Grip force control during gait initiation with a hand-held object

When walking with a hand-held object, grip force is coupled in an anticipatory manner to changes in inertial force resulting from the accelerations and decelerations of gait. However, it is not known how grip and inertial forces are organized at the onset of gait, and if the two forces are coupled in the early phases of gait initiation. Moreover, initiating walking with an object involves the coordination of anticipatory postural (e.g., ground reaction force changes) and grasping adjustments. The aim of this study was to investigate the relationship of ground reaction, grip, and inertial force onsets, and the subsequent development of the coupling of grip and inertial forces during gait initiation with a hand-held object. Ten subjects performed gait initiation with a hand-held object following predictable and unpredictable start signals. We found that ground reaction and grip force onsets were closely linked in time regardless of the predictability of the start signal. In the early period of gait initiation, the grip force started to increase prior to inertial force changes. While the strength of the coupling of grip and inertial forces was moderate in this early phase, it increased to values observed during steady-state gait after the swing foot left the ground. The early grip force increase and the coupling of grip and inertial forces represent an anticipatory control process. This process establishes an appropriate grip-inertial force ratio to ensure object stability during acceleration after foot-off and maintains this increased ratio thereafter. The results suggest that grasping and whole body movements are governed by a common internal representation.     

Evidence for a common process in gait initiation and stepping on to a new level to reach gait velocity

The aim of this study was to examine the adaptability of the gait initiation process when confronted with stepping on (SO) to a new level. Eight young adults performed gait initiation at two different speed conditions in a level walking (LW) situation and in a SO situation aimed at walking on an elevated (16 cm) level surface. As in a previous study using a single step, we found in SO a contradiction between the characteristics of anticipatory postural adjustments (APA) and gait velocity, i.e. the peak of anteroposterior velocity of the body’s centre of gravity (CG) reached at the end of the first step. In normal and fast gaits, gait velocity was similar in both situations, whereas the duration and amplitude of the APA were smaller in SO than in LW. The reduction of APA in SO allowed the forward velocity of CG at the time of foot contact of the stepping limb to be lower than in LW. This is explained by the fact that the majority of body lift, beginning at this time, required a greater increase in forward velocity than in LW. Thus, with lower APA in SO, the gait velocity could be similar in both situations. From LW to SO, the spatio-temporal patterns in the forward velocity of CG varied within characteristic phases of the movement, but in a predictable way as gait velocity changed. These results gave evidence of an adaptation of the gait initiation process for the new constraints, despite the contradiction between APA and gait velocity. The spatio-temporal parameters of the anticipation phase in SO were pre-set according to the new requirements of the task: reaching gait velocity with a body lift. Furthermore, the time for reaching gait velocity was independent of both the amplitude of this velocity and the situation. This expressed the capacity of the subjects to use in SO the same optimal conditions to reach gait velocity as in LW, i.e. essentially in a ballistic manner.     

Balance control and adaptation during vibratory perturbations in middle-aged and elderly humans

The objective was to investigate if healthy elderly people respond and adapt differently to postural disturbances compared to middle-aged people. Thirty middle-aged (mean age 37.8 years, range 24–56 years) and forty healthy elderly subjects (mean age 74.6 years, range 66–88 years) were tested with posturography. Body sway was evoked by applying pseudorandom vibratory stimulation to the belly of the gastrocnemius muscles of both legs simultaneously. The tests were performed both with eyes open and eyes closed. The anteroposterior body sway was measured with a force platform and analyzed with a method that considers the adaptive changes of posture and stimulation responses. The results showed that middle-aged people generally used a different postural control strategy as compared to the elderly. The elderly responded more rapidly to vibratory perturbation, used more high-frequency (>0.1 Hz) motions and the motion dynamics had a higher degree of complexity. Moreover, the elderly had diminished ability to use visual information to improve balance control. Altogether, despite having an effective postural control adaptation similar to that of middle-aged people, the elderly had more difficultly in withstanding balance perturbations. These findings suggest that the balance control deterioration associated with aging cannot be fully compensated for by postural control adaptation.     

