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1.  Stability Control of Grasping Objects with Different Locations of Center of Mass and Rotational Inertia 
Journal of Motor Behavior  2012;44(3):169-178.
The objective of this study was to observe how the digits of the hand adjust to varying location of the center of mass (CoM) above/below the grasp and rotational inertia (RI) of a hand held object. Such manipulations do not immediately affect the equilibrium equations while stability control is affected. Participants were instructed to hold a handle, instrumented with five force/torque transducers and a 3-D rotational tilt sensor, while either the location of the CoM or the RI values were adjusted. On the whole, people use two mechanisms to adjust to the changed stability requirements; they increase the grip force and redistribute the total moment between the normal and tangential forces offsetting internal torques. The increase in grip force, an internal force, and offsetting internal torques allows for increases in joint and hand rotational apparent stiffness while not creating external forces/torques which would unbalance the equations of equilibrium.
PMCID: PMC3394692  PMID: 22456054
Grasping; Stability; Center of mass; Motor control
2.  Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration 
Journal of biomechanics  2011;45(2):218-224.
Radial force (Fr) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit Fr tasks, and have not normalized for anatomical variance when considering Fr distributions. The goals of the present study were (i) to explore Fr during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in Fr data. Twelve subjects grasped a vertically-oriented cylindrical handle (diameter = 6 cm) and matched target upward tangential forces of 10, 20, and 30 N. Fr data were measured using a flexible pressure mat with an angular resolution 4.8 deg, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that Fr was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers’ metatacarpal heads, and that, while increases in tangential force caused significant increases in Fr for these regions, they did not significantly affect the Fr distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated Fr trends, and amplification of the test statistic. These results imply that, while subjects focus Fr primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one’s ability to draw anatomically relevant conclusions from grasping force data.
PMCID: PMC3246034  PMID: 22134182
grasp force coordination; hand and finger biomechanics; curve registration; statistical parametric mapping
3.  Grip Forces During Object Manipulation: Experiment, Mathematical Model & Validation 
When people transport handheld objects, they change the grip force with the object movement. Circular movement patterns were tested within three planes at two different rates (1.0, 1.5 Hz), and two diameters (20, 40 cm). Subjects performed the task reasonably well, matching frequencies and dynamic ranges of accelerations within expectations. A mathematical model was designed to predict the applied normal forces from kinematic data. The model is based on two hypotheses: (a) the grip force changes during movements along complex trajectories can be represented as the sum of effects of two basic commands associated with the parallel and orthogonal manipulation, respectively; (b) different central commands are sent to the thumb and virtual finger (Vf- four fingers combined). The model predicted the actual normal forces with a total variance accounted for of better than 98%. The effects of the two components of acceleration—along the normal axis and the resultant acceleration within the shear plane—on the digit normal forces are additive.
PMCID: PMC3212984  PMID: 21735245
Grasping; manipulation; motor control; modeling
4.  Females Exhibit Shorter Paraspinal Reflex Latencies than Males in Response to Sudden Trunk Flexion Perturbations 
Females have a higher risk of experiencing low back pain or injury than males. One possible reason for this might be altered reflexes since longer paraspinal reflex latencies exist in injured patients versus healthy controls. Gender differences have been reported in paraspinal reflex latency, yet findings are inconsistent. The goal here was to investigate gender differences in paraspinal reflex latency, avoiding and accounting for potentially gender-confounding experimental factors.
Ten males and ten females underwent repeated trunk flexion perturbations. Paraspinal muscle activity and trunk kinematics were recorded to calculate reflex latency and maximum trunk flexion velocity. Two-way mixed model ANOVAs were used to determine the effects of gender on reflex latency and maximum trunk flexion velocity.
Reflex latency was 18.7% shorter in females than in males (P=0.02) when exposed to identical trunk perturbations, and did not vary by impulse (P=0.38). However, maximum trunk flexion velocity was 35.3% faster in females than males (P=0.01) when exposed to identical trunk perturbations, and increased with impulse (P<0.01). While controlling for differences in maximum trunk flexion velocity, reflex latency was 16.4% shorter in females than males (P=0.04).
The higher prevalence of low back pain and injury among females does not appear to result from slower paraspinal reflexes.
PMCID: PMC2878900  PMID: 20359800
Gender; Paraspinal; Reflex Latency; Spinal Stability Control; Trunk Perturbations; Kinematics; Low Back Pain; Low Back Injury; Female; Male

Results 1-4 (4)