Wrist Velocity Research Articles

Catching a Ball at the Right Time and Place: Individual Factors Matter

Intercepting a moving object requires accurate spatio-temporal control. Several studies have investigated how the CNS copes with such a challenging task, focusing on the nature of the information used to extract target motion parameters and on the identification of general control strategies. In the present study we provide evidence that the right time and place of the collision is not univocally specified by the CNS for a given target motion; instead, different but equally successful solutions can be adopted by different subjects when task constraints are loose. We characterized arm kinematics of fourteen subjects and performed a detailed analysis on a subset of six subjects who showed comparable success rates when asked to catch a flying ball in three dimensional space. Balls were projected by an actuated launching apparatus in order to obtain different arrival flight time and height conditions. Inter-individual variability was observed in several kinematic parameters, such as wrist trajectory, wrist velocity profile, timing and spatial distribution of the impact point, upper limb posture, trunk motion, and submovement decomposition. Individual idiosyncratic behaviors were consistent across different ball flight time conditions and across two experimental sessions carried out at one year distance. These results highlight the importance of a systematic characterization of individual factors in the study of interceptive tasks.

Physical workload in neck, shoulders and wrists/hands in dental hygienists during a work-day

Physical workload was recorded by electromyography, inclinometry and goniometry for twelve female dental hygienists during authentic work. Their work was, in relation to other types of work, characterised by pronounced head flexion (90th percentile 46°), high loads on the forearm extensor muscles (90th percentile 23% and 18% of maximal EMG (MVE), for the right and left sides, respectively), average loads on trapezius muscles (90th percentile 15% and 14% MVE), average arm elevation (99th percentile 83° and 72°) and average wrist flexion and velocities (50th percentiles 17° of extension and 7.3°/s, for the right side). Manual scaling and machinery (use of ultrasonic scaling and hand-pieces) showed higher loads on the trapezius muscles, regarding muscular rest, as well as the 10th and 50th percentiles, than the other tasks, and for the forearm extensor muscles, an almost complete lack of muscular rest (0.1% time), and much higher loads regarding the 10th and 50th percentiles. Further, more pronounced head flexion and lower head and upper arm velocities were found, indicating more constrained postures for the neck and shoulders for the manual scaling and machinery. Use of ultrasonic scaler reduced the 50th percentile loads on the right forearm extensor muscles, but had no effect on the fraction of muscular rest and on the 10th percentile load. These findings are consistent with the high prevalences of musculoskeletal disorders among dental hygienists.

Bilateral assessment of functional tasks for robot-assisted therapy applications.

This article presents a novel evaluation system along with methods to evaluate bilateral coordination of arm function on activities of daily living tasks before and after robot-assisted therapy. An affordable bilateral assessment system (BiAS) consisting of two mini-passive measuring units modeled as three degree of freedom robots is described. The process for evaluating functional tasks using the BiAS is presented and we demonstrate its ability to measure wrist kinematic trajectories. Three metrics, phase difference, movement overlap, and task completion time, are used to evaluate the BiAS system on a bilateral symmetric (bi-drink) and a bilateral asymmetric (bi-pour) functional task. Wrist position and velocity trajectories are evaluated using these metrics to provide insight into temporal and spatial bilateral deficits after stroke. The BiAS system quantified movements of the wrists during functional tasks and detected differences in impaired and unimpaired arm movements. Case studies showed that stroke patients compared to healthy subjects move slower and are less likely to use their arm simultaneously even when the functional task requires simultaneous movement. After robot-assisted therapy, interlimb coordination spatial deficits moved toward normal coordination on functional tasks.

To know or not to know: influence of explicit advance knowledge of occlusion on interceptive actions

This study examined how explicit advance knowledge might influence adaptive behavior to visual occlusions. Catching performance and kinematics of good ball catchers were compared between no, early and late occlusion trials. Discrete visual occlusions of 400ms, occurring early or late in the ball's approach trajectory, were randomly interspersed between no occlusion trials. In one condition, the presence and type of occlusion were announced a priori (expected), whereas in another condition no such information was provided (unexpected). Expectation of occlusion resulted in an adapted limb transport and increased grasping time, whereas in the unexpected condition a higher peak of wrist velocity was evident for all occlusion conditions. The observed different adaptations cannot be explained by trial-by-trial adaptations alone and instead provide evidence for the influence of explicit advance knowledge in the motor response of interceptive actions.