Age-related changes in the center of mass velocity control during walking

During walking, the body center of mass oscillates along the vertical plane. Its displacement is highest at mid-swing and lowest at terminal swing during the transition to double support. Its vertical velocity (CoMv) has been observed to increase as the center of mass falls between mid- and late swing but is reduced just before double support. This suggests that braking of the center of mass is achieved with active neural control. We tested whether this active control deteriorates with aging (Experiment 1) and during a concurrent cognitive task (Experiment 2). At short steps of <.4 m, CoMv control was low and similar among all age groups. All groups braked the CoMv at longer steps of >.4 m but older subjects did so to a lesser extent. During the cognitive task, young subjects increased CoMv control (i.e. increase in CoMv braking) while maintaining step length and walking speed. Older subjects on the other hand, did not increase CoMv control but rather maintain it by reducing both step length and walking speed. These results suggest that active braking of the CoM during the transition to double support predominates in steps >.4 m. It could be a manifestation of the balance control system, since the braking occurs at late stance where body weight is being shifted to the contralateral side. The active braking mechanism also appears to require some attentional resource. In aging, reducing step length and speed are strategic to maintaining effective center of mass control during the transition to double support. However, the lesser degree of control in older adults indicates a true age-related deficit.     

Delays in auditory-cued step initiation are related to increased volume of white matter hyperintensities in older adults

An increased volume of white matter hyperintensities (WMH) on MRI has been associated with mobility impairments in older adults. The objective of this preliminary study was to investigate the relationship between the volume of WMH and the delays in auditory-cued step initiation. Eight subjects aged 75–83 years participated. The WMH volume in the corticospinal tracts and anterior thalamic radiations were summed. Subjects performed an auditory-cued stepping task that included two simple reaction time (SRT) trials and three choice reaction time (CRT) trials. SRT trials required subjects to step as quickly as possible with the right foot from a symmetric standing position to a single target position in response to an auditory stimulus. For the CRT trials, subjects stepped as quickly as possible to one of two possible locations, depending on the auditory stimulus. The time from the stimulus onset to the reaction time of the anticipatory postural adjustment (APA_RT) and liftoff (LO) of the right foot was computed for each stimulus. The mean APA_RT and LO were greater for the CRT steps compared with the SRT steps to the same location. Increases in WMH were significantly associated with larger APA_RT and LO during both SRT and CRT for both target locations. These data suggest that increased volume of WMH is associated with greater central processing time during voluntary step initiation, and highlight a possible mechanism that can help to explain how damage to white matter tracts affects mobility in older adults.     

Adaptation of the gait initiation process for stepping on to a new level using a single step

During the gait initiation in level walking, the anticipatory postural adjustments (APA) which precede heel off consist of a forward fall of the whole body and their duration depends on the intended gait velocity related to the step length. The present study examines the adaptation of the gait initiation process for stepping on to a new level. Five subjects performed a single step at natural speed in five experimental conditions. The first condition (C1) was a level walking task whereas the other (stair) conditions required stepping on to a new level (from 8 to 32 cm). The horizontal step length was the same under all conditions. Results showed that the center of mass (CM) forward velocity at the end of the APA, and also until foot contact of the leading limb, decreased from C1 to the stair conditions whereas the peak of forward velocity was similar under all conditions. Moreover, the CM forward displacement up to foot contact was smaller in the stair conditions than in C1. These results suggest the use of a sequential mode of control for the organization of the CM forward dynamics during the stair conditions. This adaptation of the gait initiation process for stepping up is examined mainly from the result that the majority of body lift, which occurred only from the beginning of the double-stance phase, involved a larger CM forward translation than in level walking. As the horizontal step length was the same in all conditions, it can be suggested that the CNS had to reduce the CM forward displacement up to foot contact in the stair conditions, in order to take into account the subsequent greater forward translation.     

The role of anticipatory postural adjustments and gravity in gait initiation

This study analyzes the anticipatory postural adjustments which precede heel-off by considering the participation of the gravitational and muscular actions about the ankle joints during the gait initiation process. The resultant moment about the ankle joints and the gravitational moment were calculated using a biomechanical model in five normal subjects for three different speed conditions. The results show that the variations of these two moments are correlated to the velocity at the end of the first step. Nevertheless, a significant variation of the ankle joints moment occurs at the beginning of the anticipatory phase, whereas the gravity effect is still insignificant. These findings show how the successive controls of the muscular actions acting during the anticipatory movement and of the gravity action acting principally during the step execution allow the subject to reach the velocity which has been initially and centrally decided, by the end of the first step.     

