Effect of body roll amplitude and arm rotation speed on propulsion of arm amputee swimmers

Only a limited amount of research has gone into evaluating the contribution made by the upper arm to the propulsion of elite swimmers with an amputation at elbow level. With assistance of computational fluid dynamics (CFD) modelling, the swimming technique of competitive arm amputee swimmers can be assessed through numerical simulations which test the effect of various parameters on the effectiveness of the swimming propulsion.This numerical study investigates the effect of body roll amplitude and of upper arm rotation speed on the propulsion of an arm amputee swimmer, at different mean swimming speeds. Various test cases are simulated resulting in a thorough analysis of the complex body/fluid interaction with a detailed quantitative assessment of the effect of the variation of each parameter on the arm propulsion. It is found that a body roll movement with an amplitude of 45° enhances greatly the propulsive contribution from the upper arm with an increase of about 70% in the propulsive force compared to the no roll condition. An increase in the angular velocity of the upper arm also leads to a concomitant increase in the propulsive forces produced by the arm.Such results have direct implications for competitive arm amputee front crawl swimmers and for those who coach them. One important message that emerges in this present work is that there exists, for any given swimming speed, a minimum angular velocity at which the upper arm must be rotated to generate effective propulsion. Below this velocity, the upper arm will experience a net resistive drag force which adversely affects swimming performance.     

Effect of leg kick on active drag in front-crawl swimming: Comparison of whole stroke and arms-only stroke during front-crawl and the streamlined position

The purpose of this study was to examine the effect of leg kick on the resistance force in front-crawl swimming. The active drag in front-crawl swimming with and without leg motion was evaluated using measured values of residual thrust (MRT method) and compared with the passive drag of the streamlined position (SP) for the same swimmers. Seven male competitive swimmers participated in this study, and the testing was conducted in a swimming flume. Each swimmer performed front-crawl under two conditions: using arms and legs (whole stroke: WS) and using arms only (arms-only stroke: AS). Active drag and passive drag were measured at swimming velocities of 1.1 and 1.3 m s⁻¹ using load cells connected to the swimmer via wires. We calculated a drag coefficient to compare the resistances of the WS, AS and SP at each velocity. For both the WS and AS at both swimming velocities, active drag coefficient was found to be about 1.6–1.9 times larger than that in passive conditions. In contrast, although leg movement did not cause a difference in drag coefficient for front-crawl swimming, there was a large effect size (d = 1.43) at 1.3 m s⁻¹. Therefore, although upper and lower limb movements increase resistance compared to the passive condition, the effect of leg kick on drag may depend on swimming velocity.     

The role of cutaneous feedback for anticipatory grip force adjustments during object movements and externally imposed variation of the direction of gravity

Grip force adjustments to changes of object loading induced by external changes of the direction of gravity during discrete arm movements with a grasped object were analyzed during normal and anesthetized finger sensibility. Two subjects were seated upright in a rotatable chair and rotated backwards into a horizontal position during discrete movements with a hand-held instrumented object. The movement direction varied from vertical to horizontal inducing corresponding changes in the direction of gravity, but the orientation of the movement in relation to the body remained unaffected. During discrete vertical movements a maximum of load force occurs early in upward and late in downward movements; during horizontal movements two load force peaks result from both acceleratory and deceleratory phases of the movement. During performance with normal finger sensibility grip force was modulated in parallel with fluctuations of load force during vertical and horizontal movements. The grip force profile adopted to the varying load force profile during the transition from the vertical to the horizontal position. The maximum grip force occurred at the same time of maximum load force irrespective of the movement plane. During both subjects' first experience of digital anesthesia the object slipped from the grasp during rotation to the horizontal plane. During the following trials with anesthetized fingers subjects substantially increased their grip forces, resulting in elevated force ratios between maximum grip and load force. However, grip force was still modulated with the movement-induced load fluctuations and maximum grip force coincided with maximum load force during vertical and horizontal movements. This implies that the elevated force ratio between maximum grip and load force does not alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits seems to be important for the economic scaling of the grip force magnitude according to the actual loading conditions and for reactive grip force adjustments in response to load perturbations. However, it plays a subordinate role for the precise anticipatory temporal coupling between grip and load forces during voluntary object manipulation.     

