Timing and magnitude of electromyographic activity for two-joint arm movements in different directions

Abstract

1. We studied electromyographic (EMG) and kinematic features of self-paced human arm movements involving rotations about the shoulder and elbow joints. Movements were initiated from various positions and covered much of the reachable work space in the horizontal plane. The attempt was to characterize robust features of the relative timing and magnitude of the EMG activity at the two joints, and to correlate them with variables related to the initial and final positions. 2. The pattern of muscle activity at each joint was typically characterized by bursts of alternating agonist and antagonist activity, comparable with the three-burst pattern associated with single-joint movements. As the spatial direction of the target was altered, the magnitude of each burst was modulated over a continuous range. Modulation down to zero activity was observed, not only for later bursts, as has been shown in some cases of single-joint movements, but for the first agonist burst as well. 3. In the preceding paper we showed that the choice of agonists (i.e., flexors or extensors) at each joint is predictable on the basis of the target direction relative to the distal segment (psi). Here, we present quantitative analyses of initial agonist EMG activity at the shoulder and elbow, which reveal that the onset-time difference between agonists at the two joints also varied systematically with psi, and so did their relative magnitude. 4. For most target directions, initial EMG activity at the shoulder preceded that at the elbow by 5-40 ms. Exceptions were observed mainly for target directions near the transitions between initial flexor and initial extensor activity at the shoulder. In these cases the initial agonist activity at the shoulder was greatly reduced or, in some cases, appeared entirely suppressed, although the later bursts were present in their usual temporal alignment with the corresponding bursts at the elbow. 5. Antagonist onset at the elbow tended to precede antagonist onset at the shoulder, but the difference in timing did not vary consistently with psi. 6. Despite the consistency of initial agonist timing between the two joints, the agonist onset-time difference was poorly correlated with the apparent difference in the onset times of shoulder and elbow joint rotations. The latter difference, which is affected by mechanics, cannot therefore be imputed directly to the CNS.(ABSTRACT TRUNCATED AT 400 WORDS)

Goal-dependent modulation of the long-latency stretch response at the shoulder, elbow, and wrist

“…Our finding runs counter to previous work demonstrating that shoulder muscles are activated prior to elbow muscles during voluntary reaching movements (Karst and Hasan 1991) and was unexpected given afferent and efferent conduction delays. Conduction velocities of ϳ50 -70 m/s (Ingram et al 1987;Macefield et al 1989) and the ϳ30-to 50-cm distance between the shoulder and wrist muscles would yield goal-dependent activity at shoulder muscles 9 -20 ms prior to wrist muscles.…”

Control of Fast-Reaching Movements by Muscle Synergy Combinations

“…We have extended those observations by showing that the characteristics of each individual muscle waveform are related to the waveforms of all the other muscles because they are organized into muscle synergies. We expect that our approach would give similar insight into the construction of the muscle patterns for arm movements in the horizontal plane (Wadman et al, 1980;Karst and Hasan, 1991;Scott, 1997) as well as the step-tracking movements of the wrist Strick, 1990, 1999). Our findings might also provide a new interpretation for the observation that the dynamic muscle torques at the shoulder and elbow during reaching movements are related almost linearly to each other, and their relative scaling changes regularly with movement direction (Gottlieb et al, 1997).…”

“…Although the muscle activation waveforms of these movements have been characterized (Wadman et al, 1980;Karst and Hasan, 1991;Flanders et al, 1994Flanders et al, , 1996, their spatiotemporal organization, simultaneously across muscles and time, has not yet been described. For each subject, we used an optimization algorithm to identify four or five time-varying synergies whose combinations explained a large fraction of the total data variation for many point-to-point movements, including those with different endpoints, different loads, or different forearm postures, as well as for more complex reaching movements in-volving velocity reversals and via-points.…”

Primary motor cortex neurons classified in a postural task predict muscle activation patterns in a reaching task

“…For example, goal-directed reaching involves agonist muscle activity to propel the hand toward the goal and antagonistic muscle activity to decelerate and stop at the goal (Flanders et al 1994;Marsden et al 1983;Wierzbicka et al 1986). The selection, onset time, and magnitude of muscle activity during reaching depend on many factors such as target and initial limb position, arm geometry, and external loads (Caminiti et al 1990;Hong et al 1994;Karst and Hasan 1991;Scott 1997).…”