energy when running on sand.     

The influence of weekly training distance on fractional utilization of maximum aerobic capacity in marathon and ultramarathon runners

This study was designed to examine the interrelationships between performance in endurance running events from 10 to 90 km, training volume 3-5 weeks prior to competition, and the fractional utilization of maximal aerobic capacity (%VO2max) during each of the events. Thirty male subjects underwent horizontal treadmill testing to determine their VO2max, and steady-state VO2 at specific speeds to allow for calculation of %VO2max sustained during competition. Runners were divided into groups of ten according to their weekly training distance (group A trained less than 60 km X week-1, group B 60 to 100 km X week-1, and group C more than 100 km X week-1). Runners training more than 100 km X week-1 had significantly faster running times (average 19.2%) in all events than did those training less than 100 km X week-1. VO2max or %VO2max sustained during competition was not different between groups. The faster running speed of the more trained runners, running at the same %VO2max during competition, was due to their superior running economy (19.9%). Thus all of the group differences in running performance could be explained on the basis of their differences in running economy. These findings suggest either that the main effect of training more than 100 km X week-1 may be to increase running economy, or that runners who train more than 100 km X week-1 may have inherited superior running economy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Specific alterations of physiological parameters in competitive race walkers

Race walking is the technical and athletic expression of fast walking and it can be considered as a type of endurance performance. The purpose of this study was to examine whether 12 weeks of a specially designed training program results in the further training enhancement of endurance performance and the related physiological parameters in already well-trained race walkers competing at the national and international level. The investigation protocol consisted of determining the maximal oxygen uptake (VO2peak) and related gas exchange values using an automated cardiopulmonary exercise system and of determining blood lactate variables (aerobic threshold - LTAer and the maximal lactate steady state - MLSS) during walking with proper technique at 8, 10, 12 and 14 km·h-1 for 4 minutes without rest in between. Thereafter, the speed on the treadmill was increased by 0.5 km·h-1 every two minutes until exhaustion to determine VO2peak. After 12 weeks of a specially designed endurance training, statistically significant increases in VO2peak (61.8±8.5 mL·kg-1·min-1 pre vs. 66.9±9.5 mL·kg-1·min-1 post training; p<0.05) and blood lactate variables (VO2-LTAer and VO2-MLSS; p<0.05) were noted. The obtained results suggest that the applied training program can improve endurance and race performance in previously well trained race walkers.     

Voluntary exercise and its effects on young SHR and stroke-prone hypertensive rats

To determine whether voluntary exercise would lower resting blood pressure in spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SP-SHR), two separate but interrelated investigations were undertaken. The studies were initiated when the animals were 28-35 days of age and after they were assigned to either activity or sedentary cages. The activity cages were connected to transducers and recorders that allowed the monitoring and calculation of frequency, duration, and running speed. The SHR group ran 3-7 km/day intermittently for 12 wk at high speeds (48-68 m/min), which resulted in heart rates in excess of 500 beats/min. When the SHR exercised, they seldom exceeded 33 revolutions/bout (37 m) with the majority being less than 22 revolutions/bout. This type of exercise training significantly lowered, but did not normalize, resting blood pressure by approximately 20 mmHg [nontrained (NT) = 185 +/- 5; trained (T) = 163 +/- 5 mmHg] while increasing maximum O2 consumption (VO2max) (NT = 78 +/- 2.6; T = 95 +/- 2.2 ml X min-1 X kg-1) and endurance run time (NT = 62 +/- 9.0; T = 286 +/- 15.0 min), respectively. Although SP-SHR exhibited comparable patterns of voluntary activity, the effects were not similar. First, after approximately 5 wk of consuming a special Japanese rat chow and a 1% NaCl drinking solution, cerebrovascular lesions occurred and deaths ultimately resulted in both exercising and sedentary groups. Second, although there was statistical evidence for a training effect (higher VO2max, longer VO2 test run times), voluntary exercise had no advantage in either male or female runners in lowering resting blood pressures or in improving their life-spans. Whereas voluntary activity wheel exercise or moderate forced treadmill exercise will lower resting blood pressures in young SHR populations, similar generalizations cannot be made with young SP-SHR rats.     