The preferred sensory direction of muscle spindle primary endings influences the velocity coding of two-dimensional limb movements in humans

The present study compares how accurately two different but close velocities of movement are discriminated by populations of muscle spindle primary afferents whether or not one takes into account the direction of the movement and the preferred sensory directions of the units (i.e., the direction of movement to which the afferents are the most sensitive). The activities of 26 muscle spindle primary endings originating from the tibialis anterior, the extensor digitorum longus, the extensor hallucis longus, and the peroneus lateralis muscles were recorded in the lateral peroneal nerve. Their responses to movements imposed at two velocities (12.5 and 18 mm/s) were analyzed. These movements were straight-line movements imposed in eight directions and circular movements in both clockwise and anticlockwise directions. The encoding of the movement velocity was analyzed in two ways. First, the mean frequencies of discharge of the muscle spindle afferents were compared for the two velocities. Second, the data were analyzed using a "neuronal population vector model." This model is based on the idea that such neuronal coding can be analyzed in terms of a series of population vectors (i.e., mean contribution of all the muscle spindle afferents within one directionally tuned muscle) and by finally calculating a sum vector. The results showed no clear and consistent difference in the response frequency of the muscle spindle afferents for the two velocities of movement compared. Rather, the most consistently significant differences between the two velocities were in the lengths of the sum vectors. It is concluded that the encoding of two-dimensional movement velocity relies on populations of muscle spindle afferents coming from the whole set of muscles surrounding a particular joint, each muscle making an instantaneous, oriented, and weighted contribution to the sensory coding of the kinematics parameters.     

Dynamic role of vision in the control of posture in man

Summary The influence of moving visual surround on the maintenance of upright posture has been studied in man during combined motion of a platform (cart) on which subjects were standing. The normal visual surround was either moving together with the cart, or with a velocity equal to cart velocity either in the same or in the opposite direction of cart motion (cart acceleration was either 0.2 m/s² or 0.05 m/s²). The changes in body pitch observed under these three conditions of visual surround motion were in the same direction as those observed when visual surround motion was given in isolation. However, their amplitude was greater, particularly when visual surround motion was in conflict with body motion.     

Predictive control of body mass trajectory in a two-step sequence

During a step, the body enters a state of falling as a foot is lifted from the ground. This has important implications for the control of body trajectory when stepping in different directions. A model has been proposed in which the central nervous system controls body trajectory by a predictive, ballistic throw of the body occurring just before the stepping foot leaves the ground. Here we investigate this model and ask how far into the future the body trajectory is predetermined by this initial ballistic throw. We measured body centre-of-mass (CoM) and foot trajectories during two-step sequences involving stepping onto illuminated targets, one for each foot. The targets were varied spatially such that leading foot placement could be dissociated from final CoM position. The results showed that the body throw was altered when stepping in different directions. However, the throw varied only with leading foot placement and not with final CoM position. Thus, provided the leading foot was placed identically, the same throw was used for steps in which body trajectories would later diverge. Furthermore, these trajectories did not diverge while the stepping foot was in the air but occurred after it touched down. The results are consistent with the ballistic stepping model. However, they suggest that the throw is limited in its predictive capacity, being concerned only with where the stepping foot is to be placed rather than final CoM position. This constitutes a one-step-at-a-time strategy that allows the body to be safely caught between steps.     

Relative contributions of visual and vestibular information on the trajectory of human gait

Seven healthy individuals were recruited to examine the interaction between visual and vestibular information on locomotor trajectory during walking. Subjects wore goggles that either contained a clear lens or a prism that displaced the visual scene either 20° to the left or right. A 5-s bipolar, binaural galvanic stimulus (GVS) was also applied at three times the subject's individual threshold (ranged between 1.2 to 1.5 mA). Subjects stood with their eyes closed and walked forward at a casual pace. At first heel contact, subjects opened their eyes and triggered the galvanic stimulus by foot switches positioned underneath a board. Reflective markers were placed bilaterally on the shoulders as the walking trajectory was captured using a camera mounted on the ceiling above the testing area. Twelve conditions were randomly assigned that combined four visual conditions (eyes closed, eyes open, left prism, right prism) and three GVS conditions (no GVS, GVS anode left, GVS anode right). As subjects walked forward, there was a tendency to deviate in the direction of the prisms. During GVS trials, subjects deviated towards the anode while walking, with the greatest deviations occurring with the eyes closed. However, when GVS was presented with the prisms, subjects always deviated to the side of the prisms, regardless of the position of the anode. Furthermore, the visual-vestibular conditions produced a larger lateral deviation than those observed in the prisms-only trials. This suggests that the nervous system examines the sensory inputs and takes into account the most reliable and relevant sensory input.     

