Keith R. Williams

A model for the calculation of mechanical power during distance running

Because widely varying estimates of mechanical power have been calculated for a given speed of running by previous investigators, the effects of various assumptions necessary for mechanical power calculations were evaluated via a segmental energy analysis using 3-D cine data from 31 well-trained subjects running overground at 3.57 ms-1. The magnitude of power calculated was very dependent upon assumptions involving the amount of between segment energy transfer occurring, the relative metabolic cost of positive vs negative work, the amount of the total negative work attributed to muscular work, the effects of elastic storage of energy and on the choice of cutoff frequency in filtering the coordinate data. Mechanical power values ranged from 273 to 1775 W depending on the particular assumptions made and computational methods employed giving efficiency ratios from 0.31 to 1.97. These results point to a need for more definitive research into the role played by energy transfer, negative work, and elastic storage of energy before confidence in measured mechanical power can be established.

The relationship between mechanical and physiological energy estimates.

The physiological energy expenditure involved in common activities such as running, walking, or cycling can be influenced by a variety of biomechanical factors. In evaluating changes in mechanical energy in order to derive a measure of mechanical power which is more directly related to metabolic energy cost, it is necessary to be able to identify the source of these changes. Factors such as concentric and eccentric muscular contractions, transfer of energy, elastic storage and reuse of energy, and joint range of motion limitations can all change the mechanical energy of a segment, but each involves a different metabolic energy expenditure. While a number of computational methods have been suggested for the calculation of mechanical power, each incorporates a different set of assumptions involving the factors just mentioned, and widely varying results for mechanical power have been obtained. The lack of definitive information concerning the relationship between mechanical and physiological energy changes limits the accuracy, meaningfulness, and usefulness of measures of mechanical power and muscular efficiency.

The Effects of Wearing High Heeled Shoes on Pedal Pressure in Women

The purpose of this study was to investigate the effects of increased heel height in womens shoes on foot pressure during walking. An increase in heel height increased the maximum peak pressure under the metatarsal heads in the forefoot, decreased the time to maximum peak pressure under the metatarsal heads, and increased the rate of loading to the metatarsals during early support. The higher pressures noted with increased heel height were accompanied by a more uniform distribution of pressure beneath the forefoot. These findings may denote increased stress to the various tissues in the foot when walking in high heeled shoes, which may contribute to deleterious orthopaedic changes. Quantitative studies need to be conducted to determine whether orthopaedic changes occur with prolonged wearing of high heeled shoes.

The mechanics of foot action during the golf swing and implications for shoe design

ABSTRACTThe forces and movement at the feet during the golf swing were studied using force platform and three-dimensional cinematography techniques, with special consideration given to implications for golf shoe design. Ground reaction forces and center-of-pressure patterns were obtained by alternat

Physiological response to cycling with both circular and noncircular chainrings.

The purpose of this study was to compare physiological variables of endurance-trained cyclists riding with four different chainring designs: round, Shimano Biopace, and two engineered ellipse designs. The ellipse designated Eng10 had the crank arm oriented 10 degrees forward of the major (i.e. longer) axis. Eighty degrees further forward, along the minor axis, was the crank arm orientation for the second ellipse, Eng90. With the major to minor axis ratio of 22.9 cm/16.8 cm (1.36), both ellipses imposed a crank angular velocity variation of 27% relative to the highest velocity assuming constant chain velocity. Best described as a skewed ellipse (i.e., major and minor axes not perpendicular), the Biopace had a major to minor axis ratio of 1.09 thus giving a crank angular velocity variation of 8%. Eleven male cyclists rode at a high (80% of maximum VO2) and a low (60% of maximum VO2) workrate using each chainring. The study was conducted over four consecutive days with the presentation order of the chainrings randomized. Open circuit spirometry was used to collect continuous respiratory data. Heart rate, blood lactate, and cadence values also were measured. None of the physiological variables including rates of oxygen consumption showed significant differences among the chainrings. Thus, the gross efficiency of cycling was not improved by any of the noncircular chainrings. For cycling events where efficiency is a determinant of performance, the noncircular chainrings do not offer any advantage over round chainrings.

