Analysis of Hand Intra-Finger Couplings During Flexion Movements in the Free Space

The anatomy of the human hand is characterized by intrinsic coupling mechanisms at the level of the tendons and bone structure. The intra-finger constraints, in particular, represent coupled movements of the joints of the same finger. Previous studies verified the existence of intra-finger couplings for circular and prismatic grasps, and hypothesized the existence of such couplings for free flexion-extension movements of the fingers without, however, quantifying them. The aim of this work was: i) to calculate subject-specific intra-finger couplings during flexion movements in the free space by exploiting a marker-based motion capture system and a validated kinematic protocol to guarantee high accuracy of the reconstructed hand kinematics, ii) to understand the effect of the hand size and of the finger on the coupling relations, and iii) to establish generalized coupling coefficients that could be used to simplify the kinematic hand model. To this purpose, ten healthy subjects performed flexion-extension movements of the fingers. Subject-specific couplings were extracted through linear regression analysis on pairs of adjacent joint angle trajectories: proximal couplings represented the relation between the Proximal-Inter-Phalangeal and MetaCarpo-Phalangeal joints for the long fingers and between the MetaCarpo-Phalangeal and Carpo-MetaCarpal joints for the thumb, whereas distal couplings represented the relation between the Distal-Inter-Phalangeal and Proximal-Inter-Phalangeal joints for the long fingers and between Inter-Phalangeal and MetaCarpo-Phalangeal joints for the thumb. The subject-specific coupling coefficients were independent from the hand size, and a difference between the distal couplings of the thumb and of the index, middle and ring fingers was highlighted. Regression analysis on the average flexion trajectories calculated on the ten participants showed a linear trend for both proximal and distal couplings (
$R^{2}>0.97$
) and small Root Mean Square Errors (1.63 deg on average). Coupling coefficients ranged 1.4 – 1.9 and 0.7 – 0.9 for the proximal and distal couplings respectively. Given its distinctive kinematic structure, the thumb exhibited a particular behaviour, as its proximal and distal couplings were the same. The extracted couplings represent normative coupling values on a population of ten individuals. The obtained results suggest the possibility of simplifying the kinematic hand model by imposing linear relations between the joints of each finger, thus reducing the number of independent degrees of freedom to one for each finger. This could be used to define input design parameters for the development of biomimetic hand prostheses and exoskeletons.