Limb Acceleration Research Articles

Effect of Speed and Gender on Rate of Velocity Development Quadriceps/Hamstrings Ratios

PURPOSE: There is speculation that the ratio of quadriceps to hamstrings rate of velocity development (RVD) differs between genders and speeds due to the male bias in limb movement speed. The purpose of this study was to determine if males and females demonstrated different lower body muscle group RVD ratios when testing across a velocity spectrum. METHODS: 119 male (n=57) and female (n=62) university kinesiology students (age 23.29±2.13 years, height 170.98±10.25 cm, weight 71.29±16.15 kg) performed 5 maximal reciprocal concentric/concentric knee extension and flexion repetitions on a Biodex System 3 isokinetic dynamometer at 13 random speeds between 30 and 400 d/s. RVD is the measured ROM traveled from 0 to the target velocity, therefore, a lower value equates to greater limb acceleration. Quadriceps/hamstrings ratios were determined by dividing peak extension RVD by peak flexion RVD and multiplying by 100. ANOVA was used to determine the differences between male and female ratios, as well as the differences of the ratios across the velocity spectrum. RESULTS: Males and females demonstrated no significant differences in peak quadriceps/hamstrings RVD ratios when compared across speeds. However, there was a significant (p <0.05) main effect for speed on the RVD ratios. As testing speeds increased, the quadriceps/hamstrings RVD ratios decreased. Ratios decreased from 76.97%+19.31 to 64.10%+6.73 between 30 and 400 d/s respectively. CONCLUSION These results demonstrate that although males had less RVD at all speeds, there was no difference in RVD ratios by gender. The data also indicate that peak RVD in the hamstrings and quadriceps increases with increasing velocity while the ratios decline. This is due to the fact that the hamstrings slow down at a greater rate than the quadriceps causing the ratio to decrease. This is contrast to the increasing ratios seen in torque development. Therefore, training protocols may need to be moderated at very high velocities to insure that hamstring exercise is within the velocity range capability.

Prolonged Vibration of the Biceps Brachii Tendon Reduces Time to Failure When Maintaining Arm Position With a Submaximal Load

Vibration reduces the amplitudes of the tendon jerk response and the Hoffmann and stretch reflexes in the muscle exposed to the vibration, yet does not alter the time to task failure when the task involves exerting a submaximal force against a rigid restraint. Because the amplitude of the stretch reflex is greater when a limb acts against a compliant load than a rigid restraint, the purpose was to determine the influence of prolonged tendon vibration on the time to failure when maintaining limb position with the elbow flexor muscles. Twenty-five healthy men performed the fatiguing contraction by maintaining elbow angle at 1.57 rad until failure while supporting a load equal to 20% of maximal voluntary contraction (MVC) force. The fatiguing contraction was performed on 3 separate days with different levels of vibration applied to the biceps brachii tendon: no vibration, subthreshold for a tonic vibration reflex (TVR), and suprathreshold for a TVR. MVC force before the fatiguing contraction was similar across the three sessions (mean of 3 sessions: 313 +/- 54 N, P = 0.83). Despite a similar decline in MVC force after the fatiguing contraction across conditions (-18.0 +/- 8.0%, P > 0.05), the time to task failure was 3.7 +/- 1.4 min for the suprathreshold TVR condition, 4.3 +/- 2.1 min for the subthreshold TVR condition, and 5.0 +/- 2.2 min for the no-vibration condition (P < 0 0.001). The average EMG of the elbow flexor muscles was similar (P = 0.22) during the fatiguing contractions. However, the fluctuations in limb acceleration at task onset were greater for the suprathreshold TVR condition (P < 0.01), but were not different between the subthreshold TVR and no-vibration conditions (P > or = 0.22). Furthermore, the difference in the SD of limb acceleration between the no-vibration and vibration conditions was correlated with the difference in time to failure for the no-vibration and subthreshold TVR conditions (P = 0.03; r2 = 0.22), but not for the no-vibration and suprathreshold TVR conditions (P = 0.90; r2 = 0.001). These findings indicate that prolonged vibration reduced the time to failure of a sustained contraction when subjects maintained limb position, suggesting that peripheral inputs to the motor neuron pool play a significant role in sustaining a contraction during tasks that require active control of limb position.

