Joint angle variability and co-variation in a reaching with a rod task

• Published in: Experimental brain research Vol. 208; no. 3; pp. 411 - 422
• Main Authors: van der Steen, M. (Marieke) C, Bongers, Raoul M
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Berlin/Heidelberg : Springer-Verlag 01-02-2011; Springer-Verlag; Springer; Springer Nature B.V
• Subjects: Arm - physiology; Biological and medical sciences; Biomechanical Phenomena - physiology; Biomedical and Life Sciences; Biomedicine; Elbow Joint - physiology; Finger Joint - physiology; Fingers; Fundamental and applied biological sciences. Psychology; Human mechanics; Humans; Joint variability; Joints - physiology; Male; Medical sciences; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor coordination; Movement - physiology; Neurology; Neurosciences; Ophthalmology; Physiological aspects; Psychomotor Performance - physiology; Range of Motion, Articular - physiology; Reaching; Research Article; Shoulder Joint - physiology; Synergy; Tool use; Uncontrolled manifold (UCM); Variables; Vertebrates: nervous system and sense organs; Vision disorders; Wrist Joint - physiology; Young Adult; Netherlands; Reaching; Synergy; Motor coordination; Tool use; Uncontrolled manifold (UCM); Joint variability; Human; Freedom degree; Coordination; Rod; Goal directed movement; Joint; Motor control; Hand; Synergism; Osteoarticular system
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-010-2493-y

The problem at the heart of motor control is how the myriad units of the neuromotor system are coordinated to perform goal-directed movements. Although for long these numerous degrees of freedom (DOFs) were considered redundant, recent views emphasize more that the DOFs should be considered abundant, allowing flexible performance. We studied how variability in arm joints was employed to stabilize the displaced end-effector in tool use to examine how the neuromotor system flexibly exploits DOFs in the upper extremity. Participants made pointing movements with the index finger and with the index finger extended by rods of 10, 20, and 30 cm. Using the uncontrolled manifold (UCM) method, the total joint angle variance was decomposed into two parts, the joint angle variance that did not affect the position of the end-effector (V UCM) and the variance that results in a deviation of the position of the end-effector from its mean (V ORT). Analyses showed that some angles depended on length of the rod in use. For all rod lengths, V UCM was larger than V ORT, and this did not differ over rod lengths, demonstrating that the arm was organized into a synergy. Finally, the variation in the joint angles in the arm as well as the degree of co-variation between these angles did not differ for the rod's tip and the hand. We concluded that synergies are formed in the arm during reaching with an extended end-effector and those synergies stabilize different parts of the arm+rod system equally.