Effect of sensory inputs on the soleus H-reflex amplitude during robotic passive stepping in humans

We investigated the modulation of the soleus (Sol) Hoffmann (H-) reflex excitability by peripheral sensory inputs during passive stepping using a robotic-driven gait orthosis in healthy subjects and spinal cord-injured patients. The Sol H-reflex was evoked at standing and at six phases during passive stepping in 40 and 100% body weight unloaded conditions. The Sol H-reflex excitability was significantly inhibited during passive stepping when compared with standing posture at each unloaded condition. During passive stepping, the H-reflex amplitude was significantly smaller in the early- and mid-swing phases than in the stance phase, which was similar to the modulation pattern previously reported for normal walking. No significant differences were observed in the H-reflex amplitude between the two unloaded conditions during passive stepping. The reflex depression observed at the early part of the swing phase during passive stepping might be attributed to the sensory inputs elicited by flexion of the hip and knee joints. The present study provides evidence that peripheral sensory inputs have a significant role in phase-dependent modulation of the Sol H-reflex during walking, and that the Sol H-reflex excitability might be less affected by load-related afferents during walking.     

Developmental changes in compensatory responses to unexpected resistance of leg lift during gait initiation

The development of the ability to integrate postural adjustments into the gait initiation process was investigated in children, using both kinematic and electromyographic (EMG) analysis. Subjects included children of 1 year of age (1-4 months' walking experience), 2-3 years of age (9-17 months' walking experience), 4-5 years of age (3-4 years' walking experience), and adults. We perturbed the balance of the children during gait initiation to determine the point at which infants begin to develop and finally master the ability to respond to external threats to balance during the gait initiation process. A magnet attached to the force platform on which the child stood was activated and served to resist the child's gait initiation (metal plaques on the soles of the shoes were attracted by the magnet) and thus served as an external perturbation during the gait initiation process. Kinematic and EMG analysis indicated that, while the ability to use preparatory postural adjustments in the gait initiation process emerges early in development, the ability to react efficiently to perturbations during gait initiation does not develop until after about 4-5 years of age. Though even the youngest age groups showed some response to the perturbation, it was highly variable, indicating its primitive form. The main response to the perturbation was a slight decrease in latency and increase in amplitude in the muscles used for push-off for gait initiation. Interestingly there was a shift in response pattern at 4-5 years of age, in both kinematic and EMG patterns. The amplitudes of the lateral and anteroposterior trunk and stance leg oscillations were significantly increased. In addition, the muscle response amplitudes (hamstrings and second quadriceps burst) of the swing leg were significantly increased and delayed (hamstring and gastrocnemius), with coactivation of agonist and antagonist muscles at the knee and ankle joint, concomitant with an exaggerated foot height of the first step.     

Influence of patellofemoral pain syndrome on plantar pressure in the foot rollover process during gait

BACKGROUND:

Patellofemoral Pain Syndrome is one of the most common knee disorders among physically active young women. Despite its high incidence, the multifactorial etiology of this disorder is not fully understood.

OBJECTIVES:

To investigate the influence of Patellofemoral Pain Syndrome on plantar pressure distribution during the foot rollover process (i.e., the initial heel contact, midstance and propulsion phases) of the gait.

MATERIALS AND METHODS:

Fifty-seven young adults, including 22 subjects with Patellofemoral Pain Syndrome (30 ± 7 years, 165 ± 9 cm, 63 ± 12 kg) and 35 control subjects (29 ± 7 years, 164 ± 8 cm, 60 ± 11 kg), volunteered for the study. The contact area and peak pressure were evaluated using the Pedar-X system (Novel, Germany) synchronized with ankle sagittal kinematics.

RESULTS:

Subjects with Patellofemoral Pain Syndrome showed a larger contact area over the medial (p = 0.004) and central (p = 0.002) rearfoot at the initial contact phase and a lower peak pressure over the medial forefoot (p = 0.033) during propulsion when compared with control subjects.

CONCLUSIONS:

Patellofemoral Pain Syndrome is related to a foot rollover pattern that is medially directed at the rearfoot during initial heel contact and laterally directed at the forefoot during propulsion. These detected alterations in the foot rollover process during gait may be used to develop clinical interventions using insoles, taping and therapeutic exercise to rehabilitate this dysfunction.     

