Expertise-dependent modulation of muscular and non-muscular torques in multi-joint arm movements during piano keystroke

The problem of skill-level-dependent modulation in the joint dynamics of multi-joint arm movements is addressed in this study using piano keystroke performed by expert and novice piano players. Using the measured kinematic and key-force data, the time varying net, gravitational, motion-dependent interaction (INT), key-reaction (REA), and muscular (MUS) torques at the shoulder, elbow, wrist, and metacarpophalangeal (MP) joints were computed using inverse dynamics techniques. INTs generated at the elbow and wrist joints, but not those at the MP joint, were greater for the experts as compared with the novices. REA at the MP joint, but not at the other joints, was less for the experts as compared with the novices. The MUSs at the MP, wrist, and elbow joints were smaller, and that at the shoulder joint was larger for the experts as compared with the novices. The experts also had a lesser inter-strike variability of key striking force and key descending velocity as compared with the novices. These findings indicated that the relationship among the INT, REA, and MUS occurring at the joints of the upper-extremity differed between the expert and novice piano players, suggesting that the organization of multi-joint arm movement is modulated by long-term motor training toward facilitating both physiological efficiency and movement accuracy.     

Motor planning of arm movements is direction-dependent in the gravity field

In the present study we analyzed kinematic and dynamic features of arm movements in order to better elucidate how the motor system integrates environmental constraints (gravity) into motor planning and control processes. To reach this aim, we experimentally manipulated the mechanical effects of gravity on the arm while maintaining arm inertia constant (i.e. the distribution of the mass around the shoulder joint). Six subjects performed single-joint arm movements (rotation around the shoulder joint) in both sagittal (upward, U, versus downward, D) and horizontal (left, L, versus right, R) planes, at different amplitudes and from different initial positions. Under these conditions, shoulder gravitational torques (SGTs) significantly varied when arm movements were performed in the sagittal but not in the horizontal plane. Contrary to SGTs, arm inertia remained constant and similar for both horizontal and sagittal planes since subjects performed arm movements with only one degree of freedom. All subjects, whatever the movement direction, appropriately scaled shoulder joint kinematic parameters according to movement amplitude. Furthermore, peak velocity and movement duration were equivalent for both horizontal and sagittal planes. Interestingly, some kinematic parameters significantly differed according to U/D but not L/R directions. Specifically, acceleration duration was greater for D than U movements, while the opposite was true for peak acceleration. Consequently, although vertical and horizontal arm movements shared a general common strategy (i.e. scaling law), the kinematic asymmetries between U and D arm movements, especially those that reflect central planning process (i.e. peak acceleration), indicated different motor intentions regarding the direction of the upcoming movement. These findings indicate that the interaction of the arm with the dynamics of the environment is internally represented during the generation of arm trajectories.     

Roles of proximal-to-distal sequential organization of the upper limb segments in striking the keys by expert pianists

Roles played by the proximal-to-distal sequencing (PDS) of the multi-joint limb in a relatively slow target-aiming task by the arm were investigated using keystroke motion on the piano. Kinematic recordings were made while experts (N=7) and novices (N=7) of piano players performed an octave keystroke at four linearly-scaled loudness levels with a short tone production (staccato) technique. The temporal relationship of the peak angular velocity at the shoulder, elbow and wrist joints showed a clear PDS organization for the experts, but not for the novices. The result thus confirmed that the PDS occurred in a slow and skilled multi-joint movement. The summation effect of segmental speed in terms of increment of the peak segmental angular velocity was equal for both groups. Similarly, no group difference was found for the total kinetic energy produced by the upper limb during keystroke. The role of the PDS in piano keystroke thus cannot be explained by the exploitation of speed-summation effect and mechanical efficiency. Compared to the novices, the experts had a longer period and a greater magnitude of deceleration at the shoulder and elbow joints while their adjacent distal joints were accelerating. These results indicated that greater inertial forces had been generated to descend the forearm as well as the hand for the experts. A dominant role of the PDS in pianists can therefore be to effectively exploit motion-dependent interaction torques at the forearm and hand, and thereby reducing muscle-dependent torques to make the keystroke more physiologically efficient.     

