01921nas a2200181 4500008003900000245007200039210006900111260004900180300001600229520133600245653001201581653001301593100001901606700001701625700001401642700001501656856006801671 2015 d00aEffect of Homogenous Object Stiffness on Tri-digit Grasp Properties0 aEffect of Homogenous Object Stiffness on Tridigit Grasp Properti a Milano, Italy, August 25th-29th, 2015bIEEE a6704 - 67073 a
This paper presents experimental findings on how humans modulate their muscle activity while grasping objects of varying levels of compliance. We hypothesize that one of the key abilities that allows humans to successfully cope with uncertainties while grasping compliant objects is the ability to modulate muscle activity to control both grasp force and stiffness in a way that is coherent with the task. To that end, subjects were recruited to perform a grasp and lift task with a tripod-grasp device with contact surfaces of variable compliance. Subjects performed the task under four different compliance conditions while surface EMG from the main finger flexor and extensor muscles was recorded along with force and torque data at the contact points. Significant increases in the extensor muscle (the antagonist in the task) and co-contraction levels were found with increasing compliance at the contact points. These results suggest that the motor system may employ a strategy of increasing cocontraction, and thereby stiffness, to counteract the decreased stability in grasping compliant objects. Future experiments will examine the extent to which this phenomenon is also related to specific task features, such as precision versus power grasp and object weight.
 
10aHaptics10aRobotics1 aGodfrey, S. B.1 aAltobelli, A1 aRossi, M.1 aBicchi, A. uhttp://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=731993105694nas a2200193 4500008004500000245008600045210006900131260004300200300001200243520507000255653001205325653001305337100001405350700001705364700001905381700001705400700001505417856006805432 2015 engldsh 00aElectromyographic Mapping of Finger Stiffness in Tripod Grasp: a Proof of Concept0 aElectromyographic Mapping of Finger Stiffness in Tripod Grasp a  aSingapore, 11 – 14 August 2015bIEEE a181-1863 a

Effective execution of a manipulation task using prosthetic or robotic hands requires that the motion and the impedance profiles of the fingers be appropriately commanded. This, however, brings some design and control challenges regarding the individual planning and realization of the finger motion and stiffnesstrajectories. It appears that the central nervous system solves for this complexity in an effective and coordinated manner which has been well-recognized under the concept of hand synergies. While the exploitation of this concept in kinematic coordinates has lead to the development of several successful robotic designs and control strategies, its extension to dynamic coordinates, such as coordinated stiffening of the fingers, remains to be investigated. Indeed, in this study we provide preliminary evidence on the existence of such coordinated stiffening patterns in human fingers and establish initial steps towards a real-time and effective modelling of the finger stiffness in a tripod grasp. To achieve this goal, the endpoint stiffness of the thumb, index and middle fingers of five healthy subjects are experimentally identified and correlated with the electromyography (EMG) signals recorded from a dominant antagonistic pair of the forearm muscles. Our findings suggest that: i) the magnitude of thestiffness ellipses at the fingertips grows in a coordinated way, subsequent to the co-contraction of the forearm muscles; ii) the length of the ellipses' axes appears to have a nearly linear relationship with the co-contraction level of the antagonistic muscle pair.

10aHaptics10aRobotics1 aRossi, M.1 aAltobelli, A1 aGodfrey, S. B.1 aAjoudani, A.1 aBicchi, A. uhttp://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7281196