Timing of muscle activation in a hand movement sequence.

Recent studies have described muscle synergies as overlapping, multimuscle groups defined by synchronous covariation in activation intensity. A different approach regards a synergy as a fixed temporal sequence of bursts of activity across groups of motoneurons. To pursue this latter definition, the present study used a principal component (PC) analysis tailored to reveal the across-muscle temporal synergies of human hand movement. Electromyographic (EMG) activity was recorded as subjects used a manual alphabet to spell a list of words. The analysis was applied to the EMG waveforms from 27 letter-to-letter transitions of equal duration. The first PC (of 27) represented the main temporal synergy; after practice, it began to account for more of the EMG variance (up to 40%). This main synergy began with a burst in the 4-finger extensor and a silent period in the flexors. There were then progressively later and shorter bursts in the thumb abductor, thumb flexor, little finger abductor, and finally the finger flexors. The results suggest that hand movements may be generated by activity waves unfolding in time. Because finger muscles are under relatively direct cortical control, this suggests a specific form of cortical pattern generation.

Drawing sequences of segments in 3D: kinetic influences on arm configuration.

Complex movements are generally thought to consist of a series of simpler elements. If this is so, how does the sensorimotor system assemble the pieces? This study recorded and evaluated sequences of arm movements to various targets placed in three-dimensional (3D) space. Subjects performed sequences consisting of single, double, or triple segments with the same first target but with different second targets. The data analysis focused on the first movement segment and evaluated hand path curvature, the hand's final approach to the first target, and the whole arm postures at the beginning and end. Although some idiosyncratic differences in approach were observed, only the final arm posture depended, in a consistent way, on which particular movement was to follow as the second segment. This provided evidence for "coarticulation" of the two segments, only at the level of arm posture, and simulations revealed that this anticipatory modification improved the energetic efficiency of the second segment. Data from movements through five consecutive triple segments (i.e., 5 triangles) were assessed to determine whether kinematic constraints, such as Donders' law, apply to repetitive drawing movements. Although such constraints could prevent the accumulation of changes in arm posture, this was not observed. Instead, in most cases, the elbow was a little bit higher at the end of each triangle than at the beginning. Taken together, the results suggest that coarticulation may facilitate the joining of two segments and the efficiency of the second movement, but does not extend over the drawing of several segments.

An evaluation of the minimum-jerk and minimum torque-change principles at the path, trajectory, and movement-cost levels.

In the present study we evaluated the minimum jerk and the minimum torque-change model at the path, trajectory, and movement-cost levels. To date, most evaluations of these models have mainly been restricted to path comparisons. Assessments of the time courses of realized jerk and torque changes are surprisingly lacking. Moreover, the extent to which the presumed optimized parameters change as a function of the duration and other temporal features of aiming movements has never been investigated, most probably because the models presuppose movement time. In order to fill this gap, we analyzed a subset of the data of an earlier experiment in which 12 participants performed leftward and rightward planar pointing movements. Hand displacements and joint excursions were recorded with a 3D motion-tracking system and subsequently evaluated by means of model-based analyses. The results show that despite a good agreement between observed paths and predicted paths, especially by the minimum torque-change model, the time courses of jerk and torque changes of observed and modeled movements differed considerably. These differences could mainly be attributed to asymmetrical properties of the time functions of slow movements. Variations of movement costs as a function of movement time and skewness of tangential velocity profiles show that, especially at high movement speed, costs increase exponentially with departures of symmetry. It is concluded that trajectory-formation models have limited explanatory power in situations that require demanding information processing during the homing-in phase of goal-directed movements. However, for slow movements, deviations from the optimal timing profiles require little extra costs in terms of jerk or torque change.