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Biomedical subjects

S L Lindstedt

Publications and source records attributed to S L Lindstedt.

At least 19 recordsLinked to original sources
PubMed2001

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.

Adaptation, Physiological↗
PubMed2001

Human aerobic performance: too much ado about limits to V(O(2)).

Human endurance performance is often evaluated on the basis of the maximal rate of oxygen uptake during exercise (V(O(2)max)). Methods for overcoming limits to V(O(2)max) are touted as means for increasing athletic endurance performance. Here, we argue that the respiratory system is well designed for delivering O(2) to meet O(2) demands and that no single factor is rate-determining for O(2) uptake. We show that V(O(2)max) can vary 5000-fold among mammals, while any limitation to O(2) delivery by a single component of the respiratory system affects V(O(2)max) by 10% or less. Attempts to increase O(2) delivery by enhancing one step in the respiratory system are shown to have little effect. Blood doping, hyperoxia and O(2) supplementation of high-altitude natives all raise O(2) availability substantially to the working muscles, but these treatments increase V(O(2)max) only minimally. Finally, we argue that O(2) uptake is only one of a number of properties important to human aerobic performance.

Aerobiosis↗
PubMed2000

Shaking up glycolysis: Sustained, high lactate flux during aerobic rattling.

Substantial ATP supply by glycolysis is thought to reflect cellular anoxia in vertebrate muscle. An alternative hypothesis is that the lactate generated during contraction reflects sustained glycolytic ATP supply under well-oxygenated conditions. We distinguished these hypotheses by comparing intracellular glycolysis during anoxia to lactate efflux from muscle during sustained, aerobic contractions. We examined the tailshaker muscle of the rattlesnake because of its uniform cell properties, exclusive blood circulation, and ability to sustain rattling for prolonged periods. Here we show that glycolysis is independent of the O(2) level and supplies one-third of the high ATP demands of sustained tailshaking. Fatigue is avoided by rapid H(+) and lactate efflux resulting from blood flow rates that are among the highest reported for vertebrate muscle. These results reject the hypothesis that glycolysis necessarily reflects cellular anoxia. Instead, they demonstrate that glycolysis can provide a high and sustainable supply of ATP along with oxidative phosphorylation without muscle fatigue.

Adenosine Triphosphate↗
PubMed2000

Eccentric ergometry: increases in locomotor muscle size and strength at low training intensities.

Lengthening (eccentric) muscle contractions are characterized by several unusual properties that may result in unique skeletal muscle adaptations. In particular, high forces are produced with very little energy demand. Eccentrically trained muscles gain strength, but the specific nature of fiber size and composition is poorly known. This study assesses the structural and functional changes that occur to normal locomotor muscle after chronic eccentric ergometry at training intensities, measured as oxygen uptake, that do not influence the muscle when exercised concentrically. Male subjects trained on either eccentric or concentric cycle ergometers for 8 wk at a training intensity starting at 54% and ending at 65% of their peak heart rates. The isometric leg strength increased significantly in the eccentrically trained group by 36%, as did the cross-sectional area of the muscle fiber by 52%, but the muscle ultrastructure remained unchanged. There were no changes in either fiber size, composition, or isometric strength in the concentrically trained group. The responses of muscle to eccentric training appear to be similar to resistance training.

Adult↗
PubMed2000

Is the spring quality of muscle plastic?

During locomotion, major muscle groups are often activated cyclically. This alternate stretch-shorten pattern of activity could enable muscle to function as a spring, storing and recovering elastic recoil potential energy. Because the ability to store and recover elastic recoil energy could profoundly affect the energetics of locomotion, one might expect this to be an adaptable feature of skeletal muscle. This study tests the hypothesis that chronic eccentric (Ecc) training results in a change in the spring properties of skeletal muscle. Nine female Sprague-Dawley rats underwent chronic Ecc training for 8 wk on a motorized treadmill. The spring properties of muscle were characterized by both active and passive lengthening force productions. A single "spring constant (Deltaforce/Deltalength) from the passive length-tension curves was calculated for each muscle. Results from measurements on long heads of triceps brachii muscle indicate that the trained group produced significantly more passive lengthening force (P = 0.0001) as well as more active lengthening force (P = 0.0001) at all lengths of muscle stretch. In addition, the spring constants were significantly different between the Ecc (1.71 N/mm) and the control (1.31 N/mm) groups. A stiffer spring is capable of storing more energy per unit length stretched, which is of functional importance during locomotion.

Animals↗
PubMed1999

Chronic eccentric exercise: improvements in muscle strength can occur with little demand for oxygen.