Internal models of limb dynamics and the encoding of limb state

Studies of reaching suggest that humans adapt to novel arm dynamics by building internal models that transform planned sensory states of the limb, e.g., desired limb position and its derivatives, into motor commands, e.g., joint torques. Earlier work modeled this computation via a population of basis elements and used system identification techniques to estimate the tuning properties of the bases from the patterns of generalization. Here we hypothesized that the neural representation of planned sensory states in the internal model might resemble the signals from the peripheral sensors. These sensors normally encode the limb's actual sensory state in which movement errors occurred. We developed a set of equations based on properties of muscle spindles that estimated spindle discharge as a function of the limb's state during reaching and drawing of circles. We then implemented a simulation of a two-link arm that learned to move in various force fields using these spindle-like bases. The system produced a pattern of adaptation and generalization that accounted for a wide range of previously reported behavioral results. In particular, the bases showed gain-field interactions between encoding of limb position and velocity, very similar to the gain fields inferred from behavioral studies. The poor sensitivity of the bases to limb acceleration predicted behavioral results that were confirmed by experiment. We suggest that the internal model of limb dynamics is computed by the brain with neurons that encode the state of the limb in a manner similar to that expected of muscle spindle afferents.

Prechtl's assessment of general movements: A diagnostic tool for the functional assessment of the young nervous system

General movements (GMs) are part of the spontaneous movement repertoire and are present from early fetal life onwards until the end of the first half a year of life. GMs are complex, occur frequently, and last long enough to be observed properly. They involve the whole body in a variable sequence of arm, leg, neck, and trunk movements. They wax and wane in intensity, force and speed, and they have a gradual beginning and end. Rotations along the axis of the limbs and slight changes in the direction of movements make them fluent and elegant and create the impression of complexity and variability. If the nervous system is impaired, GMs loose their complex and variable character and become monotonous and poor. Two specific abnormal GM patterns reliably predict later cerebral palsy: 1) a persistent pattern of cramped-synchronized GMs. The movements appear rigid and lack the normal smooth and fluent character. Limb and trunk muscles contract and relax almost simultaneously. 2) The absence of GMs of fidgety character. So-called fidgety movements are small movements of moderate speed with variable acceleration of neck, trunk, and limbs in all directions. Normally, they are the predominant movement pattern in an awake infant at 3 to 5 months. Beside a sensitivity and specificity of 95% each, the assessment of GMs is quick, noninvasive, even nonintrusive, and cost-effective compared with other techniques, e.g., magnetic resonance imaging, brain ultrasound, and traditional neurological examination.

Daily physical activity assessment: what is the importance of upper limb movements vs whole body movements?

The movement of the upper limbs (eg fidgeting-like activities) is a meaningful component of nonexercise activity thermogenesis (NEAT). This study examined the relationship between upper limb movements and whole body trunk movements, by simultaneously measuring energy expenditure during the course of the day. A cross-sectional study consisting of 88 subjects with a wide range in body mass index (17.3-32.5 kg/m(2)). The energy expenditure over a 24-h period was measured in a large respiratory chamber. The body movements were assessed by two uniaxial-accelerometers during daytime, one on the waist and the other on the dominant arm. The accelerometry scores from level 0 (=immobile) up to level 9 (=maximal intensity) were recorded. The activities of subjects were classified into eight categories: walking at two speeds on a horizontal treadmill (A & B), ambling (C), self-care tasks (D), desk work (E), meals (F), reading (G), watching TV (H). There was a significant relationship between the accelerometry scores from the waist (ACwaist) and that from the wrist (ACwrist) over the daytime period (R(2)=0.64; P<0.001). The ACwrist was systematically higher than the ACwaist during sedentary activities, whereas it was the reverse for walking activities. ACwrist to ACwaist ratio of activities E-H were above 1.0 and for walking activities (A-C) were below 1.0. A multiple regression analysis for predicting daytime energy expenditure revealed that the explained variance improved by 2% only when the ACwrist was added as a second predictor in addition to the ACwaist. This indicates that the effect of the ACwrist for predicting energy expenditure was of limited importance in our conditions of measurement. The acceleration of the upper limbs which includes fidgeting is more elevated than that of the whole body for sitting/lying down activities. However, their contribution to energy expenditure is lower than whole body trunk movements, thus indicating that the weight-bearing locomotion activities may be a key component of NEAT. However, its contribution may depend on the total duration of the upper limb movements during the course of the day.

