An Experimental Apparatus Design For Research On Finger Force Coordination Patterns Of Human Hand

Human hand is the most dexterity and sophisticated movement organ. Through the cooperation of fingers and the palm, human hand can complete all kinds of complex movements. Finger force is the mechanical information produced by each finger at the fingertip during hand interaction with the outside world. Detection of the finger force and research of its coordination patterns, as an integral part of human hand movement analysis, has an important academic and practical value in the field of medical rehabilitation engineering and bionic-body developing.This article developed an experimental system, including hardware apparatus and software interface, to study the mechanism of finger force coordination from the perspective of the human hand movement itself and from the behavioral science point of view. The hardware apparatus includes hardware circuits and mechanical devices. The hardware circuit is composed by the force measurement sensors, amplifiers and a data acquisition card, respectively, to complete the signal extraction, amplification and analog-digital conversion of the finger force information. LAA/N resistance strain sensors, provided by Nuosheng Company in Xiamen are chosen as the force measurement sensors. The amplifying circuit is mainly composed by the instrumentation amplifiers of AD620. USB6008 produced by NI Company is chosen for data acquisition. The mechanical devices are constructed by the sensor-fixing components and a support base, whose function is to provide sensors installed base and to use the slider and glides to adjust sensors'locations of the platform plane to adapt to different people's hand shapes. System software was developed in LabVIEW virtual instrument environment, which includes hardware device calibration module, finger force bias measurement module, and finger force production experiment module. The software was used to record and store the experimental data, at the same time to provide easy-to-use interface to experiment operators and to provide visual feedback to the subjects.This article conducted performance test of the above experimental apparatus, including the repeatability and linearity test, and the results show that the measuring apparatus has good repeatability (with a maximum relative variance of 2.041%) and linearity (with a maximum RMS error of 0.0059 N). After determining the effectiveness of the system, it was used in a preliminary experiment for studying the finger force coordination pattern. The experiment attempts to obtain experiment regular laws of finger force coordination pattern by measuring and analyzing the force distribution in various experiment phases of different levels of total force production tasks performed by the four fingers (not including the thumb). Seven healthy volunteers participated in the experiment. The subjects were asked to press the surface of the measuring device with all fingers being exerting force at the same time and were provided with a visual feedback of their total force. Then they coordinated their finger force outputs, after 2-second preparatory phase, to bring the total force line to reach the three target force levels (for 5N, 10N and 15N) in a linear increase manner within 1s, and to maintain the same level of force for 5s, and then to drop to the original level in a linear decrease manner in the same time of 1s, and finally to relax for 1s. The forces produced by each finger of the subjects were recorded, and the contribution of each finger to the total force was calculated.The results showed that, in all the subjects'three levels of total force production tasks, the force percentage of the index finger rose in the ramp phase, and this rising trend has continued until the maintaining phase and reached its peak in the maintaining phase. It dropped slightly in the declining phase, and re-restored to the proportion of the initial preparation phase in the relaxing phase. Percentage change trend of the middle finger is similar to the index finger, but is not as significant as the index finger. The force percentage change trend of the ring finger and little finger are in converse with the index finger. They declined in the ramp phase and reached the minimum value in the maintaining phase, while in the declining and relaxing phase, they showed a rising trend. The force percentage of the little finger changed more obviously. The results suggest that the motor nervous system can precisely coordinate the force distribution pattern of the fingers according to the different tasks performed, and the pattern change rule to some extent reflected the nervous system's control strategy or mechanism.