The role of cutaneous feedback for anticipatory grip force adjustments during object movements and externally imposed variation of the direction of gravity

Grip force adjustments to changes of object loading induced by external changes of the direction of gravity during discrete arm movements with a grasped object were analyzed during normal and anesthetized finger sensibility. Two subjects were seated upright in a rotatable chair and rotated backwards into a horizontal position during discrete movements with a hand-held instrumented object. The movement direction varied from vertical to horizontal inducing corresponding changes in the direction of gravity, but the orientation of the movement in relation to the body remained unaffected. During discrete vertical movements a maximum of load force occurs early in upward and late in downward movements; during horizontal movements two load force peaks result from both acceleratory and deceleratory phases of the movement. During performance with normal finger sensibility grip force was modulated in parallel with fluctuations of load force during vertical and horizontal movements. The grip force profile adopted to the varying load force profile during the transition from the vertical to the horizontal position. The maximum grip force occurred at the same time of maximum load force irrespective of the movement plane. During both subjects' first experience of digital anesthesia the object slipped from the grasp during rotation to the horizontal plane. During the following trials with anesthetized fingers subjects substantially increased their grip forces, resulting in elevated force ratios between maximum grip and load force. However, grip force was still modulated with the movement-induced load fluctuations and maximum grip force coincided with maximum load force during vertical and horizontal movements. This implies that the elevated force ratio between maximum grip and load force does not alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits seems to be important for the economic scaling of the grip force magnitude according to the actual loading conditions and for reactive grip force adjustments in response to load perturbations. However, it plays a subordinate role for the precise anticipatory temporal coupling between grip and load forces during voluntary object manipulation.     

The determination of drag in front crawl swimming

The measurement of drag while swimming (i.e. active drag) is a controversial issue. Therefore, in a group of six elite swimmers two active drag measurement methods were compared to assess whether both measure the same retarding force during swimming. In method 1 push-off forces are measured directly using the system to measure active drag (MAD-system). In method 2 (the velocity perturbation method, VPM) drag is estimated from the difference in swimming speed when subjects swim twice at maximal effort (assuming equal power output and assuming a quadratic drag-speed relationship): once swimming free, and once swimming with a hydrodynamic body attached that created a known additional resistance. The average drag for the VPM tests (53.2 N) was statistically significant and different from the active drag for the MAD-test (66.9 N), paired Student's t-test: 2.484, 12 DF, p=0.029. A post hoc analysis was performed to assess whether the two methods measure a different phenomenon. Based on the drag speed curve obtained with the MAD-system, the VPM-data were re-examined. For diverging drag determinations the assumption of equal power output of the 'free' trial (swimming free) vs. the towing trial (swimming with hydrodynamic buoy) appeared to be violated. The regression of the relative difference in force (MAD vs. VPM) on the relative difference in power (swimming free vs. swimming with hydrodynamic body) was: %Deltadrag=1.898 x %Deltapower -4.498, r2=0.88. This suggests that the major part of the difference in active drag values is due to a non-equal power output in the 'free' relative towing trial during the VPM-test. The simulation of the violation of the equal power output assumption and the calculation of the effect of an other than quadratic drag-speed relationship corroborated the tentative conclusion that both methods measure essentially the same phenomenon and that active drag differences can be explained by a violation of test assumptions.     

Stepping back to improve sprint performance: a kinetic analysis of the first step forwards

Using a step backward to initiate forward movement can increase force and power at push-off and improve sprint performance over short distances. However, it is not clear whether the benefit provided by this paradoxical step influences the mechanics of the first step forwards. Twenty-seven men of an athletic background performed maximal effort 5-m sprints from a standing start and employed a step forwards (parallel and split stance) or backwards (false) to initiate movement. Each sprint was started with an audio cue that also activated the timing gates. Three trials of each starting style were performed and movement (0 m), 2.5-, and 5-m times were recorded. An in-ground force plate placed at the 0-m mark measured the kinetic and temporal characteristics of the first step. Sprint times to 2.5 and 5 m were slower (p < 0.05) when a parallel start was used. No differences were seen in the normalized peak forces (vertical and horizontal) or the vertical impulse between starts, but the vertical mean force was 11 and 12% higher for the false and split starts, respectively. Surprisingly, the parallel start's impulse was significantly greater than that of the false (24%) and split (22%) styles, a consequence of the additional time spent in contact with the ground. The ground contact time, time to peak force, and time from peak force to toe-off (vertical and horizontal) were significantly longer for the parallel start. These temporal variables were also better correlated with sprint performance than any kinetic measure (0.42 ≤ r ≤ 0.75). The false start appears to be advantageous over short distances by improving push-off and the temporal characteristics of the first step.     

