Daily activities often require fast and accurate arm movements that can be performed for a long period without fatigue. Understanding which strategies result in proficiency of performance, and why, is extremely important for optimizing the efficiency of daily activities, ergonomic designs, and design of prosthetic devices. The present project uses an innovative approach that elaborates on directional biases in arm movements. Directional biases mean that arm movements performed in different directions relative to the body vary in terms of accuracy, speed, and the amount of muscle effort expended. Thus, it is possible that an increase in performance efficiency can be achieved by selecting optimal movement directions relative to the body, while moving in a variety of directions in space might be best achieved by adjusting the position of the body. Directional biases during arm movements and the conditions that influence them are explored using a novel, free-stroke drawing task in which subjects perform arm movements in as many different directions as possible in the horizontal plane. The effects of directional biases under the different movement conditions will be studied, as well as factors that may cause the emergence of directional biases. The project will generate important knowledge with respect to control strategies that make arm movements economical, accurate, fast, and resistant to fatigue. This knowledge may create a basis for a wide range of ergonomic and clinical applications, such as minimization of fatigue, prevention of injuries, work cost decreases, and development of advanced prosthetic devices and anthropomorphic robots.
Fundamental principles employed for neural control of multi-joint movements are in the core of human motor control research. A novel interpretation of these principles has been recently proposed by Dr. Dounskaia in the form of a leading joint hypothesis (LJH). The present project contributes to the development of the LJH, being driven by a prediction of this hypothesis that interaction torque (a passive torque emerging at the joints during motion due to mechanical interaction of limb segments) is exploited to decrease muscle torque for production of arm movements. Specifically, the LJH predicts a preference to perform two particular movement types during which muscle torque used for regulation of the effect of interaction torque is minimal. This prediction is reformulated for movements performed with planar rotations at the shoulder and elbow as preferences to move the hand in two specific directions. This prediction was tested with the use of a novel, free-stroke drawing task that provided freedom in the choice of movement direction for each stroke. This task required participants to make straight strokes on a table surface from a circle center to the perimeter, choosing stroke directions in a random order. Preliminary data had demonstrated that this task successfully revealed directional biases even if participants were encouraged to uniformly distribute strokes across movement directions. Thus, the prediction of the LJH with respect to the tendency to exploit interaction torque for movement production brought us to a novel, largely unexplored phenomenon of directional preferences of arm movements. Studying directional biases is important because, in addition to developing the LJH, conditions for efficient performance with minimal expenses for movement production may be revealed. This project provided comprehensive examination of the directional preferences by pursuing two specific aims: (1) to investigate behavior of the directional biases under various movement conditions (depending on movement speed, vision, load in the hand, etc.); and (2) to investigate factors that may cause the emergence of directional biases, including the interaction torque factor predicted by the LJH. The project has produced important contributions to several key issues in motor control research. First, the proposed research has created new knowledge with respect to basic principles of multi-joint movement control by contributing to the development of the LJH. Second, a novel approach has been developed to examine factors that influence planning of arm movements, and a number of potential factors has been tested. Third, the project has contributed to the development of the optimal control approach to human movements by creating a method capable to validate hypothesized, direction-dependent optimization criteria. Fourth, a method for investigation of complex, self-organized movement sequences and factors governing the process of self-organization has been developed. Collectively, the project has substantially advanced knowledge of fundamental principles of neural control of multi-joint movements. It has also contributed to the validation of the existing theories of motor control. Immense broader impact from the performed research is expected because the project has provided knowledge about conditions that increase efficiency of human motor performance (e.g. in terms of speed and accuracy) while decreasing muscle effort for movement production. This knowledge will be essential for various types of human activity involving arm movements. In particular, the results can be used for optimal organization of the working environment, prevention of injuries and fatigue, and work cost decreases. Retraining individuals to use movements in which limb properties facilitate performance can alleviate the deficiencies caused by fatigue and various medical conditions (e.g., spinal cord injury) and promote retention of individuals with arm disabilities. The obtained findings with respect to factors influencing performance of arm movements can be exploited to determine changes in these factors caused by development, aging, and movement pathologies. Also, the revealed biological principles of efficient movement control can potentially promote development of assistive robotic devices, advanced prostheses, and anthropomorphic robots. The project has provide training to students participating as research assistants and experiment participants. The results have been incorporated into both undergraduate and graduate Motor Control courses at ASU. Broad publication and presentation of the results has been provided.