Initiation of movement from quiet stance: comparison of gait and stepping in elderly subjects of different levels of functional ability

This study describes how elderly subjects initiate gait, and step from a position of quiet stance. Based on scores from selected standardized tests subjects were placed in either a high (HFL) or low functional level (LFL) group and were asked to initiate gait, step onto a 10 cm high, 1.22 m wide curb and step over a 10 cm high, 9 cm wide obstacle at a self paced speed. Stepping conditions affected the velocity of movement. It was clear that all subjects decreased initiation velocity for both curb and obstacle compared to gait initiation. Swing and stance limb acceleration ground reaction forces and EMG amplitude were modulated according to initiation velocity. Toe clearance was greater for obstacle than curb and gait initiation. Swing toe-off was significantly earlier and there was a trend for obstacle clearance to be greater for the HFL group. Those in the LFL group appear to be at a greater risk for falling due to the possible effect of slower rate of toe-off that could influence toe clearance over the obstacle.

Measures of Steadiness Are influenced by the Frequency Content of the Signals

0077 Fluctuations in limb acceleration during a steady contraction are often not associated with the fluctuations in limb position. Although the two signals often have two major peaks in the frequency range of 0–4 Hz (low frequency) and 8–12 Hz (high frequency), the low-frequency component is more prominent in the position signal. PURPOSE: To determine the influence of the low-frequency component on the association between simulated fluctuations in position and acceleration. METHODS: Position signals (20 s at 1024 Hz) were generated by summing 1,000 sinusoidal waves of varying frequency (< 15 Hz) and phase. The amplitude of each sinusoidal wave was adjusted so that the power of the low-frequency component in the position signal varied, whereas the amplitude of the high-frequency component remained constant. The position signal was double-differentiated to yield the acceleration signal. The low-frequency component was varied from 0.24 to 93.96% of the total power in the frequency spectrum. Fifty pairs of position and acceleration signals were simulated at each level of low-frequency power. Correlation coefficients between the standard deviations of position and acceleration were calculated at each level. RESULTS: The low- and high-frequency power in the acceleration signal ranged from 0.006% to 5.70% and from 86.77% to 79.56% total power, respectively. The correlation coefficients between the standard deviation of position and acceleration was distributed from −0.184 to 0.967. The correlation coefficient was high when the low-frequency component was small and it decreased continuously with an increase in low-frequency power of each signal: up to ∼ 60% in position and ∼ 0.5% in acceleration. For the signals with greater low-frequency power, the correlation coefficient was low (< 0.4) and variable. CONCLUSION: The results suggest that the association between the two measures of steadiness (fluctuations in position and acceleration) depend primarily on small changes in low-frequency power in the signals, particularly the acceleration signal.

Contribution of Inertia on Venous Flow in the Lower Limb during Stationary Gait

Blood flows toward the heart through collapsible vessels, the veins. The equations of flow in collapsible tubes in motion show a strong dependence on body forces resulting from gravity and acceleration. This paper analyzes the contribution of body forces to venous blood flow during walking on level ground. It combines the biomechanics of gait and theory of collapsible tubes to point out that body forces due to gravity and limb acceleration cannot be overlooked when considering the determinants of venous blood flow during locomotion. The study involved the development of a kinematic model of the limb as a multi-pendulum arrangement in which the limb segments undergo angular displacements. Angular velocities and accelerations were determined and the body forces were calculated during various phases of the gait cycle. A vascular model of the leg's major venous system was also constructed, and the accelerations due to body and gravity forces were calculated in specific venous segments, using the data from the kinematic model. The results showed there were large, fast variations in the axial component (Gx–Mx) of the body forces in veins between the hip and the ankle. Acceleration peaks down to –2G were obtained at normal locomotion. At fast locomotion, a distal vein in the shank displayed values of (Gx–Mx)/G equal to –3.2. Given the down-to-up orientation of the x-axis, the axial component Mx was usually positive in the axial veins, and Mx could shift from positive to negative during the gait cycle in the popliteal vein and the dorsal venous arch.

