The constrained control of force and position in multi-joint movements.

In many arm or leg movements the hand or foot has to exert an external force on the environment. Based on an inverse dynamical analysis of cycling, it is shown that the distribution of net moments in the joints needed to control the direction of the external force is often opposite to the direction of joint displacements associated with this task. Kinetic and kinematic data were obtained from five experienced cyclists during ergometer cycling by means of film analysis and pedal force measurement. An inverse dynamic analysis, based on a linked segments model, yielded net joint moments, joint powers and muscle shortening velocities of eight leg muscles. Activation patterns of the muscles were obtained by means of surface electromyography. The results show that the transfer of rotations in hip, knee and ankle joints into the translation of the pedal is constrained by conflicting requirements. This occurs between the joint moments necessary to contribute to joint power and the moments necessary to establish a direction of the force on the pedal which allows this force to do work on the pedal. Co-activation of mono-articular agonists and their bi-articular antagonists appear to provide a unique solution for these conflicting requirements: bi-articular muscles appear to be able to control the desired direction of the external force on the pedal by adjusting the relative distribution of net moments over the joints while mono-articular muscles appear to be primarily activated when they are in the position to shorten and thus to contribute to positive work. Examples are given to illustrate the universal nature of this constrained control of force (external) and position (joint). Based on this study and published data it is suggested that different processes may underlie the organization of the control of mono- and bi-articular muscles.

Longitudinal development of young talented speed skaters: physiological and anthropometric aspects.

A longitudinal analysis of a group of speed skaters was done to identify the performance-determining factors for a successful speed skating career. This paper presents both the physiological and anthropometric results of this longitudinal study. Twenty-four athletes from the Dutch National Junior Speed Skating Team were followed from age 16-17 yr to age 20-21 yr. During the development from junior to senior speed skater, a number of anthropometric and physiological variables changed. There were no differences between successful and unsuccessful speed skaters from an anthropometric perspective; consequently, it was not possible to distinguish successful from unsuccessful athletes on anthropometric grounds. The longitudinal data showed that at a younger age the successful speed skaters had similar oxygen consumption, mechanical efficiency, and power output values compared with the unsuccessful speed skaters. Later in the study, successful speed skaters distinguished themselves by the ability to produce higher power output values. There were no anthropometric or physiological relationships found in this study on which performance at the age of 20-21 yr could be predicted with measurements at a junior age.

Propelling efficiency of front-crawl swimming.

In this study the propelling efficiency (ep) of front-crawl swimming, by use of the arms only, was calculated in four subjects. This is the ratio of the power used to overcome drag (Pd) to the total mechanical power (Po) produced including power wasted in changing the kinetic energy of masses of water (Pk). By the use of an extended version of the system to measure active drag (MAD system), Pd was measured directly. Simultaneous measurement of O2 uptake (VO2) enabled the establishment of the relationship between the rate of the energy expenditure (PVO2) and Po (since when swimming on the MAD system Po = Pd). These individual relationships describing the mechanical efficiency (8-12%) were then used to estimate Po in free swimming from measurements of VO2. Because Pd was directly measured at each velocity studied by use of the MAD system, ep could be calculated according to the equation ep = Pd/(Pd + Pk) = Pd/Po. For the four top class swimmers studied, ep was found to range from 46 to 77%. Total efficiency, defined as the product of mechanical and propelling efficiency, ranged from 5 to 8%.

Intermuscular co-ordination during fast contact control leg tasks in man.