Individual differences in the biomechanical effect of loudness and tempo on upper-limb movements during repetitive piano keystrokes

The present study addressed the effect of loudness and tempo on kinematics and muscular activities of the upper extremity during repetitive piano keystrokes. Eighteen pianists with professional music education struck two keys simultaneously and repetitively with a combination of four loudness levels and four tempi. The results demonstrated a significant interaction effect of loudness and tempo on peak angular velocity for the shoulder, elbow, wrist and finger joints, mean muscular activity for the corresponding flexors and extensors, and their co-activation level. The interaction effect indicated greater increases with tempo when eliciting louder tones for all joints and muscles except for the elbow velocity showing a greater decrease with tempo. Multiple-regression analysis and K-means clustering further revealed that 18 pianists were categorized into three clusters with different interaction effects on joint kinematics. These clusters were characterized by either an elbow-velocity decrease and a finger-velocity increase, a finger-velocity decrease with increases in shoulder and wrist velocities, or a large elbow-velocity decrease with a shoulder-velocity increase when increasing both loudness and tempo. Furthermore, the muscular load considerably differed across the clusters. These findings provide information to determine muscles with the greatest potential risk of playing-related disorders based on movement characteristics of individual pianists.

Differences in the Kinematics of the Baseball Swing between Hitters of Varying Skill

The aim of this study was to determine differences in bat swing kinematics in baseball hitters of varying ability. Kinematic data for the upper and lower body were collected from 20 trained male baseball players (22.3 ± 5.3 yr, 1.82 ± 0.07 m, 83.5 ± 10.9 kg), using three-dimensional computerized motion-analysis techniques. Participants were ranked before testing based on a novel coach's rating scale and seasonal batting average. They were subsequently separated into a relatively high-caliber group of hitters (n = 10) and a relatively low-caliber group of hitters (n = 10) for comparison. Importantly, the two groups were significantly different in terms of coach's rating (P < 0.01) and batting average (P < 0.05). The results showed a significant difference in maximum bat swing velocity (P < 0.05) with high-caliber hitters having a higher velocity (36.8 m · s) in comparison with relatively low-caliber hitters (33.8 m · s). Lead elbow maximum angular velocity was significantly higher (35.9%) among relatively high-caliber hitters (P < 0.05). Angular velocity of the hip segment approached significance between the groups (P = 0.056). High-caliber hitters also had a right knee angle of 106° at ball contact, which was significantly (P < 0.05) higher than that of relatively low-caliber hitters (100°). There were no between-group differences for wrist and linear hip joint velocities at ball contact. It was established that bat swing velocity is a key characteristic of the baseball swing when identifying skill level and performance between hitters. In addition, high-caliber hitters display greater lead elbow maximum angular velocity possibly because of achieving a higher angular hip segment velocity early in the swing. It is noted that although these attributes differentiate hitters of varying skill level, future research should examine whether developing these characteristics in players of lower ability improves batting performance.

Compensatory motor control after stroke: an alternative joint strategy for object-dependent shaping of hand posture.

Efficient grasping requires planned and accurate coordination of finger movements to approximate the shape of an object before contact. In healthy subjects, hand shaping is known to occur early in reach under predominantly feedforward control. In patients with hemiparesis after stroke, execution of coordinated digit motion during grasping is impaired as a result of damage to the corticospinal tract. The question addressed here is whether patients with hemiparesis are able to compensate for their execution deficit with a qualitatively different grasp strategy that still allows them to differentiate hand posture to object shape. Subjects grasped a rectangular, concave, and convex object while wearing an instrumented glove. Reach-to-grasp was divided into three phases based on wrist kinematics: reach acceleration (reach onset to peak horizontal wrist velocity), reach deceleration (peak horizontal wrist velocity to reach offset), and grasp (reach offset to lift-off). Patients showed reduced finger abduction, proximal interphalangeal joint (PIP) flexion, and metacarpophalangeal joint (MCP) extension at object grasp across all three shapes compared with controls; however, they were able to partially differentiate hand posture for the convex and concave shapes using a compensatory strategy that involved increased MCP flexion rather than the PIP flexion seen in controls. Interestingly, shape-specific hand postures did not unfold initially during reach acceleration as seen in controls, but instead evolved later during reach deceleration, which suggests increased reliance on sensory feedback. These results indicate that kinematic analysis can identify and quantify within-limb compensatory motor control strategies after stroke. From a clinical perspective, quantitative study of compensation is important to better understand the process of recovery from brain injury. From a motor control perspective, compensation can be considered a model for how joint redundancy is exploited to accomplish the task goal through redistribution of work across effectors.