Grip and load force coupling during discrete vertical arm movements with a grasped object in cerebellar atrophy

Control of isometric grip forces during manipulation of objects is an essential feature of all skilled manual performances. Recent studies suggested that the anticipation of movement-induced loads may be a cerebellar function. We analysed grip force adjustments to fluctuations of inertial loads during discrete vertical movements with a grasped object in five patients with cerebellar atrophy and five healthy control subjects. Normally grip force is precisely adapted to the load fluctuations, in particular to the maximum load force, which occurs early in upward and late in downward movements. Both groups produced similar accelerations of the grasped object and consequently similar maximum loads. However, cerebellar patients established increased static grip forces during stationary holding of the object and increased force ratios between grip and load force at the time of maximum acceleration. These findings are congruent with earlier studies analysing grip and load force coupling in patients with cerebellar lesions. In contrast to earlier studies, we found no significant differences in the timing of grip force onset and grip force maximum relative to the onset of movement and maximum acceleration, respectively, between normal controls and four of five cerebellar patients. However, a regression analysis between grip and load forces during the load increase and decrease phases of the movement suggested deficits in the close temporospatial coupling between the two forces in all cerebellar patients. Our findings give further support to the notion that the cerebellum plays a crucial role in the forward control of grip force magnitude and timing during voluntary object manipulation. Compared to earlier studies, the increase in grip forces may be interpreted as a general control strategy to compensate for motor deficits, whereas impairments of temporal grip force regulation may occur at different degrees of dysfunction during the progression of cerebellar atrophy.     

The stability of precision grip forces during cyclic arm movements with a hand-held load

In this paper we examine the coordination of grip force and load during brisk cyclic arm movements with a hand-held object under a range of conditions. We show that, regardless of the surface texture of the object or movement frequency, grip force is modulated in parallel with load. Thus, the tight coupling between grip force and load observed in short-duration tasks such as lifting or point-to-point movements is also seen in longer-duration cyclic movements. Moreover, the gain of the relation between grip force and load remains essentially constant over time. Across conditions, we find a dissociation between the gain of the relation between grip force and load and the grip force offset. With a more slippery surface texture both the gain and offset increase, whereas increases in frequency lead to an increase in the offset but a decrease in gain. This suggests that these two parameters are under independent high-level control. We also observe that when subjects were instructed to maintain a high-baseline grip force during the movement, grip force was still modulated with load even though an increase in grip was not necessary to prevent slip. This suggests that there is an obligatory coupling between grip force and load. This coupling might be subserved by low-level mechanisms not under high-level control.     

Grip forces exerted against stationary held objects during gravity changes

 In the present study, grip forces exerted against a stationary held object were recorded during parabolic flights. Such flight maneuvers induce changes of gravity with two periods of hypergravity, associated with a doubling of normal terrestrial gravity, and a 20 s period of microgravity. Accordingly, the object’s weight changed from being twice as heavy as normally experienced and weightless. Grip-force recordings demonstrated that force control was seriously disturbed only during the first experience of hyper- and microgravity, with the grip forces being exceedingly high and yielding irregular fluctuations. Thereafter, however, grip force traces were smooth, the force level was scaled to the object’s weight under normal and high-G conditions, and the grip force changed in parallel with the weight during the transitions between hyper- and microgravity. In addition, during weightlessness, when virtually no force was necessary to stabilize the object, a low force was established, which obviously represented a reasonable safety margin for preventing possible perturbations. Thus, all relevant aspects of grip-force control observed under normal gravity conditions were preserved during gravity changes induced by parabolic flights. Hence, grip-force control mechanisms were able to cope with hyper- and microgravity, either by incorporating relevant receptor signals, such as those originating from cutaneous mechanoreceptors, or by adequately including perceived gravity signals into control programs. However, the adaptation to the uncommon gravity conditions was not complete following the first experience; finer tuning of the control system to both hyper- and microgravity continued over the measurement interval, presumably with a longer observation period being necessary before a stable performance can be reached. Received: 23 April 1998 / Accepted: 20 December 1998     

Impaired direction and extent specification of aimed arm movements in humans with stroke-related brain damage