From previous inverse dynamic analyses of human leg extensions, it was hypothesized that the underlying processes for the activation of mono- and biarticular muscles are different; the mono-articular muscles being activated when they shortened, whereas the biarticular muscles appeared responsible for the control of the external force direction. In the present study, experiments were performed on a dynamometer which was especially developed to test this hypothesis. Subjects had to exert different prescribed force vectors on a moving force-plate during leg extension, which they had intensively practised prior to the actual experiments. Of each trial, position, force and EMG activity were recorded. Net joint torques were calculated by the method of inverse dynamics and related to the EMG-patterns of the mono- and biarticular upper leg muscles to reveal whether the previously observed different roles in contact tasks might constitute a general principle in motor control. The results showed that although the action of the biarticular m. rectus femoris and hamstrings muscles was consistent in controlling the direction of the external force, the actions of the mono-articular muscles did not agree with their hypothesized role as simple work generators. The generalizability of a different control for mono- and biarticular muscles could thus not be confirmed for these tasks. They might rather reflect one out of more available strategies the CNS can use to control different contact control tasks.

Stiffness control for lower leg muscles in directing external forces.

The role of lower leg muscles is investigated during contact control tasks, in which the external force, applied by the foot on the surface, has to be controlled. Force, position and muscle activation were recorded. All subjects showed a stereotyped activation pattern to accomplish the tasks. The torques about hip and knee changed by a reciprocal activation pattern of the upper leg muscles, whereas the ankle torque remained remarkably constant as a result of a strong co-activation of the ankle muscles.

Optimisation of sprinting performance in running, cycling and speed skating.

Sprinting performances rely strongly on a fast acceleration at the start of a sprint and on the capacity to maintain a high velocity in the phase following the start. Simulations based on a model developed in which the generation of metabolic power is related to the mechanical destinations of power showed that for short-lasting sprinting events, the best pacing strategy is an all out effort, even if this strategy causes a strong reduction of the velocity at the end of the race. Even pacing strategies should only be used in exercises lasting longer than 80 to 100 seconds. Sprint runners, speed skaters and cyclists need a large rate of breakdown of energy rich phosphates in the first 4 to 5 seconds of the race (mechanical equivalent > 20 W/kg) in order to accelerate their body, and a power output of more than 10 W/kg in the phase following the start to maintain a high velocity. Maximal speed in running is mainly limited by the necessity to rotate the legs forwards and backwards relative to the hip joint. The acceleration phase, however, relies on powerful extensions of all leg joints. Through a comparison of the hindlimb design of highly specialised animal sprinters (as can be found among predators) and of long distance animal runners (as found among hoofed animals), it is illustrated that these 2 phases of a sprint rely on conflicting requirements: improvement of maximal speed would require lower moments of inertia of the legs whereas a faster acceleration would require the involvement of more muscle mass (not only of the hip and knee extensors but also of the plantar flexors). Maximal speed in cycling and speed skating is not limited by the necessity to move leg segments but rather on air friction and rolling or ice friction. Since the drag coefficients found for speed skaters and cyclists (about 0.8) are considerably higher than those of more streamlined bodies, much progress can still be expected from the reduction of air friction. Speed skaters and especially cyclists show much smaller accelerations during the start than do sprint runners. Skaters might try to improve their very first push off by developing a start technique that allows a much more horizontally directed propulsive force. The small propulsive force at the onset of a cycling sprint is due to the gearing system.(ABSTRACT TRUNCATED AT 400 WORDS)

Some fundamental aspects of the biomechanics of overground versus treadmill locomotion.

The literature shows a wide difference of opinion about the mechanical equality of, or difference between treadmill and overground locomotion. This difference in opinion is often related to the coordinate system which implicitly or explicitly is used. With help of a few theoretical examples of energy calculations this paper shows that the description of treadmill locomotion with respect to a fixed coordinate system can lead to faulty conclusions. It is concluded that as long as the beltspeed is constant a coordinate system should be used which moves with the belt. In such a system no mechanical difference exists in comparison with overground locomotion with respect to a fixed coordinate system. All differences found in locomotion patterns must therefore originate from other than mechanical causes.

Mechanical output about the ankle joint in isokinetic plantar flexion and jumping.