Advance knowledge effects on kinematics of one-handed catching

The purpose of this experiment was to examine the effects of advance knowledge on the kinematics of one-handed catching. Balls were launched from a distance of 8.4 m by a ball-projection machine with adjustable launching speed. Fifteen skilled ball catchers caught 160 balls with their preferred hand under blocked-order (4 blocks, each comprising 20 trials at 1 of 4 different ball speeds) or random-order (4 blocks, each comprising 20 trials of 4 different ball speeds) conditions. By projecting balls with different ball speeds from a fixed position, it was possible to modify the temporal constraints of the catching task. In both the blocked-order and random-order conditions, catching performance (number of catches, touches and misses) decreased with increasing temporal constraints. Analysis of successful trials indicated that this equal level of catching performance was achieved with different movement kinematics. Specifically, there was a change in movement time, latency, wrist velocity profile, and coefficient of straightness. Based on expectancy of previous trials, movement kinematics was scaled to ball speed in the blocked-order condition whereas in the random-order condition, participants exhibited a more default initial response. However, this latter mode of control was functional in that it increased the likelihood of success for the higher ball speeds while also providing participants with a larger temporal window to negotiate the unexpected temporal constraint on-line for the lowest ball speed.

Vision for Performance in Virtual Environments: The Role of Feedback Timing

This work explores how people use visual feedback when performing simple reach-to-grasp movements in a tabletop virtual environment. In particular we investigated whether visual feedback is required for the entire reach or whether minimal feedback can be effectively used. Twelve participants performed reach-to-grasp movements toward targets at two locations. Visual feedback about the index finger and thumb was provided in four conditions: vision available throughout the movement, vision available up to peak wrist velocity, vision available until movement initiation, or vision absent throughout the movement. It was hypothesized that vision available until movement onset would be an advantage over a no vision situation yet not attain the performance observed when vision was available up to peak velocity. Results indicated that movement time was longest in the no vision condition but similar for the three conditions where vision was available. However, deceleration time and peak aperture measures suggest grasping is more difficult when vision is not available for at least the first third of the movement. These results suggest that designers of virtual environments can manipulate the availability of visual feedback of one's hand without compromising interactivity. This may be applied, for example, when detailed rendering of other aspects of the environmental layout is more important, when motion lag is a problem or when hand/object concealment is an issue.

Control of aperture closure initiation during reach-to-grasp movements under manipulations of visual feedback and trunk involvement in Parkinson’s disease

The present project was aimed at investigating how two distinct and important difficulties (coordination difficulty and pronounced dependency on visual feedback) in Parkinson's disease (PD) affect each other for the coordination between hand transport toward an object and the initiation of finger closure during reach-to-grasp movement. Subjects with PD and age-matched healthy subjects made reach-to-grasp movements to a dowel under conditions in which the target object and/or the hand were either visible or not visible. The involvement of the trunk in task performance was manipulated by positioning the target object within or beyond the participant's outstretched arm to evaluate the effects of increasing the complexity of intersegmental coordination under different conditions related to the availability of visual feedback in subjects with PD. General kinematic characteristics of the reach-to-grasp movements of the subjects with PD were altered substantially by the removal of target object visibility. Compared with the controls, the subjects with PD considerably lengthened transport time, especially during the aperture closure period, and decreased peak velocity of wrist and trunk movement without target object visibility. Most of these differences were accentuated when the trunk was involved. In contrast, these kinematic parameters did not change depending on the visibility of the hand for both groups. The transport-aperture coordination was assessed in terms of the control law according to which the initiation of aperture closure during the reach occurred when the hand distance-to-target crossed a hand-target distance threshold for grasp initiation that is a function of peak aperture, hand velocity and acceleration, trunk velocity and acceleration, and trunk-target distance at the time of aperture closure initiation. When the hand or the target object was not visible, both groups increased the hand-target distance threshold for grasp initiation compared to its value under full visibility, implying an increase in the hand-target distance-related safety margin for grasping. The increase in the safety margin due to the absence of target object vision or the absence of hand vision was accentuated in the subjects with PD compared to that in the controls. The pronounced increase in the safety margin due to absence of target object vision for the subjects with PD was further accentuated when the trunk was involved compared to when it was not involved. The results imply that individuals with PD have significant limitations regarding neural computations required for efficient utilization of internal representations of target object location and hand motion as well as proprioceptive information about the hand to compensate for the lack of visual information during the performance of complex multisegment movements.