What Causes the Body to Roll in Front-Crawl Swimming?

This study was conducted to describe the kinematics of bodyroll and investigate whether bodyroll was propelled primarily by the turning effect of the fluid forces (external torque) or by the reaction effect due to the acceleration of the limbs. The performances of 11 competitive swimmers were recorded using two panning periscopes, and the three-dimensional movement of the subjects was reconstructed from digitized video recordings. The external torque acting on the whole body was determined as the first time-derivative of the angular momentum of the whole body. The reaction effect of limb acceleration was determined as the first time-derivative of the angular momenta of the limbs. Shoulder roll and hip roll angles changed synchronously with the stroke frequency but their amplitudes were substantially different, indicating that the bodyroll consisted of a roll of the entire torso and a twist of the torso. The overall contribution of the external torque was to propel bodyroll, while that of the reaction effects of limb accelerations was to resist bodyroll. These results clearly indicate that the primary source for propelling bodyroll was the external torque. Implications towards the mechanical interactions among bodyroll, stroke frequency, and forward propulsion in front crawl swimming were discussed.

Activation of the human diaphragm during a repetitive postural task.

The co-ordination between respiratory and postural functions of the diaphragm was investigated during repetitive upper limb movement. It was hypothesised that diaphragm activity would occur either tonically or phasically in association with the forces from each movement and that this activity would combine with phasic respiratory activity. Movements of the upper limb and ribcage were measured while standing subjects performed repetitive upper limb movements 'as fast as possible'. Electromyographic (EMG) recordings of the costal diaphragm were made using intramuscular electrodes in four subjects. Surface electrodes were placed over the deltoid and erector spinae muscles. In contrast to standing at rest, diaphragm activity was present throughout expiration at 78 +/- 17% (mean +/- s.d.) of its peak inspiratory magnitude during repeated upper limb movement. Bursts of deltoid and erector spinae EMG activity occurred at the limb movement frequency (approximately 2.9 Hz). Although the majority of diaphragm EMG power was at the respiratory frequency (approximately 0.4 Hz), a peak was also present at the movement frequency. This finding was corroborated by averaged EMG activity triggered from upper limb movement. In addition, diaphragm EMG activity was coherent with ribcage motion at the respiratory frequency and with upper limb movement at the movement frequency. The diaphragm response was similar when movement was performed while sitting. In addition, when subjects moved with increasing frequency the peak upper limb acceleration correlated with diaphragm EMG amplitude. These findings support the argument that diaphragm contraction is related to trunk control. The results indicate that activity of human phrenic motoneurones is organised such that it contributes to both posture and respiration during a task which repetitively challenges trunk posture.

Wrist action affects precision grip force.

When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangential to the object (load force). We investigated whether voluntary wrist actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with wrist angular speed during wrist motion in the horizontal plane, and was much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. During wrist flexion the index finger muscles in the hand and forearm increased myoelectric activity; during wrist extension this myoelectric activity increased little, or decreased for some subjects. The grip force maxima coincided with wrist acceleration maxima, and grip force remained elevated when subjects held the wrist in extreme flexion or extension. Likewise, during isometric wrist actions the grip force increased even though the fingertip loads remained constant. A grip force "pulse" developed that increased with wrist force rate, followed by a static grip force while the wrist force was sustained. Subjects could not suppress the grip force pulse when provided visual feedback of their grip force. We conclude that the extrinsic hand muscles can be recruited to assist the intended wrist action, yielding higher grip-load ratios than those employed with the wrist at rest. This added drive to hand muscles overcame any loss in muscle force while the extrinsic finger flexors shortened during wrist flexion motion. During wrist extension motion grip force increases apparently occurred from eccentric contraction of the extrinsic finger flexors. The coactivation of hand closing muscles with other wrist muscles also may result in part from a general motor facilitation, because grip force increased during isometric knee extension. However, these increases were related weakly to the knee force. The observed muscle coactivation, from all sources, may contribute to grasp stability. For example, when transporting grasped objects, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingertips from the object that must be offset by increased grip force. The muscle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wrist forces are required, such as for slow movements or when the object's total resistive load is small.