Eccentric contractions, the lengthening of muscle while producing force, are a common part of our everyday movements. This study presents a challenge to the accepted notion that eccentric work causes obligatory muscle injury while demonstrating that an increase in muscle strength, via eccentric work, can occur with little demand for oxygen. Nine healthy subjects, ages 18-34, were randomly placed in either an eccentric or a concentric training group. Both groups trained for 6 wk while progressively increasing training frequency and duration. Significant gains in isometric leg strength were seen in the eccentrically trained subjects only. While training, the oxygen consumption required to do the eccentric work was equal to or less than that required to do the concentric work. The results demonstrate that by progressively increasing the eccentric work rate, significant isometric strength gains can be made without muscle injury and with minimal increase in metabolic demand for oxygen. The potential clinical implications of an eccentric training program that uncouples skeletal muscle strength improvements from the demand for oxygen are alluring.

Adolescent↗
PubMed1998

Exercise training in chronic hypoxia has no effect on ventilatory muscle function in humans.

At the highest altitude, aerobic work is limited by environmental oxygen availability. We therefore reasoned that the hyperpnea associated with endurance training at altitude should provide a strong stimulus for adaptation of the ventilatory muscles. We measured peak inspiratory muscle pressure-flow characteristics (inspiring through graded resistors) and maximum sustainable ventilation capacity in ten permanent residents of La Paz, Bolivia (3600 m) prior to and immediately following 6 weeks of incremental endurance training. Additionally, eight local residents did no training and functioned as controls for the capacity test. While V(O2)max measured in hypoxia increased by 19% (Favier et al., 1995b. J. Appl Physiol. 78, 2286-2293.), none of the tested ventilatory variables showed significant changes. The values for the group mean slopes of maximum inspiratory pressure-flow pairs (- 10.5 vs. - 9.8 cm H2O x sec x L(-1), P=0.301; before versus after training, respectively), maximum inspiratory pressure (112.1+/-8.9 vs. 106.9+/-8.6 cmH2O, P=0.163), peak inspiratory flow (9.8+/-0.41 vs. 10.2+/-0.55 L x sec(-1) P=0.172) and the maximum volitional volume in 12 sec (43.9+/-2.4 vs. 45.6+/-2.4 L in 12 sec, P=0.133) were unchanged with exercise training. Likewise, maximal sustainable minute volume was not different between post-training and control subjects (177.4+/-7.9 vs. 165.4+/-8.4 L x min(-1), P=0.141). These data support the concept that endurance training fails to elicit functional adaptations in ventilatory muscles in humans, even when exercise is done in hypoxia.

Acclimatization↗
PubMed1998

Task-specific design of skeletal muscle: balancing muscle structural composition.

Skeletal muscle fibers are composed of three structural elements, each contributing a unique aspect of muscle function, yet each 'competing' in a sense for space inside the cell. The volume occupied by myofibrils determines the force of contraction, the volume of sarcoplasmic reticulum sets the rate of onset and relaxation of a fiber's contraction and hence contraction frequency, and the volume of mitochondria sets the level of sustained performance. The entirety of functional outcomes in muscle, from sustained isometric to high frequency contractions, and from high power output to high endurance, are all primarily attributable to shifts in the proportions (and relationships) of those three structures. This paper examines and reviews these components of muscle first to identify and summarize structure-function 'rules', and second to examine the balance between sometimes competing demands. In particular, we focus on those muscles in which power, endurance and frequency are all simultaneously high (flight muscles), and examine how muscle has 'solved' problems of space and energy demand. From these results and observations it would appear that for flight to have evolved in small animals, the double packing of inner mitochondrial membranes may be expected in animals under 50-80 g in mass, and asynchronous muscle is structurally essential for flight in small insects with wing beat frequencies above about 100 Hz.

Animals↗
PubMed1996

Minimal cost per twitch in rattlesnake tail muscle.

Sound production is one of the most energetically costly activities in animals. Minimizing contraction costs is one means of achieving the activation rates necessary for sound production (20-550 Hz) (refs 1-3) without exceeding energy supplies. Rattlesnakes produce a sustained, high-frequency warning sound by extremely rapid contraction of their tailshaker muscles (20-90 Hz) (refs 4,5). The ATP cost per twitch is only 0.015 micromol ATP per g muscle per twitch during rattling, as measured by in vivo magnetic resonance. The reduced volume density of myofibre (32%) in tailshaker muscle is consistent with contraction cost being minimized (crossbridge cycling), in contrast to the contractile costs of vertebrate locomotory and asynchronous insect flight muscle. Thus tailshaker muscle is an example of sound-producing muscle designed for 'high frequency, minimal cost'. The high rates of rattling are achieved by minimizing contractile use of ATP, which reduces the cost per twitch to among the lowest found for striated muscle.

Adenosine Triphosphate↗
PubMed1996

The whistle and the rattle: the design of sound producing muscles.