Ergonomic risk factors for the wrists of hairdressers

This study utilized a portable data logger to measure the wrist angles and forearm flexor and extensor electromyography (EMG) of 21 hairstylists. The hairstylists were divided into two groups, one with 11 barbers (9 males and 2 females) specializing in men's hairdressing, and one with 10 hairdressers (2 males and 8 females) specializing in women's hairdressing. The standard haircut task was divided into three subtasks: hair cutting, washing and blow-drying. The mechanical exposures of the overall task and subtasks were quantified to compare how subtasks, occupational groups, and gender groups differ. Experimental results show that the average time to finish a woman's haircut (51.4 min) is significantly longer than that for a man's haircut (35.6 min) ( p < 0.005). Female hairstylists had significantly greater EMG activity than male hairstylists did ( p < 0.001). The non-dominant hands of hairdressers have significantly higher overall wrist velocity than those of barbers ( p < 0.005). Analytical results suggest that the relatively higher force exertion and wrist velocity of female hairstylists combined with prolonged exposure may account for the higher rate of hand/wrist pain in female hairdressers than in male barbers.

Effect of Segmental Velocity Increments On Maximum Wheelchair Racing Speed

PURPOSE: Biomechanical characteristics in wheelchair propulsion such as range of motions, segment velocities and accelerations, and pushrim forces and moments have been broadly documented; however, the effect of segmental velocities on wheelchair racing have not been investigated. This study was to determine the effect of linear segmental velocity increments of the upper extremity on the maximum wheel speed (MWS) in wheelchair racing. METHODS: Seventeen elite international wheelchair racers with the mean age of 30.3 ± 7.0 yrs, body mass 57.5 ± 10.2 kg and wheelchair racing training 8.9 ± 6.6 yrs served as participants. Reflective markers were placed on right 5th metacarpal head (hand), lateral styloid process (wrist), lateral epicondyle of the humerus (elbow) and acromion (shoulder), as well as the right wheel. Participants were filmed at 100Hz by a three-dimensional motion system while propelling their own racing wheelchairs on a training roller at their maximum speeds. After shoulder, elbow, wrist and hand linear velocities in the drive phase were calculated, linear segmental velocity increments were derived based on the difference of peak linear velocities between two adjacent joints, including shoulder-to-elbow (SE), elbow-to-wrist (EW) and wrist-to-hand (WH) velocity increments. A multiple linear regression was employed to determine the effect of each increment on the MWS. RESULTS: Mean peak linear velocities were 2.04 ± 0.38 m/s for the shoulder, 5.14 ± 0.83 m/s for the elbow, 6.26 ± 1.15 m/s for the wrist and 7.53 ± 1.03 m/s for the hand. The mean MWS was 9.14 ± 1.72 m/s. Peak linear velocities increased from proximal to distal joints, specifically from shoulder to the elbow (from 2.04 to 5.14 m/s). It was shown 87% of the variance of the MWS was accounted for by segmental velocity increments (P < 0.01) with the greatest contribution from WH and the least contribution from EW (MWS = -21.92 + 1.67SE + 1.42EW + 2.02WH). CONCLUSIONS: It is speculated that increasing wrist linear velocity by performing more abduction/addition in the drive phase may improve the wheelchair racing speed. Supported by the Roosevelt Warm Springs Institute for Rehabilitation, Warm Springs, GA

Effects of pace and stress on upper extremity kinematic responses in sign language interpreters

Sign language interpreters suffer from high levels of upper extremity disorders and burnout due to the physical and cognitive demands of interpreting. The objective of this research was to quantify the wrist kinematics of interpreting and to assess how speaker pace and psychosocial stress influence wrist kinematics. Professional interpreters interpreted a pre-recorded lecture, while the speaking pace of the lecture varied. One group of subjects was exposed to environmental conditions intended to induce stress. Several wrist kinematic variables of interpreting exceeded previously established high risk benchmarks for development of upper extremity disorders in industrial tasks. Wrist velocity and acceleration increased significantly with pace, with increases ranging from 10.7–18.6%. Increased psychosocial stress resulted in significant increase of left hand (non-dominant) wrist velocity and acceleration, with increases ranging from 14.8–19.5%. These results provide an objective assessment of the biomechanical demands of interpreting and support earlier research into different types of work, which found deleterious effects of psychosocial stress on the biomechanical responses of the lower back.

A biologically inspired neural network controller for ballistic arm movements.