Fast actions in small animals: springs and click mechanisms

Small animals that jump or perform predatory strikes depend on much higher limb accelerations than larger animals. To overcome the temporal restrictions of muscle contraction, some arthropod muscles slowly load spring-like structures with potential energy. In flight, sound generation, jumping, or predatory strikes arthropods employ different strategies to transform muscular action to the desired movement. Click mechanisms control the frequency of oscillating spring — muscle systems while other accessory structures such as snap mechanism or latches with trigger mucles determined the stability and control the timing of the instantaneous discharge in catapult mechanisms.

The effect of velocity and gender on load range during knee extension and flexion exercise on an isokinetic device.

Limb acceleration and deceleration during exercise on an isokinetic device encounter no machine-offered resistance. The purpose of this study was to quantify the relationship between velocity and range of motion that is sustained at a predetermined isokinetic velocity, termed load range, during concentric knee extension and flexion exercise. Nine male and nine female subjects performed three maximal concentric reciprocal knee extension and flexion repetitions at 60, 120, 180, 240, 360, and 450 degrees/sec. Extension and flexion results revealed a significant (p < 0.05) increase in acceleration and deceleration range of motion while load range significantly decreased with increasing velocity. Males exhibited greater load range and less acceleration range of motion than females at 240, 360, and 450 degrees/sec, while deceleration range of motion was not different between genders at any speed. These results demonstrate an inverse relationship between isokinetic velocity and load range and suggest a need to carefully consider velocity selection when performing exercise on an isokinetic device.

The relationship between subjective knee scores, isokinetic testing, and functional testing in the ACL-reconstructed knee.

It is important to examine the functional relationships between commonly performed clinical tests and to resolve inconsistencies in previous investigative results. The purpose of this study was to determine if a correlation exists between three commonly performed clinical tests: isokinetic isolated knee concentric muscular testing, the single-leg hop test, and the subjective knee score in anterior cruciate ligament reconstructed knees. To determine if a relationship exists would be beneficial to clinicians in determining patient progression, treatment modification, and return-to-sport objective parameters. Several investigators have analyzed two of these parameters, but no one has investigated three parameters to date. Additionally, this study explored the concept of limb acceleration and deceleration during high-speed isokinetics and its relationship to function. Fifty patients were randomly selected (29 males) with a mean age of 23.7 years (range 15-52). The subjects completed a subjective knee score questionnaire that rated symptoms (pain, swelling, giving way) and specific sport function and completed an overall knee score assessment. The patients were then evaluated performing three one-legged functional tests: 1) hop for distance, 2) timed hop, and 3) cross-over triple hop. Isokinetic testing was performed on a Biodex dynamometer at 180, 300, and 450 degrees/sec for knee extension/flexion. The patients' mean value of the self-assessed knee rating was 86 points. Sixty-four percent of the patients exhibited normal limb symmetry (within 85%) on all three single-leg hop tests. Sixteen percent exhibited quadriceps strength at least 90% of the contralateral limb isokinetically. A positive correlation was noted between isokinetic knee extension peak torque (180, 300 degrees/sec) and subjective knee scores, and the three hop tests (p < 0.001). A statistical trend was noted between knee extension acceleration and deceleration range at 180 and 300 degrees/sec for the timed hop test and triple cross-over hop (r = 0.48, r = 0.49, r = 0.51, r = 0.49). No positive correlations were found for isokinetic test results for the knee flexors.

The transition between walking and running in humans: metabolic and mechanical aspects at different gradients.