The role of sensorimotor (S-M) areas in the specification of kinematic parameters for aiming movements was studied by comparing the performance of six subjects with unilateral stroke to that of matched control subjects. Rapid arm movements were made to one of four targets by rotating the forearm in a short (20 degrees) or long (45 degrees) arc of motion. Thus, the four targets represented two directions (flexion or extension) and two extents (short or long). Subjects with stroke used the arm ipsilateral to the side of the lesion. A timed-response paradigm was used to dissociate response initiation and specification. Subjects initiated movements in concert with the last of four regularly timed tones. A visual cue of the designated target was presented during the preparation interval (400-0 ms) before the last tone. Targets were presented in a fixed sequence (predictable condition) or a random sequence (unpredictable condition). No significant differences in performance were found between stroke and control groups in the predictable condition. In the unpredictable condition, subjects with stroke produced more direction errors and were less accurate in extent than the control subjects. As specification time increased to 400 ms, the frequency of direction errors attenuated less for stroke than for control groups, but the reduction in magnitude of extent errors was similar for the two groups. When specification was minimal (i.e., <100 ms), default responses were distributed equally between directions and clustered around the short extent. Further, wrong direction responses did not converge on the designated extent as specification time increased. This pattern of findings is consistent with a view of parameterization of planning and executing movements, in which direction and extent can be specified in parallel. Our results suggest that ipsilateral S-M areas contribute to the specification of an optimal motor program, particularly when imperative programming of unimanual goal-directed aiming movements is required.     

Motor control goes beyond physics: differential effects of gravity and inertia on finger forces during manipulation of hand-held objects

According to basic physics, the local effects induced by gravity and acceleration are identical and cannot be separated by any physical experiment. In contrast—as this study shows—people adjust the grip forces associated with gravitational and inertial forces differently. In the experiment, subjects oscillated a vertically-oriented handle loaded with five different weights (from 3.8 N to 13.8 N) at three different frequencies in the vertical plane: 1 Hz, 1.5 Hz and 2.0 Hz. Three contributions to the grip force—static, dynamic, and stato-dynamic fractions—were quantified. The static fraction reflects grip force related to holding a load statically. The stato-dynamic fraction reflects a steady change in the grip force when the same load is moved cyclically. The dynamic fraction is due to acceleration-related adjustments of the grip force during oscillation cycles. The slope of the relation between the grip force and the load force was steeper for the static fraction than for the dynamic fraction. The stato-dynamic fraction increased with the frequency and load. The slope of the dynamic grip force–load force relation decreased with frequency, and as a rule, increased with the load. Hence, when adjusting grip force to task requirements, the central controller takes into account not only the expected magnitude of the load force but also such factors as whether the force is gravitational or inertial and the contributions of the object mass and acceleration to the inertial force. As an auxiliary finding, a complex finger coordination pattern aimed at preserving the rotational equilibrium of the object during shaking movements was reported.     

Disruption of bilateral temporal coordination during arm swinging in patients with hemiparesis

Persistent motor deficits in the paretic arm present a major barrier to the recovery of the ability to perform bimanual tasks even in individuals who have recovered well after a stroke. Impaired performance may be related to deficits in bimanual temporal coordination due to stroke-related damage of specific brain motor structures as well as changed biomechanics of the paretic arm. To determine the extent of the deficit in bilateral temporal coordination after the stroke, we investigated how bilateral reciprocal coordination was regained after external perturbations of the arm in individuals with hemiparesis due to stroke. We used a bilateral task that would be minimally affected by the unilateral arm motor deficit. Nine non-disabled control subjects and 12 individuals with chronic hemiparesis performed reciprocal (anti-phase) arm swinging in the standing position for 15 s per trial. In each trial, movement of one arm was unexpectedly and transiently (~150–350 ms) arrested at the level of the wrist once in the forward and once in the backward phase of swinging. Perturbation was applied to the left and right arms in control subjects and to the paretic and non-paretic arms of individuals with hemiparesis. Kinematic data from endpoint markers on both hands and electromyographic activity of anterior and posterior deltoid muscles from both arms were recorded. The oscillatory period, the phase differences between arms and the mean EMG activity before, during and after perturbation were analyzed. In both groups the perturbation altered the period of the perturbed cycle in both the arrested and non-arrested arms and resulted in a change from anti-phase to in-phase coordination, following which anti-phase coordination was regained. Recovery of anti-phase swinging took significantly longer in patients with hemiparesis compared to control subjects. Stable pre-perturbed (anti-phase) reciprocal coordination was regained within one cycle following perturbation for the control subjects and within two cycles following perturbation for the patients with hemiparesis. Analysis of EMG activation levels showed that, compared to control subjects, there was significantly less activation of the shoulder muscles in response to perturbation in the patient group and the pattern of muscle activation in the paretic arm was opposite to that in the non-paretic and control arms. The finding that patients had a reduced capacity for maintaining and restoring the required reciprocal coordination when perturbation occurred suggests that stroke-related brain damage in our patients led to instability of bilateral temporal coordination for this rhythmical task.     