BackgroundIn humans, the implementation of multijoint tasks of the arm implies a highly complex integration of sensory information, sensorimotor transformations and motor planning. Computational models can be profitably used to better understand the mechanisms sub-serving motor control, thus providing useful perspectives and investigating different control hypotheses. To this purpose, the use of Artificial Neural Networks has been proposed to represent and interpret the movement of upper limb. In this paper, a neural network approach to the modelling of the motor control of a human arm during planar ballistic movements is presented.MethodsThe developed system is composed of three main computational blocks: 1) a parallel distributed learning scheme that aims at simulating the internal inverse model in the trajectory formation process; 2) a pulse generator, which is responsible for the creation of muscular synergies; and 3) a limb model based on two joints (two degrees of freedom) and six muscle-like actuators, that can accommodate for the biomechanical parameters of the arm. The learning paradigm of the neural controller is based on a pure exploration of the working space with no feedback signal. Kinematics provided by the system have been compared with those obtained in literature from experimental data of humans.ResultsThe model reproduces kinematics of arm movements, with bell-shaped wrist velocity profiles and approximately straight trajectories, and gives rise to the generation of synergies for the execution of movements. The model allows achieving amplitude and direction errors of respectively 0.52 cm and 0.2 radians.Curvature values are similar to those encountered in experimental measures with humans.The neural controller also manages environmental modifications such as the insertion of different force fields acting on the end-effector.ConclusionThe proposed system has been shown to properly simulate the development of internal models and to control the generation and execution of ballistic planar arm movements. Since the neural controller learns to manage movements on the basis of kinematic information and arm characteristics, it could in perspective command a neuroprosthesis instead of a biomechanical model of a human upper limb, and it could thus give rise to novel rehabilitation techniques.

Adolescent Baseball Pitching Technique

Document the biomechanics of the pitching motion to help provide insight about the etiology of common injuries seen in adolescent baseball pitchers. Kinematic and kinetic data for the upper and lower extremities, thorax, and pelvis were collected from 24 adolescent pitchers, using modern three-dimensional computerized motion-analysis techniques. Original information regarding forearm and wrist motion was reported in this study and were consistent with expected motions for the fastball pitch. Average excursion of motion was: pronation/supination 63 +/- 15 degrees, wrist flexion/extension 44 +/- 14 degrees, and ulnar/radial deviation 12 +/- 4 degrees. Explosive forearm motion occurred between ball release (BR) and maximal glenohumeral internal rotation (GH-MIR) with a peak pronation velocity of 2051 +/- 646 degrees.s(-1). The majority of internal/external and abduction/adduction shoulder motion was attributed to the glenohumeral motion. Internal glenohumeral rotation range of motion was 125 +/- 13 degrees and mean peak internal glenohumeral rotation velocity was 3343 +/- 453 degrees.s(-1). Thorax and pelvic motion peak velocities and accelerations occurred before the peak elbow varus moment, which occurred at 59% of the pitch cycle (PC). The peak shoulder, elbow, and wrist velocities and accelerations occurred after the peak elbow varus moment. The pelvis squared to the plate at 51 +/- 10% PC and the thorax at 59 +/- 7% PC with maximal glenohumeral external rotation (GH-MER) at 65% PC and BR at 78 +/- 3% PC. The data collected in this study were consistent with the literature, with the exception of joint velocities and moments, which were lower than those in one published study. We have established the kinematic and kinetic parameters of the adolescent baseball pitch. These measured parameters and the differences between adolescent pitchers and their adult counterparts can be used to examine and help determine the causes of the rapid increase in adolescent pitching injuries.

End-point focus manipulations to determine what information is used during observational learning

We required two groups of participants to observe an end-point model (ENDPT) while another two groups viewed a full-body, point-light model (FULL) to determine the role of relative motion information in acquisition of a multi-limb, whole-body action. One ENDPT and one FULL group also bowled a ball. Following retention, all groups observed the FULL model. The participants’ movements were compared to the model and outcome attainment was quantified. There was no difference in shoulder–elbow coordination between groups in acquisition or retention. The FULL groups replicated hip–knee coordination more accurately than did ENDPT groups in early acquisition only, with no significant differences in late acquisition or retention. Both bowling groups became more accurate at the task across acquisition, but the ENDPT group was more accurate and consistent in retention. Providing intra-limb relative motion in re-acquisition did not improve coordination for the ENDPT groups, but it did facilitate movement control (peak wrist velocity) and outcome attainment (target accuracy). The acquisition of coordination during observational learning is not only a result of copying relative motion information, but also involves copying of end-point trajectory information from the primary effector.