The purpose of this study was to compare for a group of ten subjects the mechanical output about the ankle during isokinetic plantar flexion with that during one-legged vertical jumps. For evaluation of the mechanical output the plantar flexion moment of force was related to the angular velocity of plantar flexion. The relationship for isokinetic plantar flexion was obtained using an isokinetic dynamometer; that for plantar flexion in jumping was obtained by combining kinematics and ground reaction forces. It was found that, at any given angular velocity of plantar flexion above 1 rad.s-1, the subjects produced much larger moments during jumping than during isokinetic plantar flexion. In order to explain the observed differences in mechanical output about the ankle, a model was used to simulate isokinetic plantar flexion and plantar flexion during jumping. The model represented both m. soleus and m. gastrocnemius as a complex composed of elastic tissue in series with muscle fibers. The force of the muscle fibers depended on fiber length, shortening velocity (Vfibers), and active state. The input variables of the model were histories of shortening velocities of the complexes, determined from kinematics, and active state. Among the output variables were Vfibers and plantar flexion moment. The simulation results were very similar to the experimental findings. According to the simulation results there are two reasons why at the same angular velocity of plantar flexion larger moments were produced during jumping than during isokinetic plantar flexion.(ABSTRACT TRUNCATED AT 250 WORDS)

Function of mono- and biarticular muscles in running.

In this study the function of leg muscles during stretch-shortening cycles in fast running (6 m.s-1) was investigated. For a single stance phase, kinematics, ground reaction forces, and EMG were recorded. First, rough estimates of muscle force, obtained by shifting the EMG curves +90 ms, were correlated with origin-to-insertion velocity (VOI). Second, active state and internal muscle behavior were estimated by using a muscle model that was applied for soleus and gastrocnemius. High correlations were found between estimates of muscle force and VOI time curves for mono-articular hip, knee, and ankle extensor muscles. The correlation coefficients for biarticular muscles were low. The model results showed that active state of gastrocnemius was high during increase of origin-to-insertion length (LOI), whereas active state of soleus was low during the start of increase of LOI and rose to a plateau at the time lengthening ended and shortening started. It seems that the difference in stimulation between gastrocnemius and soleus is a compromise between minimizing energy dissipation and using the stretch-shortening cycle optimally. Furthermore, it was found that the net plantar flexion moment during running reached a value of 302 Nm, which was 158% and 127% higher than the peak values reached in maximal jump and sprint push-offs, respectively. It was argued that the higher mechanical output in running than in jumping could be ascribed to the utilization of the stretch-shortening cycle in running. The higher values in running compared with sprinting, however, may lie in a difference in muscle stimulation.

Drop jumping. I. The influence of jumping technique on the biomechanics of jumping.

In the literature, drop jumping is advocated as an effective exercise for athletes who prepare themselves for explosive activities. When executing drop jumps, different jumping techniques can be used. In this study, the influence of jumping technique on the biomechanics of jumping is investigated. Ten subjects executed drop jumps from a height of 20 cm and counter-movement jumps. For the execution of the drop jumps, two different techniques were adopted. The first technique, referred to as bounce drop jump, required the subjects to reverse the downward velocity into an upward one as soon as possible after landing. The second technique, referred to as counter-movement drop jump, required them to do this more gradually by making a larger downward movement. During jumping, the subjects were filmed, ground reaction forces were registered, and electromyograms were recorded. The results of a biomechanical analysis show that moments and power output about knee and ankle joints reach larger values during the drop jumps than during counter-movement jumps. The largest values were attained during bounce drop jumps. Based on this finding, it was hypothesized that bounce drop jump is better suited than counter-movement drop jump for athletes who seek to improve the mechanical output of knee extensors and plantar flexors. Researchers are, therefore, advised to control jumping technique when investigating training effects of executing drop jumps.

Drop jumping. II. The influence of dropping height on the biomechanics of drop jumping.