Five subjects walked and ran at overlapping speeds and different gradients on a motorized treadmill. At each gradient the speed was obtained at which walking and running have the same metabolic cost (Sm) and the speed of spontaneous (Ss) transition between the two gaits was measured. Ss was found to be statistically lower than Sm at all gradients, the difference being in the range of 0.5-0.9 km h-1. The motion analysis of walking reveals that at all gradients and at increasing speed: (1) the percentage of recovery, an index of mechanical energy saving related to the pendulum-like characteristic of walking, decreases; (2) the lower limb spread reaches a limit in walking; and consequently (3) both the stride frequency and the internal mechanical work, due to limb acceleration in relation to the body centre of mass, increase much more in walking than in running. Switching to a run, although implying a higher frequency, makes the internal work decrease as a result of the lower limb spread. In this paper several influences, such as the 'ratings of perceived exertion' (RPE), on the choice of beginning to run when it is more economical to walk, are discussed. A tentative hypothesis on the determinants of Ss, which is emphasized to be a speed which has to be studied in detail but is generally avoided in locomotion, is based on a comfort criterion from peripheric afferences and is reflected by the fact that at Ss a running stride costs as much as a walking stride.(ABSTRACT TRUNCATED AT 250 WORDS)

Defining normal three-dimensional kinematics of the lower forelimb in horses

Summary Although the equine metacarpophalangeal joint is frequently damaged, there is a scarcity of data concerning the three-dimensional kinematics of this joint. Thus, the purpose of this study was to define normal three-dimensional kinematics of the distal forelimb. Six mature Thoroughbreds were adapted to a high-speed treadmill and filmed at awalk (2.0 m/s), trot (4.0 m/s), and left-lead canter (7.0 m/s). An autodigitizing videographic system was used to collect and analyze the three-dimensional kinematic data. Retroreflective markers were placed on the left forelimb at approximate centers of rotation of the carpal and metacarpophalangeal joints and at midpoints of the radius, third metacarpal bone, and hoof. The measures made during the experiment included the angle the fetlock and hoof made with the ground in three dimensions (the XZ, YZ, and XY planes), range of motion of the fetlock joint, limb acceleration and velocity, stride length, and stride frequency. The average joint angles in the XZ plane at impact were 134.6°, 140.6°, and 151.7°, and at lift-offwere 69.6°, 81.2°, and 88.9° for the walk, trot, and canter, respectively. In the YZ plane, average impact angles were 99.2°, 97.2°, and 100.3°, and lift-off angles were 96.4°, 96.6°, and 95.5° for the walk, trot and canter, respectively. In the XY plane, average impact angles were 136.4°, 130.3°, and 136.3°, and lift-off angles were 163.2°, 168.4°, and 173.6° for the walk, trot and canter, respectively. Total range of motion of the fetlock joint was 50.3° at the walk, 52.1° at the trot, and 56.0° at the canter. Average acceleration values of the fetlock joint determined at peakprotraction were -90.3, -121.9, and -201.6 and at the end of retraction were 112.0, 126.1, and 238.7 m/s 2 for the walk, trot, and canter, respectively. The average stride frequencies for the walk, trot and canter were 0.98, 1.44, and 1.88 Hz, respectively. The average stride lengths for the walk, trot and canter were 2.21, 2.72, and 3.79 m, respectively. These data will be used to build a base on normal three-dimensional kinematic patterns of the distal forelimb of the horse moving at the walk, trot, and canter on a high-speed treadmill.

Can cycle power predict sprint running performance?

A major criticism of present models of the energetics and mechanics of sprint running concerns the application of estimates of parameters which seem to be adapted from measurements of running during actual competitions. This study presents a model which does not perpetuate this solecism. Using data obtained during supra-maximal cycle ergometer tests of highly trained athletes, the kinetics of the anaerobic and aerobic pathways were modelled. Internal power wasted in the acceleration and deceleration of body limbs and the power necessary to overcome air friction was calculated from data in the literature. Assuming a mechanical efficiency as found during submaximal cycling, a power equation was constructed which also included the power necessary to accelerate the body at the start of movement. The differential equation thus obtained was solved through simulation. The model appeared to predict realistic times at 100 m (10.47 s), 200 m (19.63 s) and 400 m (42.99 s) distances. By comparison with other methods it is argued that power equations of locomotion should include the concept of mechanical efficiency.