Swimming performance is reduced by reflective markers intended for the analysis of swimming kinematics

The present study aimed to clarify whether swimming performance is affected by reflective markers being attached to the swimmer’s body, as is required for a kinematic analysis of swimming. Fourteen well-trained male swimmers (21.1 ± 1.7 yrs) performed maximal 50 m front crawl swimming with (W) and without (WO) 25 reflective markers attached to their skin and swimwear. This number represents the minimum required to estimate the body’s center of mass. Fifty meter swimming time, mid-pool swimming velocity, stroke rate, and stroke length were determined using video analysis. We found swimming time to be 3.9 ± 1.6% longer for W condition. Swimming velocity (3.3 ± 1.8%), stroke rate (1.2 ± 2.0%), and stroke length (2.1 ± 2.7%) were also significantly lower for W condition. To elucidate whether the observed reduction in performance was potentially owing to an additional drag force induced by the reflective markers, measured swimming velocity under W condition was compared to a predicted velocity that was calculated based on swimming velocity obtained under WO condition and an estimate of the additional drag force induced by the reflective markers. The mean prediction error and ICC (2,1) for this analysis of measured and predicted velocities was 0.014 m s⁻¹ and 0.894, respectively. Reducing the drag force term led to a decrease in the degree of agreement between the velocities. Together, these results suggest that the reduction in swimming performance resulted, at least in part, from an additional drag force produced by the reflective markers.     

A simple method for measuring force, velocity and power output during squat jump

Our aim was to clarify the relationship between power output and the different mechanical parameters influencing it during squat jumps, and to further use this relationship in a new computation method to evaluate power output in field conditions. Based on fundamental laws of mechanics, computations were developed to express force, velocity and power generated during one squat jump. This computation method was validated on eleven physically active men performing two maximal squat jumps. During each trial, mean force, velocity and power were calculated during push-off from both force plate measurements and the proposed computations. Differences between the two methods were not significant and lower than 3% for force, velocity and power. The validity of the computation method was also highlighted by Bland and Altman analyses and linear regressions close to the identity line (P<0.001). The low coefficients of variation between two trials demonstrated the acceptable reliability of the proposed method. The proposed computations confirmed, from a biomechanical analysis, the positive relationship between power output, body mass and jump height, hitherto only shown by means of regression-based equations. Further, these computations pointed out that power also depends on push-off vertical distance. The accuracy and reliability of the proposed theoretical computations were in line with those observed when using laboratory ergometers such as force plates. Consequently, the proposed method, solely based on three simple parameters (body mass, jump height and push-off distance), allows to accurately evaluate force, velocity and power developed by lower limbs extensor muscles during squat jumps in field conditions.     

An estimation of drag in front crawl swimming

Propulsive arm forces of twelve elite male swimmers during a front crawl swimming-like activity were measured. The swimmers pushed off against grips which are attached to a 23 m tube at 0.8 m under the water surface. The tube was fixed to a force transducer. Since at constant speed, mean propulsive force equals mean drag force this method also provides the mean active drag on a moving swimmer. The mean propulsive force at a speed of v = 1.48 m s-1 appeared to be 53.2 +/- 5.8 N which is two to three times smaller than what is reported by other authors for active drag but which is in agreement with values reported for passive drag on a (towed) swimmer who is not moving. Discrepancies with indirect active drag measurements are discussed.     

Syn vivo hydrostatic and hydrodynamic properties of scaphitid ammonoids from the U.S. Western Interior