In the literature, athletes preparing for explosive activities are recommended to include drop jumping in their training programs. For the execution of drop jumps, different techniques and different dropping heights can be used. This study was designed to investigate for the performance of bounce drop jumps the influence of dropping height on the biomechanics of the jumps. Six subjects executed bounce drop jumps from heights of 20 cm (designated here as DJ20), 40 cm (designated here as DJ40), and 60 cm (designated here as DJ60). During jumping, they were filmed, and ground reaction forces were recorded. The results of a biomechanical analysis show no difference between DJ20 and DJ40 in mechanical output about the joints during the push-off phase. Peak values of moment and power output about the ankles during the push-off phase were found to be smaller in DJ60 than in DJ40 (DJ20 = DJ60). The amplitude of joint reaction forces increased with dropping height. During DJ60, the net joint reaction forces showed a sharp peak on the instant that the heels came down on the ground. Based on the results, researchers are advised to limit dropping height to 20 or 40 cm when investigating training effects of the execution of bounce drop jumps.

Electromechanical delay during knee extensor contractions.

The purpose of this study was to investigate the magnitude of electromechanical delay (EMD) and its possible dependence on muscle type, type of contraction, fatigue, level of force, initial muscle length, and muscle contraction velocity. This was achieved using an experiment that measured voluntary knee extensor torques and surface EMG activity for a variety of different contractile conditions in seven male subjects. EMD values were obtained using a cross-correlation technique in three experimental KIN-COM dynamometer conditions of vastus medialis, rectus femoris, and vastus lateralis. In the first condition, a series of 10 repetitive submaximal (50% and 70% MVC) isometric knee extensor contractions were performed at knee angles of 90 degrees and 130 degrees extension. In the second condition, 10 maximal isokinetic knee extensor contractions were performed during passive shortening and lengthening. As such, the dynamometer was used to passively move the knee joint at 30 degrees.s-1 and 60 degrees.s-1. Both during lengthening and shortening, the contractions occurred at an angular position of 110 degrees. In the last condition, a repetitive submaximal isometric knee extensor fatigue test was performed for 100 s (150 contractions). At 10, 40, and 90 s during the time course of this fatigue test, a series of 10 contractions were recorded. To avoid a phase lag, which is introduced with one-way filtering, the EMG was processed with a bidirectional low-pass filter application. A significant main effect in EMD for the factor level of force was found. The EMD values obtained at a force level of 50% MVC were longer than at 70% MVC (107 vs 98 ms).(ABSTRACT TRUNCATED AT 250 WORDS)

The work of walking: a calorimetric study.

Experiments were designed to test the traditional assumption that during level walking all of the energy from oxidation of fuel appears as heat and no work is done. Work is force expressed through distance, or energy transferred from a man to the environment, but not as heat. While wearing a suit calorimeter in a respiration chamber, five women and five men walked for 70 to 90 min on a level treadmill at 2.5, 4.6, and 6.7 km.h-1 and pedalled a cycle ergometer for 70 to 90 min against 53 and 92 W loads. They also walked with a weighted backpack and against a horizontal load. During cycling, energy from fuel matched heat loss plus the power measured by the ergometer. During walking, however, energy from fuel exceeded that which appeared as heat, meaning that work was done. The power increased with walking speed; values were 14, 29, and 63 W, which represented 11, 12, and 13% of the incremental cost of fuel above the resting level. Vertical and horizontal loads increased the fuel cost and heat loss of walking but did not alter the power output. This work energy did not re-appear as thermal energy during 18 h of recovery. The most likely explanation of the work done is in the inter-action between the foot and the ground, such as compressing the heel of the shoe and bending the sole. We conclude that work is done in level walking.

The instantaneous torque-angular velocity relation in plantar flexion during jumping.