Kineman: A Microcomputer-based Video Digitising System for Movement Analysis

Summary This paper describes and discusses a video-digitising system which uses a suite of microcomputer programs (KINEMAN) that has been developed and used to carry out kinematic analyses of human motor action. The equipment is relatively low cost and consists of a video camera, a high quality U-matic or Super VHS videotape recorder, a monitor and microcomputer (BBC Master or PC using MS. DOS). Mouse-driven software enables the acquisition of X-Y co-ordinates for reference points on the joints of the subject which are stored on disk, then edited and converted to ‘real’ values. Further programs from the suite are used to calculate and list angles, limb velocities, and limb accelerations. It also has facilities for data smoothing, graphing, and presenting sequences of stick figures. The accuracy of the system as a means of generating two-dimensional kinematic data has been demonstrated and several studies have shown that reliable results can be obtained with minimal training and without technical assistance.

Cerebellar neuronal activity related to arm movements in trained rhesus monkeys.

1. The role of the paravermal cerebellum in controlling arm movements in monkeys trained to perform visually guided movements was investigated. Discharge patterns of extracellularly recorded Purkinje, Golgi, nuclear and unidentified cells were correlated with arm position, velocity and acceleration. 2. The discharge of thirty-seven out of fifty-two movement-related Purkinje cells and that of thirty-three out of forty-five other movement-related cerebellar neurones was more highly correlated with limb velocity (0.95 greater than r greater than 0.4) than with position or acceleration. Twelve out of fifty-two Purkinje cells and twelve out of forty-five other cells were related to both limb velocity and limb position. Three Purkinje cells were related to limb acceleration and position and no cell was related to position or acceleration alone. 3. Modulation of discharge of these cells usually preceded movement (range -240 ms, unit leading movement, to +180 ms, unit lagging; mode -36 ms; mean -15 ms). 4. Each movement-related neurone was tested for four directions of movement. All showed a preferred direction in which the correlation with arm movement velocity was highest. However, 43% (30/70) correlated at r greater than 0.4 with movements in a second direction, usually that opposite to their preferred direction. Sixty-three per cent of these neurones fired earlier during movements in the preferred than in the opposite direction. 5. It is concluded that the paravermal cerebellum may be involved in computing the velocity vectors required for achieving properly directed and co-ordinated movements of the whole arm.

Changes in the Variability of Movement Trajectories With Practice

We studied variability in movement phase plane trajectories (velocity-position relation) during movement. Human subjects performed 10 degrees and 30 degrees elbow flexion and extension movements in a visual step tracking paradigm. The area of ellipses with radii equal to one standard deviation in position and velocity was taken as a measure of trajectory variability. Trajectory variability was determined at 10-ms intervals throughout movements. Trajectory variability in both the acceleration and deceleration phases of movement decreased with practice. The average trajectory variability during deceleration was greater than that during acceleration even after extended practice (1000 trials). During practice, subjects usually increased movement speed while maintaining end-position accuracy. Trajectory variability was also related to movement speed when equal amounts of practice were given. Short duration (fast) movements had greater trajectory variability than long duration movements. Thus there is a tradeoff between movement speed and trajectory variability similar to the classical speed-accuracy tradeoff. Trajectory variability increased rapidly during the acceleratory phase of movement. The rate of increase was positively related to both movement amplitude and speed. Thus, the forces producing limb acceleration were variable and this variability was more marked in faster and larger movements. In contrast, trajectory variability increased more slowly or actually decreased during the deceleratory phase of movements. Forces involved in limb deceleration thus appeared to compensate to a greater or lesser degree for the variability in accelerative forces. The experiments indicate that the entire trajectory of simple limb movements is controlled by the central nervous system. Variations in accelerative forces may be compensated for by associated variations in decelerative forces. The linkage between accelerative and decelerative forces is progressively refined with practice resulting in decreased variability of the movement trajectory.