Vertebrate sound producing muscles often operate at frequencies exceeding 100 Hz, making them the fastest vertebrate muscles. Like other vertebrate muscle, these sonic muscles are "synchronous," necessitating that calcium be released and resequestered by the sarcoplasmic reticulum during each contraction cycle. Thus to operate at such high frequencies, vertebrate sonic muscles require extreme adaptations. We have found that to generate the "boatwhistle" mating call (approximately 200 Hz), the swimbladder muscle fibers of toadfish have evolved (i) a large and very fast calcium transient, (ii) a fast crossbridge detachment rate, and (iii) probably a fast kinetic off-rate of Ca2+ from troponin. The fibers of the shaker muscle of rattlesnakes have independently evolved similar traits, permitting tail rattling at approximately 90 Hz.

Air Sacs↗
PubMed1995

Fatigue and the design of the respiratory system.

One source of muscle fatigue may be the failure to provide the required oxygen by any step in the oxygen transport cascade or a lack of the necessary machinery to utilize that oxygen. We favor abandoning the concept of a single rate-limiting step for the concept of tuned resistors, each contributing to the overall resistance to oxygen flow. However, because some of these steps have considerably less phenotypic plasticity than others, these are the component parts of the respiratory system that must be built with adequate "reserve" to accommodate adaptive increases in the other steps (Lindstedt et al., 1988; Weibel et al., 1992; Lindstedt et al., 1994). These structures will usually appear to be over built except in those rare individual animals at the species-specific limit of VO2 in which these less malleable structures may be limiting.

Animals↗
PubMed1994

Does peak inspiratory flow contribute to setting VO2max? A test of symmorphosis.

Symmorphosis predicts that animal design is optimized in such a way that structure 'statisfies but does not exceed' functional requirements. To provide one test of this hypothesis, we examined peak inspiratory flow and its relation to maximum oxygen uptake in humans. We measured maximal forced (peak) inspiratory flow (VImax) and maximum oxygen uptake (VO2max) via cycle ergometry in well trained (VO2max > 65 ml O2.kg-1.min-1) and untrained (VO2max < 45 ml O2.kg-1.min-1) male subjects. Tests of VImax and peak oxygen uptake (VO2peak) were made while the subjects were breathing through inspiratory orifices differing in area. VImax varied as an identical function of orifice diameter in both groups of subjects. However, VO2peak was more sensitive to decreasing orifice diameter in trained endurance athletes than it was in untrained individuals. The diameter of the largest orifice that caused a reduction in oxygen uptake was over two times larger for trained than for untrained subjects, corresponding to about a four-fold difference in resistance at any flow rate. These results suggest that the structures setting VImax (airway resistance and inspiratory muscle strength) are not matched to oxygen demand (VO2max) in humans. While these structures seem to be 'over-built' and hence do not likely contribute to setting the limits to aerobic performance in most humans, they may be among the primary limiting factors in the most elite endurance athletes.

Adaptation, Physiological↗
PubMed1994

Capillary blood transit time in muscles in relation to body size and aerobic capacity.

The mean minimal transit time for blood in muscle capillaries (tc) was estimated in six species, spanning two orders of magnitude in body mass and aerobic capacity: horse, steer, dog, goat, fox and agouti. Arterial (CaO2) and mixed venous (CvO2) blood O2 concentrations, blood hemoglobin concentrations ([Hb]) and oxygen uptake rates were measured while the animals ran on a treadmill at a speed that elicited the maximal oxygen consumption rate (VO2max) from each animal. Blood flow to the muscles (Qm) was assumed to be 85% of cardiac output, which was calculated using the Fick relationship. Total muscle capillary blood volume (Vc) and total muscle mitochondrial volume were estimated by morphometry, using a whole-body muscle sampling scheme. The tc was computed as Vc/Qm. The tc was 0.3-0.5 s in the 4 kg foxes and agoutis, 0.7-0.8 s in the 25 kg dogs and goats, and 0.8-1.0 s in the 400 kg horses and steers. The tc was positively correlated with body mass and negatively correlated with transcapillary O2 release rate per unit capillary length. Mitochondrial content was positively correlated with VO2max and with the product of Qm and [Hb]. These data suggested that Qm, Vc, maximal hemoglobin flux, and consequently tc, are co-adjusted to result in muscle O2 supply conditions that are matched to the O2 demands of the muscles at VO2max.

Aerobiosis↗
PubMed1994

Exercise performance of mammals: an allometric perspective.