Torques, angular velocities, and power of the ankle joint during plantar flexion were measured in jumping experiments in order to achieve insight into shape and magnitude of the instantaneous torque-angular velocity relation in a complex movement. Twelve trained subjects performed maximal vertical jumps from a semi-squatting position with 100 degrees of flexion in the knee joint. Ground reaction force measurements and film analyses were used to calculate instantaneous torques, angular velocities, and power outputs during plantar flexion. The shape of the instantaneous torque-angular velocity was different from the well-known hyperbolic force-velocity relation for isolated muscles. Maximal power output (2499 +/- 751 [SD] W) occurred at 60% of the mean maximal torque (301 +/- 62 N X m) and 80% of the mean maximal angular velocity (970 degrees/s). The maximal power output was six times larger than the power output reported in the literature for maximal isokinetic (monoarticular) plantar flexions. Influences like storage of energy in the series elastic component of Hill's muscle model and the role of polyarticular muscles in transporting energy from knee to ankle are discussed. It is concluded that many more selective studies will be necessary before it is possible to relate intrinsic muscle properties to the performance of muscles in poly-articular complex movements.

A simulation of speed skating performances based on a power equation.

Using kinetics of aerobic and anaerobic power production as measured during supramaximal bicycle tests of five speed skaters of international level, a model of the kinetics of power production during skating is obtained. Velocity time courses of a generalized speed skater were calculated for all Olympic distances (500 m, 1000 m, 1500 m, 5000 m, and 10,000 m) by means of simulation of an equation of produced power, power dissipated to air and ice friction, and rate of change of kinetic energy of the skater. Different strategies of distribution of anaerobic energy during a race were compared. With a single equation it appeared to be possible to simulate the mean split and final times of the five distances realized during the Winter Olympics 1988 within an error which does not exceed 1.6% (mean error in final times: 0.8%). The results show that a fast acceleration (high initial power output) is crucial for the sprinting events (500 m and 1000 m). It is shown that this initial power output level is even more important than the total amount of energy available for a 500 m and 1000 m race. For the long distances the simulations show that skaters should combine a fast but short lasting start with a constant power output following the start in order to minimize air frictional losses.

The start in speed skating: from running to gliding.

The purpose of this study was to describe the push-off kinematics in speed skating using three-dimensional coordinates of elite male sprinters during the first part of a speed skating sprint. The velocity of the mass center of the skater's body VC, is decomposed into an "extension" velocity component VE, which is associated with the shortening and lengthening of the leg segment and a "rotational" velocity component Vr, which is the result of the rotation of the leg segment about the toe of the skate. It can be concluded that the mechanics of the first strokes of a sprint differ considerably from the mechanics of strokes later on. The first push-offs take place against fixed location on the ice. In these "running-like" push-offs the contribution of Vr in the forward direction is larger than the extension component Ve. Later on, the strokes are characterized by a gliding push-off in which Ve increases. In these gliding push-offs no direct relation exists between forward velocity of the skater and the extension in the joints. This allows skaters to obtain much higher velocities than can be obtained during running.

Performance-influencing factors in homogeneous groups of top athletes: a cross-sectional study.

Sport scientists have identified many factors as prerequisites for a good athletic performance in various sports. It is not clear whether these factors also influence the best performers in the homogeneous groups of top athletes selected for national teams. In this study, this issue is addressed with members of the Dutch National Junior Speed Skating Team. A total of 237 different technical, physiological, anthropometrical, and psychological parameters were collected, including many that correlated with performance in previous studies. High speed film analyses during the National Championships provided the technique parameters. A 30-s sprint test and a 150-s supramaximal test on a cycle ergometer underlie the physiological data, and questionnaires were used to measure personality traits and emotional feelings. Only trunk position and the direction of push-off (push-off angle phi) correlated consistently with skating performance in this group (r = 0.61-0.73 and r = -0.65 to -0.70, respectively). The small number of meaningful correlations means that sport scientists will have to develop more reliable methods, models, and theories to contribute significantly to knowledge useful to top athletes and their coaches.

Push-off mechanics in speed skating with conventional skates and klapskates.

