Muscle activation patterns for voluntary isometric stiffness in human index finger

Humans can voluntarily control their finger stiffness for grasping and manipulation tasks. A long standing question in biomechanics is aimed at understanding the criterion used by the cenrtal nervous system to control the motor output of human limbs. Humans are known to voluntarily control their limb posture, end-tip force and stiffness. This thesis concerns itself with controlling stiffness in isometric conditions only. This work examines the variability of voluntary isometric stiffness modulation for the index finger at constant fingertip force. Previous studies either investigated muscle synergies responsible for different force-stiffness conditions or only measured behavioral measures of stiffness. However, the variability of stiffness for constant force condition was not explained. In this study, the stiffness of the index finger was modulated while maintaining a constant isometric fingertip force at 4 different force magnitudes and 2 different force directions. The muscle activations of 7 muscles that are related to the index finger were measured using surface electromyography (sEMG) sensors. Synergies estimated from a principal component analysis (PCA) using the recorded sEMG showed that the contribution of one synergy explains 80-95% of the variation in the data. The degree of alignment was used to analyze these stiffness synergies for different force conditions. The minimum mean value of degree of alignment was found for the comparison between synergies at high forces. However, comparison of synergies at lower forces showed that the stiffness synergy varies more with the forcing direction than it does with force magnitude. These results show the existence of a stiffness synergy to modulate the stiffness for individual force direction regardless of the magnitude of force level. Although the results at higher forces do not agree with this conclusion, stiffness modulation is prominent only at lower forces. This result gives an insight into what muscle synergies are important for modulating the fingertip stiffness. It can prove useful in robotics applications to simplify stiffness modulation without explicitly calculating inverse kinematics and also in restoring stiffness modulation after hand injury