Position-Dependent Characterization of Passive Wrist Stiffness

Abstract

Because the dynamics of wrist rotations are dominated by stiffness, understanding wrist rotations requires a thorough characterization of wrist stiffness in multiple degrees of freedom. The only prior measurement of multivariable wrist stiffness was confined to approximately one-seventh of the wrist range of motion (ROM). Here, we present a precise nonlinear characterization of passive wrist joint stiffness over a range three times greater, which covers approximately 70% of the functional ROM of the wrist. We measured the torque-displacement vector field in 24 directions and fit the data using thin-plate spline smoothing optimized with generalized cross validation. To assess anisotropy and nonlinearity, we subsequently derived several different approximations of the stiffness due to this multivariable vector field. The directional variation of stiffness was more pronounced than reported previously. A linear approximation (obtained by multiple linear regression over the entire field) was significantly more anisotropic (eigenvalue ratio of 2.69 ± 0.52 versus 1.58 ± 0.39; ) though less misaligned with the anatomical wrist axes (12.1 ± 4.6° versus 21.2 ± 9.2°; ). We also found that stiffness over this range exhibited considerable nonlinearity-the error associated with a linear approximation was 20-30%. The nonlinear characterization over this greater range confirmed significantly greater stiffness in radial deviation compared to ulnar deviation. This study provides a characterization of passive wrist stiffness better suited to investigations of natural wrist rotations, which cover much of the wrist's ROM. It also provides a baseline for the study of neurological and/or orthopedic disorders that result in abnormal wrist stiffness.

The Impact of Aging and Hand Dominance on the Passive Wrist Stiffness of Squash Players: Pilot Study (Preprint)

“…It is therefore likely that both differences in sporting activities as well as the biomechanics of joints will limit the comparison of our study results. Instead, an evaluation of the magnitude of quasi-stiffness in this study (of the young players) when compared to similar studies using unskilled subjects [15,17] reveals that passive wrist stiffness is slightly higher for competitive squash players. This trend in passive wrist stiffness is likely attributed to the greater cross-sectional area of the surrounding forearm muscles and tensile strength of the passive wrist structures (ligaments, tendons, and bone geometry), which are due to increased mechanical loading of the wrist during frequent, repetitive, and intensive squash play [32][33][34][35].…”

“…The inclusion criteria were as follows; i) male, ii) 18 years or older, iii) no prior wrist surgery or injury, iv) English speaking, and v) a minimum of five years playing experience. Only male subjects were recruited to reduce the risk of data variability, as previous work had demonstrated a difference in the magnitude and direction of wrist quasistiffness between the male and female sex [14][15][16][17]. Subjects were asked not to participate in any upper-limb exercise in the 24-hours preceding their evaluation session to decrease the risk of reduced wrist range of motion (ROM) due to muscle swelling [11] and muscle thixotropic behavior [23] associated with eccentric wrist exercise.…”

“…The increased mechanical loading of the wrist associated with squash play may also explain the significant difference in the orientation of wrist quasi-stiffness between the unskilled and young squash playing groups. Nonetheless, as there is inconsistency in the orientation of wrist quasi-stiffness reported in the literature, it is possible that both differences in study subject's physical activities and study methodology contribute to the variance seen [14][15][16][17].…”

“…Previous studies have investigated the impact of study methodology [15][16][17], sex [14][15][16][17], and hand dominance [17]. A recent study [17] demonstrated that there was increased passive wrist quasi-stiffness in the dominant compared to the non-dominant upper-limb of healthy young men and women.…”

The effect of common wrist orthoses on the stiffness of wrist rotations

“…Our experimental setup and protocol followed a recent measurement of wrist stiffness without orthoses with minor adaptations (described subsequently) to accommodate the orthoses [13]. All measurements were performed using a wrist rehabilitation robot (InMotion Wrist Robot, Interactive Motion Technologies; Water town, Massachusetts) that rotated subjects' wrist in com binations of flexion-extension (FE) and radial-ulnar deviation (RUD) [14].…”