Anaerobic threshold and maximal steady-state blood lactate in prepubertal boys

To elucidate further the special nature of anaerobic threshold in children, 11 boys, mean age 12.1 years (range 11.4-12.5 years), were investigated during treadmill running. Oxygen uptake, including maximal oxygen uptake (VO2max), ventilation and the "ventilatory anaerobic threshold" were determined during incremental exercise, with determination of maximal blood lactate following exercise. Within 2 weeks following this test four runs of 16-min duration were performed at a constant speed, starting with a speed corresponding to about 75% of VO2max and increasing it during the next run by 0.5 or 1.0 km.h-1 according to the blood lactate concentrations in the previous run, in order to determine maximal steady-state blood lactate concentration. Blood lactate was determined at the end of every 4-min period. "Anaerobic threshold" was calculated from the increase in concentration of blood lactate obtained at the end of the runs at constant speed. The mean maximal steady-state blood lactate concentration was 5.0 mmol.l-1 corresponding to 88% of the aerobic power, whereas the mean value of the conventional "anaerobic threshold" was only 2.6 mmol.l-1, which corresponded to 78% of the VO2max. The correlations between the parameters of "anaerobic threshold", "ventilatory anaerobic threshold" and maximal steady-state blood lactate were only poor. Our results demonstrated that, in the children tested, the point at which a steeper increase in lactate concentrations during progressive work occurred did not correspond to the true anaerobic threshold, i.e. the exercise intensity above which a continuous increase in lactate concentration occurs at a constant exercise intensity.     

Physiological strain due to load carrying in heavy footwear.

To determine the effects of wearing heavy footwear on physiological responses five male and five female subjects were measured while walking on a treadmill (4, 5.25, and 6.5 km.h-1) with different external loads (barefooted, combat boots, and waist pack). While walking without an external load the oxygen uptake, as a percentage of maximal oxygen uptake (%VO2max) of the men increased from 25% VO2max at 4 km.h-1 to 31% VO2max at 5.25 km.h-1 and to 42% VO2max at 6.5 km.h-1. The women had a significantly higher oxygen uptake of 30%, 40%, and 55% VO2max, respectively. In the most strenuous condition, walking at 6.5 km.h-1 with combat boots and waist pack (12 kg), the oxygen uptake for the men and women amounted to 53% and 75% VO2max, respectively. The heart rate showed a similar response to the oxygen uptake, the women having a heart rate which was 15-40 beats.min-1 higher than that of the men, depending on the experimental condition. The perceived exertion was shown to be greatly dependent on the oxygen uptake. From the results a regression formula was calculated predicting the oxygen uptake depending on the mass of the footwear, walking speed and body mass. It was concluded that the mass of footwear resulted in an increase in the energy expenditure which was a factor 1.9-4.7 times greater than that of a kilogram of body mass, depending on sex and walking speed.     

Cold exposure increases running VO(2max) and cost of transport in goats

We inadvertently subjected a group of goats to 5 mo of cold exposure (mean minimum temperature less than -13 degrees C) during an experiment designed to examine the effects of training by daily running on one member of each sibling pair. During the three coldest months, the sedentary but cold-exposed goats experienced a 34% increase in maximal oxygen uptake (VO(2 max), P < 0.01) and a 29% increase in running speed at maximal (P < 0.05). When temperatures increased in the spring, both oxygen uptake and running speed decreased. We interpret these findings as evidence that cold is a sufficient stimulus to invoke the development of aerobic structures in muscle and that these structures subsequently can be utilized for the novel task of running. When the experiment was subsequently repeated without the cold exposure, running speed and VO(2 max) of trained animals increased less than in either group of cold-exposed animals. However, the cost of transport of these warm runners was lower than either group of cold-exposed animals (from 13-19%, P < 0. 0001). Thus, although aerobic capacity was increased with acclimation to severe winter weather, cold-acclimated goats operated with lower efficiency during locomotion.     

Validation of two running tests as estimates of maximal aerobic power in children

In order to validate the "Maximal Multistage 20 Meter Shuttle Run Test" by Leger and Lambert (1982) (20-MST) as an estimate of maximal aerobic power (VO2max) and to compare the results of this test with the results of a 6 min endurance run, 82 subjects (41 boys and 41 girls) aged 12-14 performed the 20-MST and the 6 min endurance run, and had their VO2max directly measured during maximal treadmill running. The 20-MST is a maximal running test starting at a running speed of 8.0 km X h-1, which is increased every minute and in which the pace is set by an audio signal. Performing the test, one runs a 20-meter course back and forth. The test result is expressed as "palier" (one palier is approximately one minute). The mean results of the 20-MST were, for boys, 8.0 palier (+/- 1.7) and for girls, 6.4 palier (+/- 1.5). The mean results of the 6 min endurance run were for boys, 1264.4 meters (+/- 160.8), and for girls, 1103.9 meters (+/- 144.7). The mean VO2max for boys was 53.2 ml X kg-1 X min-1 (+/- 5.4) and for girls, 44.1 (+/- 4.8) ml X kg-1 X min-1. The correlation coefficient between VO2max and the 20-MST was found to be 0.68 (+/- 3.9) for boys, 0.69 (+/- 3.4) for girls and 0.76 (+/- 4.4) for both sexes, and that of VO2max with the 6 min endurance run was 0.51 (+/- 4.6) for boys, 0.45 (+/- 4.3) for girls and 0.63 (+/- 5.3) for both sexes. The conclusion is that the 20-MST is a suitable tool for the evaluation of maximal aerobic power. Although the differences in validity between the 20-MST and the 6 minutes endurance run were statistically not significant (p greater than 0.05), for reasons of practicability the 20-MST should be preferred to the 6 minutes endurance run when used in physical education classes.     

