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Biomedical subjects

R Kram

Publications and source records attributed to R Kram.

At least 19 recordsLinked to original sources
PubMed1991

Carrying loads with springy poles.

People throughout Asia use springy bamboo poles to carry the loads of everyday life. These poles are a very compliant suspension system that allows the load to move along a nearly horizontal path while the person bounces up and down with each step. Could this be an economical way to carry loads inasmuch as no gravitational work has to be done to lift the load repeatedly? To find out, an experiment was conducted in which four male subjects ran at 3.0 m/s on a motorized treadmill with no load and while carrying a load equal to 19% body wt with compliant poles. Oxygen consumption rate, vertical ground reaction force, and the force exerted by the load on the shoulders were measured. Oxygen consumption rate increased by 22%. The same increase has previously been observed when loads are carried with a backpack. Thus compliant poles are not a particularly economical method of load carriage. However, pole suspension systems offer important advantages: they minimize peak shoulder forces and loading rates. In addition, the peak vertical ground reaction force is only slightly increased above unloaded levels when loads are carried with poles.

Adult↗
PubMed1991

Mechanics of running under simulated low gravity.

Using a linear mass-spring model of the body and leg (T. A. McMahon and G. C. Cheng. J. Biomech. 23: 65-78, 1990), we present experimental observations of human running under simulated low gravity and an analysis of these experiments. The purpose of the study was to investigate how the spring properties of the leg are adjusted to different levels of gravity. We hypothesized that leg spring stiffness would not change under simulated low-gravity conditions. To simulate low gravity, a nearly constant vertical force was applied to human subjects via a bicycle seat. The force was obtained by stretching long steel springs via a hand-operated winch. Subjects ran on a motorized treadmill that had been modified to include a force platform under the tread. Four subjects ran at one speed (3.0 m/s) under conditions of normal gravity and six simulated fractions of normal gravity from 0.2 to 0.7 G. For comparison, subjects also ran under normal gravity at five speeds from 2.0 to 6.0 m/s. Two basic principles emerged from all comparisons: both the stiffness of the leg, considered as a linear spring, and the vertical excursion of the center of mass during the flight phase did not change with forward speed or gravity. With these results as inputs, the mathematical model is able to account correctly for many of the changes in dynamic parameters that do take place, including the increasing vertical stiffness with speed at normal gravity and the decreasing peak force observed under conditions simulating low gravity.

Adult↗
PubMed1990

Energetics of running: a new perspective.

The amount of energy used to run a mile is nearly the same whether it is run at top speed or at a leisurely pace (although it is used more rapidly at the higher speed). This puzzling independence of energy cost and speed is found generally among running animals, although, on a per gram basis, cost is much higher for smaller animals. Running involves little work against the environment; work is done by muscles and tendons to lift and accelerate the body and limbs. Some of the work is recovered from muscle-tendon springs without metabolic cost and work rate does not parallel metabolic rate with either speed or size. Regardless of the amount of work muscles do, they must be activated and develop force to support the weight of the body. Load-carrying experiments have shown that the cost of supporting an extra newton of load is the same as the weight-specific cost of running. Size differences in cost are proportional to stride frequency at equivalent speeds, suggesting that the time available for developing force is important in determining cost. We report a simple inverse relationship between the rate of energy used for running and the time the foot applies force to the ground during each stride. These results support the hypothesis that it is primarily the cost of supporting the animal's weight and the time course of generating this force that determines the cost of running.

Animals↗
PubMed1989

A treadmill-mounted force platform.

Muscle, bone, and tendon forces; the movement of the center of mass, and the spring properties of the body during terrestrial locomotion can be measured using ground-mounted force platforms. These measurements have been extremely time consuming because of the difficulty in obtaining repeatable constant speed trials (particularly with animals). We have overcome this difficulty by mounting a force platform directly under the belt of a motorized treadmill. With this arrangement, vertical force can be recorded from an unlimited number of successive ground contacts in a much shorter time. With this treadmill-mounted force platform it is possible to accurately make the following measurements over the full range of steady speeds and under various perturbations of normal gait: 1) vertical ground reaction force over the course of the contact phase; 2) peak forces in bone, muscle, and tendon; 3) the vertical displacement of the center of mass; and 4) contact time for the limbs. In our treadmill-force platform design, belt forces and frictional forces cause no measurable cross-talk problem. Natural frequency (160 Hz), nonlinearity (less than 5%), and position independence (less than 2%) are all quite acceptable. Motor-caused vibrations are greater than 150 Hz and thus can be easily filtered.

Biomechanical Phenomena↗
PubMed1989

Stride length in distance running: velocity, body dimensions, and added mass effects.