Efficiency of food utilization during body weight gain in dormice (Glis glis)

During spontaneous body weight gain in dormice, Glis glis, progressive increases in the efficiency of food utilization as defined by weight gain (g)/food intake (g), and parallel increases in mean daily food intake were observed. Towards the end of the weight gain period, there was an abrupt drop in feeding efficiency with no significant change in food intake even when the latter was expressed relative to an index of each animal's “metabolic mass” (body weight_kg^0.62). Animals whose body weight increases followed a return to ad lib feeding after prolonged food restriction showed marked decreases in feeding efficiency from initially high values which were independent of changes in food intake. These results are discussed in relation to the sliding set point concept of body weight regulation in hibernators.     

Modulation of cutaneous reflexes in arm muscles during walking: further evidence of similar control mechanisms for rhythmic human arm and leg movements

Stimulation of cutaneous nerves innervating the hand evokes prominent reflexes in many arm muscles during arm cycling. We hypothesized that the mechanisms controlling reflex modulation during the rhythmic arm swing of walking would be similar to that documented during arm cycling. Thus, we expected cutaneous reflexes to be modulated by position in the walking cycle (phase dependence) and be different when walking compared to contraction while standing (task dependence). Subjects performed static postures similar to those occurring during walking and also walked on a treadmill while the superficial radial nerve was electrically stimulated pseudorandomly throughout the step cycle. EMG was recorded bilaterally from upper limb muscles and kinematic recordings were obtained from the elbow and shoulder joints. Step cycle information was obtained from force-sensing insoles. Analysis was conducted after averaging contingent upon the occurrence of stimulation in the step cycle. Phase-dependent modulation of cutaneous reflexes at early (~50–80 ms) and middle (~80–120 ms) latencies was observed. Coordinated bilateral reflexes were seen in posterior deltoid and triceps brachii muscles. Task dependency was seen in that reflex amplitude was only correlated with background EMG during static contraction (75% of comparisons for both early and middle latency reflexes). During walking, no significant relationship between reflex amplitude and background EMG level was found. The results show that cutaneous reflex modulation during rhythmic upper limb movement is similar to that seen during arm cycling and to that observed in leg muscles during locomotion. These results add to the evidence that, during cyclical movements of the arms and legs, similar neural mechanisms observed only during movement (e.g. central pattern generators) control reflex output. Electronic Publication     

Hemispheric specialization in the co-ordination of arm and trunk movements during pointing in patients with unilateral brain damage

During pointing movements involving trunk displacement, healthy subjects perform stereotypically, selecting a strategy in which the movement is initiated with either the hand or trunk, and where the trunk continues after the end of the hand movement. In a previous study, such temporal co-ordination was not found in patients with left-hemispheric brain lesions reaching with either their dominant paretic or with their non-dominant non-paretic arm. This co-ordination deficit may be associated in part with the presence of a lesion in the dominant left hemisphere. If so, then no deficit should be observed in patients with stroke-related damage in their non-dominant right hemisphere moving with their ipsilesional arm. To verify this, 21 right-hand dominant adults (7 who had had a stroke in the right hemisphere, 7 who had had a stroke in the left hemisphere and 7 healthy subjects) pointed to two targets located on a table in front of them in the ipsilateral and contralateral workspace. Pointing was done under three movement conditions: while not moving the trunk, while bending the trunk forward and while bending the trunk backwards. The experiment was repeated with the non-paretic arm of patients with stroke and for the right and left arms of healthy subjects. Kinematic data were recorded (Optotrak). Results showed that, compared to healthy subjects, arm-trunk timing was disrupted in patients with stroke for some conditions. As in patients with lesions in the dominant hemisphere, arm-trunk timing in those with lesions in the non-dominant hemisphere was equally more variable than movements in healthy subjects. However, patients with dominant hemisphere lesions used significantly less trunk displacement than those with non-dominant hemisphere lesions to accomplish the task. The deficit in trunk displacement was not due to problems of trunk control or sitting balance since, in control experiments, all subjects were able to move the trunk the required distance, with and without the added weight of the limb. Results support the hypothesis that the temporal co-ordination of trunk and arm recruitment during pointing movements is mediated bilaterally by each hemisphere. However, the difference in the range of trunk displacement between patients with left and right brain lesions suggests that the left (dominant) hemisphere plays a greater role than the right in the control of movements involving complex co-ordination between the arm and trunk. Electronic Publication     