PURPOSE: Personal and world records in speed skating improved tremendously after the introduction of the klapskate, which allows the foot to plantar flex at the end of the push-off while the full blade continues to glide on the ice. The purpose of this study was to gain insight into the differences in skating technique with conventional versus klapskates and to unveil the source of power enhancement using klapskates. METHODS: Ten elite speed skaters skated four 400-m laps at maximal effort with both conventional and klapskates. On the straight high-speed film, push-off force and EMG data were collected. An inverse dynamics analysis was performed in the moving reference plane through hip, knee, and ankle. RESULTS: Skating velocity increased 5% as a result of an increase in mean power output of 25 W when klapskates were used instead of conventional skates. The increase in mean power output was achieved through an 11-J increase in work per stroke and an increase in stroke frequency from 1.30 to 1.36 strokes x s(-1). The difference in work per stroke occurs during the final 50 ms of the push-off. This is the result of the ineffective way in which push-off forces are generated with conventional skates when the foot rotates about the long front end of the blade. No differences in muscle coordination were observed from EMG. CONCLUSION: A hinge under the ball of the foot enhances the effectiveness of plantar flexion during the final 50 ms of the push off with klapskates and increases work per stroke and mean power output.

Wheelchair racing: effects of rim diameter and speed on physiology and technique.

Effects of different hand rim diameters in wheelchair racing were studied with respect to physiological and technique parameters at five speed levels (N = 8 wheelchair sportsmen). In each of five subsequent 15-min exercise tests on a treadmill, a different sized hand rim was mounted to the rear wheels (0.3, 0.35, 0.38, 0.47, 0.56 m). In each test, speed increased with 0.83 m.s-1 every 3 min, starting at 0.83 m.s-1 and ranging up to 4.17 m.s-1 (slope: 0.5 degrees). Cardiorespiratory responses (ventilation, oxygen cost, heart rate, respiratory exchange ratio, mechanical efficiency) and timing data (cycle time, push time, recovery time, push angle, and work per cycle) were obtained every 3rd min, together with the movement pattern of trunk and arm segments. Clear effects of rim diameter and speed were seen for the physiological parameters (P less than 0.05). In physiological terms, D5 appeared the least beneficial, followed by D4. Moreover, increasing rim diameter had a significant effect on movement pattern of the upper arm in the sagittal as well the frontal plane of motion. However, no timing effects were seen with changing rim diameter. On the other hand, timing parameters varied markedly with speed, whereas the segmental excursions of the upper limb did not show a "speed-effect". In general, small hand rims show lower cardiorespiratory responses. This may be related to the decreased segmental excursions of the upper limb and the lower linear hand velocity. Together with a low rolling and air drag, heart rate, and oxygen cost, these are important prerequisites in racing events.(ABSTRACT TRUNCATED AT 250 WORDS)

Manual wheelchair propulsion: effects of power output on physiology and technique.

Eight wheelchair sportsmen conducted eight wheelchair exercise tests on a treadmill. Two workload strategies were followed: strategy 1--increments in speed at a constant slope and strategy 2--increments in slope at constant velocity. Thus, data on cardio-respiratory and propulsion technique parameters were obtained on two identical series of 16 speed and slope combinations. Between each two identical speed and slope combinations of strategies 1 and 2, a different workload history is apparent. A four-factor analysis of variance with repeated measures on the factors "strategy" (workload history), "speed," and "slope" was applied (P less than 0.05). No "strategy" effect was seen in the cardio-respiratory parameters (gross mechanical efficiency, ventilation, oxygen consumption, and heart rate), work/cycle, and cycle time. Thus, within the experimental set-up, workload history did not affect the parameters studied and 3-min workload periods appeared sufficiently long for experienced wheelchair users to adapt to the requirements of a given speed and slope combination. Significant effects were found on "speed," "slope," and their interaction in all parameters tested. Moreover, a comparison of two equal levels of power output, but different speed and slope, led to a significantly higher efficiency, cycle time, and work per cycle for the "low speed and high slope" combination. Push time and recovery time appeared highly dependent on speed and slope, respectively. The findings indicate that propulsion technique and cardio-respiratory parameters should not merely be studied in relation to power output, but also with respect to its constituents, speed, and slope/resistance.