Estimation of Errors in Force Platform Data

Abstract

Young, healthy varsity swimmers were studied over a period of 14 months with regard to the effect of a typical training and competitive collegiate swimming program on plasma cholesterol and phospholipids. Chemical analyses on blood cholesterol and phospholipids were made and compared for competitive and noncompetitive seasons. A dietary analysis was also made during the competitive season. In both the original and subsequent study involving the current (1960–61) varsity swimmers analyses were made of the acute effect of physical exertion on plasma cholesterol and phospholipids.

The exercise, used for conditioning varsity swimming athletes did not significantly lower blood cholesterol and phospholipids. However, this result is not necessarily at variance with the view of some investigators that exercise does exert a hypocholesterilizing effect. Several factors are discussed which are thought to have influenced ultimate results.     

BASES 2006: “From education to application”

The dependence of power on aerobic and anaerobic energy metabolism and on force production was studied in maximal leg exercise. National and international level male rowers (n = 9) performed four modified (legs‐only) rowing ergometer exercises: a progressive test, 2‐min (T2), 12‐min (T12) and 6‐min (T6) all‐out tests. In T2, significant correlations were observed between power in T2 (P_T2) and oxygen debt (r = 0.83, P<0.05) and between P_T2 and average force production (F_av) during the last 30 s (r = 0.85, P<0.05). These parameters explained 93% of the variation in P_T2. The highest correlations between power in T6 (P_T6) and physiological parameters were as follows: maximal oxygen uptake (VO₂ max: r = 0.87, P<0.01), blood bicarbonate concentration before the test ([HCO^‐ _3before]: r=0.85, P<0.05) and blood lactate concentration on anaerobic threshold (BL_anT: r= —0.82, P<0.05). Together, these parameters explained 92% of the variation in P_T6. In T12, the total power (P_T12) correlated with power of anaerobic threshold #OPP_ANT’. r = 0.95, P< 0.001) and with the highest VO₂ value in this test (VO₂ peak: r = 0.92, P<0.001). These two parameters explained 96% of the variation in P_T12.

The decrease of at least one of the force parameters during each test was taken as a sign of fatigue. The decline in force was compensated for by an increase in stroke rate at the end of T6 and T12 (P<0.01, P<0.001). Consequently, the power remained unchanged or even increased at the end of T6 and T12. The term ‘power endurance’ is introduced to describe the ability to resist and to compensate for local muscular fatigue.     

String vibration dampers do not reduce racket frame vibration transfer to the forearm

In this study, we examined the effect of string vibration damping devices on reducing racket frame vibration transfer to the forearm. Twenty participants volunteered to hold a tennis racket stationary in a forehand and backhand stroking position while tennis balls were fired at 20 m?·?s^?1 towards two impact locations, the node of vibration and the dead spot. A three-way analysis of variance with repeated measures on damping condition, impact location and stroke condition was performed on the data. The resonant frequency of the hand-held racket was found to be ～120 Hz. No significant differences in amplitude of vibration at the resonant frequency were found for the wrist or the elbow when damped and non-damped impacts were compared. Impacts at the dead spot produced greater amplitudes of vibration (P <?0.01) but no interaction between impact location and string dampers was evident. The string dampers had no effect on the grip force used or the muscle electrical activity in the forearm after impact. In conclusion, we found that string dampers do not reduce the amount of racket frame vibration received at the forearm. We suggest that string dampers remain a popular accessory among tennis players because of their acoustic effects and psychological support rather than any mechanical advantage.     

Effects of stance width on performance and postural stability in national-standard pistol shooters

Abstract

The aim of the present study was to determine whether changing stance width would result in a corresponding change in postural and/or pistol stability. Twelve national-standard male air pistol shooters performed 10 shots each at five stance widths (30 cm, 45 cm, 60 cm, 75 cm, and 90 cm). Postural stability was determined by measuring centre-of-pressure changes with a dual force-platform system. Shooting mechanics measures were determined by a NOPTEL ST-2000 optoelectronic training system. Medial-lateral centre-of-pressure excursion (F _4,44 = 7.17, P < 0.001, effect size = 0.99) and speed (F _4,44 = 77.03, P < 0.001, effect size = 3.88) were reduced as stance width decreased. Centre of gravity fine (the percentage of time held within an area the size of the ten-ring) improved during narrower stance widths (F _4,32 = 12.49, P < 0.001, effect size = 0.71). Our findings suggest that stance width affects postural and pistol stability in national-standard air pistol athletes. Moreover, the current method of suggesting a wider stance to improve shooting performance should be reconsidered and perhaps air-pistol shooters should use a 30-cm stance width to improve postural stability and shooting performance.     