Time-based prediction in motor control: evidence from grip force response to external load perturbations

An object held in precision grip creates predictable load forces on the hand during voluntary hand movement and these are associated with anticipatory modulation of grip force. Conflicting results have been obtained over whether predictable external load perturbations result in anticipatory grip force responses (e.g. Blakemore et al. in J Neurosci 18(18):7511–7518, 1998; Weeks et al. in Exp Brain Res 132:404–410, 2000). This paper investigated whether the discrepancies reflect differences in the methods used in estimating the time delay. Subjects held a manipulandum that delivered load force perturbations in the form of pulses of variable duration and interval or periodic 0.5 and 1 Hz square waves or sinusoids. The grip forces exerted by the subjects were measured. Two methods were used to assess the time delay of the grip force in relation to the load force: (1) cross-spectral analysis, (2) a single threshold method applied on time-locked averaged data. Despite a phase lag shown by the cross-spectral analysis, the threshold method revealed grip force increased 264.8±40.2 ms before the onset of the load force when 0.5 Hz square waves were used as the load force perturbation and 70.2±17.0 ms before the load force when 1 Hz square waves were used. Computer simulations indicated that the single threshold method gives a more sensitive estimate of the onset time than the cross-spectral analysis. We conclude that discrepancies in previous studies reflect differences in the methods used to assess the time-delay and that there is an anticipatory component in the grip force response to predictable external load perturbation.     

Valence-dependent brain potentials of processing augmented feedback in learning a complex arm movement sequence

ERPs in the EEG were scrutinized in learning a complex arm movement sequence with the aim to examine valence effects on processing augmented feedback during practice. Twenty-four healthy subjects practiced one session with 192 feedback trials according to an adaptive bandwidth feedback approach with a high informational level of feedback information (i.e., amplitude and direction of errors). The bandwidth for successful performance (increase of a score for a monetary competition) was manipulated to yield a success rate (positive feedback frequency) of approximately 50% adaptive to the current performance level. This allowed a variation of feedback valence unconfounded by success rate. In line with our hypotheses, the EEG data showed a valence-dependent feedback-related negativity (FRN) and a later fronto-central component at the FCz electrode as well as a P300 component at the Pz electrode. Moreover, the P300 and amplitudes in the FRN time window reduced in the second half of practice but were still dependent on feedback valence. Behavioral adjustments were larger after feedback with negative valence and were predicted by the late fronto-central component. The data support the assumption of feedback valence-dependent modulation of attentional cognitive involvement in motor control and learning.     

Predictive control of grip force when moving object with an elastic load applied on the arm

Skilled object manipulation relies on the capability to adjust the grip force according to the consequences of our movements in terms of the resulting load force of the object. Such predictive grip force control requires (at least) two neural processes: (1) predicting the kinematic characteristics of the unfolding arm trajectory and (2) predicting the load force on the object resulting, among other factors, from the arm movement. The goal of this study was to examine whether subjects can still anticipate the resulting load force on the object when the moving arm is submitted to a type of load that does not contribute to the object load. To this end, 12 subjects were asked to rhythmically move a 0.4 kg object under three different conditions. In the first condition (ARM), an elastic cord was attached to the upper arm. In the second condition, the elastic cord was attached to the object (OBJECT). In the third condition, the elastic cord was absent (NO ELAST). At the kinematic level, results showed no influence of the elastic cord on the pattern of movement of the object. At the kinetic level, cross-correlation analyses between grip force and load force acting on the object revealed significant correlations with minimal delays. In addition, grip force profiles were similar under the ARM and NO ELAST conditions, both differing from the OBJECT condition. Overall, we interpret these results as evidence that the neural processes involved in the prediction of the arm trajectory and those involved in the prediction of the load on the object held can take into account different external force fields, thereby preserving the functionality of the behaviour.     

