Timing of muscle activity during reaching while standing: systematic changes with target distance.

We examined the effects of changing target distance from within arm's length (AL) to beyond arm's length on the onsets of electromyographic (EMG) activity of non-focal muscles for a reaching task performed while standing. Two questions were addressed. First, do changes in target distance result in consistent changes in the onsets of non-focal anticipatory muscle activity of the trunk and legs in healthy subjects? Second, do changes in onsets of all non-focal muscles vary in a similar fashion in response to varying target distance? Thirteen young, healthy adults performed rapid, bilateral reaching movements to targets placed at shoulder height at four distances while electromyographic activity was recorded from muscles of the arm, trunk and legs. Ground reaction forces and arm kinematics were also recorded. The onsets of most non-focal muscles occurred prior to the onset of arm movement, and occurred progressively earlier as target distance was increased. An exception to this trend was the onset of the erector spinae muscle, which occurred progressively later as target distance was increased. These data support the notion that reaches to targets beyond arm's length involve anticipatory non-focal muscle activity that acts to transport the arm to the target rather than simply to resist the perturbation caused by the arm movement. The consistent patterns of anticipatory muscle activity observed in healthy subjects provide a template against which to compare activity patterns of non-focal muscles for individuals with potential deficits in the control of standing balance.