Timing of muscle activity during reaching while standing: systematic changes with target distance
Introduction
It is well established that muscles far removed from the focal muscle group become active during arm movements that are performed while standing [1], [2], [3], [4], [5], [6]. Some of the observed electromyographic (EMG) activity is anticipatory, i.e. it occurs in advance of the focal movement or focal muscle EMG. The anticipatory activity is presumed to play a postural role in that it prevents one from falling due to the perturbation caused by the arm movement [7]. That perturbation is a combination of the net horizontal displacement of the center of mass and the motion-dependent forces generated by the multitude of linked body segments.
Non-focal muscle activity is influenced by a number of reaching task parameters, such as reaching speed [5], [8], reaching direction [1], [4], [9], and inertial load [1], [3], [4]. Changes in these task parameters affect the magnitude and direction of the forces that perturb balance. As the direction and speed of reaching movements are varied, the non-focal muscle activity also changes.
With changes in reaching distance, too, the challenge to standing balance is varied. As reaching distance is increased, displacement of the whole body center of mass is increased [10], [11]. In addition, when the target is placed beyond arm’s length (AL), the trunk must experience a net displacement for the hand to reach the target, whereas for shorter distances, the trunk is not obligated by configuration to move.
Kaminski and Simpkins [10] made comparisons of displacements of center of pressure and center of mass across a range of reach distances. They asked nine healthy adults to stand and perform reach-and-grasp movements to a target (dowel) that was placed in front at approximately hip height. Their results support the notion that as target distance is increased beyond arm’s length, the non-focal segments act more to move the arm to the target and not solely to resist the perturbation to standing balance generated by the arm movement. Nevertheless, the kinematics of the hand remain qualitatively similar across target distance [12], [13], [14]. Kinematics have been evaluated for reaching to various target distances while sitting [12], [14] and while standing [10]. EMG patterns have been reported by Stapley et al. [11] for a whole body reaching task performed from a standing position. They asked six healthy subjects to reach, grasp, and lift an object that was placed on the floor in only two locations. We used a reaching task to multiple targets placed at shoulder height. We quantified the timing of non-focal EMG activity to characterize the manner in which the nervous system executes reaching while standing when target distance is varied in a systematic manner. By varying target distance, the challenge to standing balance is modulated by changing the magnitude of the balance perturbation and by changing the required net displacement of body segments other than the arm to successfully perform the task.
By quantifying the onsets of non-focal anticipatory muscle activity during a rapid, bilateral reaching task to varying target distances, we can address two questions. First, do changes in target distance from within arm’s length to beyond arm’s length result in consistent changes in the onset of non-focal anticipatory muscle activity of the trunk and legs in healthy subjects? If so, these patterns may serve as a template for comparison with the anticipatory muscle activity in persons with neurological conditions that may impair control of standing balance. Second, do changes in onsets of all non-focal muscles vary in a similar fashion in response to varying target distance? Similar changes in onsets of all non-focal muscles would suggest a constant postural role for those muscles across target distance. Conversely, qualitative differences in the onset patterns between non-focal muscles would suggest that such activity does not occur solely to resist a postural perturbation. We collected kinematic data of the arm and center of pressure data to provide some characterization of the modulation of the postural perturbation with target distance. A preliminary report of these results has been published in abstract form [15].
Section snippets
Subjects
Thirteen healthy subjects participated: seven female (24.3±0.7 years old) and six male (24.3±1.8 years old). Prior to participation in this study, all subjects provided informed, written consent in accordance with requirements of the Institutional Review Board at the University of Nebraska Medical Center.
Task
Subjects stood barefoot on a force platform with arms at the side of the body. From that position, they performed bilateral reaching movements to a visible target placed in front of them at
EMG onsets as target distance was varied
Most of the non-focal muscles displayed consistent EMG patterns relative to the onset of arm movement that varied in a progressive fashion as target distance was varied from within arm’s length to beyond arm’s length. These patterns are apparent in the averaged traces from a single subject in Fig. 3, and in the mean EMG onsets from all subjects in Fig. 4. Analysis of variance demonstrated a significant effect of target distance on the onsets of ES, TA, VL and SO (P<0.007).
As target distance was
Discussion
In this study, we used a reaching paradigm to evaluate the influence of target distance on onsets of EMG activity in non-focal muscles. We aligned temporal events relative to the onsets of arm movement and then averaged data over several trials. This process provided consistent, identifiable onsets of EMG activity. These systematic modulations were observable in single trials (Fig. 1), in averages over multiple trials (Fig. 2, Fig. 3), and in the mean onsets from all subjects (Fig. 4). The
Acknowledgements
We thank Dawn Dvorak, Pat Kearns, Wendy Roth, and Korie Cochran for their assistance with data collection and analysis. This study was funded by the School of Allied Health Professions, University of Nebraska Medical Center, Omaha, NE.
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