Forces applied by anterior and posterior teeth and roles of periodontal afferents during hold-and-split tasks in human subjects

Hold-and-split tasks were performed by 20 subjects (12 females and 8 males) using the right central incisors, canines, 2nd premolars, and 1st molars, respectively. Half a peanut was positioned on a transducer-equipped plate and the subject was instructed to hold the plate with the peanut between two antagonistic teeth, and not using more force than necessary. After ca. 3 s the subject was instructed to split the peanut in a natural manner. Each session consisted of a series of three in which the subject performed the hold-and-split task five times for each tooth. Thus, in total, data were obtained from 60 trials for each subject. The magnitude of the forces and the force rates used to split the peanut increased distally along the dental arch. However, the duration of the split phase was similar for the various teeth examined. During anesthesia of the periodontal ligament (four subjects), no significant changes were seen in the split phase. The forces used to hold the peanut between the teeth also increased distally along the dental arch: 0.60 N for the incisor, 0.77 N for the canine, 1.15 N for the 2nd premolar, and 1.74 N for the 1st molar. The difference in hold forces for the various teeth can be explained by the different sensitivity characteristics of the periodontal afferents innervating anterior and posterior teeth. During periodontal anesthesia, the magnitude and variability of the hold forces increased for all types of teeth, thus supporting the suggestion that periodontal afferent information is used in the regulation of the level of forces used to hold and manipulate morsels between the teeth.     

Coordination of fingertip forces in object transport during locomotion

Walking while carrying a hand-held object requires the generation of appropriate grip forces to offset the inertial forces produced during locomotion. The present study examined the interaction between grip forces and locomotion-induced inertial forces across the gait cycle. Eight subjects transported a container under three conditions: self-paced transport with and without accuracy constraints and a velocity-constrained condition. The results showed that the trunk and transported container moved in a synchronized, sinusoidal pattern during all conditions. Grip and inertial forces of the transporting hand were highly coupled in an anticipatory fashion, regardless of task demands. The inertial forces were higher and the coupling was greater in the faster, unconstrained condition. However, grip force modulation was observed even when the inertial forces acting on the container were small and applied indirectly to the container through the locomotor effects originating in the legs and trunk. We suggest that continuous grip force adjustment is used as a generalized strategy to maximize efficiency during object transport regardless of the size or origin of the inertial forces. Electronic Publication     

No stable arm preference during the pre-reaching period: a comparison of right and left hand kinematics with and without a toy present

Adult hand preference emerges from complex developmental changes in arm and hand use during childhood. Recent reports have highlighted the importance of understanding arm and hand use during the first year of life including the period before reach onset. This longitudinal study tested the hypothesis that significant right-left differences exist in pre-reaching arm movements. We examined right and left hand kinematics from 13 healthy infants during trials with and without a toy present from 8 weeks of age through the week of reach onset. Significant right-left differences were found, however there was no clear pattern within a condition or across conditions. Without a toy present, the right hand moved faster, yet ended further from midline, and displayed more movements during the Late phase compared to other phases. With a toy present, the right hand moved longer lengths, yet ended movements further away from the toy. When left and right hand kinematics were combined, previous findings of right hand kinematics alone were supported. Although infants begin adapting their pre-reaching kinematics many weeks before reach onset, we did not find evidence of a systematic right--left difference before reach onset in movements with or without a toy present. Our results, coupled with other reports, suggest hand asymmetries begin to emerge over the year following reach onset amid developmental changes both within the infant, and the physical and social environment.     

The development toward stereotypic arm kinematics during reaching in the first 3 years of life

We recorded reaching movements from nine infants longitudinally from the onset of reaching (5th postnatal month) up to the age of 3 years. Here we analyze hand and proximal joint trajectories and examine the emerging temporal coordination between arm segments. The present investigation seeks (a) to determine when infants acquire consistent, adult-like patterns of multijoint coordination within that 3-year period, and (b) to relate their hand trajectory formation to underlying patterns of proximal joint motion (shoulder, elbow). Our results show: First, most kinematic parameters do not assume adult-like levels before the age of 2 years. At this time, 75% of the trials reveal a single peaked velocity profile of the hand. Between the 2nd and 3rd year of life, “improvements” of hand- or joint-related movement units are only marginal. Second, infant motor systems strive to obtain velocity patterns with as few force reversals as possible (uni- or bimodal) at all three limb segments. Third, the formation of a consistent interjoint synergy between shoulder and elbow motion is not achieved within the 1st year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12–15 months of age and continue to develop up to the 3rd year. In summary, the development toward adult forms of multijoint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace 3–4 months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the 2nd year of life. Received: 10 April 1996 / Accepted: 28 May 1997