The strategies to regulate and to modulate the propulsive forces during gait initiation in lower limb amputees

We used the framework of motor program adaptability to examine how unilateral above-knee (AK) or below-knee (BK) amputee subjects organize the global and local biomechanical processes of generation of the propulsive forces during gait initiation to overcome the segmental and neuro-muscular asymmetry. The organization of the global biomechanical process refers to the kinematics behavior of the couple center of foot pressure (CoP) and center of mass (CoM); the organization of the local biomechanical process refers to the propulsive forces generated by the prosthetic or intact limb during the anticipatory postural adjustment phase and the step execution phase. Specifically, we examined: i) the strategy to regulate the progression velocity, i.e., to maintain it comparably when the leading limb changed from the prosthetic limb to the intact limb; and ii) the strategy to modulate the progression velocity, i.e., to increase it when gait was initiated with the prosthetic limb vs. intact limb. The kinematics of the CoM and CoP in the amputees showed the same global biomechanical organization that is typically observed in able-bodied subjects, i.e., the production of the forward disequilibrium torque was obtained by a backward shift of the CoP, followed by a forward acceleration of the CoM. However, gait initiation was achieved by using a different local strategy depending on which limb was used to initiate the step. For the regulation of the CoM progression velocity, when the gait was initiated with the intact limb, the slope of the progression velocity during the anticipatory postural adjustment phase (APA) was steeper and lasted longer, the step execution duration was shorter, and the variation of the CoM speed was lower. In other words, to regulate the speed of progression, the amputee subjects controlled the spatial and temporal parameters of the propulsive forces. In the modulation of the CoM progression velocity, when the gait was initiated with the intact limb, the amputees controlled only the intensity of the propulsive forces during both the APA and step execution phases. In contrast, when the gait was initiated with the prosthetic limb, the modulation resulted mainly from the propulsive forces generated during the step execution phase. These different strategies are discussed in terms of the subjects capacity to adapt the motor program for gait initiation to new constraints.An erratum to this article can be found at     

Aging affects the ability to use optic flow in the control of heading during locomotion

Perceived self-motion from optic flow is implicated in the control of locomotion. Aging, which affects visual perception and sensorimotor integration, may result in an inability to use optic flow to guide heading while walking. The purpose of this study was to examine whether advanced age could impact on the steering of locomotion, when changing optic flow directions were presented in an immersive virtual environment (VE). Nine young adults (21.56 ± 3.20 years) and nine older adults (66.11 ± 3.95 years) participated in the study. Subjects were asked to walk while viewing a VE through a head-mounted display unit (Kaiser). The VE viewed by the subjects was a large room displayed as an expanding translational optic flow, with the focus of expansion (FOE) located at neutral, 20° or 40° to the right or left. Their task was to walk straight with respect to the VE. Kinematic data in 3D were collected, from which the body’s centre of mass (CoM) position and heading direction were calculated. Young subjects were able to make proper heading adjustments in the VE, with respect to FOE shifts, but not older individuals. Young subjects altered their CoM trajectory so that it was oriented in the direction opposite to the FOE in the physical environment and resulted in small deviation in the VE. The older adults did not adjust their locomotor patterns in response to the different flows presented and maintained similar walking trajectories across all trials. Advanced age results in an altered control of steering of locomotion in response to changing directions of optic flow. This may be related to an impaired perception and/or use of the optic flow, or due to inherent problems in sensorimotor integration.     