AGING EFFECT ON MOVEMENT OF PREHENSION WITH OBSTACLE

In this paper, we develop an animal model of prehension movements by examining the kinematics of reaching and grasping in monkeys and by comparing the results to published data on humans. Hand movements were recorded in three dimensions in monkeys who were trained to either point at visual targets under unperturbed and perturbed conditions, or to reach and grasp 3-D objects. The results revealed the following three similarities in the hand kinematics of monkey and man. (1) Pointing movements showed an asymmetry depending on target location relative to the hand used; in particular, movements to an ipsilateral target took longer than those to a contralateral one. (2) Perturbation of target location decreased the magnitude of the velocity peak and increased the duration of pointing movements. (3) Reaching to grasp movements displayed a bell-shaped wrist velocity profile and the maximum grip aperture was correlated with object size. These similarities indicate that the macaque monkey can be a useful model for understanding human motor control.

External timing constraints facilitate performance of everyday interceptive actions in children with Spastic Hemiparetic Cerebral Palsy

We examined how children with Spastic Hemiparetic Cerebral Palsy (SHCP) perform interceptive actions they experience in daily life. Children were required to walk towards and intercept a stationary ball or a moving ball, with either their impaired or non-impaired arm. In the stationary ball condition the child was free to determine the speed of their response (internal timing), whereas in the moving ball condition there was a restricted time available (external timing). It was found that the reach movements of the non-impaired arm were different to the impaired arm, and were characterized by some of the typical movement limitations imposed by SHCP. However, there was no evidence of increased contribution from trunk motion or a lengthening of reach movement time or deceleration time. Instead, there was a coordinated change with the walking kinematics, whereby the children spent proportionately more time slowing down as they approached the point of interception when reaching with the impaired arm. There were also several differences in the response when intercepting a moving ball compared to a stationary ball. When the timing constraints were imposed externally (moving ball) rather than internally (stationary ball), children reached with a reduced movement time and deceleration time, and an increased peak wrist velocity and elbow excursion. These adaptations to behaviour were necessary to deal with the restricted time available to make the interception in the moving ball condition compared to when the ball was stationary, and reveal how children with SHCP coordinate walking and reaching when performing natural interceptive actions.

Comparison of kinematics in skilled and unskilled arms of the same recreational baseball players

We examined mechanisms of coordination that enable skilled recreational baseball players to make fast overarm throws with their skilled arm and which are absent or rudimentary in their unskilled arm. Arm segment angular kinematics in three dimensions at 1000 Hz were recorded with the search-coil technique from the arms of eight individuals who on one occasion threw with their skilled right arm and on another with their unskilled left arm. Compared with their unskilled arm, the skilled arm had: a larger angular deceleration of the upper arm in space in the forward horizontal direction; a larger shoulder internal rotation velocity at ball release (unskilled arms had a negative velocity); a period of elbow extension deceleration before ball release; and an increase in wrist velocity with an increase in ball speed. It is suggested that some of these differences in arm kinematics occur because of differences between the skilled and unskilled arms in their ability to control interaction torques (the passive torque at one joint due to motion at adjacent joints). It is proposed that one reason unskilled individuals cannot throw fast is that, unlike their skilled counterparts, they have not developed the coordination mechanisms to effectively exploit interaction torques.

Different computer tasks affect the exposure of the upper extremity to biomechanical risk factors

In order to determine differences in biomechanical risk factors across computer tasks, a repeated measures laboratory experiment was completed with 30 touch-typing adults (15 females and 15 males). The participants completed five different computer tasks: typing text, completing an html-based form with text fields, editing text within a document, sorting and resizing objects in a graphics task and browsing and navigating a series of intranet web pages. Electrogoniometers and inclinometers measured wrist and upper arm postures, surface electromyography measured muscle activity of four forearm muscles and three shoulder muscles and a force platform under the keyboard and force-sensing computer mouse measured applied forces. Keyboard-intensive tasks were associated with less neutral wrist postures, larger wrist velocities and accelerations and larger dynamic forearm muscle activity. Mouse-intensive tasks (graphics and intranet web page browsing) were associated with less neutral shoulder postures and less variability in forearm muscle activity. Tasks containing a mixture of mouse and keyboard use (form completion and text editing) were associated with higher shoulder muscle activity, larger range of motion and larger velocities and accelerations of the upper arm. Comparing different types of computer work demonstrates that mouse use is prevalent in most computer tasks and is associated with more constrained and non-neutral postures of the wrist and shoulder compared to keyboarding.