The preferred stride frequency (SF) and stride length (SL) of male recreational distance runners were measured on a level treadmill under a variety of conditions over the typical distance running speed range of 3.15-4.12 m.s-1. At a given speed, the correlation coefficients between the subjects' anthropometric variables (APV) (such as stature, leg length, and limb segment mass) and their preferred stride variables were consistently low (less than or equal to 0.36) and not significantly different from zero. As speed increased through the experimental range, SF remained nearly constant (only a 4% increase) while SL increased by 28%. The use of dimensionless velocity was shown to be no more effective than conventional methods in the prediction of a SL vs velocity relationship, but the dimensionless form of the relationship was remarkably similar to those observed for other animal species and other forms of gait. The addition of masses up to 1.1 kg at each ankle produced no significant change in SF or SL. The results indicate that factors other than APV are the primary determinants of preferred SF and SL. Since it has been shown previously that the preferred SL is usually the most economical, APV cannot be used to accurately predict or prescribe SF or SL on an individual basis.

Adolescent↗
PubMed1986

Alteration of the N-formyl-methionyl-leucyl-phenylalanine-induced response in cystic fibrosis neutrophils.

In order to determine whether cystic fibrosis neutrophils are affected in their secretory functions, lysosomal enzyme release and chemiluminescence (light emission from cells) were assayed in patients' cells and compared with those in normal control cells. We observed a decreased response of cystic fibrosis neutrophils in beta-glucuronidase release and chemiluminescence after stimulation by N-formyl-methionyl-leucyl-phenylalanine. There was no significant correlation of these results with the clinical score nor with the medical treatment. On the other hand, responses to the calcium ionophore A23187 and to opsonized zymosan showed no significant difference between normal and cystic fibrosis subjects in lysosomal enzyme release. N-formyl-methionyl-leucyl-phenylalanine receptor alterations did not seem involved in the observed effect as demonstrated by Scatchard plot analysis of N-formyl-methionyl-leucyl-phenylalanine binding to these receptors. These results clearly demonstrate a difference between normal and cystic fibrosis neutrophils in release and chemiluminescence responses to N-formyl-methionyl-leucyl-phenylalanine stimulation, a difference that might be located in the plasma membrane as both responses are membrane dependent.

Adolescent↗
PubMed1985

Mechanical and muscular factors affecting the efficiency of human movement.

This paper reviews specific examples of how energy expenditure during submaximal exercise is affected by mechanical and muscular factors. Structural biomechanical variables are discussed as a possible reason for economy differences between individuals. The practical question, "Can economy of performing a certain task be modified?" is posed. Examples of how the manipulation of a particular movement pattern results in an energetic minimum (optimal phenomena) are presented. The physiological mechanisms for these phenomena are summarized. The influence of positive vs negative work and storage of elastic energy in relation to the topic of economy and muscular efficiency is considered. The effects of athletic equipment such as footwear, track surfaces, and bicycle components on economy and muscular efficiency are presented. The prospects for improving athletic performance by improving economy are evaluated, and recommendations for future directions are made.

Bicycling↗
PubMed1985

The efficiency of human movement--a statement of the problem.

This paper is an introduction to a multidisciplinary series of papers on the efficiency of human movement. The problem is posed by citing the example of the large variation in oxygen uptake (per kg body weight) within a typical group of subjects running at the same submaximal speed. An analog of the possible causes of this variation is presented where "set points" for biomechanical, physiological, psychological, biochemical, and other factors combine in series to influence the overall energy cost. The various definitions of "efficiency" and "economy" are considered at the whole body and the isolated muscle level, and a discussion of baseline subtraction is presented. The concept of "effectiveness" is reviewed to illustrate the interaction of skilled performance and energy cost. It is recommended that the terms "muscular efficiency," "muscle efficiency," "economy," and "effectiveness" be used in their respective contexts to replace the current diversity of overlapping and, at times, confusing terminology.

Efficiency↗
PubMed1975

Cyclic AMP and cyclic GMP concentrations in serum- and density-restricted fibroblast cultures.

Mouse fibroblasts transformed by simian virus 40 (SV3T3 cells) are characterized by cyclic AMP and cyclic GMP levels, respectively, about half and twice those found in growing untransformed 3T3 cells. Density-dependent inhibition of growth is correlated with reduced cyclic GMP concentrations in 3T3 and four different density-restricted revertant lines derived from SV3T3. The levels of cyclic AMP are not increased at confluence. Upon serum restriction, serum-dependent cell lines show a greater increase in intracellular cAMP than serum-insensitive lines. Cyclic GMP levels are greatly reduced, even in serum-insensitive density revertants, but not in SV3T3. Serum readdition to all serum-dependent lines is followed by a rapid decrease in cyclic AMP and increase in cyclic GMP concentrations. The magnitude of these responses is decreased in SV3T3 and density revertants.

Animals↗