We have examined aerobic exercise performance among the mammals with particular attention to the constraints that body size places on all aspects of muscle biomechanics, aerobic energetics, tissue oxygen diffusion, cardiovascular oxygen delivery, and pulmonary oxygen uptake. Several body-size-dependent patterns emerge that seemingly govern aerobic performance in mammals, with the caveat that at any given body size there is a range of aerobic capacities, the result of natural selection operating on the size-dependent "default values" of structure and function. Among these default values, the following apparent functional clusters surface: 1. In general, concentrations and pressures (e.g., of proteins and gases) are roughly independent of body size. Inspiratory and expiratory ventilation pressures, blood pressure and the partial pressures of O2 and CO2 in lungs, blood, and tissues do not vary with body size. Likewise, concentrations of hemoglobin, myoglobin, and hematocrit are independent of body size. 2. Most volumes and capacities scale linearly with body size (i.e., as a constant function of body mass). In addition to heart, lung, and total blood volumes, important examples relevant to exercise performance are the diffusing capacities for oxygen in the lung and, apparently, in the tissues. 3. Finally, most time-dependent variables related to oxygen delivery scale allometrically with body mass; they are of shorter duration in small animals than in large ones. Biological rates, for example, Vmax of working muscle, heart and respiratory rates, and transit times of blood through the muscles and lungs, all vary roughly as the -1/5 to -1/4 power of body mass.

Animals↗
PubMed1993

Limits to maximal performance.

Body size fundamentally affects maximal locomotor performance in mammals. Comparisons of performances of different-sized animals yield different results if made using relative, rather than absolute scales. Absolute speed may be a reasonable way to evaluate the locomotor performance of an animal that must escape predators in real time. However, comparisons of metabolic power in animals of different size can only be made meaningfully on a mass-specific basis. Numerous factors associated with the mechanics, energetics, and storage of elastic energy during locomotion change with body size, which results in allometric relationships that make the energetic cost of locomotion (alpha Mb-0.3) more expensive for small mammals than for large mammals. Small mammals have lower enzymatic capacities for anaerobic glycolysis (alpha Mb0.15) and higher specific aerobic capacities (alpha Mb-0.13) than large mammals. However, the energetic cost of transport increases more than aerobic power as mammals get smaller. The higher ratio of cost to available power in small mammals may explain why they run more slowly than large mammals, as a rule. Maximum aerobic capacity is allometrically related to body size. Limits to VO2max can be imposed by mitochondrial oxidative capacity, as in goats, or by the O2 transport system, as in humans and horses. No single step in the O2 transport system can limit the flux of O2 by itself; however, in an average non-athletic species of mammal, any of the steps in the system might appear to be the weakest link. In highly aerobic athletic species, and possibly elite athletic individuals of other species (e.g. humans), the malleable elements of the O2 transport system may develop to the point that their O2 transport capacities approach that of the least malleable element in the system, the lung. VO2max is very high in such individuals, and appears to be limited by simultaneous failure of all components of the O2 transport system.

Animals↗
PubMed1992

Estimating transit time for capillary blood in selected muscles of exercising animals.

The mean minimal capillary transit time was estimated in muscles of various animals using a combination of physiological and morphometric methods. Radioactive microspheres were injected intravascularly in various animals running on a treadmill at maximum oxygen consumption rate (VO2,max) to label blood flow to individual muscles. The muscles were then removed and preserved by standard methods for electron microscopy. The volume density of mitochondria was measured to assess muscle oxidative capacity. Capillary densities in muscle cross-sections, capillary diameters and tortuosities were incorporated into an estimate of capillary volume per unit muscle mass. Mean capillary transit time (tc) in the exercising muscles was estimated by dividing mass-specific capillary volume by mass-specific blood flow. Estimates of tc ranged from values near 1 s in horse heart and thigh muscles to 0.2 s in duck gastrocnemius. The relationship between muscle blood flow and tc was hyperbolic. The experimental data indicate a limiting value of 0.2 s for transit times at very high blood flows. There was no correlation between tc and body-mass-specific VO2,max.

Animals↗
PubMed1991

Running energetics in the pronghorn antelope.

The pronghorn antelope (Antilocapra americana) has an alleged top speed of 100 km h-1, second only to the cheetah (Acionyx jubatus) among land vertebrates, a possible response to predation in the exposed habitat of the North American prairie. Unlike cheetahs, however, pronghorn antelope are distance runners rather than sprinters, and can run 11 km in 10 min, an average speed of 65 km h-1. We measured maximum oxygen uptake in pronghorn antelope to distinguish between two potential explanations for this ability: either they have evolved a uniquely high muscular efficiency (low cost of transport) or they can supply oxygen to the muscles at unusually high levels. Because the cost of transport (energy per unit distance covered per unit body mass) varies as a predictable function of body mass among terrestrial vertebrates, we can calculate the predicted cost to maintain speeds of 65 and 100 km h-1 in an average 32-kg animal. The resulting range of predicted values, 3.2-5.1 ml O2 kg-1 s-1, far surpasses the predicted maximum aerobic capacity of a 32-kg mammal (1.5 ml O2 kg-1 s-1). We conclude that their performance is achieved by an extraordinary capacity to consume and process enough oxygen to support a predicted running speed greater than 20 ms-1 (70 km h-1), attained without unique respiratory-system structures.

Animals↗