Scaphitid ammonoids were ubiquitous and significant components of the Western Interior Seaway during the Late Cretaceous. This group is characterized by a recurved hook at maturity that deviates from the juvenile whorls. Such a modification seems counterproductive to active locomotion and to manage a biologically effective orientation that facilitates efficient feeding and swimming. Virtually reconstructed 3D hydrostatic models reveal that the examined mature scaphitids had the capacity for neutral buoyancy while assuming a stable, upward-facing orientation in the water column during life. Models of juvenile Hoploscaphites nicolletii suggest that scaphitid apertures were oriented only slightly more horizontal than adults. The hydrostatic influence of sexual dimorphism was explored with the species Hoploscaphites crassus. The inflated macroconch has a lower stability and higher hydrodynamic drag compared to its microconch counterpart. The effect of shell compression was investigated by comparing H. crassus and the more compressed H. nicolletii. The latter species has a relatively high stability and much less hydrodynamic drag during movement. The mature U-shaped body chamber distributes organismal mass in a way that increases stability, and simultaneously orients the soft body so that propulsive energy is efficiently transmitted into horizontal backwards movement with minimal rocking. Swimming velocities computed from hydrodynamic drag experiments suggest that scaphitids were relatively slow swimmers with compressed forms attaining slightly higher velocities (when scaled by mass). Hydrodynamic lift was investigated with computational fluid dynamics simulations. These experiments revealed that the overall shape of the shell is responsible for significant lift in the upwards direction, which is not heavily influenced by ornamentation. This explains how a reduced soft body can overcome and manage a slightly negatively buoyant condition during life. Therefore, the seemingly cumbersome shape and orientation of the scaphitid morphotype may not have been a hindrance during locomotion.     

Locomotion of bluefish.

1. Pressure previously measured on the body surface of swimming bluefish were resolved into their backward vectorial components to allow calculation of profile drag. It was 0.18 kg at a speed of 1.8 m/sec. Tangential drag was calculated as if for a thin plate of an area equal to that of the fish. It was 0.08 kg at 1.8 m/sec. Net drag, 0.26 kg, was the sum of profile and tangential drag. 2. Thrust and drag also were calculated from the changes of acceleration measured during steady swimming, assuming that thrust took place only during the acceleration phase, whereas drag occurred during both acceleration and deceleration. This drag was 0.08 kg at a speed of 1.1 m/sec. It is compatible with the drag of 0.26 at 1.8 m/sec calculated from profile and tangential drag provided drag varies as the square of velocity. 3. The force required to produced maximal acceleration was measured during a scare. It was calculated to be 6.9 kg at a peak acceleration of 3 g. 4. The compression strength of th vertebrae was found to be approximately 20 kg per cm2, or roughly three times the force encountered during maximal acceleration. This safety factor of 3 would be reduced when the back was curved, or if opposing groups of muscles were under tension. 5. The finding that a bluefish can accelerate at 3 g and that the vertebral column is strongg enough to withstand this force indicates that the muscles and body structure of a bluefish would be able to withstand the force of gravity if the fish were otherwise equipped for terrestrial life. This fish may have evolved these strengths simultaneously with land animals. It is speculated that other fish may have evolved some degree of strength to overcome inertia and drag during aquatic locomotion, and this evolution may have been a prelude to terrestrial locomotion.     

The effect of swimmer's hand/forearm acceleration on propulsive forces generation using computational fluid dynamics

Propulsive forces generated by swimmers hand/forearm, have been studied through experimental tests. However, there are serious doubts as to whether forces quantified in this way are accurate enough to be meaningful. In order to solve some experimental problems, some numerical techniques have been proposed using Computational Fluid Dynamics (CFD). The main purpose of the present work was threefold. First, disseminate the use of CFD as a new tool in swimming research. Second, apply the CFD method in the calculation of drag and lift coefficients resulting from the numerical resolution equations of the flow around the swimmers hand/forearm using the steady flow conditions. Third, evaluate the effect of hand/forearm acceleration on drag and lift coefficients. For these purposes three, two-dimensional (2D), models of a right male hand/forearm were studied. A frontal model (theta = 90 degrees, Phi = 90 degrees) and two lateral models, one with the thumb as leading edge (theta = 0 degrees, = 90 degrees), and the other with the small finger as the leading edge (theta = 0 degrees, Phi = 180 degrees). The governing system of equations considered was the incompressible Reynolds averaged Navier-Stokes equations with the standard k-epsilon model. The main results reported that, under the steady-state flow condition, the drag coefficient was the one that contributes more for propulsion, and was almost constant for the whole range of velocities, with a maximum value of 1.16 (Cd = 1.16). This is valid when the orientation of the hand/forearm is plane and the model is perpendicular to the direction of the flow. Under the hand /forearm acceleration condition, the measured values for propulsive forces calculation were approximately 22.5% (54.440 N) higher than the forces produced under the steady flow condition (44.428 N). By the results, pointed out, we can conclude that: (i) CFD can be considered an interesting new approach for hydrodynamic forces calculation on swimming, (ii) the acceleration of hand/forearm provides more propulsion to swimmers, confirming that some unsteady mechanism must be present in swimming propulsion.     