Correcting out-of-plane errors in two-dimensional imaging using nonimage-related information

Two-dimensional imaging with a single camera assumes that the motion occurs in a calibrated plane perpendicular to the camera axis. It is well known that kinematic errors result if the object fails to remain in this plane and that if both the distance to the calibration plane from the camera and the distance out-of-plane are known, an analytical correction for the out-of-plane error can be made. Less well appreciated is that out-of-plane distance can frequently be acquired from other, nonimage-related information. In the two examples given, the mediolateral center of pressure coordinate of the foot measured from a force plate and the measured landing point of a shot put throw were used. In both cases, the resulting out-of-plane correction improved the accuracy of the 2-D kinematic data dramatically. These examples also demonstrate that the use of nonimage-related data can increase the accuracy of kinematic data without an increase in the complexity of the experiment.

Mechanical, Biomechanical and Psychophysical Study of Carpet Performance

This study investigated carpet performance by using mechanical, biomechanical and psychophysical methods. Four different carpets were tested in this study using a force platform which also served as a hard control floor. Compliance modulus was measured to characterize the mechanical property of carpets. Six college students stood barefoot on the force platform covered by a carpet sample under both eyes-open and eyes-closed conditions. The center of pressure recorded by the force platform was used to quantify the postural sway in quiet stance. Perceived comfort was evaluated in different body areas for each flooring condition. Visual environment was found to significantly influence postural sway in quiet standing, and more compliant carpets were observed in general to be associated with faster sway velocity and smaller sway area. Also, more compliant carpets provided better perceived comfort, in particular in ankles and feet.

THE RELATIONSHIP BETWEEN MASS AND ACCELERATION FOR IMPACTS ON PADDED SURFACES

It is shown analytically that when an object impacts a deformable surface, the acceleration it experiences is inversely proportional to its mass. This is because acceleration is reduced by deformation of the surface, and more massive objects deform the surface more. Since head injury is thought to depend more on acceleration than applied force, this result implies that children may be at greater risk than adults when striking a padded surface. To test the theoretical prediction, 2.69 and 7.40 kg missiles were dropped 31.8 cm onto three deformable surfaces: dry sod, moist sod, and an artificial playing surface. Impact force was measured with a force plate, and divided by missile mass to obtain acceleration. For each test surface, the smaller mass produced a larger acceleration than the larger mass, confirming the theoretical result. Additionally, load-deformation characteristics of the three surfaces were measured, and the results used to predict the effects of mass on acceleration during impact. The predicted results agreed well with the experimental data for the artificial surface (2.3% difference) and moist sod (5.7% difference), but less well for dry sod (21% difference).

Dynamic Analysis of Posture Profiles in Quiet Stance on Carpets through Fractional Brownian Motion

In this study we examine the dynamic characteristics of the center of pressure (COP) profiles in quiet stance on various carpets through the framework of fractional Brownian motion. Four different carpets plus the force platform (which served as a control floor) were tested. Six healthy young subjects stood stationary in bare feet on each flooring condition under both eyes-open (EO) and eyes-closed (EC) conditions. A stabilogram diffusion analysis (SDA) was conducted in both anteroposterior and mediolateral directions to extract the dynamic features of the COP profiles. Two approximately linear regions (short-versus long-term regions) were found for all of the flooring conditions in the double logarithmic plot of the mean-square displacements versus time intervals, implying that a two-stage control scheme exists in regulating the posture. In general, a more compliant carpet was found to be associated with a shorter transitional time interval and a lower H L value (Hurst coefficient in the long-term region) under the EO condition, but to be correlated with a higher H S (Hurst coefficient in the short-term region) and a higher H L under the EC condition. While the hard floor of the force platform significantly decreased H L from the EO to the EC conditions, the alteration of visual environments did not substantially influence H L for a compliant carpet.

Het effect van schoenen met hoge hakken op de lichaamshouding, het zwaartepunt, de driedimensionale bewegingsleer, beweging van de hiel en de bodemreactiekrachten

Als de hak van een schoen genoeg wordt verhoogd om de stand van het lichaam en derhalve het totale lichaamszwaartepunt (lzp) aanzienlijk te wijzigen, zou een compensatoire verandering van de bewegingsleer van het totale lichaam kunnen worden verwacht, hetgeen vervolgens de hoogte of impuls van de grondreactiekrachten (grk) zou kunnen veranderen.