The role of putamen and pallidum in motor initiation in the cat

The participation of basal ganglia in motor initiation was studied in six cats operantly trained to perform a ballistic flexion movement, triggered after a brief sound in a simple reaction time condition or delayed after the same sound in the presence of a tone cue. The activity of 356 neurons was recorded in the putamen and in the pallidum (globus pallidus and entopeduncular nucleus). A total of 19.4% of the neurons were not related to the conditioned flexion movement: they were either unrelated to the task (10.1%) or related to other periods of the motor performance such as trial beginning or reward delivery (9.3%). About 60% of the remaining neurons — defined as task-related — exhibited changes in firing rate that occurred, in the reaction time condition, less than 100 ms after the go signal and therefore began prior to movement onset. For most neurons, in the delayed condition, these early changes were absent, suggesting that their occurrence in the reaction time condition was not a sensory response but rather was related to the movement initiation. In addition, for many neurons these changes shifted in time, remaining time-locked to the movement. Correlations between these early changes in activity and motor parameters were demonstrated, suggesting that these changes were movement-related. For most neurons the firing levels observed during intertrial intervals and during foreperiod were similar. The mean discharge rate during the foreperiod was 19.2 impulses/s. Three patterns of activity were observed before movement: increases in discharge rate (61% of task-related neurons), transient decreases followed by increases (11%), or prolonged decreases (28%). Only minor differences were found between the characteristics of the populations of neurons recorded in the three sites under study: on average the neurons recorded in the globus pallidus were more active than the neurons recorded in the putamen or in the entopeduncular nucleus. The fact that, for certain neurons, the changes of activity prior to movement were different in reaction time condition and in delayed condition showed that the pattern of activity preceding movement might depend on the temporal requirements for motor initiation. Taken together with the motor effects obtained in the same task following GABA-receptor activation with muscimol microinjections in these structures, the present results suggest that putamen and pallidal neurons participate in the initiation of the conditioned movement under study.     

The importance of perceived relative motion in the control of posture

Two experiments investigated the role of optic flow in controlling posture. Both experiments measured postural sway in two virtual environments with different 3-D structure but the same optic flow. Observers attempted to maintain balance on one foot while viewing an object that appeared either rigid with respect to the environment or that appeared to move concomitantly with head movements. The apparent object motion concomitant with head motion was achieved by changing the perceived, but not physical, depth of the object. For both objects, the optic flow information was the same and only depth information was varied. Observers showed a decrease in stability (as measured by head sway) when viewing the object that appeared to move, suggesting that perceived relative motion, not optic flow, signals self-motion to the postural control system.     

Availability of visual and proprioceptive afferent messages and postural control in elderly adults

The ability of young and elderly adults to keep a stable upright posture while facing changes in the availability of visual and/or propriomuscular information was investigated. The two sensory sources of information were alternatively available and withdrawn, jointly and separately, during 10-s alternating sequences. Vision was modified by means of liquid-crystal goggles, and proprioception was altered by means of tendon vibration of both antagonistic ankle muscles. Elderly adults were less stable than young adults when vision was withdrawn. Both groups were greatly affected when proprio-muscular inputs were altered by vibration. Under constant visual conditions and following a propriomuscular perturbation (i.e., vibration), elderly adults were unable to take advantage of the reinsertion of propriomuscular inputs. They showed a transient, decreased stability and were unable to fully recover during a 10-s period, whereas young adults were able to rapidly integrate the information to stabilize their posture. When both propriomuscular and visual inputs were withdrawn and concurrently reinserted, the elderly adults did not show a transitory increase in the velocity of the center of foot pressure. The present results extend findings on the inability of elderly adults to reconfigure rapidly the postural set following reinsertion of sensory inputs. The results also suggest that elderly adults have difficulties in taking advantage of sensory redundancy in postural control.     

Effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense

The present study investigated the effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense in the upper extremity. Twenty-four healthy subjects (12 males and 12 females) performed a 45-min standardized mouse-operated computer task on two occasions. The task consisted of painting rectangles that were presented on the screen. On one occasion, time pressure and precision demands were imposed (more demanding task, MDT), whereas, on the other occasion, no such restraints were added (less demanding task, LDT). The order of the two task versions was randomized. Tissue oxygen saturation in the trapezius and extensor carpi radialis muscles was recorded throughout, and the position-matching ability of the wrist was measured before and after the tasks. In addition, measurements of autonomic nervous system reactivity and subjective ratings of tenseness and physical fatigue were obtained. Performance was measured in terms of the number of rectangles that were painted during the task. During MDT, oxygen saturation in extensor carpi radialis decreased (P<0.05) compared to LDT. These data were paralleled by increased electrodermal activity (P<0.05), skin blood flow (P<0.05), ratings of tenseness and fatigue (P<0.01), and increased performance (P<0.01) during MDT. Females exhibited lower oxygen saturation than males, during rest as well as during the computer tasks (P<0.01). Wrist repositioning error increased following LDT as compared to MDT (P<0.05). In conclusion, computer mouse work under time pressure and precision demands caused a decrease in forearm muscle oxygenation, but did not affect wrist position sense accuracy. We attribute our changes in oxygenation more to increased oxygen consumption as a result of enhanced performance, than to vasoconstriction.Presented in part at the 49th Nordiska Arbetsmiljömötet in Savonlinna Finland, August 25–27, 2003