Active drag related to velocity in male and female swimmers

Propulsive arm forces of 32 male and 9 female swimmers were measured during front crawl swimming using arms only, in a velocity range between 1.0 m s-1 and 1.8 m s-1. At constant velocity, the measured mean propulsive force Fp equals the mean active drag force (Fd). It was found that Fd is related to the swimming velocity v raised to the power 2.12 +/- 0.20 (males) or 2.28 +/- 0.35 (females). Although many subjects showed rather constant values of Fd/v2, 12 subjects gave significantly (p less than 0.01) stronger or weaker quadratic relationships. Differences in drag force and coefficient of drag between males and females (drag: 28.9 +/- 5.1 N, 20.4 +/- 1.9 N, drag coefficient: 0.64 +/- 0.09, 0.54 +/- 0.07 respectively) are especially apparent at the lowest swimming velocity (1 m s-1), which become less at higher swimming velocities. Possible explanations for the deviation of the power of the velocity from the ideal quadratic dependency are discussed.     

A computerized spectrophotometric instrumental system to determine the "vertical velocity" of sperm cells: a novel concept.

BACKGROUND: The presently available cell motility-analyzers measure primarily the "horizontal" velocity and there is no instrument available for "vertical" velocity measurement. This development was based on the turbidimetric method of sperm motility analysis. METHODS: Sperm was layered at the bottom of the cuvette containing buffer solution and exposed to the spectrophotometric light path at different heights to track the vertically moving sperms. The vertical movement was materialized with the development of an electromechanical up-down movement devise for the cuvette accomplished with the help of a cuvette holder-stepper motor-computer assembly. The entire system was controlled by the necessary motion control, data acquisition, and data processing software developed for cuvette movement and data analysis. RESULTS: Using goat sperm as the model a unique computer-based spectrophotometric system has been developed for the first time to determine the average "vertical" velocity of motile cells. CONCLUSIONS: Undertaking upward movement against gravity is much tougher as compared with horizontal movement. Consequently average vertical velocity is expected to be a much better identifying parameter for assessing semen and other motile cell quality. The novel instrumental system developed by us has thus the potential for immense application in human infertility clinics, animal-breeding centres, centres for conservation of endangered species, and also for research work on vertical velocity of spermatozoa and other motile cells, such as bacteria, protozoa, etc.     

Using reverse engineering and computational fluid dynamics to investigate a lower arm amputee swimmer's performance

In front crawl swimming, the hand and the corresponding forearm generate major propulsive forces. Such forces have been studied largely through experimental tests and more recently through the use of steady computational fluid dynamics (CFD). However, the effect of the upper arm on the propulsive forces has generally not been taken into consideration. An understanding of such forces is fundamental for the performance of swimmers who have an arm amputation at the level of the elbow. This study introduces the great potential offered by the multidisciplinary approach combining reverse engineering and unsteady CFD in a novel dynamic and interactive way. A complex CFD mesh model, representing the swimmer body and its upper arm, is produced. The model, including the arm rotation and a body roll movement, interacts dynamically with the fluid flow. Forces generated by the upper arm can then be investigated in great detail. In this particular study, it is found that the upper arm effectively contributes to the propulsion of the body. The propulsive force was numerically computed throughout the pull and reaches maxima of 8N. Results obtained in this study could be extended in a similar way to any other limb movement within a fluid flow.     

Starting from standing; why step backwards?

At push-off, the mass centre of gravity of the body must be positioned in front of the foot to prevent a somersault. When starting a sprint from out the standing position the use of a step backwards is necessary for maximal acceleration. The aim of the present study was to quantify the positive contribution to push off from a backward step of the leg, which seems to be counterproductive. Ten subjects were instructed to sprint start in three different ways: (a) starting from the standing position just in front of the force platform on the subject's own initiative, (b) starting from the standing position on the force platform with no step backward allowed, and (c) starting out of the starting position with one leg in front of the force platform and the push-off leg on the force platform. A step backwards was observed in 95% of the starts from the standing position. The push-off force was highest in starting type (a), which had the shortest time to build up the push-off force. The results indicate a positive contribution to the force and power from a step backwards. We advocate developing a training program with special attention to the phenomenon step backwards.     