The force–time profile of elite front crawl swimmers

Abstract

In this study, we used recently developed technology to determine the force–time profile of elite swimmers, which enabled coaches to make informed decisions on technique modifications. Eight elite male swimmers with a FINA (Federation Internationale de Natation) rank of 900+ completed five passive (streamline tow) and five net force (arms and leg swimming) trials. Three 50-Hz cameras were used to video each trial and were synchronized to the kinetic data output from a force-platform, upon which a motorized towing device was mounted. Passive and net force trials were completed at the participant's maximal front crawl swimming velocity. For the constant tow velocity, the net force profile was presented as a force–time graph, and the limitation of a constant velocity assumption was acknowledged. This allowed minimum and maximum net forces and arm symmetry to be identified. At a mean velocity of 1.92 ± 0.06 m · s^?1, the mean passive drag for the swimmers was 80.3 ± 4.0 N, and the mean net force was 262.4 ± 33.4 N. The mean location in the stroke cycle for minimum and maximum net force production was at 45% (insweep phase) and 75% (upsweep phase) of the stroke, respectively. This force–time profile also identified any stroke asymmetry.     

The 1985 C. H. McCloy Research Lecture Practical Problems in Exercise Physiology Research

Abstract

This paper examines the contribution of exercise physiology research to improvements in sports performance. By using illustrations from studies on competitive swimmers, attempts have been made to show the need to develop tests that evaluate the factors essential for success in sports. Such testing should be used to describe the strengths and weaknesses of the athlete, and to offer the athlete and coach valid information to gauge the benefits of training. Aside from the value of monitoring the adaptations to sports training, the primary role of physiological research in sports should be to solve problems that will ultimately aid athletes to achieve their full potential. This paper will conclude with examples of problem solving research and its contribution to performance.     

Estimation of the energy loss at the blades in rowing: Common assumptions revisited

Abstract

In rowing, power is inevitably lost as kinetic energy is imparted to the water during push-off with the blades. Power loss is estimated from reconstructed blade kinetics and kinematics. Traditionally, it is assumed that the oar is completely rigid and that force acts strictly perpendicular to the blade. The aim of the present study was to evaluate how reconstructed blade kinematics, kinetics, and average power loss are affected by these assumptions. A calibration experiment with instrumented oars and oarlocks was performed to establish relations between measured signals and oar deformation and blade force. Next, an on-water experiment was performed with a single female world-class rower rowing at constant racing pace in an instrumented scull. Blade kinematics, kinetics, and power loss under different assumptions (rigid versus deformable oars; absence or presence of a blade force component parallel to the oar) were reconstructed. Estimated power losses at the blades are 18% higher when parallel blade force is incorporated. Incorporating oar deformation affects reconstructed blade kinematics and instantaneous power loss, but has no effect on estimation of power losses at the blades. Assumptions on oar deformation and blade force direction have implications for the reconstructed blade kinetics and kinematics. Neglecting parallel blade forces leads to a substantial underestimation of power losses at the blades.     

Time Needed to Change the Isometric Force Production of a Response

Abstract

This study investigated the time needed to change a motor program that specified the elbow flexor muscles to gradually increase the isometric force production from 15% to 75% of one's maximum voluntary contraction (MVC). A double-stimulation paradigm was used with the restriction that subjects (N = 12) be at 15% of their MVC before the presentation of the first stimulus. Subjects reacted to the first stimulus (randomly presented) by gradually increasing their isometric force from 15% to 75% of their MVC and then reacted to the second stimulus by altering the force production in one of four ways: (a) increasing the force to the 75% level rapidly instead of gradually, (b) discontinuing the increase and maintaining the level of force attained, (c) discontinuing all force production, or (d) reversing the direction of force so that it is produced by the elbow extensors. The data revealed that more time was needed to increase the force rapidly than to perform any of the other three conditions.     

Measurement of active drag during crawl arm stroke swimming

In order to measure active drag during front crawl swimming a system has been designed, built and tested. A tube (23 m long) with grips is fixed under the water surface and the swimmer crawls on this. At one end of the tube, a force transducer is attached to the wall of the swimming pool. It measures the momentary effective propulsive forces of the hands. During the measurements the subjects’ legs are fixed together and supported by a buoy. After filtering and digitizing the electrical force signal, the mean propulsive force over one lane at constant speeds (ranging from about 1 to 2 m s^‐1) was calculated. The regression equation of the force on the speed turned out to be almost quadratic. At a mean speed of 1.55 m s^‐1 the mean force was 66.3 N. The accuracy of this force measured on one subject at different days was 4.1 N. The observed force, which is equal to the mean drag force, fits remarkably well with passive drag force values as well as with values calculated for propulsive forces during actual swimming reported in the literature. The use of the system does not interfere to any large extent with normal front crawl swimming; this conclusion is based on results of observations of film by skilled swim coaches. It was concluded that the system provides a good method of studying active drag and its relation to anthropometric variables and swimming technique.