Integrating the Physiology, Mechanics and Behavior of Rapid Running Ghost Crabs: Slow and Steady Doesn't Always Win the Race

In 1979 Bliss predicted that, "land crabs are and will undoubtedlycontinue to be promising objects of scientific research." Studiesof rapid running ghost crabs support her contention and haveresulted in several general findings relating to locomotionand activity. 1) Energy exchange mechanisms during walking aregeneral and not restricted to quadrupedal and bipedal morphologies.2) "Equivalent gaits," such as trots and gallops, may existin 4-, 6- and 8-legged animals that differ greatly in leg andskeletal (i.e., exo- vs. endoskeletal) design. These findingssupport the hypothesis that terrestrial locomotion in many speciescan modeled by an inverted pendulum or spring-mass system. 3)An open circulatory system and chitin-covered gills do not necessarilylimit the rate at which oxygen consumption can be increasedor the factorial increase oxygen consumption over resting rates.4) Interspecific and intraspecific i.e., ontogenetic) scalingof sub-maximal oxygen consumption and maximal aerobic speedcan differ significantly. 5) Locomotion at speeds above themaximal aerobic speed requiring non-aerobic contributions maybe far more costly than can be predicted from aerobic costsalone. The cost transport may attain a minimum at less thanmaximum speed. 6) The speed which elicits maximal oxygen consumptionduring continuous exercise is attained at moderate walking speedsin crabs and probably other ectotherms. Speeds 15- to 20-foldfaster are possible, but cannot be sustained. 7) The low enduranceassociated with the low maximal oxygen consumption and maximalaerobic speed of ectotherms moving continuously can be increasedor decreased by altering locomotor behavior and moving intermittently.Ectotherms can locomote at high speeds and travel for considerabledistances or remain active for long periods by including restpauses. Alternatively, intense activity with extended exerciseperiods with short pause periods may actually reduce behavioralcapacity or work accomplished relative to continuous activityduring which the behavior is carried out at a lower intensitylevel without pauses.     

Acute effect of two aerobic exercise modes on maximum strength and strength endurance

The purpose of this study was to evaluate the effects of 2 modes of aerobic exercise (continuous or intermittent) on maximum strength (1 repetition maximum, 1RM) and strength endurance (maximum repetitions at 80% of 1RM) for lower- and upper-body exercises to test the acute hypothesis in concurrent training (CT) interference. Eight physically active men (age: 26.9 +/- 4.2 years; body mass: 82.1 +/- 7.5 kg; height: 178.9 +/- 6.0 cm) were submitted to: (a) a graded exercise test to determine V(.-)O2max (39.26 +/- 6.95 ml x kg(-1) x min(-1)) and anaerobic threshold velocity (3.5 mmol x L(-1)) (9.3 +/- 1.27 km x h(-1)); (b) strength tests in a rested state (control); and (c) 4 experimental sessions, at least 7 days apart. The experimental sessions consisted of a 5-kilometer run on a treadmill continuously (90% of the anaerobic threshold velocity) or intermittently (1:1 minute at V(.-)O2max). Ten minutes after the aerobic exercise, either a maximum strength or a strength endurance test was performed (leg press and bench press exercises). The order of aerobic and strength exercises followed a William's square distribution to avoid carryover effects. Results showed that only the intermittent aerobic exercise produced an acute interference effect on leg strength endurance, decreasing significantly (p < 0.05) the number of repetitions from 10.8 +/- 2.5 to 8.1 +/- 2.2. Maximum strength was not affected by the aerobic exercise mode. In conclusion, the acute interference hypothesis in concurrent training seems to occur when both aerobic and strength exercises produce significant peripheral fatigue in the same muscle group.