计算流体力学途径揭示寒武纪乌溜期吐卓虫(Tuzoia)种间游动能力分化图景*

吐卓虫(Tuzoia)是寒武纪出现且广泛分布于全球海洋中的一类具双瓣壳节肢动物, 其软躯体性状不完备, 并不能直观判断吐卓虫是否是一种擅长游泳的生物, 因此其生态位长期存在争议。计算流体力学(Computational Fluid Dynamics, CFD)可以基于保存较好的生物骨骼化石, 直观、量化地分析深时生物运动模式, 为还原古生态提供了依据。本文采用计算流体力学模拟了加拿大布尔吉斯页岩生物群中的两种吐卓虫(T. retifera, T. canadensis)在底栖和远洋环境下的多种运动状态, 对比分析了两种吐卓虫运动流体性能、垂向灵活性的差异。流体模拟结果表明, 相同速度下T. retifera相比T. canadensis受到更小的阻力和更大的浮力。所以T. retifera具有更高游泳速度的潜力, 同时垂向移动能力更好。计算结果暗示T. canadensis营底栖游泳生活, 而T. retifera的栖息范围可以更广, 营远洋游泳生活。这表明可能早在寒武纪乌溜期吐卓虫属级分类单元内就出现了生态位的分化。     

Morphological correlates of river velocity and reproductive development in an ornamented stream fish

Adaptive phenotypic divergence can arise when environments vary in ways favoring alternative phenotypic optima. In aquatic habitats, the costs of locomotion are expected to increase with water velocity, generally favoring a more streamlined body and the reduction of traits that produce drag. However, because streamlining in fish may come at the cost of maneuverability, the net benefits of drag reduction can differ not only among habitats, but also among individuals (or classes of individuals) that rely on locomotion for different uses (e.g., males vs. females or adults vs. juveniles). We tested these predictions by exploring relationships among river velocity, body streamlining, ornamental fin size, and male reproductive condition in the steelcolor shiner (Cyprinella whipplei), a small-bodied North American cyprinid. Overall, males in peak reproductive condition (defined by the development of sexually dimorphic tubercles) had less streamlined bodies and larger ornamental fins than males in lower reproductive condition or individuals lacking these secondary sexual characters (females and immature males). There was a relationship between river velocity and body streamlining only for males in peak reproductive condition, but it was in the opposite direction of our predictions: these males were less streamlined in faster rivers. We found only weak support for the prediction that ornamental fin size would be negatively associated with river velocity. Overall, these results suggest either that drag is not an important selective pressure in these habitats, or that the sexual selection advantages of a deep body and large fin compensate any natural selection costs for C. whipplei males. This study highlights the often overlooked diversity of selective pressures acting on streamlining in fishes, and can offer novel insights and predictions allowing a more nuanced understanding of fish ecomorphology.     

The Temporal Structure of Vertical Arm Movements

The present study investigates how the CNS deals with the omnipresent force of gravity during arm motor planning. Previous studies have reported direction-dependent kinematic differences in the vertical plane; notably, acceleration duration was greater during a downward than an upward arm movement. Although the analysis of acceleration and deceleration phases has permitted to explore the integration of gravity force, further investigation is necessary to conclude whether feedforward or feedback control processes are at the origin of this incorporation. We considered that a more detailed analysis of the temporal features of vertical arm movements could provide additional information about gravity force integration into the motor planning. Eight subjects performed single joint vertical arm movements (45° rotation around the shoulder joint) in two opposite directions (upwards and downwards) and at three different speeds (slow, natural and fast). We calculated different parameters of hand acceleration profiles: movement duration (MD), duration to peak acceleration (D PA), duration from peak acceleration to peak velocity (D PA-PV), duration from peak velocity to peak deceleration (D PV-PD), duration from peak deceleration to the movement end (D PD-End), acceleration duration (AD), deceleration duration (DD), peak acceleration (PA), peak velocity (PV), and peak deceleration (PD). While movement durations and amplitudes were similar for upward and downward movements, the temporal structure of acceleration profiles differed between the two directions. More specifically, subjects performed upward movements faster than downward movements; these direction-dependent asymmetries appeared early in the movement (i.e., before PA) and lasted until the moment of PD. Additionally, PA and PV were greater for upward than downward movements. Movement speed also changed the temporal structure of acceleration profiles. The effect of speed and direction on the form of acceleration profiles is consistent with the premise that the CNS optimises motor commands with respect to both gravitational and inertial constraints.