Posture and visuomotor performance in children : the development of a novel measurement system

The aim of this thesis was to develop and test a platform which was capable of measuring the developmental trajectory of postural stability and fine motor control. Moreover, the thesis set out to explore the interdependence of these motor processes through synchronous measurement of postural and fine-motor control processes. The thesis introduces an objective, fine-motor measure sensitive enough to detect gender differences in children. This system was developed further to incorporate measures of postural sway, providing objective measures of postural performance that were capable of detecting age-dependant task-based manipulations of postural stability. Further development of the platform to incorporate low-cost consumer products allowed the cost barrier to large-scale measurement of posture to be addressed. This meant that accurate, synchronous and objective measurement of postural control and fine-motor control could take place outside of the laboratory environment. The developed system was deployed in schools and this allowed an investigation into the effect of seating on postural control. The results indicated that (a) seating attenuates the differences in postural control normally observed as a function of age; (b) postural control is modulated by task demands. Finally, the relationship between postural control and fine-motor control was investigated an interdependent functional relationship was found between manual control and postural stability development

Predicting the Effect of Surface Texture on the Qualitative Form of Prehension

Reach-to-grasp movements change quantitatively in a lawful (i.e. predictable) manner with changes in object properties. We explored whether altering object texture would produce qualitative changes in the form of the precontact movement patterns. Twelve participants reached to lift objects from a tabletop. Nine objects were produced, each with one of three grip surface textures (high-friction, medium-friction and low-friction) and one of three widths (50 mm, 70 mm and 90 mm). Each object was placed at three distances (100 mm, 300 mm and 500 mm), representing a total of 27 trial conditions. We observed two distinct movement patterns across all trials—participants either: (i) brought their arm to a stop, secured the object and lifted it from the tabletop; or (ii) grasped the object ‘on-the-fly’, so it was secured in the hand while the arm was moving. A majority of grasps were on-the-fly when the texture was high-friction and none when the object was low-friction, with medium-friction producing an intermediate proportion. Previous research has shown that the probability of on-the-fly behaviour is a function of grasp surface accuracy constraints. A finger friction rig was used to calculate the coefficients of friction for the objects and these calculations showed that the area available for a stable grasp (the ‘functional grasp surface size’) increased with surface friction coefficient. Thus, knowledge of functional grasp surface size is required to predict the probability of observing a given qualitative form of grasping in human prehensile behaviour

Object geometric properties friction-dependant functional grip area.

<p><i>Upper</i> Geometric variation in stimulus sizes: Grip surface width ‘A’, the distance between the spherical surface centre-points ‘B’ and support base width ‘C’ were varied as discussed in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032770#s2" target="_blank">Method</a> section. <i>Lower a)</i> Manually securing an object requires the frictional force to be greater than the tangential component of object weight at the interface between fingertip and object. A curved surface results in a normal reaction force direction (R_N) unique to the point at which the object is grasped. Fearing <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032770#pone.0032770-Fearing1" target="_blank">[14]</a> demonstrated that, for a stable grasp, the grip conditions should satisfy: tan⁻¹|F_t|/F_n−1μ or μF_n>|F_t|. For a stable lift, fingertip force should be applied within an angle of φ_s relative to the normal reaction force (R_N), where: φ_s = tan⁻¹μ_s. Extending this relationship in the direction of all tangential friction force directions generates a cone of friction of half-angle φ_s and cone angle ψ where: ψ = 2 φ_s. <i>b)</i> As force is applied to the curved surface at a distance d_LIM from the centreline of the radius, then the force is at an angle α to the surface normal. When α = φ_s the force lies at the limit of the cone of friction. An increase in d results in the force lying outside the cone of friction and unstable grasp. Thus φ_s, and d_LIM are linked to the coefficient of static friction μ_s such that an increase in μ_s extends the functional area which can be grasped to achieve a stable grasp.</p

Kinematic profiles for stop and ‘on-the-fly’ prehension movements.

<p><i>Upper</i> A velocity profile typical of a stop movement: 1, the hand is in the transport phase with the wrist IRED reaching peak velocity. 2, as the hand and fingers approach the object the hand velocity drops below the threshold velocity (Vth) and remains below threshold velocity or stops for a period (T_DW). 3, upon successful application of the grip, both the wrist and object markers move in unison as part of a second distinct movement. 4, movement complete – hand and object velocity tends to zero. Time to Peak Speed (tPS) is defined as the time between the wrist marker moving above Vth and achieving peak speed. Movement time is defined as the time elapsed between the wrist marker achieving Vth and the object marker achieving Vth, here represented in the stop movement scenario. <i>Lower</i> A velocity profile typical of a ‘fly-through’ movement: 1, the hand is in transport phase toward the object. 2, as the fingers contact the object, the wrist IRED velocity is maintained above the threshold velocity (Vth) as the object is gripped. 3, the hand and object continue to move in unison while the wrist IRED velocity remains above the threshold velocity. 4, movement complete, hand and object velocity tends to zero. Movement time is defined as the time elapsed between the wrist marker achieving Vth and the object marker achieving Vth, here represented in the fly-through movement scenario.</p

Proportion of ‘on-the-fly’ movements as a function of surface texture.

<p>The mean coefficient of static friction was 1.31, 0.76 and 0.44 for the high, medium and low friction object surface textures respectively (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032770#s2" target="_blank">Methods</a>).</p